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,

 

[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).