Typical Split Consumer Unit
Single & 3 Phase MCB's
Plug in RCD
RCBO (Combined RCD and MCB)
RCD Protected Socket Outlet
RCD (Residual Current Device)
A safety device designed to protect against personal electric shock.
Fuses and MCB’s protect only the installation and final appliances; they are not suitable for personal protection from electric shock. Many believe that it is voltage that can kill whereas in actual fact it is the current that causes a shock and can prove fatal.
The actual amount of current that can cause death is very small. In fact its just 50mA (milliamps), much less than the 3A fuse you may have fitted in your plug.
To simplify, just 1/20th of an amp can kill an average person.
Modern RCD’s whether as part of a consumer unit, industrial/commercial distribution system or fitted as a plug in device for use on appliances such as mowers and drills are rated at 30mA. Look for this rating to be sure of personal electric shock protection. Some high risk installations such as caravans may have 10 or 20mA RCD's fitted to give even higher levels of protection.
How do they work?
A simple description of their operation would be to refer to them as many electricians do as a current balanced trip. Most people appreciate conventional current flows from Live (Phase) to Neutral. The RCD monitors the current flowing through the Live (Phase) conductor and returning back down the Neutral conductor. These should be equal or balanced if the circuit is correct. For example if 5A is flowing through the Live (Phase) then 5A should be flowing through the Neutral. The rating of the RCD of 30mA is the allowable difference between the Live and Neutral. Should this be exceeded then the RCD will trip.
How do I know mine will work when it’s needed?
It is vitally important to regularly check the operation of all RCD's. A plug in type RCD should be checked each and every time by plugging in and operating the test button prior to use. For those contained within split fuseboards again check regularly by operating the test button and ensure that as per the IEE Regulations that the whole installation and RCD operating times are tested using the correct instruments at least every 5 years by a qualified electrician.
My house only has a regular fuseboard with rewireable/cartridge fuses or MCB’s. Is this OK for personal protection?
No. Rewireable Fuses, Cartridge Fuses and MCB's only offer protection for the installation and appliances and not personal protection. You should use plug in 30mA RCD’s for any appliances and extension leads that may be used outside as a minimum but it is strongly recommended that you have fitted a new split fuseboard that contains a 30mA RCD for all socket outlets and other high risk areas such as kitchens, electric showers, jacuzzi's, hot tubs etc by a qualified electrician.
My house is protected by a 100mA Current Balanced Trip. Is this sufficient for personal protection?
No. 100mA trips were commonly installed in houses where an earth was not provided by the local electrical company such as in remote countryside areas so an earth electrode in the form of a rod was driven into the ground and is used as the earth path. The 100mA RCD does afford better protection for the installation than fuses or MCB's alone but not personal protection. You should use plug in 30mA RCD’s for any appliances and extension leads that may be used outside as a minimum.
My house has a Voltage Operated Trip.
Old Voltage Operated Trips commonly marked 'ELCB' (Earth Leakage Circuit Breaker) should be replaced with a 30mA RCD as soon as possible. They are prone to failure and nuisance tripping especially during thunderstorms. The way to identify an ELCB is by looking for green or green and yellow earth wires entering the device. They rely on voltage returning to the trip via the earth wire during a fault and afford only limited protection to the installation and no personal protection at all. You should use plug in 30mA RCD’s for any appliances and extension leads that may be used outside as a minimum.
Does the same apply for Industrial and Commercial premises?
Most certainly. Portable appliances and extension leads are more likely to be left in an unsafe condition particularly if used by multiple personnel in the workplace. If a socket outlet is to be used for portable appliances it should be protected by a 30mA RCD. 3 phase devices are also available if required. The new 17th Edition IEE Regulations came into force in 2008 and require all socket outlets rated under 32A that could be used by untrained or unskilled persons, be 30mA RCD protected.
What's the difference between an RCD and an RCBO?
30mA RCD's afford only personal protection as explained above and not circuit overcurrent protection which would be provided by individual MCB's or fuses. A 30mA RCBO is a combined RCD and MCB unit which protects individual circuits from overload and also affords personal protection.
How do I upgrade?
Personal and overcurrent protection and the design of such should be entrusted to a qualified electrician. The electrical safety of your property and maybe even your life depends on the correct selection, installation and testing of the installed safety devices and correct earthing arrangements and is not a do it yourself exercise.
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