Published:
4 August 2008
Category:
Q&A
Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day, about electrical testing and how loop testers work, is answered by Kew Technik (Socket & See):
Question: Most loop testers plug into an AC supply to get their power source. Therefore, do loop testers pass AC current for so many cycles to obtain the AC volt drop and thereby calculate the impedance? Or do they pass DC current to thereby calculate the resistance?
Answer: All the leading brands of instruments claim to measure loop impedance - but the simple truth is that they DO NOT!
The method used to test is to insert a resistor between phase and earth, measure the volt drop and extrapolate the result in ohms. As the resistor is effectively in series, the power factor is unity, so impedance cannot be measured (it is swamped by R).
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