Earth leakage explained

Jan 20, 2024

Earth leakage is electric current that finds its way to earth via an unintended path. There are two categories: unintentional earth leakage, which results from faulty insulation or equipment, and intentional earth leakage, which is a consequence of the way equipment is designed. It seems strange to "design in" earth leakage, but it's sometimes unavoidable – IT equipment, for example, often produces some earth leakage, even when it's working properly.

Whatever the source of earth leakage, it must be prevented from causing electric shocks. This is usually done by using either RCDs (residual current devices) or RCBOs (residual current circuit breakers with overcurrent protection). These measure the current in the line conductor and compare it with the current in the neutral conductor. If the difference exceeds the mA rating of the RCD or RCBO, it will trip.

Most of the time, earth leakage works as intended, but sometimes an RCD or RCBO keeps tripping for no apparent reason – this is "nuisance tripping". The best way to tackle this is with an earth leakage clamp meter, such as the Megger DCM305E. This is clamped around the line and neutral conductors together (but NOT the protective conductor!) and it measures the earth leakage current.

To decide which circuit is causing nuisance tripping, turn off all the MCBs in the consumer unit and position the earth leakage clamp around the mains cables. Turn on each circuit in turn. If one causes the earth leakage to increase significantly, this is likely to be the problem circuit. Next decide whether the leakage is intentional. If it is, some form of load spreading or circuit splitting will be needed. If it's unintentional leakage – the result of a fault – the fault must be found and repaired.

Don't forget the problem might be a faulty RCD or RCBO. To check, do an RCD ramp test. If it's a 30 mA device – the most common rating – it should trip somewhere between 24 and 28 mA. If it trips at a lower current, it probably needs replacing.