Can the upstream circuit breaker be 16A TP and downstream circuit breaker 16A TPN? Is there an error?

Question

Can the upstream circuit breaker be 16A TP and downstream circuit breaker 16A TPN? Is there an error?

Answer 1

I agree with Steve....He has it right about the upstream breaker and downstream breakers, and fault detection. Its not wrong...just maybe not the way we do it.

I'm a engineer as well

Answer 2

This combination can be used, but it will not provide the appropriate selectivity or discrimination between fault locations.

Basically, any fault that would trip the downstream breaker will also trip the upstream breaker.  If the upstream breaker happens to serve other loads in addition to the 16A TPN circuit, you can be fairly certain that the upstream breaker will trip first for any fault on the 16A TPN circuit.

This is considered a miscoordinated protection system, because it trips more circuits than necessary to remove the fault.  If the upstream and downstream breakers serve only one load, then this is often an accepted compromise where more than one switching device is required for regulatory or safety compliance on a single circuit.

Do not be distracted by the 'n' designation of the downstream breaker.  The neutral part of the 3 pole + neutral device (the TPN) only has significance if the downstream load contains single-phase connections (or in some other particular arrangements regarding neutral-to-earth connections that most likely do not apply here).

Despite the miscoordination, if the 16A circuit breakers provide adequate protection for the conductors and load, then this error is a nuisance more than a hazard, as long as the 'overtripping' does not present a further safety risk.  However, it still should be corrected if more than the one load is served..