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Cheap protection circuit for testing PCBs in the office?

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treez

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Hello,
I was today powering a 150W offline LED light in the office. It’s a non isolated design, no transformer.
The PCB is on an earthed heatsink, but is insulated from it with a thin insulating sheet. There is no earth connection to the PCB itself.
Today it blew up, but strangely didn’t blow the 1.6A fuse in the AC line upstream of it. Some series resistors overheated and blew up (went open), so it just stopped working. The problem is, the blow up turned off the office electricity supply, and everyone’s computer went off.

Do you agree that installing an RCD wouldn’t have helped as the current couldn’t have flowed through earth (since earth isn’t taken to the PCB).
Do you know of any cheap protection units that I can put in my mains cable so it doesn’t trip the office electricity supply every time something blows up?
 

Hi,

Could a (dedicated) miniature circuit breaker from AC line to any circuits being tested afford a simple solution?
 
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And....an isolation transformer.
 
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Unlikely to trip a MCB without burning the 1.6A fuse at the same time. Most likely it's an earth fault and a RCD in the switchboard has tripped.

In this case an additional protection is useless because an earth fault will always trip the switchboard RCD, too. Isolation transformer is the only way to avoid it.
 

Thanks, so somehow the RCD tripped even though earth isnt wired to the PCB?.....earth is taken just to the heatsink, on which the pcb sits...(with thermal layer between pcb and heatsink)
 

Did you check the heat sink isolation strength with high pot tester?
 

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