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In our previous articles, we looked at different certifications such as CSA certification, and NFPA 70: certification. Today we will dive deep into UL certification.

There are times when seeing a logo or mark on a particular product gives you renewed belief and confidence in its authenticity. The truth is that these aren’t just seals or logos. They convey useful information and tell us that a particular product or component has been fully certified.

Some builder and manufacturers aren’t sure of the importance of this seal and why they need them. The truth is that it is very difficult to blame them. They simply don’t know what these seals stand for and that is what we will be addressing in this article. We would be opening the eyes of manufacturers on how these seals benefit them and what they actually mean.

The truth is that it isn’t inexpensive to certify a product. However, it is important to state that a certified product is one that has been seriously tested for safety and security. If you want to maintain the loyalty of your customers and workforce, do not compromise on the safety and security of your products. It is often the difference that exists between catastrophic failure and long-term success.

It is not uncommon to see UL marks on home appliances and industrial equipment such as electrical panels, furnaces, alarm systems, glass, sprinklers, fire extinguishers, circuit breakers, and some other products and components.

It isn’t every product that will appear on this list but there is an incentive to do so. In this article, we will be discussing the importance of UL certification for both businesses and consumers. We will also be explaining how UL Recognized and UL Listed are different so that you can make informed decisions.

What does UL Certification mean and why is it important?

UL is an acronym for Underwriters Laboratories. It is a third-party certification company founded in 1894. UL was founded in Chicago and they are responsible for ensuring the safety and security of products in the manufacturing industry. They so not only test and certify products, they also set industry standards that manufacturers and builders must comply with when innovating new products. For instance, about 14 billion UL marked products made it into the global marketplace last year. UL is basically a safety organization responsible for setting standards for new products in the industry. They check and test these products to ensure compliance with the set standards. When a product is properly tested, things like proper wire size and maximum permissible current are confirmed. UL testing also helps to confirm that the building and construction of a product follows the best safety standards. 

There is a misconception that the Underwriters Laboratories tests all products by themselves. Actually, they don’t. They sometimes give a builder or manufacturer the authority to test the product by themselves using the UL stamp. Their role will simply be to follow up with this manufacturer to ensure proper guidelines are followed and complied with. This is one of the benefits of UL certification to businesses and manufacturers.

UL approval is broken down into various tiers. Actually, there are three tiers – UL listed, UL classified, and UL recognized. Although there is so much similarity between these tiers, some differences still exist. Knowing these basic distinctions will help you to figure out what is best for your business. It will also give you insight if you are just a consumer of UL products. Therefore, knowing the difference is very important.

In this text, we will focus on the two most basic differences you should know about, i.e. UL listed vs. recognized.

UL Recognized, what does it mean?

The fact that there is no seal of approval to end products is the biggest difference existing between UL recognition and UL classified or certified. The focus of UL Recognition Service is more on parts and machinery that make other products. Therefore, UL Recognition Service is less consumer-facing. In simpler terms, what UL Recognition service does is to certify that a component that exists within a larger mechanism meet UL standards. It is commonly found in factories as circuit boards or power supplies for powering other machinery. It guarantees the safety and efficiency of machinery that workers and employees use. It also gives companies the necessary impetus to strive for more sustainable practices. Since UL Recognized focuses on components, Underwriters Laboratories ensure the equipment is properly housed. To shield the product from chemicals or liquids during the processing of production and manufacturing, additional installation precautions are put in place. It is relatively easier to attain UL Recognized stamps than UL Listed stamps. The reason is that manufacturers are at liberty to choose which components are certified within a bigger system. You can apply UL Recognition certification to any component part. It can also be applied to barrier materials. You would have to get a UL Listed approval seal if you want the UL logo on a stand-alone product.

UL Listed, what does it mean?

UL Listed is a very common term. The seal is very recognizable and can be found in so many consumer goods. The main difference between UL Recognized and UL Listed isn’t in the certification itself. The discrepancy is in the type of product being certified as well has the impression the consumer has of it. While UL Listed is for standalone products, UL Recognition is for components. Although they are both industry standards for safety and security, they are not used for the same products and they aren’t used in the same capacity. This explains why it is easier for people to recognize UL Listed since it ensures the safety of consumer-ready products that can be found in the market. UL Listed certification involves more testing. Therefore any product with this seal has been rigorously tested by UL and has been approved to meet nationally acceptable safety and sustainability standards. Also, a product with this seal is reasonably free from risk of fire and electric shock. It also guarantees the longevity and safety of household items under normal wear and tear subject to frequent use. This is of immense benefit to both the business and consumer.

Difference between CE and UL certification

There are some logo seals that are similar to the UL in other countries and one of such is CE. CE is an organization that sets standard for the electronic industry in the EU. CE is an acronym for” Comformite Europeenne” which is a French word that translates to “European Conformity”. The only difference between UL and CE is the country it is old or manufactured. With CE, companies can sell anywhere within the European Union.

Why should I buy UL products or get UL certified?

UL certification and recognition is attractive for businesses because it gives consumers renewed confidence in the product when they see the UL stamp on it. For instance, a consumer may decide to buy a circuit breaker because it carries the UL mark.

Due to UL certification, a company can innovate its safety standards and adopt industry best practices. When a manufacturer is certified by UL, it shows that the business is committed and dedicated to the safety of its final product consumers. UL certification is also useful for insurance purposes.

In our previous articles, we looked at different regulators and standards in the electric industry. We looked at the CSA certification, briefly analysed the NFPA 70 electrical industry code, and discussed the CSSR. Let us explore the Underwrites Laboratories.

Underwriters Laboratories is a regulatory and compliance company within the manufacturing industry saddled with the responsibility of setting rigorous standards for parts and components to meet. The standards set by UL ensure security, quality, confidence, and safety at every manufacturing stage.

There are guidelines that specify the type of tests to be conducted and metrics to be met. This gives employees total control over monitoring and regulating performance level.

In this article, we will be analysing UL standards for industrial electrical controls.

UL 508A

Since it was confirmed as a regulatory body, underwriters’ laboratories have come up with over 800 safety standards. These industry requirements are broken down into categories so that the search process is easier and simpler. The UL standard for electrical controls is outlined in the UL 508A section. This section outlines parameters for controlling and designing control panels so that they can be used in the industry. There are two ways of installing control panel. It can either be installed inside an enclosure which will make it“enclosed type” equipment. The panel can also be arranged onto a mounting panel which makes it“open type” equipment. This will look such that the assembly will be positioned inside an enclosure. Some common assumptions for both methods of installation are:

• They will be installed by users in ordinary locations
• The voltage the system will operate from wouldn’t exceed 1,000 watts.
• The installation location’s ambient temperature is 104^oF or less.
• The installation will follow guidelines from the National Electric Code

UL 508A isn’t limited to basic industrial control equipment alone, it also covers industrial control panel enclosures and control panels that are made specifically to provide support for flame safety supervision over the equipment types listed below:

• Fountain control
• Combustible fuels
• Crane controls
• Hoist controls
• Elevator controls
• Refrigeration equipment
• Irrigation systems control
• Load management components
• Air conditioning equipment

Industrial machinery such as metalworking machine tools, power press controls, and plastic injection moulding machinery are also covered in the standards.

As far as industrial control panel equipment are concerned, those installed with the aim of providing control of permanent electrical equipment found in the systems outlined below are fully covered.

• Irrigation systems
• Automatic car washes
• Pulp and paper processing
• Packaging material handling systems
• Residential and commercial swimming pools and spas found in-ground
• Water and wastewater treatment facilities
• Burner and burner management systems

Requirement of components

The assembly of two or more power circuit components are included in industrial control parts found under 508A. These may be:

• Terminals
• Enclosures
• Motor controllers
• Wiring
• Circuit breakers
• Fused disconnect switches
• Overload relays
• Timers, buttons, controllers, and switches

They may also be made up of control circuit components like control relays, push-buttons, pilot lights, etc. The assemblies may also be made up of a combination of power and control circuit components, as well as other associated terminals and wiring.

Although, 508A covers an extensive list of components and control panels, it doesn’t cover evaluation of equipment controlled by the controlled panels such as:

• Heaters
• Motors
• Lighting
• Loads linked to other circuits

Furthermore, consideration and evaluation of the control panel as a separate unit is done. It is also important to state that the evaluation by Underwriters Laboratories doesn’t cover the adequacies of the protective devices and controls contained in the control panel. These components are designed and saddled with the responsibility of supervising and monitoring the performance of equipment or controlled loads. In this instance, there is a UL standard for the complete piece of utilization equipment that users are expected to comply with.

Pre-fabricated structures, buildings, as well as other platforms supplied with industrial control panels are not covered under UL 508A.

Structure of the standard

There are four sections included in the full UL 508A. They include:

Part 1: General Use

Part 2: Specific Use

Part 3: Specific Component Requirements

Part 4:  Short Circuit Current Ratings

Related standards by Underwriters Laboratories for industrial controls

UL 508A isn’t the only standard for designing control panels albeit it is the most direct of all. Other common and popular ones are:

• UL 218: Standard For Fire Pump Controllers
• UL 294: Standard For Access Control System Units
• UL 845: Standard For Motor Control Centres
• UL 864: Standard For Control Units and Fire Alarm System Accessories
• UL 891: Standard For Switchboards
• UL 916: Standard For Energy Management Equipment
• UL 924: Standard For Power Equipment and Emergency Lighting
• UL 1037: Standard For Anti-theft Alarms and Devices
• UL 1640: Standard for Portable Power-Distribution Equipment

Why the UL Listing mark is significant

During the inspection of industrial control panels, electrical inspectors reference UL 508A. If equipment has this mark, it is an indication that it has been fully tested and has passed certification by an independent third-party. It means the equipment is safe and appropriate and meets all required UL standards. Before a purchase is made, the buyer and municipal inspection authority must look for this mark. A control panel will be UL-marked only if it includes UL-listed components. If a particular component is UL-marked, it shows that several samples of that component has been evaluated and tested by the UL. Therefore, it passes all safety and security requirements. The tedious and rigorous testing process makes sure that there are no concerns with any part or piece of the system as far as safety and security is concerned.

508A Certified enclosures

When an enclosure that meets the high standards set by Underwriters Laboratories has been purchased or selected, it is an indication that the control panel is compliant and very safe. This is done to guarantee the safety of end users and the general public. It tells people that the control panel in question adheres fully to local and national electrical codes. As a manufacturer of control panels, you must do all you can to get this certification because it is very credible. From time to time, UL officials visit these providers for periodic checks and inspection. The regular visit and audits helps to put control panel manufacturers on their toes so that all regular standards will be met during the production of control panels.

Requirements for general enclosure

Under the UL 508A standard, this section is divided into both outdoor-specific enclosure type and general enclosure type. For the general requirement, proper testing is in place so that all components include:

• Grounding
• Corrosion protection
• Adequate spacing
• Conduit bushings
• Support and securement of live parts
• Insulation materials and barriers

Requirements for outdoor-specific enclosure

Control panels with outdoor-specific enclosures must comprise components tested to ensure:

• Disconnect switches
• Field wiring
• Internal wiring
• Appropriate markings
• Brand circuit protection
• Live parts accessibility
• Ventilation openings
• Adequate enclosure openings
• Enclosure environmental control devices
• Bonding

Changes made to UL 508A in 2005

The first edition of UL 508A was issued in 2001 and since that time, it has undergone a few changes. These changes and innovation are important to keep up with latest trends and improve the testing and verification process. In September 2005, the UL 508A was revised and this revision affected manufacturers of control panels all over the world. The summary of this change was for manufacturers and builders of control panels to include a short-circuit current rating on all control panel assemblies. Manufacturers found it difficult at first to meet up with this important safety consideration.

A short note on UL 698A

The UL 698 A is the Standard for Industrial Control Panels Relating to Hazardous (Classified) Locations. This requirement for testing and verification covered control panels that were planned for general industrial installation. They could range from unclassified locations featuring safe circuit extensions to the hazardous and classified locations listed below.

• Class I, Division 1
• Class I, Zone 0 and Zone 1 AEx
• Class II, Division 1
• Class III, Division 1
• Zone 20 and Zone 21 AEx

The components are expected to operate under the atmospheric conditions listed below:

• Nominal barometric pressure of one atmosphere’
• Ambient temperature of 104^oF or lesser
• Oxygen concentration of 21% or less

UL 698A Control panel types

• Circuit breakers
• Motor controllers
• Overload relays
• Fused disconnect switches
• Associated wiring terminals
• Related control devices such as push-buttons, control relays, timers, selector switches, and pilot lights

Features of UL 698A control panels

Industrial control panels under this category may consist of the following features:

1. Electrical or entity parameters
2. FISCO (Fieldbus Intrinsically Safe Concept) applications
3. Internal interconnection of multiple barriers
4. External interconnection to a particular apparatus

There are some panels that aren’t included under UL 698A. They include:

• Gas or vapour analysis equipment
• Electrostatic devices, systems, or circuits
• Equipment that has process line

In our previous articles, we looked at the NFPA standards as well as comparing the NEMA and IEC standards. We also discussed important things that anyone should know about SCCR. However, let us look at the CSA certification.

They say you cannot judge a book by its cover but in the electrical world, it is possible to judge the label of an electrical control product. If you intend to purchase an electrical product, be sure that what you are purchasing has a stamp of approval from a regulatory or certifying body within the industry.

Therefore, you must be able to recognize labels and know which one to look out for. There are different types of labels and certifications. There is the CSA certification and UL listing. Are they different? What do they mean and which one applies to the purchase that has been made?

We will be discussing these labels thoroughly so that you will be able to understand the dichotomy that exists between them.

What does the UL mark mean?

UL means Underwriters Laboratories. The headquarters of this organization is in Norfolk, Illinois. UL has been around for a very long time. This certifying body was founded in 1894 to be specific. OSHA identifies it as a Nationally Recognized Testing Laboratory. Some years before UL was founded, electrical distribution systems came on board bringing utility power to offices, factories and homes. They did this with new components that needed to be tested and this was what birthed Underwriters Laboratories. When UL started, they tested fire doors and extinguishers before moving on to warehouse inspection which they still do till today. Up until 2012, Underwriters Laboratories was a non-profit outfit. Today, they currently have facilities in 46 different countries and there service is available in about 104 countries. 

Underwriters Laboratories issues security certificate for several products across various industries.

It is important to state that it isn’t every electrical product that will feature the UL mark. For instance, even if a galvanized screw doesn’t have the UL mark, you need not worry. That said, the UL mark isn’t optional in several industrial applications, it is mandatory albeit the certification is only applicable to the United States of America. There are several certifying bodies that test electrical products so that they meet international standards in the United States of America and the UL isn’t one of them. If an electrical product or component carries the UL mark in the United States of America, it means it has been verified by a third-party to meet the standards set in the United States of America. This is why we recommend that you only shop from a verified supplier.

UL mark variations

There isn’t just one type of UL mark. Therefore, you must understand what each one stands for so that you can make an informed decision. The most common examples are C/UL mark and C/UL/US mark.

UL Component Marks

UL Recognized marks is a sub-division of UL labels. This mark is commonly used when smaller scale components contained in a larger control panel system are tested. The uses of UL Recognized marks are discretionary and it isn’t compulsory that components feature this mark. If you need to find out more info about a specific part, you can do this by referencing the CCN (Control Category Number) which can be found close to the mark. With the CCN, you can get information about the specific product category, catalogue numbers that are associated with it, part restrictions (if

What is the importance of CSA-Listing?

 The acronym CSA means Canadian Standards Association. However, it is not a private testing body. As you can deduce from the name, the group is based in Canada and they offer certification for both electrical and mechanical products. They offer certify other products that are regarded as high-risk. CSA was accredited in 1992 by OSHA. Unlike the mark or label from Underwriters Laboratories that is restricted geographically, CSA is a global standard. Any product or component that has been CSA-listed has passed so many rigorous and extensive tests. These tests must meet the standards set by ANSI (American National Standards Institute), NSF International (previously referred to as National Sanitation Foundation), and UL.

CSA mark variations

Just like there are varieties of UL marks, we have varieties of CSA marks too. You will come across them as you research further. Some of them include:

• General CSA Mark – The general format is a simple CSA mark or logo. If a product has this mark, it means the product has been well tested and it meets Canadian standards.
• CSA/US Mark – In this instance, the CSA logo will have “US” at the bottom. It is a sign that the product or component was tested with an agreement with UL in place. Therefore, it meets all standards for safety and security in the United States of America. It simply means it is certified to be used in the United States of America.
• C/CSA/US Mark – In this instance, the CSA mark will have both a “C” and “US”. It means the component has been tested for use both in Canada and the United States of America.

Which should I work with or choose?

It isn’t that one of these certifications is better than the other. Actually both are recognized and identified as NRTLs by OSHA. They both conduct tests against standards that are recognized across the industry such as NEC (National Electric Code) and CEC (Canadian Electric Code).

The UL isn’t more reputable or recognized than the CSA and vice versa. The only dichotomy is based on the jurisdiction they cover.

Trustworthy and certified electrical control products

Do not waste time on unsafe, inferior, and inefficient components as you search for the right parts for your electrical control system. When you choose components and parts that are industry-certified, you wouldn’t have anything to worry about because they have been vigorously tested for performance and security. To reduce your stress and worry, we recommend that you work and collaborate with a partner and supplier that deals in CSA-listed and UL-verified parts.

Therefore, no matter whether you are buying a circuit breaker, an electric motor, or a switch, make sure that it is UL certified.

We previously discussed whether a circuit breaker can be used as a disconnect switch, and went forward to explore a fused disconnect switch. However, today we will look on how to size a disconnect switch depending on the task.

Many people think that a disconnect switch is complicated. Actually, there isn’t really any complication involved. It is simply a on/off switch that disrupts the flow of electric power to an equipment or electrical enclosure. Disconnect switches are also referred to as the main switch in several applications.

Disconnect switches are easy to use but tricky to install or setup especially for beginners. You would have to be careful about determining the correct size the switch should be. Size in this context does not refer to the physical dimension or measurement of the switch but its current rating.

In this article, we will be running you through how to size a disconnect switch for the next project you want to embark upon.

Understanding the requirements from IEC and NEC

One of the reasons for sizing a disconnect switch properly is because it is required by industry regulators. IEC stands for International Electrotechnical Commission and according to IEC 60204-1:2016: Safety of Machinery – Electrical Equipment of Machines – Part 1: General requirements, every time a main power supply is installed, it must be accompanied by a manually-operated disconnect switch.  There are design considerations to make for this switch. For instance, the switch must have a way of indication its off and on state such as an O/I label. Another design consideration is that there must be an accessible handle on the disconnect switch so that it can be manually operated easily.

According to IEC standards, all connected conductors must be promptly disconnected as soon as the disconnect switch is flipped to the “off” position. The disconnect switch should also feature an associated padlock so that the power can be locked by operators.

The NEC code and standard for disconnect switches postulates that there must be a visible disconnect means in any permanently-wired or large equipment. This code is applicable to attic exhaust fans, HVAC unit, and some other major household equipment. This means of disconnecting the large equipment must be a device that appears on the NEC list. This device must also have the ability to disconnect the same amount of power as the equipment installed. For instance, a motor that is rated 3-horsepower will need a disconnect switch rated 3-horsepower or more.

A large number of disconnect switches are fused albeit there are some that are non-fused. According the NEC code, the selected switch must feature the appropriate fuse size to fit the equipment.

Having known this, let us dive into how the disconnect switch sizing process can be approached.

1. Be sure of your power requirement and needs: If you have doubt about the type of disconnect switch that works best for a particular application, find out what your power requirements are before proceeding further. The direct switch used for direct current (DC) power is not the same as that used by alternating current (AC) power. Do not make the mistake of interchanging them even though you may be tempted to do so. A disconnect switch that is configured for alternating current power will not accept direct current power and vice versa. The IEC code also breaks down how these switches can be utilized into various categories for AC and DC power application. Some of the common types are:
2. AC Power: Frequent Operation Utilization Categories – Utilization categories for frequent AC power operations include AC-20A, AC-21A, AC-22A, and AC-23A. The United States of America does not permit the application of AC-20A. AC-20A and AC-20B are used for the connection and disconnection of power under load conditions. AC-21A and AC-21B are used for moderate overloads and for switching resistive loads. AC-22A and AC-22B are used for moderate overloads, inductive loads, and for switching mixed resistive loads. AC-23A and AC-23B are used for switching motor loads and other loads that are highly inductive.
3. DC Power: Frequent Operation Utilization Categories – Utilization categories for frequent DC power operations include DC-20A, DC-21A, DC-22A, and DC-23A. DC-20A and DC-20B are used for connecting and disconnecting power under no-load conditions. DC-21A and DC-21B are for moderate overloads and switching resistive loads. DC-23A and DC-23B serves for switching highly inductive loads like series motors.
4. DC Power: Infrequent Operation Utilization Categories – Utilization categories for infrequent DC power operation include DC-20B, DC-21B, DC-22B, and DC-23B.
5. Calculate current and voltage requirements: Look at the nameplate of the equipment for information about its current and voltage rating. If the electric power rating is not measured in horsepower, do the conversion by dividing the wattage by 0.7457. This will give you the electric power rating in horsepower. Also, when a disconnect switch is installed, its contacts must be sized so that it is able to withstand the electrical energy and charges that flow through it. If you are dealing with higher currents, you’d need stronger contacts to provide adequate support. If you neglect this, it can lead to overheat of the system.
6. Consider the type of application: Is the disconnect switch for a single-motor application or combined load application? The answer to this question will give you an idea of the type of switch needed. The National Electric Code has clearly stated the requirement for each type of application. If it is a single motor, the installed disconnect switch must meet the criteria listed below:
7. The ampere or current rating must be a minimum of 115% of the motor rating when it is at full load.
8. The horsepower or electric power rating must be greater than or equal to the rated motor horsepower. This requirement is only applicable to installers whose disconnect switch is horsepower-rated.

To determine the size of disconnect switch needed for a combined load application, you must analyze each load separately rather than collectively.  This could mean cumulating all simultaneous individual loads that exist in that circuit. The chosen disconnect switch must meet the following criteria:

• A minimum of 115% of the equivalent full-load current.
• It must meet the equivalent horsepower rating.
• Choose a method for mounting: After meeting all technical requirements, you must also focus on aesthetics. Where the switch should be installed and how it should be mounted are important decisions that you must make. There are different mounting options and one of them is the four-hole mounts which is the most common mounting style. It is square-shaped patterns of mounting disconnect switches such that there is a hole for screws on all four corners. However, if the disconnect switch isn’t a big one, you may opt for the two-hole mount style.
• Do a research on disconnect switch handles: The disconnect switch handle can also be made a thing of beauty. The chosen style depends on what you prefer. However, you mustn’t jettison the technical functions of the handle and be too fixated on looks. For example, some user handles can be locked while some cannot be locked if you want to be able to use the disconnect switch to restrict users from electrical enclosures, you should take this feature seriously.

In our previous articles, we explored the history and evolution of PLCs, ad looked at how PLCs will transform the future. Today we are going to compare PLCs and microcontrollers.

The interest in microcontrollers and their related components are increasing steadily due to how open-source electronic prototyping platforms are widening the possibilities of digital device development.

In this industry, so many people confuse microcontrollers with their counterparts, PLC’s (Programmable Logic Controllers) and it is difficult to blame them because the industry is nuanced.  Although they look similar, Programmable Logic Controllers are stronger in capacity.

In this article, we will be analyzing the dichotomy between Programmable Logic Controllers and microcontrollers. We will be discussing the usefulness of each system as well as the purpose they serve. We will also look into their unique features and characteristics.

What is a microcontroller?

A microcontroller is nothing but a computer system. However, it is not the same as a personal computer or laptop albeit it shares the same features – a Central Processing Unit (CPU), Random Access Memory (RAM) for storage of variables, input and output devices for ease of communication, as well as Program-loading capabilities.

Important characteristics of a microcontroller

1. Low-power: A microcontroller will only use about 50 milliwatts of electricity when plugged into a wall socket. However, a general purpose computer will use up to 50 watts.
2. Embedded: A microcontroller is usually inside another device so that its features and actions can be controlled. This explains why some people refer to a microcontroller as an “embedded microcontroller”.
3. Task-dedicated: Microcontrollers concentrate on one task and run a specific program. The program will be in Read-Only Memory (Memory). Hence, it isn’t subject to change.
4. Dedicated input/output devices: A microcontroller will have both a dedicated input and output device such as LED or LCD display. It can also accept input from the device it controls and send signals to other components. For example, if the microcontroller is connected to a television, it will accept inputs from a remote control and display the output on a television screen.
5. It is small and economical: Microcontrollers aren’t pricey. There are small and portable microcontrollers that can still do the job.
6. Rugged and sturdy: Most microcontrollers have the ability to withstand tough conditions such as high heat.

Microcontrollers in manufacturing

Microcontrollers are used in a host of mainstream applications for obvious reasons. One of this is because they are cost-effective. In previous years, they were mostly used in robotic and automation applications. However, their functionality has expanded such that they are now been used in industrial applications.

What is a Programmable Logic Controller?

Programmable Logic Controllers are simply larger-scale microcontrollers. It is a digital computer for industrial use. Programmable Logic Controllers are rugged and are suited for tough manufacturing processes. They are commonly used in assembly lines, robotic and machine learning devices, as well as functions requiring high-reliability control.

The operating principle of a Programmable Logic Controller is such that information is accepted from input devices and connected sensors. This date is then processed and an output is generated based on parameters that have been previously set. It is possible for a Programmable Logic Controller to be used for recording and monitoring run-time data such as analyzing operating temperature or machine line productivity.

Characteristics of a Programmable Logic Controller

Let us look at some of the features and characteristics of a PLC that sets it apart from a microcontroller.

1. Communications: Programmable Logic Controllers may need to connect to another type of system not just an I/O device. For instance, there could be need for recorded application to be exported from the PLC by a user and sent to a system that supervises and controls the acquisition of data.
2. Human Machine Interface: HMI makes it possible for humans to interact quickly with a Programmable Logic Controller.
3. Inputs/outputs: Programmable Logic Controllers are made up of Central Processing Units (CPU) that does the job of storing and processing program data. However, there is need for an input and output module for connection to the rest of the machine. These modules transfer information to the Central Processing Unit so that certain results can be triggered. The input and output controls may either be digital or analogue. They include switches, sensors, lights, valves, relays, drives, and meters.

PLC’s vs. Microcontrollers

You must consider the scenario in which either of these is to be applied before choosing any. Factors to consider before making a decision include:

1. Input/output compatibility: After determining the number of inputs and outputs required, confirm and verify that the microcontroller meets this requirement. Do not forget to factor in the type of inputs and outputs.
2. Mounting and housing: Because of the way microcontrollers are constructed, mounting and housing aren’t always easy. The user may need to make terminals and provide their own power if an external device will be connected.
3. Watchdogs and operating systems: Although microcontrollers aren’t pricey, what the user gets depends on what he pays for. The user may have to program the features that are needed. You’d have to build some simple applications for a Programmable Logic Controller as well. However, the good thing is that they work well within common programming environments.

Industrial conditions to consider

• Corrosion: Programmable Logic Controllers are usually coated to reduce the amount of exposed parts. Their wires are also made from corrosion-resistant materials.
• Shock and vibration: Programmable Logic Controllers have the ability to cope with extreme shock and vibration common to warehouse environments. A microcontroller can also be used albeit special mounting considerations must be made.
• Noise: There are different sources of noise in an industrial environment. Electronic noise emanating from magnetic fields is very common. If you buy an inexpensive or inferior microcontroller, it could develop a fault as a result of significant interference. A Programmable Logic Controller doesn’t have this issue; it can withstand electronic noise without problems.
• Temperature levels: Microcontrollers are best suited for temperature-controlled environments. Also, there are a few rugged ones that have the ability yo withstand lower and higher temperature ranges. The good thing about Programmable Logic Controllers is that they can be installed in an outdoor enclosure where they are exposed to extreme temperature levels without an issue.

Standards for testing

If you are not sure of how to test for the aforementioned characteristics, we suggest you go by the Underwriters Laboratories (UL) or International Electrotechnical Commission standards.

In our previous articles we looked at different rating standards and regulatory bodies and how they differ. We compared NEMA and IEC controls, looked at NEMA electrical enclosure’s ratings, and looked at the major differences between NEMA and IEC product designs. Today we will dive into something else.

You must know the most recent guidelines and safety requirements if you are someone that works with electrical controls.

The fact that there are so many regulatory requirements and compliance boards makes it difficult to keep track of latest safety guidelines. Two of the most popular regulators that set standards as far as the production of electrical equipment is concerned are NEC (National Electric Code) and NFPA (National Fire Protection Association). Funny as it sounds, there are still so many electricians and electrical engineers that do not understand the meaning of these rules and what they entail.

In this article, we will be discussing the NFPA and NEC. We will also be taking a look at NFPA 70 and the area it covers.

NFPA 70 and National Electric Code

Before we dive deeper, it is important to establish that NFPA 70 is officially known as the “National Electric Code”. Therefore, we can use the terms National Electric Code (NEC) and NFPA 70 interchangeably. NFPA 70 was established in 1987.  The aim of NFPA 70 was to provide a guideline for electrical engineers and designers to follow during the installation of electrical systems. To be honest, it isn’t easy to understand NFPA 70 in its entirety. It is written with experienced and technically gifted electricians and engineers as the target audience. These people have the ability to understand popular jargons in the industry.

It was approved as an American National Standard by the American National Standards Institute and this explains why it is often written as ANSI/NFPA 70. It is commonly referenced by builders in the United States of America and they adopt it into their local building codes. If you live or work in these places, you must comply with these guidelines.

What exactly is NFPA 70?

Almost all the states in the United States of America have adopted NFPA 70 in one way or the other. NFPA 70 is made up of ten informative annexes and nine chapters. In some places or locations, some sections of NFPA 70 may be modified or omitted.  They may also decide to add a few things to it depending on the practices that are acceptable within their locality. From time to time NFPA 70 is usually revised just like all other industry standards. The periodic revision of NFPA 70 takes place every three years. What this means that there are regular updates to it even if its core elements remain unchanged. NFPA 70 is basically a compliance guideline for professionals in the electrical world.

The most recent revision of this guideline was done in 2017. It was called “NFPA 70-2017 – NFPA 70 National Electrical Code, 2017”. The reason for this frequent update is so that reliability and relevancy can be improved. NFPA 70 is aimed at providing practical safeguarding practices to shield both humans and property from electrical hazards.

When any change is made to NEC 70, it isn’t usually inconsequential whether it is one line that is removed or a single sentence is added. These changes must be clearly stated and easy to understand as it could go a long way to shape acceptable practices and procedures. For example, one of the update in the revised version released in 2017 was the definition for cable routing as seen in a section of Chapter 1.

The previous definition was that a cable routing assembly was a structural system for routing and supporting optic fibre cables, communications wires and cables, as well as data cables used in communication equipment and information technology.

In the revised definition, the list was updated with Class 2, Class 3, and Type PLTC cables.

NFPA 70 major change

Another important change made was to arc-flashed warnings should be marked by operators. An entire section was added in the revised edition released in 2017. This modification required application of a permanent label to service equipment by users as long as the rating of these equipments exceeds 1,200 amperes. Users were also mandated to add useful data on each label. They include:

• Nominal system voltage
• Date of label application
• Service overcurrent protective device clearing time

NFPA 70 covers the removal and installation following key components:

• Raceways, electrical conductors, and equipment
• Conductors and equipment linked to the electrical supply
• Raceways and optical fibre cables for both private and public buildings, e.g. substations, parking lots, carnivals, and yards.
• Communucation and signalling conductors, raceways, and equipment.

NFPA Chapters

As an industry professional, you do not necessarily have to read all the chapters of the NFPA 70. You can simply flip through the documentation until you get to the chapter that pertains to what you are working on. Chapter-by chapter breakdown of NFPA 70 is as follows:

• Chapter 1 – General
• Chapter 2 – Wiring and production
• Chapter 3 – Wiring materials and methods
• Chapter 4 – Equipment for general use
• Chapter 5 – Special occupancies
• Chapter 6 – Special equipment
• Chapter 7 – Special conditions
• Chapter 8 – Communications systems
• Chapter 9 – Tables
• Informative Annex A to Informative Annex J

What each chapter entails

The introductory chapter is to whet the appetite by describing the scope and purpose of NFPA 70. It also talks about the rules generally as well as enforcements.

Chapters one to four is all about electrical installation guidelines. There isn’t a consideration for use cases or specialized requirements. These chapters include rules and definitions for general purpose equipment, electrical installations, circuit, and circuit protection, as well as wiring methods and materials.

Chapters’ five to seven are nothing but supplemental guides. They shed more light on other chapters. These chapters cover special equipment, special conditions, and special occupancies such as those that pose a huge risk to several persons.

Chapter eight can be referenced as a standalone albeit the requirements in other chapters must be considered. It also covers extra requirements that are only peculiar to communications systems.

NFPA 70 E, what is it?

This is another standard published by the NFPA. It details acceptable practices for protection of electrical workers and their workforce from dangerous occurrences like arc flashes, and other electrical hazards. NFPA 70E revolves around safety unlike NFPA 70 that focus on installation and design.

It provides insight on how electrical workers should inspect and operate electrical equipment so that they can function optimally.

NFPA 70E must be studied by anybody that will likely be exposed to electrical hazard of any form.  The following do not necessarily have to follow NFPA 70E.

1. Vehicles such as aircrafts and ships
2. Railway electrical systems for operating rolling stock
3. Railway electrical systems for signalling and communications
4. Electrical installations that are under the exclusive electric utility control

NFPA 70 B, what is it?

We have already established the fact that NFPA 70 E revolves around electrical safety in the workplace. On the other hand, NFPA 70 B is all about best practices for maintaining electrical equipments. NFPS 70B was first published in 1968 and there is an electronic version for web users.

NFPA 70 B isn’t only about electrical equipment; it also covers electronic equipment as well as systems and equipment used in communications. NFPA 70 B can be referenced in the following locations:

1. Industrial plants
2. Institutional buildings
3. Commercial buildings
4. Large residential complexes

There are thirty-five chapters and sixteen annexes in NFPA 70B.

In our previous articles, we discussed on how to select an overcurrent device, how circuit breakers combine thermal and magnetic protection, and the different types of relay protection devices. In this article, we are going to look at trip curves.

Trip curves are also called Time Current Curves. In this article, we will be opening your eye to what they are used for and why they are important.

What does UL mean?

UL stands for Underwriters Laboratories. When it was founded in 1894, it was called Underwriters Electrical Bureau. This body was created with the main aim of testing electrical products against fire outbreak. Circuit protection devices are part of the electrical products UL was created to test independently.

What are circuit protection devices?

Circuit protection devices are electrical devices created primarily to shield electrical appliances and components from destruction when overload, ground fault, or short circuit occurs. Damage or destruction of electrical equipment, injury to persons, or fire outbreak can occur as a result of overloaded power outlets, lightning storms, or a surge in electrical current. With a circuit protection device in place, this can be averted because power will be cut off from the circuit.

What is a trip curve?

A trip curve is just a way to graphically represent how a circuit protection device is expected to behave. There are different forms and types of circuit protection devices. They include miniature circuit breakers, fuses, supplementary protectors, moulded case circuit breakers, overload relays, motor protection circuit breakers, air circuit breakers, and electronic fuses. To obtain this curve, simply plot an overcurrent device’s interruption time at a specific level of current. There are different types of trip curves. The popular ones are B curve, C curve, D curve, S curve, Z curve, and K curve. B curve is best for resistive circuits (e.g. wire, cable, and control circuits) while D curve is meant for loads that are highly inductive and capacitive such as transformers and motors. S curve is also intended for loads that are highly inductive such as light filaments and control circuits. The K curve is also best for highly inductive loads like transformers and motors.

Why different trip curves or time current curves?

If a circuit breaker doesn’t trip quickly, equipment damage may occur. It may also cause bodily harm and fire outbreak. Also, it must not trip to fast so that nuisance or false trips can be avoided. This explains why electrical engineers must size circuit breakers appropriately so that inrush current can be compensated for. According to NEMA, inrush current refers to the “momentary current transient that occurs immediately (within half an AC cycle) after contact closure.”

An example of the effect of inrush current is when the lights in a house dims when a motor start up.

The main reason different trip curves are needed is to provide balance for the appropriate amount of overcurrent protection when a machine operates optimally. Nuisance or false tripping will occur when a circuit breaker with a trip curve that trips very fast is chosen. On the other hand, you wouldn’t want to work with a circuit breaker that trips too late as it may spell doom for your electrical equipment or machinery. So, there has to be a balance.

The X-axis on the trip curve is a representation of multiple of the circuit breakers operating current. The Y-axis depicts the time it takes for tripping to occur. Important components of the trip curve include:

1. Thermal trip curve: This refers to the bimetallic strip’s trip curve. The bimetallic strip is for smaller overcurrent conditions.
2. Magnetic trip curve: This represents the solenoid or coil’s trip curve. The solenoid reacts to larger overcurrent conditions which may be more than 10X the operating current. An example is a short-circuit condition.
3. The ideal trip curve: This is a representation of the bimetallic strip’s desired trip curve. The difficult in determining the exact tripping point of the bimetallic strip is not unconnected with its organic nature.

What is the operating principle of a miniature circuit breaker?

You would understand trip curves better if you have an understanding of the basic operating principle of an overcurrent protection device or miniature circuit breaker.

A miniature circuit breaker has the ability to serve as a two-in-one circuit protection device because of its components and parts. It has a magnetic coil/solenoid and two bimetallic strips. The bimetallic strip protects the circuit in small overcurrent conditions which may be 10X the value of the operating current. Basically, the bimetallic strip is made up of two strips of different metals (each metal has a different coefficient of expansion). When heat is applied, the rate of expansion of these metals is different. When overload happens, the bimetallic strip will be heated which will cause it to bend. This movement causes the actuation of a trip mechanism which leads to opening of the circuit. What a bimetallic strip does is to convert change in temperature to mechanical movement or displacement.

The solenoid kicks into gear when higher overcurrent occurs as a result of short circuit. It usually exceeds 10X the operating current. The magnitude of this current leads to the generation of a magnetic field. Hence, the internal piston is moved quickly so that the actuator mechanism can be tripped for the circuit to break. For a miniature circuit breaker, the most popular trip curves are B, C, and D.

Wrapping up!

The role of a circuit breaker is to shield electrical equipment and wires from damage when electrical overload, ground fault, or short circuit occurs. With proper circuit protection in place, when sudden electrical surge, lightning storms, and overloaded power outlets occur, personal injuries, fire, and damage of equipment can be avoided.

What a trip curve does is to predict the behaviour or reaction of a circuit protection device in slow and fast overcurrent conditions. Therefore, you must do all you can to choose the correct trip curve for your application so that nuisance tripping can be avoided.

In our previous articles we looked at contactors and types of contactors, overload relay and overload protection as well as everything that you should know about relays. In this article, we will be looking at SCCR.

What does SCCR mean?

SCCR is an abbreviation for Short-Circuit Current Rating. In the 2017 NEC (National Electric Code), SCCR is defined as: “the prospective symmetrical fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining any damage exceeding defined acceptance criteria”.

In layman’s terms, it is the highest short-circuit current that an electrical unit can withstand without the risk of shock or electrical hazard. An electrical panel’s SCCR rating depends on the SCCR rating of each component in that panel. SCCR is a very important subject of discussion as far as the installation of electrical panels and industrial equipment is concerned.

The importance of SCCR is not unconnected with the fact that it is the maximum short-circuit fault current that equipment can withstand without causing electrical hazard. If the SCCR is exceeded, it will lead to catastrophic failure of the component and equipment. According to the NEC, if an industrial control panel contains only a control circuit component, there is no need to mark it with an SCCR. However, if it contains a power circuit branch circuit over-current protective device, you must mark it with SCCR based on the over-current protective device’s interruption rating.

A comparison of power circuit and control circuit

The power rating of the control circuit is lower than that of the power circuit. The power circuit is usually rated at 240VAC – 600VAC, 3 phase power. The control circuit is rated as low as 5volts. Although the control circuit is isolated from the power circuit, it controls how it is operated. We can liken the power circuit as the brawn of a control panel while the control circuit is its brain.

Calculating Short-Circuit Current Rating

In recently produced electrical components, the Short-Circuit Current Rating is usually marked on the component itself. This marking is done by the product manufacturer.

1. Identify the power circuit’s component: Common power circuit components include
2. Fuse holders
3. Circuit breakers
4. Overload relays
5. Disconnect switches
6. Motor controllers (contactors)
7. Power distribution blocks or terminal blocks
8. Determine the Short-Circuit current rating for all the components in the power circuit: A component’s SCCR value can be found on the component itself. For clarity, you can follow the manufacturer’s instruction.
9. Determine the Short-Circuit current rating for each branch circuit: The lowest SCCR value from any of the components serves as the branch circuit SCCR.
10. Determine the feeder circuit’s SSCR
11. You must consider over-current protection devices of feeder circuit
12. Find out the lowest value of component circuit or branch circuit.

All in all, you must remember three things when complying with Short-Circuit current rating. They are:

• The control panel’s Short-Circuit current rating is the minimum Short-Circuit current value of any of the branch circuits or components
• You must determine the available fault current at the facility of the user where installation of electrical panel will be done. This can be determined by the manufacturer or installation contractor.
• The electrical panel’s Short-Circuit current value must equal or exceed the available fault current.

A contactor is an electrical device that allows us change the state of an electric circuit. Hence, it is seen as a different type of relay. The only dichotomy between a relay and contactor is that a contactor is best suited for applications with higher currents capacity. Contactors are usually compact and can be mounted easily. They are made up of multiple contacts and are usually normally open. The most common application of a contactor is for controlling electric motors.

There are different types and sizes of contactors and each type has its peculiar features and qualities. Contractors can break electric current ranging from a few amperes to thousands of amperes. In terms of size, we have handheld contactors and those that measure up to a yard or meter.

Contactors are commonly used for high current load and are most popular for their impressive ability to handle current over 5000 amperes. A contactor also gives us control over arcs produced by heavy motor currents.

Components of a contactor

The three main components of a contactor are:

1. Coil or Electromagnet: This is a very vital contactor component.  The coil or electromagnetic is responsible for providing the driving force for closing the contacts. This coil or electromagnet is shielded in an enclosure.
2. Enclosure: There are enclosures in contactors just like all other electrical devices. The enclosure shields the contactor from unauthorized or unapproved physical contact. It also serves as a means of insulating the contactor. This enclosure can be made from several materials such as nylon 6, polycarbonate, thermosetting plastics, Bakelite, and polyester. The job of the enclosure is also to protect the device from moisture, dust, oil, bad weather, and explosion hazard.
3. Contacts: Contacts are also crucial components of a contactor. It is that part of the contactor that carries currents. There are different types of contacts – power contacts, contact springs, and auxiliary contacts. The role of each type of contact is unique.  

Principle of operation of the contactor

The electromagnet is excited by the current that flows through the contactor. As a result of this, a magnetic field is created which causes the armature to be moved by the core of the contactor. The circuit between the fixed and moving contact is completed by a normally closed contact.  Hence, current will be able to flow through the contact to the load. The circuit will be open and de-energized when current is removed. These contacts open and close rapidly.

Types of contactor devices

1. Manual controller: The manual controller was developed because of the risks and dangers associated with the knife blade switch. It is like an upgrade on the knife blade switch because it has features that cannot be found in the knife blade switch. Some of these features are:
2. It is safe to operate
3. It isn’t as big as the knife blade switch
4. It features single break contacts and not double break contacts
5. It is properly encased

The manual controller’s buttons is physically attached to the controller. Hence, it is not operated remotely.

1. Knife blade switch: This was extremely popular in the late 1800’s. it is often regarded as the first contactor used for the control of the electric motors. A knife blade switch is made up of a metal strip. Also, it had a lever for changing the state of the switch. It either had to be pulled down or pushed up. There were so many issues with this method of switching and one of such is that the contacts wear out frequently and easily. Also, there was always a risk or chance of arcing because it was not possible to open or close the switch very fast. As years went by, there was a need for larger currents because there was an increase in motor sizes. This posed a risk as a high current carrying switch had to be operated. It was this serious safety concern that led engineers to produce manual controllers.
2. Magnetic contactor: it is an advanced contactor that can be operated remotely. This one operates electromagnetically and there is no need for human intervention. Therefore, an operator isn’t exposed to the risks and dangers associated with operating a contactor manually. The opening and closing of the circuit only needs small control current. It is commonly used in industries.

A contactor’s life expectancy

There is an inverse relationship between the rate of opening and closing of the contacts and the life expectancy of the contactor. What this means is that if the contacts are opened or closed frequently, the lifespan of the contactor will be reduced and vice versa. Additional heat is usually generated when the contacts are opened or closed because of the creation of an electric arc. As these arcs are continually produced, the contact surface will be destroyed.

Contactor standards

Common contactor standards are NEMA (National Electrical Manufacturers Association) and IEC (International Electrotechnical Commission). IEC is a global standard for contactors while NEMA is commonly used in North America and the United States of America.

Application of contactors

1. Lighting control: Contactors are used for controlling large lighting installations centrally. A typical example is a retail building or an office tower. For this purpose and to reduce power consumption, a contactor with two operating coils is used. This contactor is called a latching contactor. Each of these operating coils are responsible for opening or closing power circuit contacts.
2. Electric motor starter: It is possible to use a contactor as a magnetic starter. What a magnetic starter does is to provide power to an electric motor. A contactor is an important component of an electric motor starter because it shields it from overload condition, under-voltage, power cut-off, and over-current.

Industrial applications and equipment make use of electric motor. Electric motors are also crucial to how vehicles toys, and electronic devices operate. What an electric motor does is to convert electrical energy to mechanical energy. Electric motors are either AC or DC powered. Common devices that make use of electric motors are extruders, fans, blowers, and cranes.

Induction motor

Another name for induction motor is synchronous motor. It is an AC electric motor that is used in so many industries. The operation of an induction motor is based on the principle of electromagnetic induction. Hence, they feature armortisseur windings. The rotating field of the stator is responsible for producing the electro-magnetic field in the rotor. There are two types of induction motor – single-phase induction motor and three-phase induction motor.

Three-phase induction motors

Three-phase induction motors are widely used. They are found in so many industrial and commercial applications. Three-phase induction motors are extremely popular because of their impressive qualities such as speed regulation, rugged construction, and the fact that there is no commutator. Three-phase induction motors are made up of rotor and stator.

• Stator: This component is usually stationary in the induction motor, i.e. it does not move. It houses the rotor’s cylindrical core and features slotted stampings which carries three-phase windings.
• Rotor: This is the motor’s rotating component. It is made up of laminated cylindrical slots that have copper or aluminium conductors. The rotor is the motor’s shaft. Rotors can either be classified as slip-ring, squirrel cage or phase-wound. Squirrel cage motors are the most popular.

Squirrel cage induction motor

If an induction motor is equipped with a squirrel cage, we call it a squirrel cage induction motor. It is called a squirrel cage because of its resemblance to the rotating cylindrical cage found in a hamster cage. There are different sizes of squirrel cage induction motors. Its size ranges from a single kilowatt to thousands of horsepower. Squirrel cage induction motors are popular because of their ruggedness, simplicity, and the fact that they have constant speed in different sizes of load. The squirrel cage induction motor also comprises of a rotor and stator.

Overload protection

Overload occurs when the electric motor draws more current than it is designed to take. Often times, overload leads to the damage of motor windings and causes overheating of the motor. This is why it is extremely important to protect the motor and its branch circuits. Motors and their branch circuits can be protected from overload condition with an overload relay. An overload relay is part of the motor starter and it shields the motor by keeping a close watch on the current flowing into the circuit. The overload relay will trip by itself if it senses that the current is going over the top. When it trips, it operates an auxiliary contact that interrupts the circuit. As a result of this, electrical power will no longer be supplied to the motor hereby preventing destruction cause by overheat or overload. We can reset the overload relay manually. However, there are some that resets automatically. After the reset, the electric motor can be restarted.

How does an overload relay work?

The motor and overload relay are connected in series. This means the same current that passes through the motor will pass through the overload relay. If excess current goes through the overload relay, it gets tripped. This tripping leads to the creation of an open circuit between the motor and the source of electrical power. As earlier explained, the reset of the overload relay can be done manually or automatically after which the motor can be restarted.

Overload relay types

Common types of overload relay include bimetallic overload relay, eutectic overload relay, and solid state overload relay. Let us discuss each of them exhaustively.

1. Solid state overload relay: They are also called electronic overload relays. In this type of overload relay, current measurement is done electronically. Solid state overload relay is available in various design patterns albeit the features and characteristics are the same. Solid state overload relays can be installed easily due to the heaterless design. This design makes it insensitive to temperature changes. Hence, it doesn’t trip unreasonably. Another impressive quality of solid state overload relays is that they offer better protection from phase loss than bimetallic or eutectic overload relays.
2. Eutectic overload relays: Eutectic overload relays are made up of eutectic alloys (contained in a tube), tripping mechanism, and a heater winding. Basically, it can be described as a combination of two or more materials allowed to solidify at a certain temperature. The tube that houses the eutectic alloy gets heated by the heater winding when overload occurs. This heat causes the alloy to melt and leads to the release of the ratchet wheel. When the ratchet wheel is released, it turns and causes the closed auxiliary contacts to open. It is possible to manually reset a eutectic overload relay after it trips. To do this, simply press the reset button which can be found on the cover of the relay.  
3. Bimetallic overload relay: bimetallic overload relays are made up of bimetallic elements or bimetallic strips which are a combination of metals with dissimilar coefficient of expansion. The coefficient of expansion of one of the metals will be greater than the other. Heat is applied to the bimetallic strip from a winding and each metal expands according to its coefficient of expansion. Since, the expansion will be dissimilar; the bimetal will bend towards the metal whose coefficient of expansion is lower. The bending of the strip causes the actuation of an auxiliary contact mechanism and leads to the opening of the overload relay.

Tripping of overload relay

There is an inverse relationship between the tripping time of an overload relay and the current that flows.

You can read about electronic timing relays and the mechanism of automotive relays in our previous articles.