Main and sub distribution boards - Circuit Breakers

I was about to respond to this when Mike's response popped up. However I will still add to what he has said.

Regarding 1, I am not sure what you mean by ‘check manually’. The way to check is to look at the current-time characteristics of the two breakers and see if there is an overlap (where an overlap indicates lack of discrimination). As the discrimination is achieved either gestcurrent or time (or both) the fact that both are 25A breakers and both MCBs (and hence unlikely to have adjustments) suggests to me that you do not have any discrimination between the two (and if they are identical breakers you can be certain of this). It is possible to get current injection equipment to test this but unlikely to tell you anything you can't see from the current-time graphs. This does ignore the fact that the upstream breaker will have a higher current than the downstream breaker by virtue of the other circuits in operation, but at fault current levels this will probably not make that much difference.

Regarding 2, replacing the breaker with a lower rating CB is an option, but only if you have the option of doing so. If you have a 20A load then putting in a 10A or 16A breaker is not practical. Also don't forget that if you have a motor the starting current shouldn't be tripping the breaker so a 16A flc motor may need a larger breaker rating than 16A.

I am not aware of any standard which limits the number of CBs on a secondary distribution board. It is normally down to how many circuits are needed and the size of the board (that is, how large you can make it to fit in the space available) as you can only fit in as many CBs as there is space available, which may drive you to multiple DBs.

For the rest, I will bow to the wisdom of mapj1….

Alasdair

I was about to respond to this when Mike's response popped up. However I will still add to what he has said.

Regarding 1, I am not sure what you mean by ‘check manually’. The way to check is to look at the current-time characteristics of the two breakers and see if there is an overlap (where an overlap indicates lack of discrimination). As the discrimination is achieved either gestcurrent or time (or both) the fact that both are 25A breakers and both MCBs (and hence unlikely to have adjustments) suggests to me that you do not have any discrimination between the two (and if they are identical breakers you can be certain of this). It is possible to get current injection equipment to test this but unlikely to tell you anything you can't see from the current-time graphs. This does ignore the fact that the upstream breaker will have a higher current than the downstream breaker by virtue of the other circuits in operation, but at fault current levels this will probably not make that much difference.

Regarding 2, replacing the breaker with a lower rating CB is an option, but only if you have the option of doing so. If you have a 20A load then putting in a 10A or 16A breaker is not practical. Also don't forget that if you have a motor the starting current shouldn't be tripping the breaker so a 16A flc motor may need a larger breaker rating than 16A.

I am not aware of any standard which limits the number of CBs on a secondary distribution board. It is normally down to how many circuits are needed and the size of the board (that is, how large you can make it to fit in the space available) as you can only fit in as many CBs as there is space available, which may drive you to multiple DBs.

For the rest, I will bow to the wisdom of mapj1….

Alasdair