Fires in PLCs


Thread Starter

Gerry Coates

I am investigating a fire in a switchboard that included a SLC 500 in one panel. The fire seemed to have originated in one rack near the power supply module. Everything was correctly fused and protected. I have examined one of these modules fairly thoroughly and while the case could burn, I'm having some difficulty seeing a potential ignition source. Any ideas, or past experiences that might help?

Gerry Coates
E-Mail: [email protected]
The "fire" could have been the result of a phenomenon called "arcing-ground-fault." It is caused by the inability of circuit protective devices to detect the fault-current level. It is evident by the appearance of spattered or melted metal, damaged enclosure materials, as well as badly charred insulating materials.

What is the source voltage level? What is the panel's supply source protection: fuses? or circuit breakers?

Phil Corso, PE
Trip-A-Larm Corp.
Deerfield Beach,FL

Michael Griffin

A possibility may be that the fire was caused by excessive heat due to an overload. A fuse may not always be able to protect the circuit against overload in complex devices. While a short circuit is fairly easy to protect
against, overload conditions require the fuse to blow before enough heat has been generated to create a fire. If a component of the system has a positive temperature coefficient, the current may actually fall to within normal operating range as the device continues to self destruct.
If the module you mentioned had a defect, there is a lot more scope for things to go wrong. In this case the cause could have been a short
internal to the module. This defect could have been due to module design, a manufacturing defect, or some conductive debris (e.g. a wire strand or metal shaving) getting inside.

If the fuse is in an enviroment with large temperature changes, this can be even more difficult. This is particularly a problem in for example, automotive systems which can be technically impossible to protect completely. It is impossible to design a fuse which works correctly at both -40C and +40C and still be guaranteed to protect everything (if anyone
thinks they know how to solve this problem with a conventional fuse, I would certainly like to hear it).

If you were using miniature circuit breakers, the problem is much worse. Most of these devices have such wide tolerances and long maximum operating times that they provide little real protection. This is why for example, these are not considered to be protective devices under the Canadian Electrical Code (instead they are classed as "supplimentary protectors"). This is something to keep in mind for anyone designing
equipment for the Canadian market.

I have seen terminal blocks blacken and melt (although not burn) because of improper tightening. The heat was caused by a high resistance joint (low contact pressure), not by excessive current. In this case, the current would likely have been *below* normal - not something a fuse can do anything about.

One of the reasons why control systems are normally housed in a metal enclosure is precisely because of this problem. The protective
measures you take will prevent fires most of the time. When they do occur however, the enclosure contains the fire.

If there is enough left of the module, a careful examination by someone who knows what they are looking for may reveal some clues. Perhaps the manufacturer could examine it and give you a report. I am sure we would all be interested in hearing about the final results of your investigation.

Michael Griffin
London, Ont. Canada
[email protected]