Thread: Fuses
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Old 29th Jan 2011, 9:53 pm   #37
Red to black
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Default Re: Fuses

By G Barham:
Quote:
As I mentioned earlier... don't forget the discrimination.

There are curves published for all types of fuses and MCB's. Basically the curve is a trace of current verses disconnection time and competent designers can look at the curves (normmally using both curves on one graph) and see if there are any overlaps.
Very true, See:

Quote:
To ensure discrimination is a very complicated matter, particularly where an installation includes a mixture of types of fuse, or of fuses and circuit breakers. Manufacturers' operating characteristics must be studied to ensure discrimination. As a rule of thumb where fuses or circuit breakers all of the same type are used, there should be a doubling of the rating as each step towards the supply is taken.

When fault current is high enough to result in operation of the protective device within 40 ms (two cycles of a 50 Hz supply), the simple consideration of characteristics may not always result in correct discrimination and device manufacturers should be consulted.

When RCDs are connected in series, discrimination between them is also important, the rule here being that a trebling in rating applies with each step towards the supply
Extract taken from one of my electrical documents, as I thought this would be useful, and to dispel some myths.
Emphasis mine to highlight the point

This is why for example, a 5 Amp plug/spur (BS1362) fuse may blow and the 3 Amp plug fuse downstream may not, or they may both blow, ie. a 3Amp plug fuse will not discriminate against a 5 amp plug

cheers,
Baz
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Last edited by Red to black; 29th Jan 2011 at 10:08 pm.
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