Discrimination

[Farmelectrics]

We have a test bench in one of the buildings that I can't attend at mo so trying to diagnose over phone not good I no but we have a rcd socket on a work bench were we plug in to test faulty appliances that have been sent back to the company no why does it sometimes trip the breaker at the board and not the socket on the work bench I have asked them to install new rcd socket with test button and let me now how it goes but the rcd socket should trip first and not the breaker at db your thoughts please

 

[Darkwood]

Why do you think that!

If you have earth leakage the RCD rightly should pick it up first.. but if the fault was a short L-E then either device would be subject to the time curve of the fault value (0.1s) thus pot luck... if not both together.
If it was L-N short the RCD may trip still with the high magnetic shock of the large current.

The nature of the fault will determine discrimination, a circuit has to be well designed to full discrimination would have circuit O/L - S/C - E leakage protection at the bench and the upstream device would hold regardless of fault donstream of socket..... this can rarely be achieved with small incoming supplies.

 

[Engineer54]

Do you know what the protection is upstream of the work bench RCD socket?? Could well be a RCBO, it may even be tripping an RCD at the supply DB/CU. In either case there would be no discrimination between the two 30mA devices. You wouldn't even get guaranteed discrimination with a 100/300mA upstream, unless the upstream device was an S type RCD...
In any case you can't really diagnose a fault or possible fault over the phone, it takes test equipment to diagnose/analyze what going on....

Other than that, take notice of Darkwoods post above.

 

[Geordie Spark]

Discrimination is defined by one Muslim Doctor's Receptionist demanding - and being granted - three consecutive weeks holiday in July thus denying the other receptionists who are not Mulim any holiday during that month.

Now tell me just who is being discriminated against here?

 

[Farmelectrics]

Wonderfully put dark wood thank you wasn't trying to fix fault over phone just trying to help a friend out other side of the country and your well put information has helped

 

[Farmelectrics]

E54 don't no that info don't think he did it's a rented warehouse and they have to wait over an hour for the property's spark to turn up to switch back on not ideal but it is what it is thanks