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Topic Title: Fuse sizing & Thermal Protection
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Created On: 11 April 2013 07:51 AM
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 11 April 2013 07:51 AM
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rcs2

Posts: 6
Joined: 25 July 2008

Hi Folks,

Just found out that some bright spark (no pun intended) has fitted a 200A fuse in a 100A Red Spot holder. The equipment is a 3-phase induction motor with FLC of 90A. The previous fuse was a 100M160 and, aparently kept blowing. The cable is a 50mm armoured with max current of 151A (table 4D4A in regs).
The guy is saying that even though the 151A cable is being 'protected' by a 200A fuse it's still safe. There is a 2-phase O/L protection (as expected with motors) but i don;t think that's good enough.

Any thoughts?
 11 April 2013 09:56 AM
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AJJewsbury

Posts: 11295
Joined: 13 August 2003

If there's downstream overload protection, then it's just a matter of ensuring the fuses provide fault (short circuit and earth fault) protection. You'll need to do the calculations (knowing the fuse characteristics and loop impedances as well as conductor size and cable type - see regs 434.5.2/543.1.3), but it doesn't sound implausible on the face of it. (By way of example, in a domestic ring circuit a 1.5mm2 c.p.c. is usually more than adequately protected by a 30A fuse).

- Andy.
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