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Power switch failing permanently conductive

Posted by PRZ 
Power switch failing permanently conductive
December 29, 2016 05:50AM
I got yesterday a very troubling incident.
I have two printers, the big one equipped with a physical emergency shutdown (ESD) switch (red mushroom button), the other smaller one NOT equipped with an ESD.
This second printer was supplied till three days ago with a mobile switchable plug, in order to have the possibility to unpower the machine with a switch nearby the printer.
I modified this machine to have a more permanent system and a power switch on the machine itself. This power switch cut the mains (230V) supply of the two power supplies (one supply for the printer board, the other for the bed). There is no other possible path than this switch to power the machine.
After a few hours of printing in 4 ou 5 sessions, I shutdown the machine, but the fan was still audible. A few on/off test shown that whatever the switch position, the power was still there.
The switch have became permanently conductive (and the plug is not easily accessed).

The rocker switch is a twin two poles bought in Hong-Kong on ebay, with Faston connectors, indicated to be 2x AC 250V, 15A capable. This model (from another seller): [www.ebay.com]

Current through this switch is below 2A (bed power is 120W).

Tests confirmed that the AC power was there whatever the switch position, however, unplugging the connectors and testing with a multimeter, the switch was working. Replugging connector, it was working again, till a new failure conductive after a few operations. Pushing gently the faston connectors shutdown the switch (while off). A test on the second contact row of the same switch failed in the same manner after just a few operations.

This is very troubling and call for the installation of an emergency shutdown button whatever the machine.
My big machine is equipped with an ESD because its bed is supplied by the mains (230V).
The smaller machine with the failing switch have a bed supplied in 12V and is less dangerous, but being unable to shutdown power can drive to serious problems either for maintenance or in case of heating incident.

We all buy low quality stuff for printers, but a switch failure for a duty much lower than the capability was unexpected. We probably shall care more for all mains equipment supply, being the switches or fuses.

Edited 1 time(s). Last edit at 12/29/2016 05:52AM by PRZ.


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Re: Power switch failing permanently conductive
December 29, 2016 10:34AM
... there were some conductivity problems with mains switches, where the manufacturer used so high loads of graphite in the plastic housings, that they went conductive too eye popping smiley

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Re: Power switch failing permanently conductive
February 24, 2017 08:29AM
This usually happens when the movable contacts in the switch make poor connection with the fixed contacts, arcing occurs and welds the contacts together, a sharp tap normally frees them off but that is not at all acceptable.
The ratings given are, of course, Chinese Amps and Volts which, just like the Chinese mAh ratings for batteries and lumens output for LED torches are wildly over-optimistic winking smiley

You need a better quality switch....or better still an isolator!
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