Protect expensive semiconductors with pure silver

Paul Boughton

Aside from its natural aesthetic beauty, silver has unique electrical properties which make it a natural ally for semiconductors. Semiconductor fuses have an ‘i squared t’ term for instantaneous alternating current (AC) over time, which is quicker than conventional fuses when there is a short circuit.

In a semiconductor fuse, the elements are usually made of pure silver strips with notches, several of which may be in parallel, depending on the current rating of the fuse. They are enclosed within an insulating tube or ceramic body, which is filled with pure quartz sand.

Short-circuit current is typically more than 20 times normal full-load current and the heating effect is so rapid that damage to silicon controlled rectifier (SCR) semiconductors can occur within milliseconds - well inside the duration of an AC half cycle.

A conventional miniature circuit breaker (MCB) can’t limit the peak current permitted to flow in a semiconductor circuit, so a semiconductor fuse must be used instead.

Electromagnetic forces are dependent on the square of the instantaneous current and may produce mechanical damage to equipment.

If short circuit currents are allowed to flow unchecked, after the mechanical damage to the components of the circuit, melting of circuit conductors can occur and be followed by arcing between the molten fragments. This can cause fires and hazards to personnel as well as the further destruction of the electrical system.
Semiconductor fuses are available with voltage ratings up to 12.5kV, and with rated currents up to 10kA. High-speed semiconductor fuses open very rapidly under short circuit conditions thus providing excellent protection in case of short-circuit faults. Using an upstream contactor instead of a semiconductor fuse will not fully protect an SCR in a short circuit situation because it’s not fast enough, and will allow the SCR itself to act as a very expensive fuse instead.

Power controller manufacturer CD Automation always suggests to customers that they have adequate ‘quick-blow’ fuses and, if not purchased through us, have a fuse semiconductor that is rated at 25 per cent more than the power controller rating (amps) but has an i squared t value that is at least 20 percent less than the power controller i squared t value.

This value is important to ensure that the fuse blows before the thyristor or SCR semiconductor. Devices such as circuit breakers and general purpose fuses, while sufficient and necessary for load and installation protection, do not provide adequate protection for a semiconductor device.

CD Automation  specifies semiconductor fuse protection for our SCRs, and whether or not we supply it, the i squared t value has to be 20 per cent less than the SCR. If we supply the semiconductor fuse, the customer has the assurance that it complies with CE and UL standards both in Europe and North America.

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