Coding question - 8.5kw shower, B32

[brianmoooore]

Running the shower hotter doesn't make it use more power; it just means the water flow rate is less.
That doesn't apply to longer showers, of course, but an EICR is a snapshot in time, just like your car's MOT test.
Doesn't stop you adding comments if you think something might deteriorate in the future.

 

[andyb]

I’m sure I’ve completely misunderstood this thread, and I’ve read through it a couple of times, but I have had a couple of scotches.

We’re saying that, and I’m taking everything written as correct, that we have a cable rated at 47a, protected by a 32a mcb and we’re looking to give it a code 2? Have I got this right?

The other way round I would understand, but what’s the problem?

 

[brianmoooore]

Cable is fine. The problem is that the load is slightly greater than 32A, so it's the (obsolete) MCB that is being queried.

 

[Pretty Mouth]

I’m sure I’ve completely misunderstood this thread, and I’ve read through it a couple of times, but I have had a couple of scotches.

We’re saying that, and I’m taking everything written as correct, that we have a cable rated at 47a, protected by a 32a mcb and we’re looking to give it a code 2? Have I got this right?

The other way round I would understand, but what’s the problem?

Not exactly. The cable is fine for the load, it's not a concern (or at least it won't be once i dig it out from the loft insulation).

The problem (or perhaps not) is that the breaker is rated lower than the load. Could, as @loz2754 suggests, repeated overload of the breaker over the long term cause damage to it that might result in danger?

I suspect not, but I don't know that for certain, hence the question

 

[andyb]

Well there's certainly a discussion to be had I suppose, but if we're talking about an EICR I don't see a problem, and I don't understand why you would look for damage to a cable being utilised below it's ccc.

Like I said, I've had a scotch, or 2, and I probably shouldn't be anywhere near a keyboard at the moment.

I'll have a look in the morning.

 

[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.