Using Fuses Effectively

A Communicator Reprise: May 2012

In today’s electronics, fuses play a very important role that is frequently misunderstood, with the result that expensive equipment is often not fully protected, and this can result in expensive repair bills. The history of fuses is as old as the use of electricity and probably goes back to the time of the first short circuit! At first, fuses were simple open-wire affairs, but around 1890 Edison enclosed the wire into a lamp base to make the first enclosed fuse. By 1904 the Underwriters Laboratories had introduced specifications covering fuse size and ratings to meet the safety standards of the period. In 1927 Littelfuse started making the first of their range of low amperage fuses for the budding electronics industry. Since that time many new types of fuses have appeared on the market, some with very special characteristics for particular types of protection. Today the choice is extremely large and protection can be provided inexpensively and the risk of expensive repair bills reduced.

Purpose of a Fuse

A fuse is a device which is wired into an electrical circuit to prevent excessive current flowing when a fault occurs. On overload the wire forming the fuse element will heat up and melt (‘blow’) thereby breaking the circuit, interrupting current flow, preventing damage from excessive current to the remaining circuits. It is the electrical equivalent of a "safety valve".
The most important characteristic to the user is the current rating which, unfortunately, 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 its 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 three main groups of fuses:

1. slow-blow (or anti-surge),
2. normal quick acting, and
3. very fast acting

There is also a fourth type known as time delay fuses. 
Each of these types 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.
Let’s take a detailed look at each type. The blowing time in seconds plotted against percentage overload for the three main types of fuse mentioned above is shown.


It can be seen that up to 100 per cent overload there is very little difference between the three types. But if we take a current overload of say 500 per cent 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 ‘release the smoke’ in expensive semiconductors under fault conditions.
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.
Ah, you may be thinking, let's 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.

Construction

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 delay and 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.
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. The size of fuse may vary but there are a number of standard sizes and the most common are the  Standard metric which is 20mm long by 5mm diameter and the US standard of 1 x .25 inch diameter.  Many other sizes are available ranging from 5mm long to over 200mm. 
Particularly in automotive applications there are also ‘blade’ type fuses (see graphic below), and of course there are a number of other styles available for special purposes.

Fuse Characteristics

There are two main characteristics which will concern the Amateur 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 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-HRC fuses (more common in Amateur Radio 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.

Fusing Speed

As we have seen, 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.
Time delay fuses will react to long term overload currents but will withstand transient surges without harm; 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. Time delay types 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 the lower current ratings. Both the "spring" and the "blob" type are time delay fuses.
So far we have mentioned only the current rating of fuses, but they also have a maximum voltage rating. This voltage rating has no effect on the current rating but is important nevertheless. 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. 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 at their current rating at all voltage levels up to their maximum. 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. But. if in doubt, keep within the voltage ratings given by the manufacturers.

Mobile Fuse Applications

In one sentence: Place the fuse as close to the battery as possible in both the positive and negative line.

That’s good advice because if for any reason the positive wire’s insulation is damaged and the wire touches the chassis or engine (a hot manifold is a frequent cause of this problem) then it will blow the fuse if it is close to the battery but not if it is between the radio and the short.

It’s also good practice to place a fuse in the negative line.  Many hams think that is unnecessary because you will not cause a short if the negative wire touches the metal of the vehicle.  So why place a fuse in this line? In many installations the negative wire goes straight back to the negative terminal of the battery.  If the battery cable develops resistance between the cable and the body of the vehicle by rust or corrosion or the wire itself corrodes to the point that it is not a good conductor this type of installation can cause problems. When the engine is being started a lot of current is being drawn from the battery and the wiring to the mobile radio is not designed to handle any where that much current.  Simple Ohm’s law will tell you that the maximum current will flow through the path of least resistance and if that path happens to be through the negative wire of the radio to the negative terminal of the battery then that is the where the most current will flow. Frequently the unit is not grounded well at the mounting bracket but the shield side of the coax makes a good ground by the antenna mount.  In that case the current for the starter will attempt to flow through the coax shield to the coax connector on the radio then on to the negative wire to the battery. If that wire is not fused the coax shield will smoke. If the radio is grounded at the mount, the negative wire to the battery is not big enough to handle the load and it will smoke.  Either way there is a fire danger.

This type of hookup is not recommended.  If you do this and the fuse blows you may not know it is blown because the radio finds sufficient contact between the mounting bracket and/or the antenna ground to continue to operate.  The resistance between the battery and the vehicle chassis does not have to be high enough that the vehicle will not start to cause this phenomenon. The antenna ground and the mounting bracket are not designed to be the negative source for the DC power of the radio and it will cause more problems than you can imagine. Tracing the source of these problems can drive you crazy. The suggestion is therefore to run the ground wire to the chassis of the vehicle. Use an eye terminal with one outer locking lock washer between the head of the screw and the lug and another between the lug and the chassis.  Scrape the paint off the place where the lug will come in contact with the metal. Run the screw down tight but do not strip it out.