28-07-2016, 10:41 AM
The only time my bench RCD tripped (N-E short on a bit of IT gear bought second hand), it also took out the house one. Both are 30mA types. Obviously both detected the fault and were able to fire the release solenoid before the power to the bench one was removed or the fault (as "seen" by the house one) was removed - they probably went on the same cycle.
That shows that there's no point even hoping for discrimination based on current, as when faults occur, they are likely to be more than 30 or even 100mA. But time-based discrimination works, and that's used where I work...
I think that if I was fixing one of these old record players for a customer, I'd fit a plug-top RCD (e.g., http://www.screwfix.com/p/masterplug-rcd-plug/44855). This adds a reasonable and practicable layer of safety while not actually modifying the original equipment. If you do modify the equipment, then it has to confirm to current safety standards - not those that existed at the time. So that would involve an isolation transformer at the very least. A modern one - not something found in the junk stores!
I'd also get the customer to sign some sort of disclaimer - not because I think it'll help me (it won't!), but because I hope it would get the customer to think really carefully before going ahead with the work.
That shows that there's no point even hoping for discrimination based on current, as when faults occur, they are likely to be more than 30 or even 100mA. But time-based discrimination works, and that's used where I work...
I think that if I was fixing one of these old record players for a customer, I'd fit a plug-top RCD (e.g., http://www.screwfix.com/p/masterplug-rcd-plug/44855). This adds a reasonable and practicable layer of safety while not actually modifying the original equipment. If you do modify the equipment, then it has to confirm to current safety standards - not those that existed at the time. So that would involve an isolation transformer at the very least. A modern one - not something found in the junk stores!
I'd also get the customer to sign some sort of disclaimer - not because I think it'll help me (it won't!), but because I hope it would get the customer to think really carefully before going ahead with the work.
