This Question of the Day, concerning the prospective fault current in an electrical installation, (referring to Regulation 612.11 of BS7671: 2008, and page 52 of Guidance Note 3), has been answered by both the ECA and SELECT (Theme – Electrical Testing):
Question: When measuring prospective fault current, I understand it is to be measured at every relevant point under both short circuit and earth fault conditions and the highest of these values recorded. From what I understand, the term 'every relevant point' means every point in the installation where protective devices are required to operate, and this includes the origin.
If I have measured prospective fault current at the consumer unit (origin), am I right in saying that this is where the highest prospective fault current will occur? (i.e. because the resistance of conductors would be relatively low at the origin, and gradually higher as we move further into the final circuits and away from the origin)?
So, if I have measured prospective fault current at the origin, this will be the highest prospective fault current that can occur in the installation, and if this measured value at the origin is equal to (or less) than the breaking capacity of every protective device in the installation, we have compliance for the whole installation. Is this correct?
Answer by ECA: Yes, you are correct. Please remember, for installations with more than one source of supply, or which have a standby generator or UPS, their respective PFCs would need to be checked and recorded also.
Answer by SELECT: Yes, you are correct. The breaking capacity of the overcurrent protection device should normally be greater than the prospective fault current measured, calculated or determined to achieve compliance.
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