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Topic Title: ze too high for a D type
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Created On: 23 April 2015 06:46 AM
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 23 April 2015 06:46 AM
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OllieM81

Posts: 1
Joined: 25 July 2014

I need to fit a 50a type D mcb for a compressor. Only problem is the Zdb is 0.29ohm. Rated Ze for mcb is 0.18. The db is fed from a 4core SWA and that is used as the cpc. Ze at the intake is 0.03ohm. Running a separate earth will be an expensive and very awkward job. So my question is can I just furnish the circuit with an RCD to achieve the disconnection time?
 23 April 2015 08:17 AM
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phantom9

Posts: 1757
Joined: 16 December 2002

I would check your figures. Table 41.3, section (c), max Zs for 50A type D is 0.44 ohms (5s disconnection time).
 23 April 2015 08:23 AM
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JZN

Posts: 789
Joined: 16 November 2006

Yes as Phantom says above you can use 5 seconds for the disconnection time (even though it's not for distribution) as the circuit is greater than 32 Amps. I had a case like this recently and had to look it up!

John
 23 April 2015 09:51 AM
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davezawadi

Posts: 3954
Joined: 26 June 2002

It is also not a good idea to use RCDs on motors, the starting current pulse often gives tripping trouble. If you must use one for some good reason, then you need a rated current of 5 - 10 times the running current to be sure of avoiding trouble, perhaps a bit less with a star delta starter. Even time delayed RCDs can suffer badly with pulse load currents, there may be a make out there which is completely reliable, but I have not found it (and I often have difficult loads and RCDs).

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 23 April 2015 09:59 AM
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Jaymac

Posts: 35
Joined: 24 September 2009

I came across this (http://www.shepleyelectricalse.../Max%20ZS%20Table.pdf) on google.

The figures shown are different from those in the Regs tables.

Are they manufacturers figures? Which are you advised to go by.? The Regs I should think

Regards

Jaymac.
 23 April 2015 10:31 AM
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AJJewsbury

Posts: 16088
Joined: 13 August 2003

The figures shown are different from those in the Regs tables.

Look pretty close to the pre-AMD 3 figures to me (for BS 1362 and B type MCBs at least). What differences have you spotted?
- Andy.
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