Although fuses are probably the simplest of electronic components they still manage to cause a certain amount of confusion and misunderstanding, and it is probably a case of "familiarity breeds contempt". A fuse is simply a piece of (usually) fine wire which normally allows an electric current to flow, but it overheats and melts if an excessive current should flow through it breaking the circuit and cutting off the current.
Fuses have a current rating in amps (A) or milliamps (mA). A milliamp is simply a subdivision of an amp, and is one thousandth of an amp. Confusion can a rise because a lot of electrical and electronic equipment will have an information plate or handbook which states a power consumption of a certain number of watts, rather than a current consumption of the appropriate figure. Power takes into account both supply voltage and supply current, and it is simply the product of the two. In order to find the supply current it is merely necessary to divide the power in watts by the supply voltage. A 1000 watt electric fire operating on the 240 volt mains supply, for example, would have a current consumption of 1000 ÷ 240 = 4.17 amps. However, with (say) a 50 volt supply and a power consumption of 1000 watts there would be a current flow of 1000 ÷ 50 = 20 amps! Fuses must therefore have a rating in current and cannot be given a meaningful power rating unless supply voltage is also specified.
Although it is generally believed that the current rating of a fuse is the nominal current at which it will blow, it is, in fact, the highest nominal current that the fuse can withstand without blowing. In practice a fuse will normally take a current of about 40% to 90% more than its rated current indefinitely without being damaged. This means that, for instance, a fuse which is to operate at a normal maximum current of 1.2 amps could be a 1 amp type. In fact a 1 amp component would give better protection than a 1.5 amp type and should not be prone to blowing superfluously. However, it is probably best not to under-rate a fuse by more than about 25% as this could easily result in the fuse frequently blowing unnecessarily and becoming something of a nuisance.
It is undesirable for a fuse to have a rating very much greater than the nominal maximum current flow it will have to take as this could conceivably result in a significant current overload occurring without the fuse blowing. In practice there may be no alternative to using a fuse having a rating perhaps 50% or more above the ideal figure simply because fuses are only available in a limited number of values and the required value may well fall between two of these. In practice, this is not of great importance because a serious fault will usually produce a current increase of several hundred per cent and will rapidly blow a fuse having a marginally high rating. This is just as well since fuses for mains plugs are only generally available in two current ratings; 3 amps (for loads of up to about 720 watts or so) and 13 amps (for equipment having a power rating of between about 720 watts and 3120 watts).
There are just three normal fuse sizes, and these are 20mm, 1in and 1¼in. These sizes simply refer to the nominal length of the cartridge in which the fuse is housed. One inch types are only used in mains plugs, while 20mm and 1¼in types are fitted in items of equipment.
Both 20mm and 1¼in types are available as quickblow and antisurge types, but 1in fuses are only available as the quickblow type. A quickblow fuse is probably what most people would consider to be a 'normal' type, and as its name implies, is one which blows as rapidly as possible when an overload occurs. In electronic terms a quickblow fuse does not really live up to its name in many cases, and with an overload of around 100% it can actually take several seconds or even minutes for the fuse to finally blow! The time taken for the fuse to blow reduces very considerably as the overload margin is increased, and would typically be only around ten milliseconds (one hundredth of a second) with a 1000% overload. Even this may be too long to prevent expensive electronic components being damaged, and some pieces of electronic equipment, such as power supplies, normally have fast acting electronic overload protection rather than conventional fusing.
Antisurge fuses are used in circuits where in normal operation there are likely to be brief surges of current that are well in excess of the normal current flow. An antisurge fuse is deliberately made slow acting so that substantial overloads which only last a few tens of milliseconds do not blow the fuse. Overloads lasting longer than this will do so. The normal and brief surges are therefore allowed to pass without unnecessarily blowing the fuse.
If you examine an antisurge fuse having a glass cartridge you will notice that it does not have the usual straight filament, but instead it has the appearance of a spring with a straight section in the middle. This can be clearly seen in one of the photographs. Actually the fuse is formed by two spring-like pieces of metal which are joined at the straight central section, and it is this join that acts as the fuse. When an overload occurs the two 'springs' absorb heat from the central section and prevent the fuse from blowing. However, they soon heat up and become inefficient at taking heat from the central section which then overheats and blows. With a brief overload the current will drop back to an acceptable level before this occurs and, as required, the fuse will not blow.
Antisurge fuses are not used a great deal, and are primarily employed in power supply circuits which have a large smoothing capacitor that represents a virtual short circuit when the supply is switched on. However, after an initial heavy surge of current the capacitor is charged up and thereafter only a relatively small current flows. An antisurge fuse ignores the initial current surge, but acts fast enough to protect the mains transformer and (probably) the rectifier components as well if a prolonged overload occurs.
Feature by Robert Penfold
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