Robert Welch asks this question about a single-phase circulation pump installation and contactor loop measurement values. It has been answered by Schneider Electric (Theme – Electrical Control):
Question: I have two small single-phase circulation pumps, which required having loop measurements taken. This was taken at the local rotary isolators and gave readings around 19 - 20 ohms.
I traced this down to the resistance through the contactor on the live conductor, replacing the contactor with a like-for-like brand (Telemecanique). This has reduced the loop on the one pump to 7.35 ohms, but this is still too high for the 6A BS88 fuse, which requires a maximum Zs of 6.9 ohms. The loop for both pumps, with the contactor taken out of circuit, is 0.3 ohms; at the control panel it is 0.15 ohms.
How can I reduce this further so that it is in line with the regulations?
Answer: I don't know which contactor you are using, but our (TeSys) LC1D range has a very low impedance/pole. Of course, has you go higher in the range, the impedance will get lower and lower.
If you need to associate an MCB, there is also information about the loop Zs of our GV2 range.
You can also use the Schneider Electric website (www.schneider-electric.co.uk), where our entire main catalogues, technical publications and brochures are available to be used and can be downloaded free.
For any web queries, please use [email protected] or call 0870 608 8 608.
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