correct mcb sizing on sub board??

mmurphy1970

Hi guys just quick question, i have a main board and a sub board in my warehouse/office. the cable going to the sub board is 10mm and theres a 50a breaker protecting it(which seems ok). but then at the sub board there is a 63a switch fuse. it doesnt seem right that a 50a is supplying a 63a. would it be worth changing as i'm gettin a sparks in to do work soon anyway? thanks

Jonny1384

There is no need to replace the 50a I normally fit a 32a MCB at the fuse board in the house running a feed out to a shed. That MCB is to protect the cable going to the sub board. Then have a 63a protecting the sub board.

grousedogtom

It's known as discrimination, where you have a 50 amp in the house the next fuse or mcb further down the line should be smaller than the fuse or mcb feeding it or equal so just put a 50 amp neoz fuse in the warehouse switch fuse unit

Dardania

Jonny1384 wrote: »

There is no need to replace the 50a I normally fit a 32a MCB at the fuse board in the house running a feed out to a shed. That MCB is to protect the cable going to the sub board. Then have a 63a protecting the sub board.

A 32A MCB or 40A sounds right to me to protect the supply going out to the sub board, presuming you have a 63A MCB for the main board? I would reckon a C type 40A would be best... What size are the breakers feeding from the sub board - 20A RCBOs? If they're type Bs, it should work fine - I'm not at my PC otherwise I could check with curves software

There's no need for an MCB or fuse on the incomer to the sub board if the supply cable is providing the protection.

Also, see this for discrimination info: http://www.voltimum.co.uk/news/9289/cm/q---a-of-the-day---discrimination-information.html

mmurphy1970

thanx for replys, yes well there is a 63a main switch fuse at main board which then feeds 50a breaker for the sub board,sub board it only has 4 or 5 20a rcbo and few mcbs i tink. its only lighting and sockets its supplying. its the discrimination i was thinking about, it has been working fine but going by rules and regs the sub board should have 50a fuse or lower. i understand its not always done as my own shed has 63a switch fuse supplyed by 32 a breaker in house. just curious ye're thoughts if its kinda techinally right or wrong

Dardania

mmurphy1970 wrote: »

thanx for replys, yes well there is a 63a main switch fuse at main board which then feeds 50a breaker for the sub board,sub board it only has 4 or 5 20a rcbo and few mcbs i tink. its only lighting and sockets its supplying. its the discrimination i was thinking about, it has been working fine but going by rules and regs the sub board should have 50a fuse or lower. i understand its not always done as my own shed has 63a switch fuse supplyed by 32 a breaker in house. just curious ye're thoughts if its kinda techinally right or wrong

It's not ideal, but its not unsafe. If there's a fault in your office, it could possibly bring down the main board

If you want to look into it more, this is the sw to simulate : http://www.soft.schneider-electric.com/curve-direct.html

mmurphy1970

will do thanks for the info

2011

Dardania wrote: »

A 32A MCB or 40A sounds right to me to protect the supply going out to the sub board, presuming you have a 63A MCB for the main board?

+ 1

There's no need for an MCB or fuse on the incomer to the sub board if the supply cable is providing the protection.

+ 1

I would agree, but would add that there should be a single point of isolation on all distribution boards including the sub-distribution board.

If protective devices are of the same type (such as B type MCBs or BS88 fuses) discrimination is normally achieved when the rating of the upstream device is at least 1.5 times that of the next downstream device.

A 32 or 40A MCB (B type or C type) should provide the required discrimination (with the 63A main fuse) under overload conditions. However it most probably won't under short circuit conditions. In most cases fuses blow far quicker than MCBs can operate once the fault current is very large as it is under short circuit conditions. I saw this happen last year when an upstream 250A fuse blew when we had a short across busbars within a sub-distribution board that was protected by a 63A MCCB.

M cebee

that seems surprising
i don't ever recall a 63amp neozed blowing on a domestic short circuit
anyhow

i know the short-circuit currents would be higher in industrial installation's

frankmul

This may be of interest
http://www.aerco.co.uk/images/sub_sub_categories/Elfa-Plus-technical-MCB(1).pdf
Informatin on discrimination between protection devices from page 17 on. More for industrial that domestic.

Dardania

frankmul wrote: »

This may be of interest
http://www.aerco.co.uk/images/sub_sub_categories/Elfa-Plus-technical-MCB(1).pdf
Informatin on discrimination between protection devices from page 17 on. More for industrial that domestic.

very handy reference guide