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Topic Title: Connecting a compressor
Topic Summary: Omitting overload protection on plug cable
Created On: 21 October 2013 12:19 PM
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 21 October 2013 12:19 PM
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Imhidingunderyourfloor

Posts: 42
Joined: 13 September 2007

Hi,

Been asked by a regular client to put a plug and cable on a compressor. It needs a 16A 3-phase supply and min 2.5mm conductor csa.

Supply is 63A interlocked 5 pin 3-phase socket. He doesn't want to replace the 63A socket with a fused unit if possible.

So apart from getting him to replace the socket or power supply arrangements I am considering the following and would really appreciate some feedback...

Omit overload protection on socket cable and use 4 or 6mm SY connected to a 63A plug. I don't think terminals will accept 10mm.

The compressor has a thermal overload built-in.

Any views please...
 21 October 2013 12:26 PM
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Avatar for OMS.
OMS

Posts: 19668
Joined: 23 March 2004

Originally posted by: Imhidingunderyourfloor

Hi,

Been asked by a regular client to put a plug and cable on a compressor. It needs a 16A 3-phase supply and min 2.5mm conductor csa.

OK - sounds reasonable

Supply is 63A interlocked 5 pin 3-phase socket. He doesn't want to replace the 63A socket with a fused unit if possible.

OK - again, sound reasonable

So apart from getting him to replace the socket or power supply arrangements I am considering the following and would really appreciate some feedback...

Omit overload protection on socket cable and use 4 or 6mm SY connected to a 63A plug. I don't think terminals will accept 10mm.

Why ? - overload protection can be source or load end or any point in between. The overload settings will generally protect a 2.5mm2 cable, but you need to check.

Then you consider the source end CPD, and with knowledge of fault level, decide if the 2.5mm2 flex is thermally protected for short circuit and earth fault protection (assuming you have a suitably low Zs at the compressor or you have an RCD



The compressor has a thermal overload built-in.

Any views please...


Sound perfectly feasible to me - but you need a few checks on that thermal overload protection unit as to what its characteristics are

Regards

OMS

-------------------------
Failure is always an option
 21 October 2013 12:34 PM
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Imhidingunderyourfloor

Posts: 42
Joined: 13 September 2007

Thanks,

I only suggest using the 4 or 6mm cable as I have it on board. The 2.5 mm is manufacturers minimum recommendation but not supplied with the machine.
 21 October 2013 01:10 PM
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AJJewsbury

Posts: 11462
Joined: 13 August 2003

What's protecting the 63A socket?

For instance if it's a 63A BS 88-2 fuse, the 2.5mm2 would probably be OK if the measured Zs & L-N loop <= 0.34 Ohms, or 0.24 Ohms for a BS 88-3. For an MCB, you'd probably need manufacturer's energy let-through data.

I guess another option (might be reasonable if they wanted to move the compressor about and plug it into a random socket elsewhere) would be a 16A socket with a DIN rail enclosure (where you could install 16A fuses or MCB) fed from a short flex with a 63A plug on the end - which would then act as a 63A - 16A adaptor and could be moved around as needed without having to modify the 63A circuits or get an electrician to check Zs etc every time.

- Andy.
 30 October 2013 06:16 PM
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HarryJMacdonald

Posts: 254
Joined: 15 May 2002

Last time I read the regs, the SC protection had to be upstream but the OL protection could be anywhere on the circuit so 2.5mm² probably OK (see post above) . Compressors have a nasty habit of blowing fuses on start up so go for the largest fuse the SC calcs allow for.

Edited: 30 October 2013 at 10:26 PM by HarryJMacdonald
 31 October 2013 01:48 PM
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Avatar for davezawadi.
davezawadi

Posts: 2656
Joined: 26 June 2002

10mm flexible (SY) does fit 63A BS4343s!

-------------------------
David
CEng etc, don't ask, its a result not a question!
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