How to test RCD and earthing of a temporary supply.

[Ed Davies]

With RCDs with the test button going line to earth, if there's a broken earth connection won't that make the cases of any connected equipment live while the button is pushed? Not liable to directly kill anybody with the resistor in the way but might startle somebody with indirect consequences.

[Jeremy Harris]

21 minutes ago, Ed Davies said:

With RCDs with the test button going line to earth, if there's a broken earth connection won't that make the cases of any connected equipment live while the button is pushed? Not liable to directly kill anybody with the resistor in the way but might startle somebody with indirect consequences.

 

The test button built-in to an RCD/RCCB doesn't usually connect a resistor to PE, it connects to the neutral on the other side of the sense coil.  This means it can test for functionality without needing a working earth connection.

[Ed Davies]

Right, but what about when the test buttons which do go to PE? The subject of this thread is testing on a temporary supply which might be to a caravan or site equipment with possibly dodgy TT wiring so having a caravan frame at 230V, albeit through a resistor, is not out of the question with a tester of the type @MJNewton and you have.

[Jeremy Harris]

49 minutes ago, Ed Davies said:

Right, but what about when the test buttons which do go to PE? The subject of this thread is testing on a temporary supply which might be to a caravan or site equipment with possibly dodgy TT wiring so having a caravan frame at 230V, albeit through a resistor, is not out of the question with a tester of the type @MJNewton and you have.

 

The difference is between routine functional testing of the RCD, which only tests the functionality of the RCD/RCCB/RCBO itself, and installation testing, where using a tester that creates an imbalance by shunting to earth more or less tests the real-world protection the RCD/RCCB/RCBO provides for the circuit.

 

The evidence available suggests that routine testing of RCB/RCCB/RCBOs tends to increase their reliability (perhaps by just exercising the mechanism and reducing the risk of mechanical jamming?).  Testing an installation isn't such a regular exercise, typically it would be done every three to five years, rather than every six months for RCD/RCCB/RCBO functional testing.  The installation test is more realistic, as it simulates what might happen when leakage develops between line and earth, so it will reveal a very high resistance, or open circuit, PE, as when tested like this the RCD/RCCB/RCBO won't trip.  The routine functional test won't ever test that; it will only show whether  the RCD/RCCB/RCBO is working as designed (more or less - it doesn't test the time taken to disconnect).