Dave Osborne asks a question about a high prospective fault current across phase and neutral. It is answered by the NICEIC (Theme – Fire & Safety):
Question: I wonder if you can assist me with an issue I have never come across before?
In measuring the prospective fault current between the phase and neutral, I had a reading of >19.9. The result across the phase was - circuit protective conductor (CPC) 8.7. Does this mean that there is a fault in the neutral side? If so, is there something I should be doing, or do I need to contact the electricity board?
Your help with this matter will be much appreciated.
Answer: In this instance your peak short circuit current reading obtained is nearly twice the measured peak earth fault current and indicates a concern at that point. This could be an indication that the electrical supply cable or transformer may have a fault with it.
Therefore, in the first instance, the distribution network provider should be consulted and made aware of your test results for clarification.
If the substation transformer is in close proximity to the installation, then readings may be higher than expected. However this is likely to be reflected with both readings.
You have indicated that your test equipment did show over range on the test - if the DNO confirms that the supply cable and transformer are safe and adequate, then you may need to confirm with your test equipment manufacturer whether your instrument is capable of carrying out the PFC test and any repair that may be required to your tester - if indeed suspected to be faulty for any reason.
HSE guidance and recommendation is that measurement of fault currents of this magnitude is not recommended and the industry guidance is to safely calculate the PFC likely on the installation.
Regards,
NICEIC Technical.
To see many more Q & A in Voltimum UK's Experts Area, please click on the link:
www.voltimum.co.uk/consult.php?universe=consult.index.questions