Instantaneous Trippin/Discrimination

[davesparks]

So my understand now is if you feed a outbuilding on a 63A and have a 50A shower on the Submain board this would also not achieve discrimination?(Just an example).
Maybe if I Install a 63A type C instead of the 50A it would then achieve discrimination?

No a type C won't help. The type of the mcb relates to its ability to handle inrush current and not its behaviour under short circuit fault conditions.

At a guess you'd need the distribution circuit to be at least 80A protected by a bs88 fuse to be able to discriminate with a 50A mcb.

in some installations it is not possible to achieve full discrimination where the company fuse just isn't big enough to be able to discriminate with. For example on a standard domestic 100A supply it's unlikely to be possible to achieve full discrimination on a larger circuit.

 

[Ian1981]

A type c will trip at between 7-10 times it's rated current between 0.1-5 seconds which is why it's max zs is lower than that of a type b and is normally associated which inductive loads

 

[Nightmare1308]

That's why I use a TYPE C due to its inductive loads, as the PSU has 100A DC breakers within it fed via the -48v DC system.
Is the calculation in the regs book for this in chapter 43
• 434.5.2
- t=k2xS2 / I2
just so I can check next time I do a circuit design it won't / Prevent happen again.

Another question i want to do training on this now so would I have to do a 2396 course?

Thanks again all.

 

[Ian1981]

That's why I use a TYPE C due to its inductive loads, as the PSU has 100A DC breakers within it fed via the -48v DC system.
Is the calculation in the regs book for this in chapter 43
• 434.5.2
- t=k2xS2 / I2
just so I can check next time I do a circuit design it won't / Prevent happen again.

Another question i want to do training on this now so would I have to do a 2396 course?

Thanks again all.

For disconnection times lower than that given in bs7671 manufacturers let through energy needs to be obtained so that the cable can withstand the fault current.
I2t <k2S2
Where k is the material operating temp and S is the CSA of the cpc.
The formula you have stated will give max time the cable will withstand the fault current before damage occurs

 

[Nightmare1308]

That's great thanks, I'll make sure I note thease for future reference.
Appreciate everyone's help and time.
Thanks for not roasting me