Earth loop measurements are essential in verifying the safety of electrical installations. However, the subject is plagued with misconceptions and misleading claims from test equipment manufacturers. Getting it right requires an understanding of the methods used and their limitations.
One of the common problems with loop testing is that a high current flowing to earth will cause the circuit breaker to trip. One solution is to bridge out the breaker, but don’t forget that it will then be inoperative and the site a potential danger. You also need to remember to remove the bridge afterwards. So, all in all, this isn’t an ideal solution.
Another approach is to use a DC signal to ‘lock’ the trip closed. This usually works well but users should know that it can leave the trip locked and the circuit unsafe for some time after testing. This non-trip feature is also less effective with some types of circuit breaker.
Other manufacturers use low current test methods which do not risk tripping standard 30mA breakers. These are generally less prone to tripping but may not always give a stable answer. They are also more sensitive to the impact of other devices on the same circuit.
A further approach, from Martindale, uses a high-frequency medium current pulse, which does not cause tripping and allows inductive and capacitive effects to be incorporated, giving a better real-world solution. And this is an important consideration because many meters are specified just for laboratory use, so their performance will be very different in the real world where inductance, capacitance and noise are present in many complex ways. Specifications quoted for laboratory use are unlikely to be true in the real world.
All manufacturers recommend that testing is done with no appliances connected so you get true readings without interference. Though, manufacturers’ specifications permit a wide variation in results. Indeed, many manufacturers do not specify the accuracy of their meters below 0.25 or 0.5 ohm.
One common and dangerous misconception is that standard socket testers carry out an earth check – but this is not correct. From a safety viewpoint, it is vital to understand that all these conventional socket testers do is ensure that earth, live and neutral are correctly connected. They will spot the transposition of earth and live or live and neutral, or a disconnection. However, it is impossible for any device to identify that earth neutral reversal. Also, although they verify the presence of the earth, they do not indicate its quality.
Our EZ650 Socket Tester Kit with Earth Loop Impedance can aid you with making sure that you have everything you need for socket testing and is supplied in a compact, soft carry case.
For more detailed listing on our Earth Loop Impedance products, click here.
Paul Wilson
UK & Ireland Sales Manager
Martindale
AN: Socket testing safety
Martindale CP501, BZ101, EZ150 and EZ2500 socket testers