Coding question - 8.5kw shower, B32

[loz2754]

Small overloads of long duration are specifically mention in the Regulations as something to be avoided.
The way an MCB works for overloads of this nature is by heating a bimetallic strip. When this strip heats up it bends. Repeatedly causing metal to bend and relax induces stress into the metal. Eventually it will break. Thus the MCB will no longer function correctly and could lead to it not responding to an actual overload, which could in theory start a fire.

 

[VWWM 1]

Small overloads of long duration are specifically mention in the Regulations as something to be avoided.
The way an MCB works for overloads of this nature is by heating a bimetallic strip. When this strip heats up it bends. Repeatedly causing metal to bend and relax induces stress into the metal. Eventually it will break. Thus the MCB will no longer function correctly and could lead to it not responding to an actual overload, which could in theory start a fire.

Very good point for this thread but a ring will get consistently high loads especially when it had appliances connected for similar times if not for longer.... the issue is I see is if the Pullcord or switch is breaking down as a safety issue.

 

[brianmoooore]

I don't think there's many on here who think this overloaded MCB shouldn't be coded. C2 or C3 is the question.
As I wrote earlier in the thread, the shower cable needs a clamp meter applied to it, to see just what current it draws. Its theoretically just under 34A, but if the shower is a good distance from the CU, the voltage drop in the cable might reduce the current enough to drop it down to or below 32A

 

[telectrix]

Nominal UK voltage is 230V, so if that's the shower in question, it's rated at 7.8kW, which equates to a shade under 34A, so not overloading a 32A MCB by any great amount. Still an overload though, so needs coding.

this is it, it's not that the shower will draw more current @ 240V<,just that it's kW rating will be higher than at 230V. the current drawn being I = V/R. and P = V x I.

 

[andyb]

I suppose it falls foul of 433.1,

Coordination between conductor and overload protective device
Every circuit shall be designed so that a small overload of long duration is unlikely to occur. 433.1.1

This is a small overload which will probably never cause the MCB to operate, so code 3 for me.

 

[Pretty Mouth]

Thanks for all your comments so far, as always greatly appreciated.

I'm currently inclined to go for a C3 myself. I see this set up regularly, sometimes with >10kw showers on a 32A breaker, and have yet to see any visible problems as a result. TBH I wouldn't have given it this much thought, but it'll be a board change if C2.

In a optimistic mood, I've fired off a round of emails to various manufacturers asking if using their MCBs in this way would cause any damage long term, so we'll see what they say.

 

[mattg4321]

C3 for me all day long.

 

[loz2754]

C3 for me all day long.

You will be nice and clean?

 

[Pretty Mouth]

Well fair play to the manufacturers, all but 1 of them I contacted got back to me on this. Interesting to note the diverse replies I received. This is the email I sent out:


Good morning,

I have a technical query regarding your MCBs.

An MCB is subjected to a small overload of short duration (eg. a B32 MCB subjected to 40 A for 10 minutes), but not long enough to operate the MCB. This happens repeatedly, say once or twice a day for 25 years.

Could being used this way eventually damage the MCB?
In what way would the MCB be damaged?
If the MCB did fail as a result of the above, would it fail safe (open circuit) or closed?

Kind regards,
PRETTY MOUTH


And here are the replies:


CHINT:

The situation you are talking about would not cause any long term damage.

In the real world the only real way to damage an MCB is to have a dead short. This can create 1000’s amps of current for a split second until the MCB trips.

This is called the kA rating. Most of our MCBs are either 6 or 10 kA (6000 or 10000). A 6ka rated MCB with survive a 6000A short circuit and still be useable in the future.



HAGER:

Using an MCB in this way would not be a good design practice, and may over time be damaging to the MCB, I can not say if it fails it will fail open or closed.

Importantly you should consider is the circuit correct to carry this extra current, cable size? And possible damage to this cable over time.

This is possibly a shower circuit with an upgrade to a higher wattage shower but using the original breaker and cable.



WYLEX:

Please see below from our test lab.

Although it appears that the mcb is operating satisfactorily without further information it’s not possible to do more than make recommendations.

The 32 A mcb is being subject to an overload which is thermally stressing it and possibly any cable between the mcb and the load. It would be better if the mcb was replaced by a 50 A mcb which would then be operating well within its capabilities.

Could being used this way eventually damage the MCB?

Yes.

In what way would the MCB be damaged?

As the mcb is being thermally stressed the mouldings could be being burnt, when the contacts operate they could be subjected to degradation due to the higher current producing more arcing.

If the MCB did fail as a result of the above, would it fail safe (open circuit) or closed?

The worst outcome would be that the mcb would fail, which could cause a fire. If the mcb is in an insulated consumer unit this could cause a house fire.



BRITISH GENERAL:

The product has been designed to accommodate this load demand for the periods as given in the BS7671 regulations and should not affect the performance in anyway.

It is more prolonged overloading or fault operations that could reduce the products life expectancy as there is a limit as with all manufactures on how many times the product can withstand this before it fails typical.

For the manner the unit would fail we would expect it fail in the off position. But this would depend on the fault that had occurred at the time. This is why now in domestic situations an RCD is required for all circuits.

I have attached a copy of our data sheet which includes the operations switching and mechanical operation values.


The BG data sheet says 'Endurance Operations : Electrical 4000, Mechanical 10000.

Crabtree also got back asking which range I was referring to, and contactum didn't reply.

 

[Deleted account]

Chint and BG I think give the best factual response. They don’t start quoting regs or cable sizes.

They stick to the question put to them.

good response from others though.

I think in future I will be putting C3 for this one.

 

[loz2754]

I think these responses back up my thoughts that it should be a C2.

 

[pc1966]

Here I would query the "long duration" of a small overload. MCB curves allow for trip times from about an hour all the way down to the 10ms or so magnetic trip time.

To me a long duration is then something lasting longer than an hour that won't trip the MCB but is above the ratings. For any sane use of a shower I doubt it would be on for more then 30 minutes once or twice a day, so unless you already see signs of thermal stress, I would say C3 as poor design but not ever likely to be running for more than the max design trip-time of a MCB.

We allow diversity on cookers for rating the cable and MCB on a similar basis, it is in that region of overload only for several minute but below the MCB's thermal time-curve until the thermostats start cycling and the cooker demand drops down.

Had it been something like an immersion heater that could well be on for hours at a time then I would think it merits C2.

 

[loz2754]

Here I would query the "long duration" of a small overload. MCB curves allow for trip times from about an hour all the way down to the 10ms or so magnetic trip time.

To me a long duration is then something lasting longer than an hour that won't trip the MCB but is above the ratings. For any sane use of a shower I doubt it would be on for more then 30 minutes once or twice a day, so unless you already see signs of thermal stress, I would say C3 as poor design but not ever likely to be running for more than the max design trip-time of a MCB.

We allow diversity on cookers for rating the cable and MCB on a similar basis, it is in that region of overload only for several minute but below the MCB's thermal time-curve until the thermostats start cycling and the cooker demand drops down.

Had it been something like an immersion heater that could well be on for hours at a time then I would think it merits C2.

I guess that's reasonable thanks for your input.

 

[brianmoooore]

In the real world, I still think that if a clamp meter was put on the supply to this shower, you'd find that the actual current is considerably less than the simple theoretical one.

 

[Pretty Mouth]

In the real world, I still think that if a clamp meter was put on the supply to this shower, you'd find that the actual current is considerably less than the simple theoretical one.

I'll try to remember to do this when I return for remedials, it'll be interesting to see what the actual current drawn is.

Well, I'm settled on a C3 now, which will delight the tight fisted old landlord no end, as he won't have to shell out for a new CU.