We recently asked our users to share their concerns and questions about some of the changes coming through in the 18th Edition of BS 7671. Here, Paul Norris from Eaton responds to a question regarding RCD selection.
This Q & A is one of the thousands posted in our Technical Expertise area and answered by our Voltimum Experts.
Question:
As there are different types of RCDs to be used for certain load types & their characteristics, the question arises as to RCD Types in series. i.e. Downstream & Upstream RCDs relative to their protected loads.
RCDs Type ‘A’ and ‘F’ will function safely with smooth DC residual currents present, up to 6mA for a Type ‘A’ and up to 10mA for a Type ‘F’ but they do not detect smooth DC. Therefore they must not be used upstream of Type ‘B’ RCCBs. To the best of my understanding, some RCD-in-series combinations that are not recommended are
1. A type ‘A’ RCD should not be fitted upstream of a type ‘F’ or ‘B’ RCD.
2. A type ‘AC’ RCD should not be fitted upstream of a type ‘A’, ‘F’ or ‘B’ RCD.
3. A type ‘F’ RCD should not be fitted upstream of a type ‘B’ RCD.
This would be because the operation of the downstream RCD which has been selected for the load’s characteristics, could be impaired by the addition of the wrong type of upstream RCD, as in the various combinations above.
My question is, in the examples above, if any of those upstream RCDs are Time Delayed, would that allow them to be used?
e.g. Could an RCD type ‘A-S’ be used upstream of RCD types ‘F’ or ‘B’?
Answer:
Paul Norris, Eaton
No, an RCD type ‘A-S’ should not be used upstream of RCD types ‘F’ or ‘B’. Using an S type (time delay) RCD does not change the guidance on coordination of type AC, A, F & B RCDs, the same guidance applies for non-delay and delay (S type) RCDs.
For further reading on this, and similar topics, we have recently published the following articles:
Consumer Units and RCDs in the 18th Edition BS 7671
Selecting the right MCB or RCBO