Discuss Instantaneous Trippin/Discrimination in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

Reaction score
8
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.
 
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
 
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
 
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.
 
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
 
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.
 
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.
 
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.
 
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.
 
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.

Instantaneous Trippin/Discrimination {filename} | ElectriciansForums.net

 
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.?
 
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.
 
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
 
Last edited:
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.
 
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
 

Reply to Instantaneous Trippin/Discrimination in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock