Electrical Contracting News (ECN) September 2017

SPECIAL FEATURE TEST & MEASUREMENT out a ramp test using a multifunction installation tester. A 30mA RCD or RCBO should operate somewhere between 24 and 27mA. If it operates at a lower current, then it may be the source of the problem and should be replaced. Where does the leakage current come from? The electrical contractor must establish whether it’s from unintentional earth leakage or intentional leakage current. fl owing in these conductors, which is the earth leakage current. Because no direct connections are needed to the circuit under test, 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. As each circuit is switched on in turn, it will often be found that one causes an unexpected increase in the earth leakage current. This circuit is likely to be the source of the nuisance- tripping problem. Sometimes, however, life is not quite so easy, such as when nuisance tripping is the result of an intermittent fault. In this case, a clamp meter with a peak hold function can be helpful; this can be left in place to monitor a suspect circuit until a trip occurs and will show the maximum earth leakage current that has fl owed 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 ‘Resolving nuisance tripping can be diffi cult, but an earth leakage clamp meter can make it easier.’ Intended or unintended? Assuming the fault is not with the RCD or RCBO and that the problem circuit has been located, the next step is to decide whether it results from unintended earth leakage – from a fault in the circuit or the equipment connected to it – or from intended earth leakage. If it is unintended, 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 results from high intentional leakage current, it may be possible to employ circuit splitting. Here some of the devices with high leakage currents are transferred to different circuits so 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

Electrical Contracting News (ECN) September 2017 | Page 54