Protective Devices for Control Equipment, Part 2
In Part 1, we dealt with fuses and circuit breakers. He're we'll look at overvoltage and transient overload protection.
Overvoltage ProtectionEquipment can be damaged by voltage surges and spikes. Sources vary. Power factor capacitor switching, utility switching and lightning are some. They are unpredictable and can come and go while you blink. It takes special instruments that record the time, duration and peak values to catch such transients.
A simple form of protection uses high-energy, transient-clamping devices placed in parallel across the AC power source. When presented with instantaneous voltages higher than a certain value, they behave like a short circuit and clip transient spikes down to a value equal to the device's clamping voltage. The metal oxide varistor (MOV) is one such device. A pair of zener diodes in series, back to back, is another. Different clamping voltages and joule ratings are offered, so you need to buy a device with a joule rating greater than the clamping voltage times the expected transient current times its duration. Clamping cannot be achieved by the device alone. It is done by the voltage divider action of the internal impedance of the power supply and the incremental impedance of the clamping device when conducting.
You will sometimes see the ashes of such devices that have received excessive joules due to the low impedance of the AC power source. You can choose a higher joule rating, or you can increase the source impedance by addition of a series inductor.
Purpose built protection devices use current-limiting series inductors and other circuitry that takes account of low source impedance and duration of the transient.
Silicon controlled rectifiers (SCRs) can fail when line voltages exceed their maximum forward blocking voltage or their peak inverse voltage rating. In the normal inverse-parallel connected pair used for AC control, each one can be arranged to protect its threatened partner by switching on and passing the transient power on to the load. The SCR gates are connected to sense the instantaneous line voltage and switch on when it is excessive.
Transient OverloadsSome SCR-controlled heaters are subject to sudden overloads that can clear when conductive material hits the heater and quickly falls away. Examples are:
- Silicon carbide heaters in glass melting furnaces where conductive molten glass splashes the silicon carbide heater elements occasionally, causing a flashover.
- The metal workpiece in an induction heater accidentally touching the bare copper induction coil.
Current-limiting circuits are not fast enough to catch this kind of sudden short circuit. The transient overcurrent will sometimes blow your fast I2t fuse. If the fault clears with no damage to the SCR, the process can continue. If the process suffers because you have to replace fuses, you have recourse to two other methods of protection: chop-off and crowbar.
I will talk about these as well as other devices in my next column. I also will offer a few comments on flow failure protection and arc-fault breakers.