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Topic Title: compliance with fault protection
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Created On: 14 November 2012 09:41 PM
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 14 November 2012 09:41 PM
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davettherave

Posts: 38
Joined: 06 May 2012

could anyone confirm, fault protection of the following,

40a bs88-2 supplying fixed load via 1.5mm mineral 4 core 3phase.

lowest measured pfc is 605a
 14 November 2012 11:09 PM
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John Peckham

Posts: 7614
Joined: 23 April 2005

What is the Ze/Zs at the feed end of the cable and how long is the cable? Is that 4L1.5 or 4H1.5?

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 15 November 2012 10:26 AM
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AJJewsbury

Posts: 11768
Joined: 13 August 2003

lowest measured pfc is 605a

605A - presumably that's at the load end of the cable. Thinking of worst case (cables cold when measured) and potentially warm before a fault, then we need to adjust - in the lack of any more information I'll use the 80% rule (probably not entirely right as the pyro could be running hotter than 70-degrees, but will do to be going on with) - so let's say 484A.

That's still off the appendix 3 charts - so <<0.1s disconnection time. Really we should look up the manufacturer's energy let-through, but if the adiabatic works for 0.1s, then we're safe anyway.

Presuming (again looking for the worst case) that the pyro is PVC sheathed or exposed to touch, k=115

So energy let-though is (less than) I²t = 484² x 0.1 = 23,426 A²s
and cable withstand: k²S² = 115² x 1.5² = 29,756.25.

So it looks OK to me.

Being that it's a fuse (rather than an MCB), my presumption of a lower fault current (equating to a higher Zs) should be safe as for fuses, energy let-though doesn't increase with fault current.

Do check both my assumptions and maths though!
- Andy.
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