Electrical fire due to specification error

Laughton Electronics This is the description of a serious mishap:   the damage that was done, the long-overlooked origin of the problem, and what was ultimately done in response.

melted wiring

closeup of melted wiring

closeup of melted wiring

I've attended to many machine malfunctions, and this is one of the ones that involves smoke. When this Kugler book-binding machine started belching billows from its electrical panel, the machine operators scrambled for the main power switch! Then they called the in-house Maintenance staff, who — in a state of shock, I think — called me. The news filled me with dread. As I drove to their plant I wondered grimly whether the machine would be laid up for hours, days or...

A cursory inspection showed we had been lucky. Certainly it was an ugly-looking mess, but the damage was localized in one area of the main electrical panel. We were spared the nightmarish prospect of melted wiring in the cables that thread throughout the machine.

The problem behind the problem

The meltdown had been triggered by a shorted bridge rectifier. But that sort of thing is not terribly extraordinary. Why did this particular failure result in fireworks?

What I found is that, in one portion of the machine's 24 VDC power supply, an inadequate gauge of wire had been used. (This was the portion that melted.) The circuit is deceptively simple, and it would be an easy oversight to make. And in ordinary circumstances the error is harmless and produces no symptoms.

Referring to the schematic below, it is easy to see that the 6 Amp output is protected by an appropriate circuit breaker. What is less obvious is that the current on the input side of the bridge rectifier might be far greater than the 6 amp maximum on the output.

Ordinarily this would not be the case, but a shorted diode in the bridge allows pulsating-DC current simply to flow between the transformer secondary and the bridge — independently of the 6 Amp breaker.
DC power supply schematic

What this means is that the conductors connecting the secondary to the 6 Amp bridge (red arrow) are not protected against overload, except indirectly by the breaker on the transformer primary. That protection is limited. The primary breaker has a rather heavy rating since it supports the total transformer capacity; ie, both output circuits.

(In my opinion the transformer is adequately protected. The two output circuits share a single, high-capacity secondary winding capable of sustaining an overload until the primary breaker trips. However, if a separate, thinner gauge secondary winding had been used for the 6 Amp supply, then that secondary would require its own protection.)


Everyone was relieved when I restored the machine to service in only a few hours. Diagnosing the shorted rectifier was a no-brainer, of course. But I also upgraded the bridge input wiring with heavy-gauge conductors — a detail that the manufacturer overlooked.
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