Instantaneous Trippin/Discrimination

[Nightmare1308]

Hi,

This may sound stupid and I may be missing something but here goes.
I have been pulled by a third party company regarding discrimination of a 50A Type C 60898 3ph MCB feeding a Submain which then has another distribution board feeding a PSU (which has 2 AC circuits and a DC consumer unit) protected by a 32A Type C 60898 3ph MCB.
The part I'm struggling with is how does this not achieve it as the 32A has 320A instantaneous tripping current and the 50A is 500A (0.1-5sec)(BS7671 Page 326).

Could someone point me in the right direction regarding the regulations and thoery please.

I'm expecting to be schooled here as I think I'm missing something simple.

Thanks in advance.

 

[telectrix]

the breaker ratings are too close to each other to achieve discrimination. replace the 50A with a BS88 fuse.

 

[The Ghost]

too close

I am not questioning your statement. I am curious in what way are they too close? The time curve? Or the rating? I can be stupid as well you know, and just want to tease a little more info on this.

 

[Nightmare1308]

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

 

[Ian1981]

Hi,

This may sound stupid and I may be missing something but here goes.
I have been pulled by a third party company regarding discrimination of a 50A Type C 60898 3ph MCB feeding a Submain which then has another distribution board feeding a PSU (which has 2 AC circuits and a DC consumer unit) protected by a 32A Type C 60898 3ph MCB.
The part I'm struggling with is how does this not achieve it as the 32A has 320A instantaneous tripping current and the 50A is 500A (0.1-5sec)(BS7671 Page 326).

Could someone point me in the right direction regarding the regulations and thoery please.

I'm expecting to be schooled here as I think I'm missing something simple.

Thanks in advance.

What if your zs at your 32 amp mcb was say 0.45 then you will have a fault current of 511 amps which will trip the 50 amp mcb.
Remember your max zs for the 32amp type c mcb is only 0.68 ohms

 

[Ian1981]

Fuses or mccb are usually used to achieve discrimination of protective devices

 

[Nightmare1308]

Ian,
Where would I find this information as I checked page 58 showing me,
32A Type C - 0.68
50A Type C - 0.44
Why would the MCB not achieve it.
230/0.68 = 338 (max 320 page 326)
230/0.44 = 522 (max 500 page 326)
Why would they design a fault current the MCB Can't manage?
can you dumb it down as NICEIC confused me saying different.
I appreciate what you are saying with the MCCB and Fuses, just feel I've seen this wrong my whole life from previous people.

 

[Ian1981]

Ian,
Where would I find this information as I checked page 58 showing me,
32A Type C - 0.68
50A Type C - 0.44
Why would the MCB not achieve it.
230/0.68 = 338 (max 320 page 326)
230/0.44 = 522 (max 500 page 326)
Why would they design a fault current the MCB Can't manage?
can you dumb it down as NICEIC confused me saying different.
I appreciate what you are saying with the MCCB and Fuses, just feel I've seen this wrong my whole life from previous people.

If your fault current is close to the fault current required to trip a 50 amp mcb (500amps) any where downstream of the 50amp mcb then this is the first mcb that the earth fault loop impedance will encounter on the loop and thus will operate the 50amp mcb

 

[telectrix]

an example i had of this ( on a small scale in this case ) was a blown lamp in a vivarium. it took out it's 5A fuse in the plug top, but also tripped the 20A MCB in the CU. it's basically that the fault current is > that needed for the higher rated OCPD.

 

[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.