Clamp down on earth leakage

19 December 2017

Thanks to changes in the IET Wiring Regulations over recent years, earth leakage protection is now part of almost every new low voltage electrical installation. Sometimes, however, this protection can cause problems of its own. What’s going on and what can be done about it? Peter Wade of Megger has the answers.

Most electrical contractors have experienced that anguished phone call from a customer saying that they have a circuit breaker that keeps on tripping for no apparent reason. And most contractors will know at once that it’s much more likely to be an RCD or RCBO that’s tripping rather than an ordinary MCB. In short, it’s a typical case of “nuisance tripping”. So what’s to be done? Let’s start to answer this by looking at a few earth leakage basics.

Surprisingly perhaps, the IET Wiring Regulations (BS7671) don’t include a definition of earth leakage current, but there are two definitions that are related. Leakage current is defined as “Electric current in an unwanted conductive path under normal operating conditions”, and protective conductor current is defined as “Electric current appearing in a protective conductor, such as leakage current or electric current results from an insulation fault.” From these definitions, it’s clear that earth leakage current is simply current that finds its way to earth via some unintended path.

But where does this leakage current come from? The first possibility is unintentional earth leakage. This is usually the result of an insulation fault that allows current to leak from the live circuits to earth, hopefully via the protective conductor. The second possibility is sometimes referred to as “intentional leakage current”. This results from essential design features in equipment connected to the supply.

It might seem strange to “design in” earth leakage, but sometimes this is unavoidable. The operation of filter circuits in computer or television power supplies relies on capacitors connected to earth, and although the impedance of these capacitors is high, at 50 Hz, they still allow a small “leakage” current to flow. Later we’ll see that, when it comes to tackling nuisance tripping, it’s important to distinguish between unintended and intended earth leakage.

Because excessive earth leakage current is potentially hazardous – the leakage current might, in the worst scenario be flowing to earth through someone’s body rather than through the protective conductor – the IET Wiring Regulations now insist that protection is provided on almost all circuits. In most cases, this takes the form of an RCD (residual current device) or an RCBO (residual current circuit breaker with overload protection).

Both of these devices work by monitoring the current flowing in the line conductor (or line conductors in a three-phase system) and comparing this with the current flowing in the neutral conductor. The difference between these currents is the earth leakage current and, if it exceeds the sensitivity (often called the mA rating) of the device, the device will trip and open the circuit. RCDs and RCBOs used in domestic installations typically have a sensitivity of 30 mA, although devices with higher or lower sensitivity are sometimes used in special applications.

Earth leakage protection usually works very well but, as we’ve already noted, there are cases where it trips repeatedly, apparently for no good reason – this is nuisance tripping and, as anyone with experience of it will confirm, it is aptly named.

Sorting out cases of nuisance tripping can be difficult, but a tool that makes it much easier is an earth leakage clamp meter. When clamped around the line and neutral conductors in a circuit – but NOT the protective conductor – such an instrument will show the difference between the currents flowing in these conductors which, as we’ve already seen, is the earth leakage current. Since it’s a clamp meter, no direct connections are needed to the circuit under test, which means this form of testing is fast, easy and safe to carry out.

Earth leakage measurements can be made with a clamp meter in almost any location where the line and neutral conductors are separate from the protective conductor. Typically locations are at the mains incoming connections, at each of the outgoing circuits from a distribution board, including the sub-main circuits, at intermediate points on radial circuits and at the connected equipment.

A useful technique for locating troublesome circuits in domestic and small commercial installations is to turn off all the MCBs and RCBOs in the consumer unit, then position the clamp meter around the incoming mains cable. If each circuit is then turned on in turn, it will often be found that one causes an unexpectedly large increase in the earth leakage current. This circuit is very likely to be the source of the nuisance-tripping problem.

Sometime, of course, life is not quite so easy, as is the case when the nuisance tripping is the result of an intermittent fault in a particular circuit. In such a case, a clamp meter with a peak hold function can be very helpful, as this can be left in place to monitor a suspect circuit until a trip occurs, and will retain the indication of the maximum earth leakage current that has flowed in that circuit.

In challenging cases, it’s also worth remembering that nuisance tripping can be caused by an overly sensitive RCD or RCBO. This can be checked by carrying out a ramp test; facilities for which are provided by most modern multifunction installation testers (MFTs). A 30 mA RCD or RCBO should operate somewhere between 24 and 27 mA. If it operates at a lower current, then it may be the source of the problem and it should be replaced.

Assuming that the fault is not with the RCD or RCBO and that the problem circuit has been located, the next step in tackling nuisance tripping is to decide whether it results from unintended earth leakage – that is, a fault in the circuit or the equipment connected to it – or from intended earth leakage. If it is unintended earth leakage, the fault must be found and repaired. Typically, this will involve safely isolating the faulty circuit and carrying out insulation tests on the circuit itself and the equipment connected to it.

If it is found that the nuisance tripping is a result of high levels of intentional leakage current, a different approach must be adopted. Most often this will take the form of circuit splitting, where some of the devices with high leakage currents are transferred to a different circuit or circuits so that the earth leakage in each circuit is kept to a level where it won’t cause problems.

Correctly used, RCDs and RCBOs do an excellent job of protecting all of us from the perils of electric shock. There’s no doubt, however, that when nuisance tripping occurs, these devices start to make their presence felt in a most annoying way. Hopefully this article has explained how these situations occur and how they can be addressed without too much trouble – always provided, of course, that the right test equipment is always at hand!