Using Type D MCB for discrimination

[Michaelwgroves]

I've just been on phone with Hager designing domestic distribution board design. Engineer advised using Type D 63A in distribution board supplying CU to give better discrimination. Looking at tables anything over 10A on CU might also trip Type D, but it does give a better chance.
So I need to make sure this is okay, I've found reference to 16th edition Regulation 413-02-08. But this has been moved in 17th edition and I can't find anything. Has anyone got any pointers and thoughts about using Type D breakers for domestic distribution panel?

 

[westward10]

Have you checked out the maximum Zs on a D, 63A.

 

[DPG]

Wouldn't a fuse be better? You would get your discrimination then.

 

[Ian1981]

Well using a type D mcb @ 63 amp would give a maximum zs of 0.35 ohms for a maximum disconnection time of 5 seconds being a distribution circuit, that’s without the 0.8 correction factor
Typically it’s very difficult to achieve your zs values using type D for lower rated mcbs using 0.4 seconds as their maximum disconnection times as the typical zs values are Uo x cmin/ In x 20

I would suggest using a bs 88 fuse for protection of your submain tho.

 

[Octopus]

Its an interesting thread this ...................... there are so many submains in service supplied from standard type B MCB's .............. including one in my shed.

But if this is protection from the DNO fuse to the CU what do you do if the DNO fuse is 60A

 

[Ian1981]

 

[Strima]

Its an interesting thread this ...................... there are so many submains in service supplied from standard type B MCB's .............. including one in my shed.

But if this is protection from the DNO fuse to the CU what do you do if the DNO fuse is 60A

One of our customers has some telecoms modules with no discrimination. Had a 16 amp C type trip out and it also took out the distribution circuit in the generator room and the main incoming 160 amp MCCB.

Luckily the DNO 200 amp fuses never broke a sweat...

 

[Leesparkykent]

You are only going to achieve partial discrimination at the very best. It's just carp design from the onset. Multiple 60898's in series for a supply to a shed isn't really going to be a drama if it trips and takes out the upfront Circuit breaker. Put the same situation in a factory where taking out the upfront circuit breaker could lead to loss of production and consequently loss of thousands of pounds...then you have a real problem. Also worth considering that due to lack of discrimination you may not be able to minimise disruption in the event of a fault.

 

[buzzlightyear]

Engineer advised using Type D 63A in distribution board supplying CU to give better discrimination

best with bs88 60amp has Ian Gallagher 1981 says.

 

[buzzlightyear]

MW, is this for Buckingham palace you working on in 5 years time
time lord!

 

[Leesparkykent]

Its an interesting thread this ...................... there are so many submains in service supplied from standard type B MCB's .............. including one in my shed.

But if this is protection from the DNO fuse to the CU what do you do if the DNO fuse is 60A

pick a 100A one up from the wholesalers and put it in place of the 60A one...play dumb if the DNO ask anything

 

[buzzlightyear]

pick a 100A one up from the wholesalers and put it in place of the 60A one...play dumb if the DNO ask anything

and rollover on the floor like a dog .

 

[Michaelwgroves]

Wouldn't a fuse be better? You would get your discrimination then.

Distribution board start to get big an expensive with fuses, unless you can recommend something smaller and more cost effective?

 

[Michaelwgroves]

Well using a type D mcb @ 63 amp would give a maximum zs of 0.35 ohms for a maximum disconnection time of 5 seconds being a distribution circuit, that’s without the 0.8 correction factor
Typically it’s very difficult to achieve your zs values using type D for lower rated mcbs using 0.4 seconds as their maximum disconnection times as the typical zs values are Uo x cmin/ In x 20

I would suggest using a bs 88 fuse for protection of your submain tho.

That's the answer I was looking for...... well actually not what I wanted but that kills this option dead. Looking at old EIC Zs up to 0.9, typically around 0.6

 

[DPG]

Distribution board start to get big an expensive with fuses, unless you can recommend something smaller and more cost effective?

Fuse holder module??

 

[Michaelwgroves]

But if this is protection from the DNO fuse to the CU what do you do if the DNO fuse is 60A

DNO fuse 100A

 

[buzzlightyear]

DNO fuse 100A

or 6in nail !

 

[Rocboni]

Fuse holder module??

You only get standard size fuseholders up to 32A any bigger and its a double module, so the busbar won't fit.

What not individual switch fuses?

 

[Michaelwgroves]

Fuse holder module??

Interesting, got any links I can look at?

 

[buzzlightyear]

What not individual switch fuses?

did tell him pike !