What is a Solid State Relay device?

Answer from Joe (Technician)

A solid state relay also known as SSR, is an electrical switching device. It switches off and on when a small amount of voltage is passed across its control terminals. It can be used to load either Alternating Current (AC) or Direct Current (DC) and serves the same functions as the Electromechanical Relay except for a few differences.

Packaged solid state relays can switch current up to about a hundred amperes (100A) and they do this by the help of power semiconductors such as Thyristors and Transistors. Compared to electromechanical relays, they have faster switching speed and generally last longer because they do not have contact parts to wear out. However, solid state relays have a lower capacity to withstand momentary overload and also have a higher level of resistance to the “ON” state, in contrast to the electromechanical relays. Also, unlike the electromechanical relays, SSR devices provide limited switching arrangements.

Obviously, there are sharp contrasts between Solid State Relays and the Electromechanical Relays. Solid state relays have a few advantages over the electromechanical relay devices. Below are a number of these advantages:

  • They have no moving or contact parts that can wear out or build up carbon, thus they have the tendency to last longer than electromechanical relays.
  • It has a totally silent and bounceless operation.
  • They are generally smaller in size than similar electromechanical relay specifications with slimmer profiles which allow for tighter packing.
  • The amount of time an optical solid state relay device takes to switch ON/OFF is usually between a few microseconds to milliseconds. Thus they have a faster switching speed range than electromechanical relays.
  • They are safer, especially in explosion prone areas or environments because they do not create any sparks while switching ON/OFF.
  • No matter the amount of usage, the output resistance of SSR devices stays the same.
  • Compared to electromechanical devices, they have lower sensitivity to environmental and physical conditions such as humidity, external magnetic or electromagnetic force fields, mechanical shock, vibration, etc.

Despite this number of advantages, solid state relays have their fair share of disadvantages. These include:

  • Due to their faster switching speed, they have the tendency to switch spuriously because of sudden but short-lived changes in voltage. Also, due to the presence of body diodes, they take longer to recover from these short-lived voltage changes. This amount of time is called Transient Reverse Recovery (Trr) time.
  • While electromechanical relay devices have the tendency to fail open, solid state relays generally fail shorted on their outputs.
  • For gate charge circuits, solid state relays require an isolated bias supply.
  • Unlike electromechanical relays, some SSR devices are affected by polarity.
  • When closed or OFF, solid state relays have a higher resistance which generates more heat and produce more electrical noise.
  • When open or ON, they have lower resistance and a reverse leakage of current which can cause distortions in waveforms.

Answer from James (Technician)

A solid state relay device is a switching device. It is very similar to the electromechanical relay and it can load either AC or DC.

All solid state relay by square D here.

What is an under-voltage control relay used for?

Answer from Joe (Technician)

Primarily, a relay is used to detect abnormalities in a circuit and send signals to the circuit breaker to break the circuit. The undervoltage relay works in this way. If the amount of voltage in a circuit is less than it should be, it sends signals for the circuit to be broken or disconnected.

Answer from James (Technician)

Power systems usually have 3 phases and the voltage in these phases can fluctuate or destabilize due to excess load. This can happen when a large motor or system is started which draws more current or when the capacitor connected to it trips which causes a decrease in the power factor. Whenever the capacitor trips, there is a further decrease in the power factor which would make the load draw more current which would in turn cause more trips.

When this happens, the system eventually collapses. If an undervoltage relay is present, some feeders will be tripped and a total collapse of the system will be avoided.

Answer from John (Technician)

As its name suggests, an undervoltage relay trips the circuit breaker during under voltage occurrences. If for instance, the voltage drops 70% of the rated voltage, the undervoltage relay will automatically activate, trip the breaker and hold it until the voltage supplied gets to 85%.


In electricity, when electric power is made to flow in a reverse direction, backfeeding is said to take place. Circuit breakers and electric cables have the ability to permit the flow of current across different axis. To backfeed an electric panel, the main breaker must be placed strategically as part of a long line of breakers instead of the usual pattern of putting it in a separate position. To distinguish between the main breaker and other breakers in the arrangement, the main breaker is usually labelled.

The ‘maximum amps per stab’ for a main breaker that is backfed should not exceed the maximum value permitted by the product manufacturer. It is a convention to have this value written on a data plate. This logic explains why the slots on the outer portion of a main breaker are left empty and vacant.

So, to validate and inspect a main breaker, look out for the following:

  1. Is the main breaker conspicuously labelled?
  2. Is there a mechanical security for the device?
  3. Does the cumulative amperage value of the main breaker and neighbouring breaker exceed the maximum permissible value outlined by the product manufacturer on the data plate?

An electric panel that has a regular main breaker can also allow the flow of current in a reverse direction. However, there must be an extra level of security in place for such to happen. This is to help eliminate the possibility of the timing of the main and breaker and generator backfeed breaker coinciding.


The panel boxes are formally known as ‘electrical enclosures’ and have two major functions which are to offer protection to the user of the equipment from shock and protect the components inside the equipment from environmental disturbance.  Other things may also be required like spreading of heat, reduction of interference of electromagnetic radio frequency and preventing the equipment from getting damaged by electrostatic discharge.

However, if the panel does not offer shock protection to the user or the hands o children curious enough to touch or protecting the internal components from environmental disturbance, it is due for a change. In the picture above, it’s clear that the panel needs to be changed. There are holes caused by rust at the side, the disconnect lever mechanism is spoilt and the wire connections are corroded because of water interference.

If the corrosion on a panel cabinet is not much, it should be examined to know why it’s rusting, where it’s coming from and then fixed to prevent advancement of the corrosion. For instance, if rain leaks down a service mast, the panel box can be damaged alongside the circuit breakers if nothing is done about it. quickly. In the photo below, we can see two areas where rain can get down the service mast: where the weather head cap is missing and where there is damage to the boot at the point where the mast enters the roof. Trace the trail made by the rust-colored stains of water drops which makes it easy to spot. If a panel is kept in a closed space with pool equipment or pool chemicals like chlorine can bring about increase in corrosion.


Protection from shock vs. protection from excess current.

These two devices have different ways they protect a home electrically. The purpose of a circuit breaker is to interrupt the flow of electricity in a circuit if the current flowing through it is more than it was designed to handle. If the flow of current is too high for a wire especially if the cross-section cannot handle it, it will heat up more than normal and end in a fire outbreak. The breaker goes off when the current surpasses what is written on the switch. Hence, breakers are basically designed to protect against fires.

GFCI receptacles protect the occupants of a house against shock and are used in wet parts like the bathrooms, kitchen, garage and exteriors. They keep tabs on the current going out of the receptacle in comparison with the current coming in. if there is a leak of current out of the circuit [a ground fault], in about 25 milli-seconds, the GFCI device in the receptacle shuts down.

It is true that there are different paths through which current can leave a circuit but it could pass through the body of a human to the ground as well. Due to the fact that water conducts electricity and wet parts of the house tend to fuel the connection to the ground through a person in contact with a surface that is alive electrically, GFCIs are usually required here.

Another device that looks like the GFCI receptacle exists but does not have slots for a cord. At the top of the page on the left is a photo showing one known as a ‘dead front GFCI’s.


Every circuit breaker protects against excess current. This is to say that if electricity flowing in the circuit is more than the amperage of the breaker, it will shut down [trip] and break the connection in the circuit so that the wiring or equipment it is used for would not overheat and start a fire in turn.

If a breaker has a small coloured button [which may be green, white, red or yellow] marked TEST, it has two protective purposes. See the picture below showing older 1980’s GCFI breaker.

Breakers are of two basic types, namely:

1. GFCI (Ground Fault Circuit Interrupter)

2.  AFCI (Arc Fault Circuit Interrupter).

 Pushing the test button senses the defect it was made to fight against and shuts down the breaker. It then props the switch in between the ON and OFF position if it still works well. Push the switch to the OFF position first and then back to the ON position to reset the breaker. If it doesn’t work when you push the test button, the device doesn’t work anymore and is not offering the protection it was made to give.

The work of the GCFI breaker is to provide protection against ground faults

[basically, current flowing off course and may shock you]

. these are the breakers used to protect circuits or spa tubs and pools including receptacles in places that are often wet like kitchens, garages, bathrooms and exteriors. Today, panels do not often have GCFI-breakers in them because the protection GCFI provides can be gotten from GCFI receptacles that have TEST and RESET buttons in the middle and can be stationed in the wet places around the home.

What a AFCI-breaker does is to offer protection against arc faults [when sparks are created across a space in a wire caused by bending or breaking which can be along one wire or two adjacent to each other]. Since wire arcing is known as a major cause of fire outbreaks, AFCI breakers have been in demand for circuits in bedrooms since the beginning of the century. These days, they are required to be stationed as protection for every receptacle in a house that is up to 120 volts. Since 2008, an updated version has been the requirement and it’s called the CAFCI [Combination AFCI].


Over the years, the method of use and application of Arc Fault Circuit Interrupters have undergone massive evolution. From the first requirements reeled out by the National Electrical Code (NEC) in 1999, there has been frequent and continuous changes. At first, Arc Fault Circuit Interrupters were meant to be used in bedrooms. However, every code cycle since that time has forced electrical engineers to up their game.

The 2008 guidelines released by the National Electrical Code made provisions for the extended use of Arc Fault Circuit Interrupters in some other parts of the house. This new recommendation meant that this technology could now be used in living rooms, libraries, recreation rooms, dining rooms, and other habitable locations. The decision made by the National Electrical Code is a logical one as it meant more safety for home owners especially if you consider the number of fire accidents caused by overheating of cables or electric arcs. The numbers were staggering and so much money was lost by property owners. As part of the 2008 directives, the National Electrical Code (NEC) made it mandatory for electrical engineers, builders, electrical contractors, and house owners to use Combination Arc Fault Circuit Interrupters which is an upgrade on the conventional Arc Fault Circuit Interrupters (AFCI). The NEC made this decision because Combination Arc Fault Circuit Interrupters provided more cover as it is able to detect both series and parallel electrical arc faults. On the other hand, the conventional Arc Fault Circuit Interrupters are only able to detect parallel arc faults. In 2014, the National Electrical Code decided to include kitchens and laundry rooms on the list of places where Arc Fault Circuit Interrupters can be installed. Although extended deployment of Arc Fault Circuit Interrupters has increased home safety and reduced occurrences of fire accidents, there are a few concerns about the accompanying cost. Normally, it is cheaper and less expensive to install standard circuit breakers in homes and offices than it is to install Arc Fault Circuit Interrupters. Notwithstanding, the benefits derived from using Arc Fault Circuit Interrupters outweighs the cost incurred especially if you factor in the fact that Arc Fault Circuit Interrupters helps prevent death and injuries.


Protection against excess current is provided by every breaker in an electric panel. This means that if the current flowing through the circuit is excessive, which could bring about overheating of the wiring and in turn cause a fire, the breaker goes off and cuts the electricity. The switch of the breaker then stays in between the ON and OFF positions after it goes off and has to be turned to the OFF position before being taken back to ON to repower the circuit.

Breakers that have TEST buttons like the green ones seen in the pictures above are ACFI or GCFI and have two protective functions. It is advised by the manufacturers that the TEST button is punched regularly [say monthly] to check that the second protective unction is still active.

The acronym GCFI means Ground Fault Circuit Interrupter. This simply means that the circuit is interrupted [goes off] by it when there is a ‘’ground fault’. Its purpose is to protect the occupants of a house from shock in a wet place like the kitchen and bathroom where electricity can pass through your body to places that are wet and connected to the ground material. In a house, wall receptacles provide GFCI protection with TEST and RESET buttons on them. For appliances like a Jacuzzi tub which has its receptacle buried deep in a space underneath the tub, it’s best to keep the GCFI protection in the panel for easy access to test and reset the GCFI-protection [or do both]. GFCI breakers were usually in the panels in homes built before the1980’s. this was because there weren’t wall GCFI’s then.

ACFI means Arc Fault Circuit Interrupter. It shuts down the circuit when there is sparking known as arcing at any point in the wiring. There can be arcing either in parallel [between a bent/broken piece of wire and one adjacent to it] or in series [between a part of a bent/broken piece of wire and another].

As a reaction to the research done by CPSC [Consumer Products Safety Commission] in the year 2000, ACFI-breakers were required for wall receptacle circuits in bedrooms because it was proven that every year about 40,000 fires were caused by home electrical wiring, majorly faults of arcing. When there is arcing, high temperatures are created and can begin to burn combustibles like wood, carpet and paper that are close by.

In 2008, the requirement was changed for most of the wall receptacles to incorporate breakers in new homes and the standard is now a new kind of ACFI-breaker known as the combination ACFI or the CAFCI which takes into cognizance arcing in both series and parallel. But the ACFIs of old only shut down for arcs in series.

Sometimes, it’s hard to know the kind of breaker you’re seeing because the markings that differentiate ACFI from GCFI breakers with test buttons are hidden by a cover plate [electricians call it a ‘deadfront’]. Just note that it is a GCFI if it serves a kitchen, laundry room, bathroom, garage or exterior receptacles, basically, any place that is usually wet.


Under varying conditions, a circuit may not be able to be reset when there’s been a cut in the breaker. These three are the most common:

  1. When the breaker goes off: This could either be because the test button has been punched on the AFCI or GFCI breaker or because there is an excess flow of current [that is to say that the switch is stuck in between ON and OFF on a panel that is new]. If you punch the switch again and try to push it back to the ON position in trying to restart it, it would just go back to the in-between position. What you should do is take the switch completely to the OFF position before pushing back to ON to get the circuit reset. Check out a photo below.
  2. If the switch is taken to OFF and then taken to ON and it immediately gets stuck in between the two positions, the circuit has been “short-circuited” this is to say that an appliance load has caused a little problem thereby creating another path for current to flow uninterrupted. The breaker shuts down again due to the excess current. Dial your electrician’s number now.
  3. One other thing that can cause a breaker to trip is when there are excess appliances and lights sucking more current than was intended for the breaker. When electric current is too much, steady overheating of a thermal link in the breaker happens and if this is the case, you can reset the breaker but it would go off again after a while. To test this analysis, reset the breaker after you’ve disconnected some electrical points from the circuit. If the breaker doesn’t go off again after some minutes, then the issue was the excess load on the circuit or one appliance drawing too much current than was intended for the breaker. An example is a portable space heater. A local electrician, Craig Eaton says that sometimes, you can’t get the breaker to reset till it has cooled a bit and that waiting a little for the breaker to reset and it does, it also shows that the issue was circuit overload.

If you succeed in resetting the breaker but find that the receptacles are all dead or just a few still works, it may be that there is a GCFI receptacle in the house that has tripped. The full meaning of GCFI is Ground Fault Circuit Interrupter and they protect wet part of the house from causing shock like the bathrooms, kitchen, garage, the exterior and the laundry. Two buttons can be found in the middle of a receptacle marked TEST and RESET. The RESET button usually pops out more than the TEST button when a GCFI device has tripped inside the receptacle. Sadly, it’s not always easy to discover so it’s wise to punch the RESET button and watch if it goes inside. If it does, then there was a trip.

GCFI receptacles were expensive when there were still new in the 1980’s and 1990’s. Since having one GCFI receptacle in one location after the electric panel of a number of receptacles all around the house protects the other ones around it, people building houses utilized this fully then. So, one GCFI receptacle placed in a bathroom or the garage offers protection to the other receptacles in the bathroom or garage and exterior receptacles in the house. Hence, it’s wise to check every GCFI receptacle in the house to see if one of the receptacles stays dead before contacting an electrician. That’s after you’ve checked all the circuit breakers in the electric panel. Often, there are cases where a receptacle dies in a non-wet location like a sun room or dining room after a GCFI receptacle trips in a farther location in the house. Therefore, check every GCFI for any dead receptacles even in a dry room.


The National Electrical Code [NEC] did not cite anywhere that GFCI should be used for shower lights and exhaust fans in the bathroom but a large number of manufacturers advise that it be used. This may be as a result of the necessity placed on the manufacturer to obtain a UL listing for the appliance. Seeing as it is instructed by the NEC in 110.3(B) that any equipment listed or labelled should be amounted and used exactly as the label instructs. This means the NEC requires it indirectly.