Discrimantion

[Cj24]

Hi all, if u have one of those little 40amp garage boards obv in your garge with a 40amp main switch in it then u have 6amp for lights n 16amp for socket what size mcb would u say it should be on back at main board, it's an in 2.5 armoured

 

[Octopus]

Distance? Anticipated load? You're the designer.

 

[nickblake]

As murdock has said but its a 2.5mm SWA so realy the maximum you can have is a 20 amp installation methods also come into account

 

[alanl]

It isnt discrimination. That is when you have 2 RCD's or RCBO's in series, and you have to discriminate between them so they dont trip together if a fault develops.

The cable from the CU should be protected by an appropriate MCB. 20A would be good for 2.5mm T+E.
16A and 6A would be fine at the sub CU, as although they add up to more than 20A, in real life we have Diversity, so they are unlikely to try to use 20A, so a 20A MCB would be fine at the main CU end.

Alan.

 

[Cj24]

Sorry was bit vague, it's about 15m and load there just 2 56watt tubes then 2 double sockets, I was thinking a 20amp breaker but will the 40amp rcd main switch come into play first before the 20amp at main board

 

[telectrix]

no, the 40A RCD main switch is not a breaker. the 40A refers to the maximum design current of the RCD.

 

[TPES]

Sorry was bit vague, it's about 15m and load there just 2 56watt tubes then 2 double sockets, I was thinking a 20amp breaker but will the 40amp rcd main switch come into play first before the 20amp at main board

Yes. The RCD will come into play first if a fault occurred. 20a will be overload protection

 

[Chr!s]

Hi all, if u have one of those little 40amp garage boards obv in your garge with a 40amp main switch in it then u have 6amp for lights n 16amp for socket what size mcb would u say it should be on back at main board, it's an in 2.5 armoured

1st Question, do you really need discrimination between the feeder and final circuits?

If yes then you need to consider the types of discrimination.

Overload discrimination and short circuit discrimination.

You then need to look at what devices will be in the circuit. RCD'S, MCB's or FUSE'S.

For RCD'S you will require time discrimination, do you need 30 mA RCD protection for the feeder if so you will loose your Earth fault discrimination.

MCB - MCB Overload discrimination is not a problem, though short circuit discrimination is not so easy.

Far easier is to have:

Fuse - MCB This set up is far better where partial or total discrimination is required for Overload and short circuit discrimination.

Chris

 

[KINKY]

A 16A MCB will not discriminate against a 20A MCB.

 

[zedess]

2.5 swa ....20a mcb....simples!

 

[KINKY]

2.5 swa ....20a mcb....simples!

The 16A MCB protecting the sockets will not discriminate with the 20A MCB feeding the board.

Say there was an accident using a power tool in the garage, 20A MCB feeding the board trips instead of the 16A MCB, taking out all the lights aswell, could be potentail for a dissaster.

Assuming changing the cable is not an option I would put the sockets on a 10A MCB all tho again some 10A & 20A MCB will not discriminate.

Graph shows 16 & 20A MCB protection curves.

 

[telectrix]

or he could use a c type 20A ( Zs being low enough )

 

[Cj24]

Hi, can somebody please explain discrimination for me, because I thought it was breaking a circuit down via different size breakers so certain breakers will trip before others causing minimal disruption to other circuits, for example lamp posts fed from an external lighting board on an rcd has a fuse in the cut out so putting a smaller fuse in the cut out will mean that lamp post will blow it's fuse before tripping the rcd n all other lamp posts

 

[telectrix]

Hi, can somebody please explain discrimination for me, because I thought it was breaking a circuit down via different size breakers so certain breakers will trip before others causing minimal disruption to other circuits, for example lamp posts fed from an external lighting board on an rcd has a fuse in the cut out so putting a smaller fuse in the cut out will mean that lamp post will blow it's fuse before tripping the rcd n all other lamp p osts

that is correct if you are talking of, say a 20A MCB followed by a 6A, but 20A and 16A are too close in their ratings. tripping time tolerances mean that the 20A may break before the 16A.

 

[zedess]

So..dig up the 2.5swa..replace with....6.00mm swa!..stick a 40a mcb on it....to avoid discimination..yes?....or do we live in the real world ..put a 20a mcb on it and know fine well that a 16a mcb will go first in the case of a problem!!..or put it another way, the 2.5mm cable is in the ground..what size breaker do u use ?