RCD Test Button

[Vitesse]

I got called to a domestic property today as the RCD 80amp 30ma kept tripping and wouldn't hold under load nor would the test button work. I removed the busbar to remove all circuits from the RCD and the test button works ok again, so it appeared the RCD was ok.

Tracked down the offending circuit causing the RCD to trip, in this case the ring final, which wasn't exactly difficult as it only happened when you plug something in. Carried out IR testing and found a neutral & earth short.

Plan was to split into the ring and IR test back to the CU but I spotted a odd looking FCU in the kitchen that didn't look like it was doing anything and it wasn't, apart from the face plate screw cutting through the neutral and then onto the earth that hadn't been sheathed properly. IR test again all ok, RCD tested fine. Job done. Really pleased sorted on opening just one box.

I understand the workings of an RCD with opposing magnetic fields cancelling each other out in normal operation, therefore the sensing coil picks up no magnetic field. I can't get my head around the test button though. I know it adds impedance across the neutral and live when pressed by introducing a resistor, which in normal operation is fine but I can't understand how in a fault scenario with no load, but earth leakage is occurring that it doesn't cause the RCD to trip?

Any ideas as it is bugging me?

Tony

 

[Marvo]

The test button has it's own associated coil that when the button is pressed it causes a small current flow in this coil which causes sufficient imbalance for the tripping mechanism to operate. The test button will work as long as there is power on the supply side of the RCD and regardless of current flow on the load side.

 

[Vitesse]

The test button has it's own associated coil that when the button is pressed it causes a small current flow in this coil which causes sufficient imbalance for the tripping mechanism to operate. The test button will work as long as there is power on the supply side of the RCD and regardless of current flow on the load side.

Thanks Marvo I understand that but why in this fault situation with earth leakage from neutral to earth would it not operate? With no load on the circuit the RCD would hold but the test button would have no effect?

 

[Leesparkykent]

Thanks Marvo I understand that but why in this fault situation with earth leakage from neutral to earth would it not operate? With no load on the circuit the RCD would hold but the test button would have no effect?

With the neutral earth fault, the test current which normally flows to earth will partially flow through the neutral earth fault and return via the neutral.

 

[Vitesse]

With the neutral earth fault, the test current which normally flows to earth will partially flow through the neutral earth fault and return via the neutral.

Ok so are you saying Lee just to clarify. That the test current flowing to earth via the fault path will not cause the remaining test current a sufficient imbalance in the test sensing coil to cause the relay to trip?

 

[Farmelectrics]

the joys of neutral earth faults

 

[ian.settle1]

Test button works via the resistor which has one end connected to the neutral incoming terminal of the RCD and the other end to the live outgoing terminal of the RCD so that when you press the test button you complete the circuit allowing a current to flow through the incoming live sensor coil and resistor to the neutral terminal thus causing an imbalance between the live and neutral sensing coils allowing the RCD to trip.

 

[Farmelectrics]

very well put ian