Switch principles:AC and DC switching

AC and DC switching

The type of current that is being switched has a very considerable effect on the life of the switch contacts. If we confine ourselves to currents that are not signals, then the switch may have to handle either AC or DC. Vhen contacts are made, the type of current is almost immaterial, because there is very little sparking across contacts that are rapidly coming together. Any sparking or arcing takes place almost exclusively when contacts are being separated. This is because conditions for sparking or arcing are ideal at the instant when two contacts are separating. At this instant, the gap between the contacts is very small, so that only a small voltage difference is needed to make current cross the gap. In this respect, sparking means an intermittent discharge between the contacts, and arcing is a continuous discharge.

Arcing is a much more damaging effect, because very high temperatures can be generated locally at the points where the arc originates. In addition, the air itself between the contacts will be heated to very high temperatures, and this has the effect of ionizing the air, making it conduct and so encouraging the arcing, which will then continue even when the contacts are separated by considerably more than the normal sparking distance.

A DC supply is much more likely to cause arcing than an AC supply, given the same conditions of contact opening speed. This is because the voltage across the opening contacts will remain fairly steady, whereas for an AC supply the voltage will drop to zero in each half-cycle. If the opening time of a switch used for AC is reasonably short, then the contacts will have opened appreciably while the voltage across them is, on average, low. In addition, the effect of metal transfer (see below) will be more serious when DC is switched. This difference is reflected in the very different ratings that apply for AC and for DC use of any given switch. One example is a miniature mains rocker switch rated at 250 V 4 A AC and 28 V 4 A DC, so that the effect of using DC is effectively to derate the switch operating voltage by a factor of nine times. This is not at all unusual, although the DC voltage rating might be increased if the current rating were decreased.

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