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Can I use any make MCB B40 on my Crabtree C/U ?


 
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zappastix

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PostPosted: Sat May 15, 2010 10:17 am Reply with quote

icon_confused.gif Hi

I need to change my MCB B32 to MCB B40 for a new shower.

The C/U is around 12 years old.

Will any make of MCB fit my Crabtree Star Breaker C/U ?

For instance the BusBar?

Many thanks.

Stephen
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ban-all-sheds

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PostPosted: Sat May 15, 2010 10:25 am Reply with quote

zappastix wrote:
I need to change my MCB B32 to MCB B40 for a new shower.
  1. What size cable is it?

  2. Do you know what its current carrying capacity is given how it is installed?

  3. How will you verify that its R1+R2 is OK for a breaker of increased rating?

  4. Is the circuit RCD protected?

  5. electrics:part_p


Quote:
Will any make of MCB fit my Crabtree Star Breaker C/U ?

For instance the BusBar?

No. Ones other than the originals might fit, but it is absolutely not the case that any and all will fit.
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zappastix

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PostPosted: Sat May 15, 2010 11:28 am Reply with quote

Thanks ban-all-sheds.

I just wanted to know if there are any physical differences between MCB's (B rated).

This is because I have a long journey to ScrewFix/B & Q and don't want to find out there are differences and a different brand would not fit.
I wanted to get he job done this weekend. If I have to order online then I'll do that.

In answer to your concerns;

I've spent hours looking into this.

The cable is 6mm.

6mm twin and earth cable is rated at 47 Amps at best. At 240V (real, not nominal).

The old shower was 8.5kW.

The MCB for the old shower is B32.

We have lived here for 7 years and the shower has never tripped.

90% of the cable is in an airy/large loft.

Showers up to 8.5kw may carry a 6mm cable as long as the shower unit is within 18 meters of the fuseboard. Fuses and switches should be rated at 40 amps.

It is because it seems the person who fitted the MCB for the shower didn't seems to fit the correct rating MCB, is why I want to change this for B40.
In saying this - maybe Regs years ago were different.

My electric oven is on a MCB B32 running on 6mm cable - no problem there.

Thanks.
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ban-all-sheds

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PostPosted: Sat May 15, 2010 12:43 pm Reply with quote

zappastix wrote:
I just wanted to know if there are any physical differences between MCB's (B rated).

There are.


Quote:
6mm twin and earth cable is rated at 47 Amps at best. At 240V (real, not nominal).

And 23.5A at worst.


Quote:
90% of the cable is in an airy/large loft.

The other 10%?
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Taylortwocities

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PostPosted: Sat May 15, 2010 2:41 pm Reply with quote

zappastix wrote:


My electric oven is on a MCB B32 running on 6mm cable - no problem there.



Yes, but the oven won't be pulling 32amps and if it did then not for too long as it has thermostats and stuff.

Electric showers are wham 37amp all the time until you turn them off devices so overloads and tripping of mcbs are likely unless the circuit is designed properly.

What is the R1+R2 for the 6mm circuit?
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STI

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PostPosted: Sat May 15, 2010 4:08 pm Reply with quote

TTC genuine interest and wanting to learn and understand. If i have understood your last response correctly why does knowing what the R1+R2 is, which i understand is a continuity check between phase and earth, impact/effect/influence the choice of MCB.
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ban-all-sheds

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PostPosted: Sat May 15, 2010 4:13 pm Reply with quote

Because if it is too high a L/E fault will not be cleared quickly enough by the MCB.


http://www.diynot.com/forums/viewtopic.php?p=1602798#1602798

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STI (15 May 2010)
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STI

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PostPosted: Sat May 15, 2010 4:30 pm Reply with quote

Thanks BAS, I need to read that link in detail to get my head around the inter relationships of the Zs , Ze etc but with this topic example it will make more sense.
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ban-all-sheds

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PostPosted: Sat May 15, 2010 4:38 pm Reply with quote

Zs = Ze + (R1+R2).

Or Ze + (r1+r2) for a ring final.
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Spark123

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PostPosted: Sat May 15, 2010 5:09 pm Reply with quote

Zs = Ze + (r1+r2)/4 for a ring final circut icon_wink.gif
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STI (15 May 2010)
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ban-all-sheds

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PostPosted: Sat May 15, 2010 8:02 pm Reply with quote

Got them the wrong way round.... icon_evil.gif icon_redface.gif icon_cry.gif
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STI (15 May 2010)
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STI

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PostPosted: Sat May 15, 2010 8:22 pm Reply with quote

Thanks to both. A further question, if the calculated Zs result is different to the actual measured result, does it signify a problem or is it ok as long as the actual result is within the limits of the tables.
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Taylortwocities

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PostPosted: Sat May 15, 2010 10:47 pm Reply with quote

Measured and calculated are often slightly different. Its OK as long as within the EFLI limits. And don't forget the rule of thumb.
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STI (16 May 2010)
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