Constructors Corner.
In modern electricity and electronics, fuses play a very important role which is frequently misunderstood, with the result that expensive equipment is often not protected fully. This can result in expensive repair bills, injury or even DEATH.
It is not my goal to baffle you with science or maths but it can be a difficult problem for the home constructor to chose the correct rating and type of fuse for a particular project. If this article goes some way to help your understanding of this small but vital component then I'll be happy.
The history of fuses is as old as the use of electricity and probably goes back to the time of the very first short circuit!
A fuse is a device which is placed into an electrical circuit to prevent excessive current flowing under fault conditions. On overload the wire forming the fuse element will heat up and melt (or blow) and interrupt the current flow, preventing damage from excessive current to the remaining circuits. It is, if you like, the electrical equivalent of a safety valve.
Possibly the most commonly abused safety device in the home! One main purpose of a UK plug fuse is to melt before the equipments' cable does, thereby reducing the risk of fire. This gives extra protection in line with the house fuse or circuit breaker box. A more important reason for a fuse is that it is less likely that you'll receive an electric shock in the case of a major equipment fault. If you use the highest possible rating (13 Amp) the equipment will still work but you will have little or no protection from fire or personal injury. I have often received second hand electrical equipment (even from shops that should know better, there are laws governing the sale of electrical equipment) that have a 13 Amp fuse fitted in the plug, even though it requires a 3 Amp one!
Mains plug fuses are generally available in 3 ratings: 3, 5 and 13 Amp (A). If you know where to shop you can also find them in 1, 2, 7 and 10A ratings. With those extra ratings you are able to "fine tune" your protection but usually you will be using the 3, 5 or 13A sizes.
All electrical devices should be marked with the correct plug fuse rating or power consumption, so look for this first. It is usually on a rating plate, sticker or label on the device or on a label on the plug or cord. I will use the power rating in Watts for my examples, as this is what is usually quoted, some items may be rated in KW or Kilowatts, the KW is 1000 Watts, so a 3KW electric heater is 3000 Watts (12.5 Amps).
NOTE: The marking or label on the plug itself usually tells you which fuse is supplied with the plug, not necessarily which fuse should to be fitted!
If a different rating of fuse is required for a special reason, for example: the normal power consumption is 700 Watts 3A fuse) but at switch on 1000 Watts (about 4 amps) is required then the fuse Amperage is usually noted on the rating plate. If the rating plate only has power consumption noted then go with the ratings below:
How did I arrive at those figures? Well it's easy really, the formula is: Maximum Wattage divided by 240 (that's the UK line voltage, other countries alter to suit).
NEVER bridge a fuse holder with wire, electrocution is not pleasent and the smell of burnt flesh hangs in the air for ages!
The characteristic of most importance to the home constructor is the current rating and, regretfully, this rating is often misunderstood. The current rating of a fuse is established by the manufacturer after a series of tests under controlled conditions. This enables the manufacturer to publish a set of specifications for his product which design engineers can use to decide which is the correct type of fuse for a particular circuit. In order to understand the current rating of a given fuse it is important to know the conditions under which this rating was achieved. There are 3 main groups of fuses:
Each of these types of fuse will protect a circuit from excessive continuous current, but act very differently under surge or short time conditions. The fitting of the wrong type could mean no protection at all is being provided, or fuses that keep "blowing" for no apparent reason.
Slow Blow fuses are characterised by permitting an overload in-rush or surge current to flow through the fuse without interrupting the circuit or blowing the fuse. Such fuses, however, will blow in response to relatively moderate constant current overloads. They are important for protecting circuits of various types that have large surge currents when a power is first connected, but shortly afterward these devices reach normal operating conditions and use a relatively steady flow of normal current considerably lower than the surge current. Several types are available. For example, one type has what looks like a spring inside the barrel and these will stand up to surges of around ten times the normal rating for 75 milliseconds. Another type has a "blob" in the middle of the fuse element and this type has a reduced surge capacity, typically ten times rated current but only for 25 milliseconds. They have a very low resistance and can be used in enclosed places as there is little self-generated heat but they are only available in lower current ratings.
A Quick Acting fuse is designed to react both to short and long term overload conditions. They are very robust in construction and will withstand shocks and vibration. But they do tend to have a higher resistance and the voltage drop caused by this may be a problem in some applications. This higher resistance also means that more heat is produced and this must be effectively dissipated.
Normal fuses fall between the 2 types above. They will suffer a slight overload for a short period of time, longer than Fast Acting but nowhere near as long as Slow Blow.
Let's look at each type. The blowing time in seconds reletive to the percentage overload for the three main types of fuse mentioned above is shown left. It can be seen that up to 100 percent overload there is very little difference between the three types. But if we take a current overload of say 500 percent we can see that the fast acting fuse blows in 0.001 seconds (a millisecond) and the slow-blow in about 2 seconds with the normal acting fuse at about 0.01 seconds. Quite a considerable difference between the three types! In fact the ratios (taking our normal acting fuse as the reference) work out at one tenth of the time for our fast acting fuse and 200 times longer for our slow blow type. A very big difference indeed and more than enough to decide the fate of expensive semiconductors under fault conditions.
If we now look at the table on the right we can see how the temperature also has an effect on the current rating. As the ambient temperature becomes lower the amount of current required to blow a fuse becomes higher and this can make a considerable difference to the blowing times under surge conditions. In fact taking our slow blow (B) and normal (A) fuses we can see that the two curves actually cross over at low temperatures. This could require a change of ratings for low and high temperature operation. Although this curve applies to the continuous current rating the overload performance may also be affected. When deciding on a suitable fuse under extreme temperature conditions it is important to carry out a series of tests with simulated faults to check the actual results in practice. This is unlikely to affect the home constructor as most domestic equipment is operated at or around 20°C with possible lower temperatures of 0°C (cold room on a frosty morning), so the "normal" manufacturers rating can be used for all practical conditions but if you are a hardened SOTA operator, for example, it is as well to know that the current rating may be different at extremes of temperature.
You may think to yourself "OK, I'll use a fast acting fuse all the time and be safe." Regretfully this is not practical as many circuits have a high surge current when first switching on, or switching to change operating conditions.
Let us now consider the construction of a typical cartridge fuse. First it has to have a body or barrel and this is normally made of glass 
or ceramic 
material. The barrel will have some form of
termination at each end, usually brass or copper which has been plated to prevent corrosion. The fuse element will be connected between the two end terminations and enclosed within the barrel, it will consist of a single wire in the case of a quick acting fuse, or may be one or more wires arranged in a specific way for anti-surge types. Sometimes a filler is used to modify the action of the fuse and this may be sand or quartz powder. This filler will absorb the energy of the arc when the current is interrupted by fuse failure.
The housing is designed to resist the pressure generated if the overcurrent vaporises the fuse element, provided the voltage across the fuse does not exceed its rating. For this reason, it is important to replace a blown fuse with the correct construction type as well as rating. I have seen glass fuses that have shattered on failure because it should have been a cermic fuse used for that application. "Like for like" is a fair rule of thumb, but only if you can be sure that nobody has already inserted the wrong type.
The physical size of the fuse may vary but there are a number of standard sizes and the most common are the:
(magnified for clarity).
Fuses are of course marked in some way as to type and ratings, normally on one or both of the end caps and in addition there may be an indication of one of the many standards that the particular fuse complies with, e.g. BS, SEMKO, etc.
Thhough the two main characteristics which will concern the home constructor and these are maximum continuous current rating and the surge rating of slow-blow types, the rupturing capacity of a fuse may also be important and for completeness is mentioned here. A high rupturing capacity (h.r.c.) fuse is capable of interrupting currents in the order of thousands of amperes. It would have a ceramic body and also contain an arc-quenching medium. Non-h.r.c. fuses (more common in home constructed equipment) do not have an arc-quenching medium and are only suitable for surge currents up to about 50 amps. With higher currents than this they would be very likely to explode when they blow.
The voltage rating has no effect on the current rating but is important. When a fuse blows an arc is developed between the two ends of the broken fuse element and if the voltage across these ends is high enough, the arc will be maintained and the current will not be interrupted. This condition could result in considerable damage to the equipment, even melting of the fuse body and / or fire. Arcs are readily produced in high voltage circuits or where inductive loads are being used and in these conditions the voltage rating of a fuse must not be exceeded.
Fuses can be used for their current rating at all voltages up to their maximum voltage but AC rated fuses do not offer the same arc protection at DC as a DC rated fuse, the AC voltage drops to 0V during its' cycle giving an oppertunity for the arc to drop whereas DC does not.
The interruption of true DC currents (voltage does not pass through zero) is more difficult than that of AC currents. In the presense of large time constants and relatively small prospective currents, safe interruption of DC currents can only be achieved by fuses specially designed for DC applications. However if using an AC fuse on a DC circuit you could use an AC fuse that has a rated voltage of at least double the DC voltage present.
When it is known for certain that although the circuit has a high voltage present the power available is limited, it is possible to use a fuse at a higher voltage than that for which it is rated. This is common practice in domestic electronic equipment and quite safe. However, if you have any doubt, keep within the voltage ratings given by the manufacturers.
