Protective Devices for Control Equipment, Part 3
In Parts 1 and 3 [links at bottom of page], I dealt with fuses and circuit breakers. Here I'll continue my look at transient overload protection; then, a few comments on flow failure protection, thermal cutoffs and arc-fault circuit breakers.
As I noted in my last column, current-limiting circuits are not fast enough for some sudden short circuits. 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 cool down because you had to replace fuses, you have recourse to a method called chop-off.
Chop-Off. Chop-off is a feature that can head off a destructive overload, save your fuse and let you continue operating right away provided the fault clears itself (figure 1).
The Crowbar Circuit. Sounds crude? You bet. The crowbar circuit is the equivalent of dropping a low-resistance metal bar across the power lines just downstream of the fuse (figure 2). In place of a bar, an extremely robust SCR pair is connected across the lines and triggered into conduction immediately when the load current hits a preset value. This quickly blows the fuse -- not the crowbar SCR or the load SCR -- and spares the circuit downstream from damage. Unlike chop-off, crowbar action works instantly without waiting for the next zero crossing. You waste a fuse and have to put up with a shut down, but you head off further damage.
Other DevicesThermostat. Common types are, in ascending order of cost: bimetallic, liquid-filled expansion type. Use them on motor windings; SCR heat sinks; inside control enclosures, ovens, plastic machinery, gearboxes, hot air ducts and in heat transfer fluids. Speed often is not a requirement because of the slow changes in most thermal processes. So here, bimetal and liquid-filled thermostats are acceptable. Bear in mind that a leaky liquid-filled thermostat is fail-safe low, not fail-safe high; i.e., it senses that the temperature is lower than it actually is. Neither of these can be validated without removing it and taking its temperature up through its trip point. Also the validation procedure is slow -- and inaccurate if you are impatient.
Electronic Thermostat. For response speed and ease of installation, consider a thermocouple-actuated electronic thermostat. This is virtually an on/off temperature controller with added features such as it stays tripped after the temperature has returned to normal. It can have a manual-acknowledge-and-reset capability that you cannot cheat, and its action can be validated quickly and frequently without taking the temperature up to the trip point.
You can consider backup of this kind to be mandatory protection in processes where your regular controller or its power-controlling device could fail and allow the temperature to run away.
Flow-Failure Protection. There are processes where liquid, gas- or airflow can stop and result in overtemperature. Air ducts or heat exchangers are examples. Use a flow switch or thermostat -- or both -- set to the minimum safe flow or maximum safe temperature. Connect them to cut off the heat source when they trip.
Thermal Cutoff or Thermal Fuse. This is small non-resettable temperature-sensitive device that interrupts the circuit when its environment temperature exceeds its temperature rating. One design uses an internal organic pellet that undergoes a phase change allowing spring-activated contacts to part. Another design uses a metal link that melts and breaks the circuit when the predetermined temperature is reached. A disadvantage of these devices is that, unlike the electronic thermostat, you can only verify their action by heating them to their trip point after which they are not reusable.
Arc Fault Circuit Breakers. Arc fault circuit breakers have already made their way into home wiring. In addition to tripping on overcurrent, they will trip on sensing the noisy signature of an arc fault current caused by a bad connection. Current of normal magnitude passing through an intermittent open circuit acts like an arc welder, so it has good igniting potential. Let's look at the worst-case figure on, say a 40 A 240 V load. A bad connection making kissing contact of say 1 mm2 could deliver 2.5 kW into that very small area. It could be a lot worse than 2.5 kW if you had an inductive or a tungsten filament load.
Home wiring is a first priority market for breaker manufacturers because there is a lot of combustible material around, teamed up with a lot of unsupervised and ill-maintained wiring, appliances and connections.
Though industrial plants, aircraft and ships are more strictly regulated than homes, they can all benefit from arc fault protection.