Short circuits and danger from overheating in circuits

No electrical appliances and machines are designed for use at very high temperatures. Motors, generators, and transformers all have coils of wire with currents passing through them: too much heat damages their insulation. In addition to it this heat is really lost energy.

We have already learned that the less resistance there is in a circuit, the greater the amount of current carried through it. Thus, the lines that carry large amounts of current are designed with large cross-section areas so that the heat loss is reduced to a minimum. Have you ever, noticed the size of a trolley-bus wire? It is very large. It must be large because of the great amount of current which it has to carry.

Overloaded wires are found to overheat, and if the overload is too great, the rubber and cotton insulation burns up and then the circuit is readily shorted.

A short circuit is any connection which allows the current to pass through a shorter, easier, less resistant path than that through the apparatus which is connected to the circuit.

In some cases short circuits only prevent proper operation of the equipment, at any rate, they are considered to be dangerous and damaging. A short circuit may be very damaging if a large amount of current is involved.

Frequently, if small diameter wires are heavily overloaded, the very wire catches fire. In order to prevent overloading of a circuit which might cause a fire, we use a protective device by means of which the circuit is broken, at once, as soon as it is overloaded.

In general, to protect a circuit from too much current in installations up to 500 V, a short piece of wire with a low melting point is inserted in it. As a matter of fact, a fuse is nothing more nor less than a piece of lead or lead-alloy wire. Due to it, appliances are protected from short circuits. It melts at a comparatively low temperature and breaks the circuit. In order to reestablish the circuit, a new fuse should be inserted, at once but, of course, after the damaged circuit is turned off by means of a switch.

As to the lines designed to carry large currents, they mostly have special automatic protection instead of fuses. Either fuses or some other protective measures are always required in order to keep the electric circuit from too much current.

Changing the resistance of a circuit is one of the methods of controlling the flow of current in the circuit. The above could be achieved, for instance, thanks to the rheostat. The latter is an adjustable resistor. As for the resistor, it is a device offering high resistance and used in an electric circuit both for protection and control. It is usually a wire wound in a coil, a carbon rod or even a weak solution of acid in water. The rheostat is wound with a wire having a much higher resistance than that of a copper wire.

It stands to reason that we could also both increase the resistance of the rheostat by making the diameter of the wire smaller and decrease its resistance by making the diameter of the wire larger. The reason is that the larger the cross-section area of the wire, the greater the possibility for the movement of the electrons.

At any rate, one should always keep the diameter of the wire large enough to prevent overheating in case the current is flowing in the circuit.