The Association of Manufacturers and suppliers of Power Systems and ancillary equipment


Should all generating sets be fitted with an RCD, and what tripping current level should be used?

Any advice offered regarding the incorporation of Residual Current Devices (RCD) – sometimes referred to as Earth Leakage Circuit Breakers (ELCB) - within an electrical protection scheme, must include the need for care and responsibility.

RCD’s provide safety-protection for personnel, while fuses and circuit breakers provide safety-protection for equipment.

The human body can tolerate a mild electric shock, with evidence supporting the long established fact that a 30 mA RCD offers excellent personnel protection. It is for this reason that circuits supplying electrical equipment which will be touched by people must have an RCD within that distribution circuit’s protection package.

So, homes, offices and workshops will all have their 240 V single phase circuits protected by a 30 mA RCD. Therefore, when installing a generating set to provide power for such applications there are two aspects which must be determined. The first one is a fundamental need to determine if a 30 mA RCD is a part of the existing electrical distribution system. The second is to ensure that the generating set is installed with a suitably referenced electrical supply configured to provide: Live (L) and Neutral (N) conductors along with ensuring the N point at the ac generator is connected to a suitable and approved Earth (E) connection associated with the applications existing distribution system. Without this N and E circuit any faulty equipment’s leakage current cannot be detected by the RCD.

For such an application there is an additional technical consideration. Having two 30 mA RCDs in series will often promote an issue of nuisance tripping, as would be the case if a single phase output from a generating set was fitted with an RCD, and the proposed application’s distribution network also has an RCD as part of its installed electrical protection package.

It is for this reason that RCDs are available with higher leakage current detection devices. So RCDs with a designed trip of 100 mA and 300 mA are available along with RCD units with an adjustable trip level which enables a trip level to be set appropriate for the intended application.

It has become common practice for certain generating sets to be fitted with 100 mA RCDs which monitor all three phase currents and the returning N current.

These generating sets have been identified as most likely to operate and deliver a three phase electrical output (typically 415 V L-L-L, along with 240 V L-N) and are most likely to be used for applications which will consist of a mix of three phase industrial type loads along with single phase loads e.g.; office / computer type equipment and power tools.

Technical points to be considered.

The inclusion of a 100 mA 3ph RCD must be considered as a most beneficial addition to the normal array of electrical protection equipment incorporated on a generating set. BUT IT DOES NOT PROVIDE PERSONNEL PROTECTION where the 30 mA limit has been set and is covered by numerous Regulations, Codes and Standards.

It does overcome the issue of nuisance tripping which is often encountered when operating two in-series 30 mA RCDs, which can promote the risk of personnel protection devices being disabled.

The use of a 100 mA RCD can provide an excellent warning indication of a weak insulation system in piece of electrical equipment being supplied by the generating set. But as the RCD does not have the capability to identify which piece of equipment, this warning is often disregarded. However, a good technician can use the tripping RCD to help pinpoint the affected circuit and in turn the faulty equipment on that circuit.

Conclusion.

The responsibility for the initial selection and then the safe and effective installation of a power generation equipment package for any application must always be based on a range of considerations.

Appropriate questions need to be asked for each application to determine the suitability of a generating set’s installed RCD. This must be part of a stipulated Company Policy which includes a set routine of investigation to determine the characteristics of the local sites electrical distribution system and then an appropriate installation of the generating set. This process will demonstrate due care; along with the determination of responsibility should the switchboard to which the generating set is to be connected not having appropriate personnel protection.

FAQ 1013

Are you a member? If you are a member, you can use this form to ask AMPS.

If you are not an AMPS member click here to find out more about how to join.