Many contractors find earth leakage confusing, but Peter Wade of Megger is ready to help with a useful introduction to this important subject.
What is earth leakage? The IET Wiring Regulations (BS 7671) covers two related definitions. They say that leakage current is “electric current in an unwanted conductive path under normal operating conditions.” And that protective conductor current is “electric current appearing in a protective conductor, such as leakage current or electric current resulting from an insulation fault.” Put simply, this means that earth leakage is electric current that finds its way to earth via an unintended path.
Earth leakage can be divided into two categories. The first is unintentional earth leakage, which results from faulty equipment or insulation. The second is intentional earth leakage, which occurs as consequence of equipment design. For example, IT equipment and high-frequency fluorescent lighting systems often produce small amounts of earth leakage even when they’re operating correctly.
Earth leakage protection methods
Whatever the source of earth leakage, it must be prevented from causing electric shocks, either by making provision for automatic disconnection of the supply (ADS), or by using isolated power systems (IPSs) that incorporate insulation monitoring devices (IMDs). The first method is the most common and is usually based on RCDs (residual current devices) or RCBOs (residual current circuit breakers with overcurrent protection).
RCDs and RCBOs monitor the current flowing in the line conductor(s) and comparing this with the return current in the neutral conductor. If there is a difference that exceeds the sensitivity setting (mA rating) of the RCD or RCBO, the device will trip and open the circuit, as shown below.
Measuring earth leakage current
Most of the time, earth leakage protection works just as intended, but “nuisance” tripping can occur, where the RCD or RCBO trips for no apparent reason. The best way to tackle this sort of problem is to measure the earth leakage current, which is most easily done with an earth leakage clamp meter.
This is clamped round the line and neutral conductors only (NOT the protective conductor) and will measure the difference between the line and neutral currents, which is the earth leakage current (see image at top of page).
This test can be performed at various places in an electrical installation: at the incoming cables, at each outgoing circuit from a distribution board, at midpoints on a radial circuit where live and neutral conductors are accessible, and at connected equipment.
The procedure for deciding which circuit is causing nuisance tripping in a domestic or small commercial installation is to turn off all the MCBs in the consumer unit, and position the earth leakage clamp around the mains incoming cables. Turn on each circuit in turn until a circuit is found that causes the measured earth leakage current to increase significantly. This is likely to be the problematic circuit.
The next step is to decide whether the leakage is intentional or unintentional. If it’s intentional, some form of load spreading or circuit splitting will be needed. If it’s unintentional – the result of a fault – the fault will need to be located and repaired.
This procedure will allow most nuisance-tripping problems to be tackled, but the problem could also be an oversensitive RCD or RCBO. To check this, perform an RCD ramp test. Most 30 mA RCDs and RCBOs will operate somewhere between 24 mA and 27 mA; if they operate at a lower current, this could be the source of the nuisance tripping problem.
Hopefully, this article has shed a little light on earth leakage and helped contractors to ensure installations are safe, and to tackle nuisance tripping which, as the name suggests, can be really annoying!