Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, concerning a domestic 100A electrical installation and RCBO protection, is answered by the NICEIC:
I have read that a type-tested consumer unit backed up with a 100A main fuse will suffice for up to 16.5kA, regardless of the breaking capacity of the protective devices.
Answer: You have, no doubt, come across the guidance originally provided within Engineering Recommendation P25/1, that indicates at a service tee-off of a public supply network, the typical maximum value of 16kA may be assumed for a single-phase supply where the distributor has provided a service cut-out rated up to 100A.
Typically, consumer units to BS EN 60439-3 Annex ZA are able to withstand the fault current for Ipf levels up to 16kA, where the distributors fuse is a type II, BS1361 fuse rated at no more than 100A (33kA breaking capacity).
Although individual protective devices within the consumer unit may not interrupt the high fault current, the devices should withstand the current until the distributor's fuse operates.
This arrangement is permitted by Regulation 536.1 - Co-ordination of protective devices - General, which states that a lower breaking capacity is permitted if another protective device (a back-up protective device) having the necessary breaking capacity is installed on the supply side and the characteristics of the devices are suitably co-ordinated such that the energy let-through of the upstream device does not exceed that which can be withstood without damage by the downstream device.
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