Thread: Fuses
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Old 8th Jan 2007, 2:09 pm   #3
pmmunro
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Join Date: Jul 2004
Location: Dundee, UK.
Posts: 1,797
Default Re: Fuses

David,

Ceramic bodied fuses are usually High Breaking Capacity (HBC) designs which are constructed to withstand the very considerable energies which can be released when a fault occurs in a circuit connected to the mains supply.

A typical domestic 13A socket can have an earth loop impedance of less than 1 ohm, giving a prospective fault current measured in thousands of amps.

A glass fuse SHOULD NOT be used as a mains fuse as it can break open with very considerable force, certainly enough to damage a fuseholder and potentially be dangerous. Despite this some equipment manufacturers did use glass fuses as mains fuses in the past.

The time characteristics depend on the application and what is to be protected. The common types are:

FF - semiconductor fuses; very fast characteristics, intended to protect devices such as thyristors, triacs and some rectifiers. VERY expensive and only justifiable where the cost of replacing the device is greater than the fuse which can be as much as £20.

F - general purpose fuse with "Fast" characteristic. Used where high surge or inrush currents are not likely to occur.

M - medium fast characteristic able to withstand moderate surges. Unusual in small fuses as used in electronic equipment.

T - Also known as "Slow-blow" or "Anti-surge" fuses. These use one of a number of techniques to give the fuse the ability to withstand currents in excess of the rated value for a short period. Typically used with loads such as transformers, smoothing capacitors and incandescent lamps.

Miniature fuses, as used in electronic equipment, normally conform to standards which specify an operating current of 1.6 times the rated current. Manufacturers publish sets of curves which show how long it will take for a fuse to blow for a range of currents. Times range from several hours to "instantaneous" - a few milliseconds.

I would suggest that you select a range of values typical of the type(s) of equipment that you intend to work on - for mains radios this might be 100mA, 250mA, 500mA, 1A, 2A and 2.5A in glass, type (F), 1A, 2A and 2.5A in HBC (these generally have some surge resisting capacity) and 250mA, 500mA and 1A in type (T).

There are two sizes in common use; 20 x 5mm and 1.25" x 0.25".

Then there are fuses to BS 1362 (Plug top fuses) only available as HRC ceramic types. For these 3A and 13A are the essentials, but 5A is useful for equipment with switch-mode power supplies such as computers and monitors. Other values are available, but not at all common.

"Selection box" kits are available and probably represent a good range if bought from a reputable supplier. You will probably be surprised at the cost of these but find that this works out to be no more expensive than buying the fuses separately.

There is much more to fuse technology then "just a wire that melts" and there are good books on the subject - see also manufacturers' web sites.

Peter M. Munro
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