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Topic Title: 110v and rcd,s again!
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Created On: 21 July 2013 01:18 PM
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 21 July 2013 01:18 PM
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hertzal123

Posts: 302
Joined: 26 August 2007

Further to an earlier post,in a large workshop with many 110v extension leads available,would it not be wise to always fit rcd,s on the supplies as it would not be practical to estimate the max zs of the of the circuits?
Regards,Hz.
 21 July 2013 01:49 PM
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slittle

Posts: 3510
Joined: 22 November 2007

I disagree.

BS7671 doesn't make us take into account the fact the user may plug 30 extension leads together. We can calculate and measure the Zs at the 110v socket on the wall in the same we can do with 13A sockets on a ring final.

We don't RCD them just in case someone plugs a 100m lead in do we ? (I know we RCD them for other reasons but 110v is one way of mitigating that risk)


Stu
 21 July 2013 02:06 PM
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alancapon

Posts: 5768
Joined: 27 December 2005

I agree with Stu. BS7671 ends at the 110V socket in this case. Connecting several extension leads together is poor practice and may need additional socket outlets closer to the point of work, or the work area relocated closer to the existing outlets. With 110V the voltage to earth will either be 55V or 63.5V, depending on whether the step-down transformer is single or three phase.

Regards,

Alan.
 23 July 2013 01:28 PM
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AJJewsbury

Posts: 11462
Joined: 13 August 2003

It seems to me that there's a mis-match between the traditional UK 110V CT system and CENELEC's approach.

Given equal sized L and c.p.c. it's relatively easy to show <50V at the point of the fault (half of 55 or 63.5V), but the BS 7671 requirement for not having to worry about ADS (411.3.2.5) - presumably inherited from HD 60364 (?) - requires the voltage of the source to collapse to 50V or less.

Maybe something for JPEL/64 to have another think about next time...

- Andy.
 24 July 2013 05:36 PM
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OldSparky

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can i ask why you would want an rcd on 110V system
 24 July 2013 05:44 PM
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AJJewsbury

Posts: 11462
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can i ask why you would want an rcd on 110V system

Me or BS 7671?

Me - I don't particularly.

BS 7671 however seems to demand 5s ADS disconnection time for RLV systems - which, given the limited fault current provided by the transformer and often long circuit lengths - can sometimes be hard to achieve using overcurrent devices alone.

- Andy.
 24 July 2013 05:44 PM
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Parsley

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Joined: 04 November 2004

Length of cable runs may make complying with table 41.6 difficult I guess.

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