Rewireable Fuses

 

RCD (Residual Current Device)

 

A safety trip 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 50,000th of an amp can kill an average person.

Modern RCD’s whether as part of a split 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.

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.

A very common accident resulting in injury or death from electric shock is where a mower cuts through the flex while cutting the lawn. In most cases this will not cause the fuse to blow in the plug and therefore leaves the cut end of the flex live. If the user then inadvertently picks up the bare end they can receive a fatal electric shock or be badly shaken and burnt, but if an RCD is fitted at the moment the user comes in contact with the Live end the device senses that current is flowing through the Live but not returning through the Neutral (In fact flowing through the user to Earth) and trip at 30mA therefore affording personal electric shock protection to the user.

How do I know mine will work when it’s needed?

It is vitally important to check the operation of an RCD. Firstly check regularly using the test button provided on the RCD. For those contained within split fuseboards again check by operating the test button first 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. This only offers protection for the installation and the appliances and not personal protection. You should use plug in 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 electric showers, jacuzzi's, hot tubs etc by a qualified electrician.

My house is protected by a 100mA Current Balanced Trip so this not sufficient for personal protection?

Correct. 100mA trips were commonly installed in houses where an earth was not provided by the local electrical company such as in remote countryside areas. The 100mA did afford better protection for the installation than just fuses but not personal protection. You should use plug in 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 usually marked 'ELCB' (Earth Leakage Circuit Breaker) should be replaced with an RCD as soon as possible. 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 and afford limited protection to the installation and no personal protection. You should use plug in 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 as 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. When the new 17th Edition IEE Regulations come into force in June 2008 all socket outlets used by untrained or unskilled staff will need to be 30mA RCD protected.

What's the difference between an RCD and an RCBO?

RCD's afford only personal protection as explained above but not circuit overcurrent protection. An RCBO is a combined RCD and MCB unit which protects the circuit against overload and also affords personal protection.

 

Top of page