Discrimination

[Skyba]

A quick question on discrimination. Now in an ideal world we'll always have a larger breaker closer to the board and work down but I'm on a job where I have a 32A feed already in place and the customer wants a new board putting in and the lighting circuit replaced. Now I'm putting in a bog standard RCD garage board and there will be a 6A lighting and a 32A ring on it. Now because the main switch will be rated higher than the feed do you think adding a type C or a time delay mcb at the main board will suffice? Also can you get the retrofit wylex plug ins in time delay or type C??


Go easy on me, I'm only just setting out to do domestic I'm used to shiny new industrial buildings where everything is installed as per regs with extra wiggle room and no faffing about.

 

[Skyba]

Oh and replacing the feed to the board is not an option (wish it was but ta too much work due to the position of the garage)

 

[telectrix]

forget the switch rating, it;s irrelevant. you have a 32A MCB feeding the supply to the garage, to comply with discrimination, fit a 20A or 25A MCB to your RFC in the garage.

 

[Andy78]

What size and type cable do you have feeding this garage board ?
Why does the rating of the MCB protecting this feed have to change due to the rating of the main switch in the garage DB ?

 

[Skyba]

Cheers tel, by the way your inbox is full lol.

I did think that the mcb for the circuits would provide the discrimination but was unaware if this would be "bad practice" even though it is safe.

 

[telectrix]

the problem is that if you fit two 32A MCBs in series, then a fault could trip the one in the house CU before the one in the garage. it's not really safe, though, client may have to struggle in the dark to go back to the house in the dark, will clear inbox now, cheers.

 

[Skyba]

Yeah this was my thinking of fitting a time delayed mcb at the board but as you said if I rate the garage mcb lower then it would be a much better solution.

As much of a pain in the arse all this old school domestic can be I'm loving it really, I'm trying to use my brain again, all my learning will start flooding back to me, metal munching in huge industrial sites has drained my brain for too long.

 

[Guitarist]

Yeah this was my thinking of fitting a time delayed mcb at the board but as you said if I rate the garage mcb lower then it would be a much better solution.

As much of a pain in the arse all this old school domestic can be I'm loving it really, I'm trying to use my brain again, all my learning will start flooding back to me, metal munching in huge industrial sites has drained my brain for too long.

Could you elaborate on that please.

 

[Skyba]

Well if I fitted a type c or a delayed trip at the source this would mean that if there was a 32a breaker in the garage and one at source the delay would ensure that the 32a in the garage would trip out rather than the one at the main DB.

 

[Lucien Nunes]

Does the expected demand require a 32A RFC? If not, you don't have a problem, make it a 20A circuit. If so, the best solution hinges on the rating of the submain. If its size / install method / ambient temp etc allow you to beef up the breaker in the main board, then do so. If you can't increase it, or despite the increase you still can't get discrimination, downsize the MCB for the RFC as a second-best solution.

Incidentally a type C will not discriminate relative to a type B on overload.

 

[Engineer54]

Well if I fitted a type c or a delayed trip at the source this would mean that if there was a 32a breaker in the garage and one at source the delay would ensure that the 32a in the garage would trip out rather than the one at the main DB.

There is no such animal as a time delayed MCB, so where you got that one from i have no idea!! I doubt very much if you'll ever pull 16A in a garage setting let alone 32A. So a 16A radial will be adequate in the majority of domestic garages....

 

[Guest111]

There is no such animal as a time delayed MCB, so where you got that one from i have no idea!! I doubt very much if you'll ever pull 16A in a garage setting let alone 32A. So a 16A radial will be adequate in the majority of domestic garages....

Depends on the number of heaters in there,these pot plants like it hot and humid you know:shades_smile:

 

[Engineer54]

Depends on the number of heaters in there,these pot plants like it hot and humid you know:shades_smile:

In that case the 32A will be for the lighting circuit, to cater for all those heat lamps!!....

 

[Guitarist]

Well if I fitted a type c or a delayed trip at the source this would mean that if there was a 32a breaker in the garage and one at source the delay would ensure that the 32a in the garage would trip out rather than the one at the main DB.

As Eng has said, no such thing as a time-delayed MCB as such. Take care with terminology

 

[Darkwood]

Your unlikely to ever achieve discrimination in a domestic set-up at most you may gain partial discrimination, having a C type upstream of a B type will not afford you discrimination either .... a short circuit is just as likely to take out a 16amp mcb as it is to take out the front 'say' 40amp mcb... they only attain partial discrimination up to a certain fault current so above that they exists no discrimination regardless of type and as most fault currents are 1000amps + then only true discrimination would exist between a 6amp mcb and a 63amp mcb if my memory serves me correct.