An RCD test button could save your life: How to Safety Test

1) What is an RCD exactly?

An RCD is a clever little device that monitors electrical current flow through our wiring system. When it detects that there is electrical current missing, such as would be the case for somebody receiving an electric shock, it turns off the electrical supply to the circuit or circuits that it is monitoring and prevents any harm from coming to the occupants of the property. 

WARNING SCIENCE BIT:  An RCD measures electrical current leaking to earth (earth leakage) by detecting the imbalance in electromagnetic field strength between Line and Neutral. When the magnetic field strength discrepancy is great enough the RCD will trip. 

Residual Current Devices have many names that include GFCI (Ground Fault Circuit Interrupter), RCBO (Residual Current Breaker with Overload), RCCB (Residual Current Circuit Breaker), RCB (Residual Current Breaker), RCD (Residual Current Device), AFDD (Arc Fault Detection Device but normally contain an RCD).

2) Different types of RCD

2.1) Fixed RCDs

A fixed RCD is the type that would be found in a consumer unit or electrical panel. They are hardwired into the brains of the electrical system and often protect several circuits at once. They are often supplied with the consumer unit and installed by electricians when the unit is fitted. It is sometimes possible for an electrician to retrofit these devices into a consumer unit or electrical panel but compatibility of the RCD needs to be checked with the manufacturer of the assembly. (536.4.203) to ensure that everything will function as intended. 

Care should also be taken to ensure that no single RCD device is likely to see more leakage under general operating conditions than is intended (531.3.2) by the device. 

(Picture: An RCD with test button next to a circuit breaker)

2.2) Portable RCDs

A portable RCD can often be found on electric lawnmower cables along with other equipment such as wet saws and outdoor apparatus. These devices are intended to perform the same operation as fixed RCDs but can be used anywhere the equipment is taken. 

These devices are particularly useful for protecting the people using the equipment in various different properties. A gardener for example, may visit many homes each week that may not have an RCD device installed in the electrical system so the gardener is protected whilst using their equipment outdoors. 

2.3) Plug Socket Outlet RCD

Where older electrical installations do not have upfront or fixed RCD protection, electrical socket outlets with built in RCD protection can sometimes be installed to offer the advantages of RCD protection without the requirement for changing the consumer unit or electrical panel. 

Each socket outlet will be protected individually and in the event of a device trip, the rest of the electrical circuit will remain unaffected. This is particularly useful in commercial applications where the loss of a complete circuit could result in data loss, risk to life or financial damage. 

If we think of a hospital for example, we would not want the cleaner unwittingly using a faulty vacuum appliance which then caused all the life support equipment to fail when the upfront RCD tripped the power to the entire circuit!

2.4) AC Type, A type, B Type, F Type, S type?

Amongst the various physical types of RCD device mentioned above, there are also different tripping types of RCD. Depending upon the demands of your electrical installation, the electrical appliances in the property and the risks of electrocution, your electrician will select the correct type of residual current devices that are needed. 

An AC type RCD will trip where there are AC electrical currents in circuit but may NOT trip if there is DC present in sufficient amounts.

An A type RCD will trip where there are pulsing DC currents (and smooth DC up to 6 mA (milli amps)) in the circuit. They will also manage the job of the AC type device. 

A type F RCD will trip where there are differing residual currents from very few to 1000 Hz (frequency with which the current alternates between a positive and negative portion of the wave) such as may be the case with variable speed drive motors. These devices will perform the operations of an A type as well.

A B type RCD will trip where there are smooth DC currents up to 10mA present in the circuit. This device will also perform the function of the F Type RCD.

3) Why test an RCD?

Testing of an RCD device is absolutely vital to ensure electrical safety. Without regular testing of an RCD Device, we have no idea if there is protection against earth faults and electrical shocks. Regardless of the type of RCD, the test button should be pressed once every 6 months to make sure that the electricity supply disconnects following the push-button test. 

RCD devices have been installed in your electrical installation instead of other protective measures and without their known safe operation there is a risk that your electrical system does not have the correct protective measures in place and operational to protect you. 

4) Self-Test (Partial Test)

In order to self- check the RCD device the test button should be pressed once firmly and quickly then immediately released. If the device fails to trip then the device is likely to be faulty and an electrician should be called to confirm if the RCD switch needs replacing. 

This test should be performed on a regular basis (once every 6 months) to ensure that the device turns off and disrupts the flow of electricity to the electrical circuits that it protects. 

5) RCD test Requirements UK BS7671 (2024)

Full Test (Electrician Must Perform)

Under the BS7671 wiring regulations (2024) the requirements for testing RCD devices that have been installed for additional protection are as follows:

Regardless of RCD type, effectiveness is deemed to have been verified where an RCD disconnects within 300ms with an alternating current test at its rated residual operating current (I delta N) 

https://electrical.theiet.org/wiring-matters/years/2022/91-july-2022/changes-to-rcd-testing-in-bs-76712018plusa22022/ 

What does this mean? 

It means that an electrician should check with an RCD -Tester that has been recently calibrated and checked to see if the RCD device trips off quickly enough to prevent electric shock. 😊 Whatever the type of RCD, your electrician should be able to test it for you in accordance with the RCD testing requirements. 

(Picture: A typical Multi-Function Tester that can check RCDs)

6) How are RCD test Results Recorded and Verified?

RCD test results should be recorded on the appropriate electrical certification document for the work that is being carried out. This may be a MEIWC (Minor Electrical Installation Works Certificate), an  EICR (Electrical Installation Condition Report) or an EIC (Electrical Installation Certificate). 

The model and serial number of the RCD tester or other RCD test instruments must also be recorded on the paperwork such that it can be referenced at a later date if needed. 

These records of RCD testing are useful to refer to as the electrical installation ages. Whilst they are a helpful record to prove the maintenance of the electrical installation, they may also show any deterioration in the devices over time. 

The trip time of the device should be checked against the regulations and the manufacturers instructions to ensure compliance. 
Regular RCD testing is vital to ensure that the RCD safety switches do not become jammed over time and continue to offer the highest level of protection possible to the whole house or property. 

7) What does RCD protection cost?

Typically, the installation of various types of RCDs is inexpensive. Depending upon the electrical equipment that is installed in the property, the overall amperage of the circuit breakers that are required to be covered and the local wiring regulations, costs can vary from as little as £150 for a single device replacement up to £1000 or more for a new consumer unit with the very latest circuit protection. 

A far more accurate means of obtaining a price is to ask a local electrician to check your installation and see what various types of RCDs might be required.

8) Common Questions Answered

8.1) Can an RCD become faulty?

Yes, an RCD can absolutely go faulty. There are a number of reasons for this:

Failure from excessive tripping – RCD devices are meant as a life saving device and are not meant to last forever. These devices will only trip so many times before they fail completely and refuse to turn on again. 

Overloaded RCD burnt out – An RCD will not trip if there is an overload (Unless RCBO or AFDD) and as such, the overload will simply burn out the device internally causing failure. When this occurs, the device is not going to trip when it senses electric shock occurring and should be replaced. Care should be taken by your electrician when installing new RCD devices to ensure that the rating is sufficient to cope with the electrical demands of your installation.

(Photograph showing an RCD that burnt out through overload and loose connections)

Internal mechanism has failed – The internal components of the RCD can corrode, rust, crack, split and fail. There are several moving components inside an RCD device and they are just as susceptible to failure as any other mechanical parts. If the device is not tripping then is should be replaced.

8.2) What is the most common cause of an RCD tripping?

The most common cause is actually a faulty appliance causing the tripping problem. Unplug all of your electrical appliances from the plug sockets including extension leads then try resetting the RCD device. If it fails to turn back on then our article may help you further. 

8.3) Why won’t my RCD reset after test?

The most likely reason that the RCD will not turn back straight away is that the device is trying to turn on too much. 

Try turning off the other trip switches first. Each RCD protects several circuit breakers which are usually situated immediately next to the RCD device. Turn these off then try turning the RCD ON. If the RCD turns on then turn each of the circuit breakers back on with a few seconds pause in between each one. 

Sometimes reinstating power to a whole load of circuits at once is too much and can make the RCD think there is an earth leakage fault.