Instantaneous Trippin/Discrimination

[davesparks]

This is what they have said but I can't find it in the regs book where it shows this?
As the graph on page 326 shows a clear gap?
Could you explain more into this as I now don't want to have the same mistake again?

Thanks

It's not in the refs book, you need to consult the manufacturers information.
As a general rule of thumb a 60898 mcb wont fully discriminate with another 60898 mcb regardless of rating or type.

 

[Nightmare1308]

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?

Only problem is now I would like to install a MCCB OR BS88 but there is no space to install anything new where the Distribution board is at the origin.

I no this would only ever be a problem if there was a short circuit at the Submain (32A) but it's all relevant.

Well I've been educated today and appreciate your help on this.

 

[telectrix]

Only problem is now I would like to install a MCCB OR BS88 but there is no space to install anything new where the Distribution board is at the origin.

you can replace a 60898 with a din rail fuse carrier.

 

[Nightmare1308]

Is my other method correct still?
I like that and I just checked they do 3ph ones aswell.
For purpose of isolation of that Submain would I need to isolate whole consumer unit (which isn't permitted) to remove fuse or do they have a method where it won't arc under load.
I know you are ment to isolate the load first but thinking of a non skilled persons turning it off.?

 

[davesparks]

Isolate the load before removing the fuse or replacing it.

 

[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