Reply to post: Far from unique

Exsparko-destructus! What happens when wand waving meets extremely poor wiring

Mike 137 Silver badge

Far from unique

At one site I worked on, there was an intermittently occupied environment that became hazardous once or twice a day. it was equipped with a hazard warning siren that went off each time, a few minutes before the event. But the safety system was "home built".

A colleague had installed a critical relay (the one that supplied power to the warning signal) on a panel by gluing its top to the panel instead of wiring in a socket and plugging it in. Then he soldered wires to the pins of the relay. Consequently it would be very hard to replace. He wired the relay contacts so they directly fed a massive capacitor that was used to carry on powering the warning for some time after the mains were switched off for safety reasons. So every time the warning signal was triggered a humungous current rushed into the capacitor through the relay contacts.

Quite soon the relay (and thus the warning signal) ceased to work. When I investigated I found the contact arms (originally springy) had softened with the repetitive heating until they could be bent like tin foil. Analysis of the power source suggested a current in excess of 50A for up to some 100ms through a pair of 2.5A rated contacts each time the warning was triggered. The silly thing was that the hazard warning siren only took a few milliamps, so the addition of a single resistor to limit the current into the capacitor would have prevented the problem entirely.

This, as in the example reported here, is a combination of Dunning Kruger and lack of interest in the outcome.

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