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Topic Title: 9.5kw Shower on 32A Breaker
Topic Summary: 10mm cable used
Created On: 08 May 2009 10:18 pm
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 08 May 2009 10:18 pm
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GJH

Posts: 512
Joined: 24 January 2008

Evening everyone

TFI Friday!

Been to a job tonight where the customer wants to replace his shower.

The existing shower is a 9.5KW on a 10mm T&E protected by a 32A type C MCB on the RCD protected side of a split board.

It has been in for 4 years since the house was rewired.

I don't know how the breaker is holding in as 9500KW divided by 240v is 39A!!

Is it because the shower is not being used on full load?
 08 May 2009 10:27 pm
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micjamesq

Posts: 777
Joined: 23 January 2009

Because a 32A Type C BS EN 60898 breaker will allow for a current draw of over 40A for an indefinite period of time. Have a look at the time/current characteristics for this device.

It would even be happy to work up to 50A for 1000 secs enough time for a shower

Regards

-------------------------
E & OE

Edited: 08 May 2009 at 10:31 pm by micjamesq
 08 May 2009 10:40 pm
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GJH

Posts: 512
Joined: 24 January 2008

micjamesq

Thanks very much! That explains it.

Regards

GJH
 09 May 2009 07:29 am
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kaichung

Posts: 459
Joined: 02 December 2003

It seems like bad design - a type C should not be used for a resistive (i.e. shower) load, a Type B is more suitable - Why wasn't a B40 installed instead - the 10mm cable seems suitable for the B40 breaker???
 09 May 2009 08:29 am
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AMN

Posts: 645
Joined: 29 June 2007

Its okay to use C types as long as the disconnection times are met, just a bit unconventional in a domestic. Is the circuit RCD'd as well ? A power shower after all is not a purely resistive load anyway.

AMN
 10 May 2009 03:53 am
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GaryMo

Posts: 1214
Joined: 09 May 2007

Originally posted by: kaichung
Why wasn't a B40 installed instead - the 10mm cable seems suitable for the B40 breaker???


How did you come to that conclusion without knowing the installation method of the 10mm2 cable?
 10 May 2009 06:48 am
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broadgage

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Joined: 07 August 2007

A "C" type MCB is of course less liable to trip on starting or inrush currents, and this has led to a widely held, and totally wrong,view that C types are also less liable to trip on overloads. Almost every handyman and kitchen fitter knows that any B type that trips regularly, should be replaced with a C type.

It is also widely believed that "C" stands for commerecial, and that such MCBs are "better" or "heavy duty" or in some other way superior.

My employers, who should know better, once instructed me to replace a B20 with a C20 because "it keeps tripping, and they should have used a C type anyway in an office"
The real reason for the trips was a load of about 27 amps !
 01 June 2009 04:40 pm
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Baz

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I know it's safe but it don't comply. We should not be using the extended tripping time as a method of over running CB's.
 01 June 2009 04:56 pm
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AJJewsbury

Posts: 17795
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but it don't comply

In what way does it not comply?

We should not be using the extended tripping time as a method of over running CB's

C types have exactly the same tripping times as B types. It's just that they require a somewhat higher fault current to operate 'instantaneously' - which will be provided for whenever Zs is within spec.

- Andy.
 01 June 2009 06:11 pm
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ajelectrical

Posts: 1517
Joined: 26 June 2007

Originally posted by: GJH

Evening everyone



TFI Friday!



Been to a job tonight where the customer wants to replace his shower.



The existing shower is a 9.5KW on a 10mm T&E protected by a 32A type C MCB on the RCD protected side of a split board.



It has been in for 4 years since the house was rewired.



I don't know how the breaker is holding in as 9500KW divided by 240v is 39A!!



Is it because the shower is not being used on full load?



No, it might be that even with the shower on full load it is only drawing around 32Amps, If you were to put the shower on full, and measure with a clamp ammeter across the phase at the shower (or neutral) you may well find it doesn't draw what you would expect from calc.

Obviously an unacceptable situation iether way, so I would take an R1+R2 or R1+RN of the circuit as it is, this(using on-site-guide tables) can be used to give you an accurate length of the circuit. You can then do a proper calc using all other info to find out if the 10.00 will suffice.

-------------------------
Andrew. But I don't want you to test anything. I just want the board changing !!
 01 June 2009 06:21 pm
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Phillron

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Originally posted by: GJH



Is it because the shower is not being used on full load?



If it made it through the winter,the summer would be no problem to the little blighter
 01 June 2009 06:51 pm
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industryspark

Posts: 571
Joined: 23 October 2006

In reality the shower will operate fine and no nuisance tripping should really occur.

However, what about regulation 433.1.1

The operating charachteristics of a device protecting a conductor against overload shalll satisfy

(i)The rated current or current setting of the protective device(In) is not less than the design current(Ib) of the circuit.
 01 June 2009 06:58 pm
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OMS

Posts: 22864
Joined: 23 March 2004

But is the CPD actually required to offer protection of the conductor from overload - a shower is a fixed load so we could just provide protection from short circuit faults.


Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 01 June 2009 07:10 pm
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industryspark

Posts: 571
Joined: 23 October 2006

Well, i suppose in 433 we are talking about protection against overload current, and as the shower is fixed load i guess your right OMS

Can probably discount 433.1.1 from the argument then.
 01 June 2009 07:22 pm
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ajelectrical

Posts: 1517
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Originally posted by: OMS

But is the CPD actually required to offer protection of the conductor from overload - a shower is a fixed load so we could just provide protection from short circuit faults.





Regards



OMS


Just because the item is fixed to the wall, doesn't mean it can't develop a small overload of long duration. Especially if it is a 'power shower' with an integral pump.
I still think 433.1.1(i) applies in this instance.

-------------------------
Andrew. But I don't want you to test anything. I just want the board changing !!
 01 June 2009 08:26 pm
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Paul1966

Posts: 1538
Joined: 21 December 2004

Originally posted by: AJJewsbury

but it don't comply


In what way does it not comply?


Using a 32A MCB, whether type B or type C, where the expected load is in excess of 39A.
 02 June 2009 09:41 am
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OMS

Posts: 22864
Joined: 23 March 2004

Just because the item is fixed to the wall, doesn't mean it can't develop a small overload of long duration. Especially if it is a 'power shower' with an integral pump.


Although on balance it's more likely to be a bog basic 9.5kW shower that almost certainly presents a fixed maximum load which is only present for limited duration - even with a shower pump on board the motor characteristics are not likley to be significantly overloading unless the impeller is of an unusual type

So the 10.0mm in my opinion is protected from overload in all circumstances and is likley to be protected from short circuit/earth fault in all circumstances by the Type C MCB

Would I be happy running about 40A through a 32A device - not really but in the grand scheme of things, not really a problem

I'm not sure how we got from fixed load to fixed to the wall though Andrew

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 02 June 2009 10:08 am
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AJJewsbury

Posts: 17795
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Using a 32A MCB, whether type B or type C, where the expected load is in excess of 39A.

Ah, yes! (I was thinking it was part of the C-type/B-type debate!)

- Andy.
 02 June 2009 11:22 am
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kj scott

Posts: 2144
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Although both B and C 60898 devices will hold the load; what does the manufacturer state the operational temperature to be at the device terminals under such conditions?
 02 June 2009 11:30 am
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OMS

Posts: 22864
Joined: 23 March 2004

Well the cable temperature would be around 45C - so assuming a house full of teenage daughters the breaker temperature would be unlikley to be above this - probably well within the manufacturers suggested 60C limit. With more normal useage, the temperature wouldn't rise much above ambient

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
IET » Wiring and the regulations » 9.5kw Shower on 32A Breaker

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