Protecting circuit breakers in a boom wrecker truck - EDN

2022-05-28 14:18:21 By : Mr. XJ Fiber

In the 1980s, I was tasked with figuring out why a million-dollar boom wrecker truck had burned up. The origin of the fire had already been determined; it began in one of the tool boxes. I retrieved the specifications for the wiring harness and switch panel, which was built by a subcontractor.

A 5 A automatically-resetting thermal circuit breaker feeding #18 AWG wire shouldn’t be a problem. Next, I retrieved a switch panel from the stockroom and discovered problem #1: none of the circuit breakers had any markings on them. This switch panel had six circuit breakers rated from 5 to 50 A. How in the world does the subcontractor know which breaker is which? Suspecting incorrect parts, I made a load bank, got a rheostat and ammeter, and went to work.

Sure enough, problem #2 was that the breaker protecting the tool box wiring tripped only after holding 50 A for a few minutes. With both the problem and the obvious solution in hand, I call the supplier.

Now problem #3 rears its ugly head; the supplier knows this circuit breaker is rated at 50 A. All the circuit breakers are rated at 50 A. For more than a decade, the supplier has been building circuit breakers exactly the same way. Dozens of boom wreckers mounted on very expensive trucks, often totaling over a million dollars for the two, have #18 AWG tool box lighting protected with a 50-A circuit breaker. And not just any circuit breaker, but the one that automatically resets itself when it’s cooled down. Okay, now we have a big problem.

Do you have a memorable experience solving an engineering problem at work or in your spare time? Tell us your Tale.

Time for problem #4—the supplier flat out refuses to change the circuit breakers to the values on the drawing to which he is supposedly building these switch panels because “then they might trip.” Yes, because they are supposed to trip. That is literally the only reason they are there. Tripping is their sole purpose in life. However, nothing I say gets through, because this man has built these switch panels for us for over a decade “without a single problem.”

Temporarily stymied but not yet out of the game, I escalate this to my boss, the head of engineering. He and the purchasing agent get the supplier on the phone. No success. They escalate to the owner of the company, a highly-competent engineer himself. Nope, nothing doing. Finally, they throw up their hands and give in.

I revise the design and write the service bulletin to add in-line fuses behind the four circuit breakers that weren’t designed to be 50 A. That’s right, we now had properly-sized in-line fuses to protect the wire right behind 50-A circuit breakers whose job was, apparently, to protect the supplier.

Sometimes inertia is stronger than common sense or good engineering practice.

Since leaving the wrecker industry, Mike Whitfield has spent 27 years working as an electrical designer and is part owner of an AEC engineering company doing electrical design for buildings.

So many examples today of this type of…. – “stubbornness” to consider any change (polite) ? – unwilling to learn or consider others thoughts on a subject (still, polite)? – idiocy .. (accurate, but shows an attitude unlikely to help change/improve the situation)

The author’s understandable solution .. represents how we get so many of the broken systems in our lives today.

If the out-of-spec safety-related subsystem was the direct cause of an expensive fire, would the financial consequences of a lawsuit not motivate the supplier to bring their product into compliance with their own drawing?

That is how one would expect the process to flow: Insurance company investigates, finds cause of fire, determines cause of fire is not reasonable, writes check then sues manufacturer to be reimbursed. Manufacturer then sues subassembly supplier. Instead, the process stopped after “writes check”. However, the risk of the next insurance carrier deciding not to simply eat a million dollar loss certainly inspired us to fix the product in spite of the subassembly supplier.

We got this comment on EDN’s Twitter page: Maybe both of you are right. Why not replace the 50A cb with either a 50A fast-acting cb, or 35A time-delayed cb? There is not much information here, but I am guessing that the starting current must be around 50A.

Sorry, didn’t realize I had another comment. There was no appreciable starting current surge; this was simply toolbox lighting. These were tiny, xenon-buffered incandescent lights mounted in the wrecker body’s toolboxes, designed to light only the interior of the toolbox. Depending on how many tool boxes a wrecker body had, the running current might be anywhere from less than one amp to almost two amps, with an essentially equal starting current. The original designer’s specified 5A circuit breaker was correctly sized, although one could quibble over whether #18 AWG is ever appropriate in a tool box. The wire was in a plastic armored harness though, suspended by clips above the top perimeter door flange in an area relatively (though obviously not completely) safe from physical damage. And had the breaker been appropriately sized, shorting the wire would have meant only tripping the breaker and a wiring repair.

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