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What is an RCD

A Residual Current Device or RCD is a fast-acting, safety trip switch or safety device that reduces the risk of a fatal electric shock. The RCD will measure the current passing through the device which will then flow around the circuit, in an ideal world this would be the same value when the current returns to the RCD through the neutral path on completion of the circuit. However, this is unlikely to be the case when a circuit is in operation therefore the current must be flowing elsewhere other than the neutral. This phenomenon is known as Residual Current hence the term RCD or earth leakage as the currents only other path will be to earth. All circuits in a domestic setting must now be installed with this safety device as of BS 7671:2008. (Wiring regulations)

How to recognize if an RCD has been installed

You should be able to locate an RCD as they all have a pushbutton either marked with the word “Test” or the letter “T” this button should trip the switch on the RCD and isolate the supply to the circuit/circuits it is feeding. It is recommended to be pressed either quarterly or every 6 months depending on the manufacturer’s instructions (as pictured) if it fails to trip you should contact an electrician as the device will no longer operate under fault conditions. Payne Electrical recommends to operate this Test switch when changing the clocks biannually when the time goes forward or falls back one hour. 

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What an RCD or RCBO looks like

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RCD Type A.jpg



An RCD is usually twice the width of an average circuit breaker however some older models can be even larger. An RCD and will feed more than one circuit.

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fusebox RCBO.jpg


An RCBO as described below tends to be half the size of an RCD although some older models can be the same size but are not very common. An RCBO should feed only one circuit.

The difference between RCDs and RCBOs

The problem with RCD's is that the device cannot protect against overload or short circuit faults like a fuse or a miniature circuit breaker (MCB) does.

An RCD will protect several circuits which are also protected by one MCB per circuit. Whereas an RCBO (Residual Current Breaker with Overload protection) is a combination of an MCB and an RCD within the same device.  RCBO boards are slightly more expensive, however offer numerous advantages:

1. RCBOs are a smaller and can be fitted in a more compact consumer unit which may be necessary when there is limited space available for an install. 

2. If there is ever a fault with the wiring which needs fixing it will make it easier, quicker and therefore cheaper for an electrician to find and repair.

3. If an RCD detects a fault it will trip, cutting the power to every circuit it is protecting. An RCBO will only cut the power to the one circuit it is protecting rather than all of the circuits protected in by RCD.

4. As BS 767 is constantly evolving there has been talk of making RCBOs a mandatory requirement.

One analogy which is often used is to imagine a heating circuit fed via boiler the where the voltage is the pressure and the current is comparable to the flow of water. If the same volume of water doesn’t return to the boiler there must be a leak somewhere within the circuit an RCD will trip in this scenario in order to prevent a fire risk or electric shock. This is known as additional protection and all circuits in a domestic setting must now be installed with this safety device as of BS 7671:2008.


A Plumbers Analogy


Unfortunately, there will be times when “nuisance tripping” will occur several reasons include:

  • The degradation of cable over time causing the insulation to deteriorate and the current being able to flow through the insulation to the earthing conductor within a cable.

  • A faulty appliance plugged into a circuit.

  • Some electronic equipment is actually designed to have a small value of earth leakage to operate. So too much of this equipment can cause nuisance tripping.

  • The path could be through the structure of a building to earth if a cable has been damaged which can cause fires.

  • However, the worst-case scenario would be through livestock, a pet or a human body.


As a small amount of earth leakage is unavoidable the value of current permitted before an RCD will trip as set out in BS7671 (The wiring regulations) is 30mA. At this level in the event of a human coming into contact with a live part it is unlikely to cause death via electrocution. It is a requirement in BS7671 that all RCDs should trip when tested at its rated sensitivity within 300 ms. When tested at five times, ie, 150 mA for a 30 mA device, it should trip in 40 ms.

Nuisance tripping

Before Calling an Electrician

There are two things which you can try and rectify nuisance tripping before calling an electrician which are:

  1. Turn off and unplug all the appliances that are plugged into the socket outlets throughout the building as it may be a faulty appliance.

  2.  If this fails turn all the breakers at the consumer unit off so the switches are down. Then turn the RCD on, if it trips you will need to call an electrician. However, if it stays up turn each individual breaker on (upwards) one at a time you may be then able to identify the faulty circuit as when you turn the faulty circuit back on the RCD will trip. This will not solve the issue but at least you will be able to use the rest of the circuits protected by that RCD while an electrician is able to arrive.

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