Q & A of the Day – How to test for RCCB/fuse malfunction?

Question: I would like to seek clarification as to whether the following interpretations are correct:

1. The loop impedance of a circuit would determine whether the protective device (circuit breaker or fuse) would disconnect within a particular time (as per table in BS7671) when there is a fault (i.e. short circuit).
2. If this particular circuit is also protected by an RCCB, this would provide earth fault protection. For the purpose of testing the functioning the RCCB, an RCCB tester would suffice to determine whether the RCCB is functioning correctly.
3. Therefore, the loop impedance would have no effect on the functioning of the RCCB tripping during earth fault. But the loop impedance would have an effect, whether the circuit breaker or the fuse would trip - if there is a short circuit.
4. Despite testing the loop impedance of the circuit, it is only showing the appropriate compliance to BS7671 with regards to the protection of indirect contact.

However, is there any way of testing whether there could be malfunction of the tripping mechanism of the circuit breaker or fuse, where it doesn't trip despite being tested to comply with the BS7671? I have seen cable heating up until it burned, but the protective device did not trip - even though the test records show compliance.

Answer:

1. The table to which you refer to is probably 41.3. This relates to earth fault loop impedance.
2. Correct.
3. For earth fault loop impedance values relating to residual current devices (RCDs / RCCBs)) refer to table 41.5.
4. There is no safe method of checking short-circuit operation - trust the device to be manufactured to the appropriate standard. Refer to Regulation 133.1.1. Further, refer to the time/current characteristics of protective devices in Appendix 3 of BS7671:2008.
   

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