MCB discrimination ...

[Soulsurfer]

Hey I had a callout referred by a lad working for me yesterday where one of the old dimplex CXL I think it is storage heater with an on peak convector heater built in had a fault and took out the property supply.

He had attended the call and said that the B16 Hager at the board had not tripped at off peak or on peak but found that in the riser to the other flats that the B63 Hager had indeed tripped next to the 4 pole switch and so taken the entire on peak / general service feed to the flat consumer unit out.

I would've thought that if there was a high enough fault current or short that it would have taken the 16A before the 63A at source ?

I will be going there tomorrow but just wondering if anyone has had an issue such as this before ? Thanks,

 

[Leesparkykent]

Discrimination between mcb's is typically hard to achieve. Having two in series like you have on this job is poor design. IIRC the the upstream MCB would roughly need a current rating of around 100A to discriminate with the 16A MCB if they are both type B...It would be better to have a fuse as the protective device for the distribution circuit..I would discuss the benefits of changing what ever the existing set up is to a switch fuse.

 

[Soulsurfer]

Discrimination between mcb's is typically hard to achieve. Having two in series like you have on this job is poor design. IIRC the the upstream MCB would roughly need a current rating of around 100A to discriminate with the 16A MCB if they are both type B...It would be better to have a fuse as the protective device for the distribution circuit..I would discuss the benefits of changing what ever the existing set up is to a switch fuse.

Thanks Lee, seems to be a very used method over here unfortunately, 4 pole main switch controlling both supplies and then a 63A mcb each side isolating the line to the boards. He said consumer unit is a mess too and apparently a weird kind of stranded mains cable so I think must be split concentric to the flat board. Thanks again

 

[Darkwood]

Just for future knowledge, you would need a 160amp MCCB to give full discrimination against a standard MCB board with max 63amp ratings, anything below that only gives partial and at the fault currents we tend to see then partial means pot luck as either mcb is likely to trip first, if I recall a 63amp mcb only gives full discrimination on 6 and 10amp mcb's if fault current low enough, again above that and you might as well flip a coin as to which series device will trip first.

 

[Soulsurfer]

Just for future knowledge, you would need a 160amp MCCB to give full discrimination against a standard MCB board with max 63amp ratings, anything below that only gives partial and at the fault currents we tend to see then partial means pot luck as either mcb is likely to trip first, if I recall a 63amp mcb only gives full discrimination on 6 and 10amp mcb's, again above that and you might as well flip a coin as to which series device will trip first.

Brilliant thanks you guys. Checked today, stripped and cleaned and tested elements etc. All alright today !

 

[jackhammerJIM]

You won't get discrimination between a 6 and 63 .

 

[Ian1981]

All depends on the fault current really as a fault current of 315 amps or higher will trip a 63 amp type B mcb.
In fact a current of 189-315 amps may do it.

 

[Darkwood]

You won't get discrimination between a 6 and 63 .

Can you cite you claim here, you will get partial discrimination anyway because of the different ratings but Ill beg to differ on your claim of no full discrimination between 63 an 6 as I assume that is what you meant.

 

[jackhammerJIM]

yes partial discrimination but what good is that ?

 

[Darkwood]

yes partial discrimination but what good is that ?

Partial discrimination can be fine if your fault current is sufficiently low enough but like my post said, in most cases the fault current is large enough to mean it's a game of pot luck unless of course you have full discrimination, Ill ask again can you cite any charts/info to back up your claim that a 63 amp mcb will not fully discriminate to a 6 amp - I have some very useful discrimination charts from that say otherwise.

 

[Risteard]

No two BS EN 60898 circuit breakers of any rating will reliably discriminate.

 

[bigspark17]

Approach owner and look to install fuse & fuseholder in place of mcb. Obviously checking max zs complies.

 

[Darkwood]

No two BS EN 60898 circuit breakers of any rating will reliably discriminate.

Unless you have the manufacturers assurance that they will, the above statement is a broad generalisation, quality of product can offer better discrimination, in reflection I should have made that clear in my post but a low fault current means you cannot make a sweeping statement like has been although like I said under your average fault current then we have partial.

 

[jackhammerJIM]

Yes i have seen a fault blow a 63 amp mcb protecting a sub main . The fault was on a piece of 1mm protected by a 6 amp mcb .

 

[Leesparkykent]

I have a Schneider chart that shows discrimination is achieved between 6A and 63A type B circuit breakers up to a short circuit current of 250A.