Earth Leakage Protection

You may meet them as RCDs (Residual Current Devices), ELCBs (Earth Leakage Circuit-Breakers), CBRs (Core Balance Relays) or GFIs (Ground Fault Interrupts) but whatever your background and whichever TLA[1] (Three Letter Abbreviation) you use, earth leakage devices are not only a crucial component in your electrical safety scheme, they are frequently also required by regulation. Let's look at what they do, how they do it and how we can use them to improve safety without causing too many unnecessary disruptions to what we do.

Whilst overload circuit breakers are designed to protect the power distribution system, earth leakage devices are only useful for protecting people from electrocution. An overload circuit breaker is designed to interrupt a circuit when the current exceeds a preset limit. In a thermal type breaker, a small over-current (due to overloading) can be tolerated for a prolonged period before tripping, although a large over-current (due to a short circuit) will trip fairly quickly. A magnetic type breaker will trip very quickly once its threshold current has been reached. As the current necessary for fatal electrocution is less than a couple of Amps for a duration of less than a couple of seconds, an overload breaker offers virtually no protection from electrocution. Earth leakage protection devices are designed to trip for fault currents between 10 and 100 mA and for interrupt times between 40 and 100 milliseconds after a fault current is sensed.

Earth leakage devices are based on the principle that the amount of current entering a device should be exactly the same as the amount of current leaving the device and that any discrepancy is due to current flowing somewhere that it shouldn't, which is a bad thing. If an electrical appliance such as a luminaire is working properly all electrons entering the luminaire will flow in down the active wire, around through the filament of the lamp and out again down the neutral wire[2]. If there is a fault in the wiring which somehow allows the body of the appliance to become connected to the active line, then when someone touches the appliance, by being elecrocuted they provide a path to earth for the current which is not via the neutral; this is the situation for which an earth leakage protection device is employed. An overload circuit breaker will not care that a small, albeit fatal, additional current is now flowing.

In a protection device an earth leakage current is sensed by comparing the active and neutral currents flowing through a circuit, usually by the simple means of measuring the current induced in a coil through which both of the wires have been passed. Under normal conditions, as there are identical currents in both wires and the currents will be flowing in opposite directions, there will be no current induced in the coil. The current induced in the coil by a fault condition can be used to trigger an interruption to the circuit.

In the best of all possible worlds every electrical circuit would be protected by an earth leakage device, but of course there is a down side to their use: firstly they're not cheap, but then again safety rarely is; more importantly they're subject to nuisance tripping. It seems that there are a number of circumstances where there may be an imbalance between the active and neutral currents without any safety implications. Some inductors, particularly the ballasts of discharge lamps and the chokes on phase-control dimmers (the ones in all of your dimmer racks), quite commonly have a small and often variable amount of earth leakage; put several of them on the same circuit, protected by the same earth leakage device, and you have an unscheduled blackout just waiting for the most inconvenient moment[3].

The ideal way to avoid a major production catastrophe is to place each device in your electrical system on its own rather costly earth leakage protected circuit ensuring that only the faulty device will be disconnected. A more cost-effective alternative may be to place devices with known leakage on to a circuit which has an earth leakage protection device with higher activation threshold or one designed specifically to work with the ballasts of discharge light sources. Some devices may actually be so reduced in operational effectiveness that they are better installed on circuits not fitted with an earth leakage device. These variations are often only possible in fixed installations and must always be done in consultation with an electrician or electrical engineer who is fully cognisant with the safety and operational requirements of the situation.

Earth leakage protection is an important part of the electrical safety scheme for any system, although it may present some implementation difficulties where there are conflicts between the safety ideal and the operational reality. It is vital for us to make certain that the system that we are using is configured to meet the best possible balance of needs.

[1] Ok, so one of the list is a four letter abbreviation. The correct term for these is ETLA (Extended TLA).
[2] Or the reverse of this, depending on which half-cycle of the mains we are examining

[3] The memory of losing all lighting on a live outside broadcast due the leakage in the ballast of a discharge followspot is still crystal clear decades later.

by Andy Ciddor
© copyright The Kilowatt Company