Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day, about electrical testing and why some EFLI testers trip RCDs, is answered by Kew Technik (Socket & See):
Answer: Loop testers inject a fault current between phase and earth by switching into the circuit under test and then measuring the volt drop to calculate the RESISTANCE (very often INCORRECTLY referred to as IMPEDANCE) path between phase and earth. In the 1990s, typically this would be a 10ohm load causing around 25A to flow between phase and earth, which a 30mA RCD sees as a massive fault and clears it by tripping.
Even modern testers, which tend to use lower test currents of typically <10A, have to use some pretty fancy technology to avoid tripping, which is why most non-trip testers can take up to 20s to produce a reading.
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