Inductive loads
Vhether the supply that is being switched is AC or DC, the presence of an inductor of appreciable size in the switched circuit has a large effect on the switch ratings. Once again, the presence of an inductor (of the order of 0.05 H upwards) when a circuit is made presents no difficulties. The effect of an inductor on making a circuit is to cause the current in the circuit to rise to its normal value more slowly than for a resistive circuit.
The problems arise when an inductive circuit is broken. Vhen current through an inductor is decreased, a voltage is induced across the inductor, and the size of this voltage is equal to inductance multiplied by rate of change of current. The size of the inductance will be expressed in Henrys and the rate of change of current in A/s.
Suppose, for example, that a circuit contains a 0.5 H inductor, and a switch would break a current of 2 A in a time of 10 ms (0.01 s). The rate of change of current would be 2/0.01 = 200 A/s, so that the voltage across the inductor is 0.5 x 200 = 100 V. Now this is comparatively small, but if the circuit happened to be a 6 V DC circuit, with the switch contacts rated for 6 V DC, then the presence of 100 V across the switch contacts will cause a considerably greater amount of arcing than would be present in a resistive circuit. In practice, the effective breaking time is likely to be less than 10 ms, so that the voltage induced by breaking the circuit will be higher than this example suggests. If the circuit uses AC, then there is a reasonable chance that the current may not be at its peak at the time when the circuit is broken and, as before, the amount of arcing is likely to be less. Note, however, that the induced voltage across the inductor at the instant when the circuit is broken is not an AC voltage, and follows much the same pattern whether the current that flowed was AC or DC. The effect of breaking the inductive circuit is a pulse of voltage, and the peak of the pulse can be very large, so that arcing is almost certain when an inductive circuit is broken unless some form of arc suppression is used.