Thread: Fuses
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Old 8th Jan 2007, 9:20 pm   #7
pmmunro
Octode
 
Join Date: Jul 2004
Location: Dundee, UK.
Posts: 1,797
Default Re: Fuses

G8DLG,

I'm a bit surprised by your statement "there's not a lot of current to be got from the mains, especially AC". Maybe I'm misunderstanding your meaning.

The earth loop impedance at the origin of a comsumer's installation may be determined, by inquiry (asking the supply authority), by measurement, or by calculation (usually only practical for a system using a local generator).

For a new installation none of these may be practical and a guide figure of 0.35 ohms is used. That would give a prospective short circuit current (rms) of 230/0.35 = 657A. So in this case my statement of "thousands" is perhaps a little high. However, I have measured systems, close to a substation where the ELI has been less than this, so maybe it's not too great an exaggeration.

The normal consumer unit MCB is rated for 6 KA, even for 6A types.

Even 657A is a very high current for a 5 x 20mm fuse and the wiring of consumer electronic equipment.

Many electricians have had unfortunate experiences involving flying molten copper and steel and burnt pliers or screwdrivers. Such experiences are very convincing concerning the effects of releasing significant amounts of energy in a short time.

The important point is the suitability of fuse construction techniques for a given application.

Quoting "Fuses for Electronic Applications", Littlefuse, 1991,

Breaking capacity is defined as - "A characteristic of a fuse is the maximum value which can be interuppted by that fuse under specified conditions. A simple non-filled glass fuse, as used in electronic circuitry for example, has limited interrupting capacity.

If such a fuse is used for mains protection it may be subjected to an excessive current in the event of a fault. The fuse COULD EVEN EXPLODE and become a FIRE HAZARD. For such an application, fuses with a higher breaking capacity MUST be used, which normally contain a filler material".


(My capitals for emphasis).

For a 20 x 5mm fuse to IEC 60127, the breaking capacity (not the current at which the fuse opens) is ten times the rated curent or 35A, whichever is the lesser for glass and 1500A for the HBC type.

When you state "especially AC", do you have in mind large storage batteries?

Some of our correspondants with telephone exchange experience will have many stories to tell about the current available from "the battery".

If you really want to know, consult the title quoted above or "Electric Fuses", A. Wright & P.G. Newbery, IEE, 1995.

Peter M. Munro
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