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.

 

[Ian1981]

Hager

 

[Darkwood]

@Ian1981

Why have you posted a chart of energy let through against ampage?
The charts we need are time/current curves for all 3 types of mcb.
Given the 63amp is probably a submain supply we can use 5-seconds fixed load, as we are discussing a 6amp mcb then it is unlikely we need to factor 0.4 in as it makes little difference to the outcome.
63B-6B has FD up to approx 250amps
63C-6B has FD up to approx 610amps
63D-6B has FD up to approx 1300amps

The only thing to be concerned with when using a C or D is that zs is still met, but the majority of installs out there are domestic so we can use the charts directly been single phase in most cases (no need to calculate 400v fault current)

Now after putting all this I personally wouldn't cascade any BS60898 as it is poor design from the off as a rule of thumb but what limits us is the limitations of domestic supply size meaning we often see it done more by ignorance that understanding I might add, it is quite plausible to use other forms of front end fusing like a BS88-II to achieve better discrimination but knowledge of the front end device and its type are needed here as we tend to only get partial between these two devices although submains circuits tend to be less prone to nuisance tripping like when a lamp pops especially if they are not BS60898.

 

[Octopus]

So that's thousands of installs across the UK that are non compliant then ....

 

[Darkwood]

Thousands! - more like millions, well most of them.

Full discrimination for fault currents has never been easy to achieve in domestic given the limited supply rating, discrimination should be calculated and applied where possible, the only negative here is wrong device tripping and in a domestic environment it is not the biggest concern, commercial and industrial though can have big consequences, production hits and even have an effect on the customer base so that is when you need to ensure you get the best discrimination possible and where due to limitations of supply then the customer should be informed and given the options if any available, I had one customer relocate due to this very issue as it was effecting the running of his business.
To note here we are only talking about fault current discrimination and not an overload discrimination, if I remember rightly you need to apply a x2 factor to ensure mcb's are overload discriminant.

 

[Ian1981]

@Ian1981

Why have you posted a chart of energy let through against ampage?
The charts we need are time/current curves for all 3 types of mcb.
Given the 63amp is probably a submain supply we can use 5-seconds fixed load, as we are discussing a 6amp mcb then it is unlikely we need to factor 0.4 in as it makes little difference to the outcome.
63B-6B has FD up to approx 250amps
63C-6B has FD up to approx 610amps
63D-6B has FD up to approx 1300amps

The only thing to be concerned with when using a C or D is that zs is still met, but the majority of installs out there are domestic so we can use the charts directly been single phase in most cases (no need to calculate 400v fault current)

Now after putting all this I personally wouldn't cascade any BS60898 as it is poor design from the off as a rule of thumb but what limits us is the limitations of domestic supply size meaning we often see it done more by ignorance that understanding I might add, it is quite plausible to use other forms of front end fusing like a BS88-II to achieve better discrimination but knowledge of the front end device and its type are needed here as we tend to only get partial between these two devices although submains circuits tend to be less prone to nuisance tripping like when a lamp pops especially if they are not BS60898.

 

[jackhammerJIM]

63B-6B has FD up to approx 250amps

This is not useful given the max zs for a b63 is 0.55 ohms

230 / 0.55
= 418 amps

 

[Darkwood]

Just looked them up in my Schneider books for a standard C60 mcb - mcb discrimination. The values in post 17 are a tad out but were done from memory

63b - 6b is 250amps as I stated (I agree this is a pointless value given max zs but was just showing the relative fault levels)
63c - 6b is 500amps
63d - 6d is 1200amps

I actually see mcb 60898 cascading in industrial and commercial all the time, like I said I wouldn't ever do this as a 60amp BS88-II would provide full discrimination for the lower end mcb's and partial if higher assuming supply size is limited, 60898 are prone to filament lamps nuisance tripping where any BS88 are not do to the nature of operation. (on a plus, filament lamps are now not as common).