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Topic Title: Mistake
Topic Summary: Cable sizes, RCBO's and Showers
Created On: 21 February 2013 09:21 AM
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 21 February 2013 09:21 AM
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Thripster

Posts: 553
Joined: 22 August 2006

I've made a mistake. Old Mira Elite shower (which I think is 9.8Kw), existing 6mm² cable and new Wylex mini CU. After having fitted the CU, remembered that Wylex do not make a 45A MCB/RCBO. I know I have to go back to correct the situation but it got me thinking..........

To put in a 40A RCBO would probably work for the time that it takes to have a shower but would be bad design as there is a continual small overload (on the RCBO) - using the nominal voltage of 230. However, the measured voltage is 243 giving 40.30 Amps - would work but still to be frowned upon.

But then, what about a 50A RCBO? 6mm² is rated at 47A, clipped direct - so this again would be bad design. But wait a minute - under what circumstances would a shower cause a continual small overload? The shower, at maxiumum setting and in normal use, is never going to use more than 42.60 Amps (at 230V nominal) - so the cable size/method is fine and overload current irrelevant. The purpose of the RCBO in this case (besides it's RCD function) would be to protect against fault current only and as long as Zs is sufficiently low to cause tripping within the regulated times then earth fault current is covered (which would in any case be negated by the faster tripping time of the RCD part of the RCBO). We are left with short circuit current between line and neutral - how is this covered? Notionally, then I guess that if the MCB part of the RCBO trips quickly enough for earth fault current then the short circuit current is going to cause it to trip much faster being as we are talking a lower resistance fault path. So, what am I missing? Am I having a bad day?

Thanks and regards
 21 February 2013 09:30 AM
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impvan

Posts: 705
Joined: 07 September 2005

The 50A rcbo would not be bad design, it's perfectly compliant. Both overcurrent scenarios are covered:-

The rcd and (subject to loop) mcb element will cover the short-circuit condition.
The characteristics of the load will cover the overload condition.

It's in the BRB - protection from overload by nature of the characteristics of the load.
 21 February 2013 09:55 AM
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jcm256

Posts: 1792
Joined: 01 April 2006

Don't forget the 50amp ceiling switch, bad things do happen had to dig a cartridge fuse (Hager) out of its melted holder, it was only 30amps. Whole unit needed replaced with proper shower RCD/50amp MCB unit. Must be the cold water at this time of year, replaced, 4off 45amp ceiling switches seized or loose connections.


http://www.google.co.uk/#hl=en...68145&biw=1008&bih=579
 21 February 2013 11:00 AM
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Thripster

Posts: 553
Joined: 22 August 2006

Thanks folks......it is me being a wally. 433.3.1 (ii) applies in BGB. Not enough sleep and locked into overload & fault protection mode.

Regards
 21 February 2013 12:31 PM
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broadgage

Posts: 1244
Joined: 07 August 2007

An electric shower or other heating element will use MORE current at a HIGHER voltage and not less as suggested in the O/P.

Shower ratings are usually at 240 volts, not 230, presumably as this gives a more impresive figure.

If the rated current IS specified at 240 volts, then at 230 volts it will use less current.
If the rating is at 230 volts, then at 240 it will use more current, and at 253 even more.

Still should be fine on a 50 amp MCB though, as even under worst case conditions the running current wont exceed the cable rating.
 21 February 2013 01:36 PM
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AJJewsbury

Posts: 10963
Joined: 13 August 2003

We are left with short circuit current between line and neutral - how is this covered?

434.5.2 - i.e. the same procedure we use for sizing c.p.c.s in 543.1.3.
- Andy.
 21 February 2013 02:14 PM
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stateit

Posts: 2088
Joined: 15 April 2005

Originally posted by: Thripster

I've made a mistake. Old Mira Elite shower (which I think is 9.8Kw), existing 6mm² cable and new Wylex mini CU. After having fitted the CU, remembered that Wylex do not make a 45A MCB/RCBO.


Regardless of the previous advice, which is useful..., why not populate the Wylex 'mini CU' with MK (or other manufacturer's) modules that are of the desired rating?

-------------------------
S George
http://www.sg-electrical.com
 21 February 2013 02:31 PM
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geoffsd

Posts: 258
Joined: 15 June 2010

If the 9.8kW is quoted at 240V then this is 39.1A at 230V.

If you rate the MCB using nominal voltage then 40A is acceptable.
 21 February 2013 06:13 PM
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Thripster

Posts: 553
Joined: 22 August 2006

Yes, you are quite right - higher voltage more power - hope this (obvious) mistake doesn't detract from the discussion and that you can forgive my bad day.

Regards
 21 February 2013 07:24 PM
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tillie

Posts: 754
Joined: 03 May 2006

Hi , am I right in thinking that you would find the line to neutral fault current and use the equation t = K2 x S2 / I2 , and then compare the resulting figure with the graphs in App 3.

Regards
 21 February 2013 07:30 PM
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davettherave

Posts: 38
Joined: 06 May 2012

I would rate mcb at 50 otherwise any lower will probably suffer heat effects. dont forget the shower is a fixed load and will not overload above this, as long as fault protection is met and additional protection by rcd?

any help mate ?
 22 February 2013 09:37 AM
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AJJewsbury

Posts: 10963
Joined: 13 August 2003

Hi , am I right in thinking that you would find the line to neutral fault current and use the equation t = K2 x S2 / I2 , and then compare the resulting figure with the graphs in App 3.

For fuses, yes. For MCBs you'll almost certainly be in the magnetic ("instant") trip, which will be off the bottom of the BS 7671 appendix 3 graphs - so you need to refer to the manufacturer's energy let-though data. Work out k²S² for you cable and compare with the worst-case I²t from the manufacturer for your range of possible L-N fault currents (which for MCBs/RCBOs is most likely at the supply end).

- Andy.
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