RCD not tripping?

[sellers]

Installed a new CU this morning. Both RCD's test out perfectly. circuits however do not. a 0.51k ohm short on one circuit N-E. But still the rcd doesn't trip?

 

[ProDave]

Simple ohms law.

If there's a resistance and a voltage you get a current.

You know the resistance is 510 ohms, but what's the neutral - E voltage?

If it's TNC or TNCS probably very low, say 1V. In which case the leakage is only 2mA, not enough to trip the RCD.

If you had that resistance from L-E it most certainly would trip the RCD.

Either way, you ought to be looking for the cause of that leakage.

 

[sparkytim]

Some high resistence N-E faults can disable RCD.

 

[Riggy]

Some high resistence N-E faults can disable RCD.

Don't understand this sorry, can you explain please?

 

[sparkytim]

If high resistence L-E and N-E fault on same circuit seen by the RCD but no imbalance seen so RCD does not trip, but also any other fault current diverted thru fault and equalized so RCD incapacitated.

Had this on a Mobile home installation,TT 30 m A incomer. Someone had cut off supply cable to a shed and just left buried in the ground.

Could not get RCD to trip under test, untill the faulty cable was disconnected.

On testing faulty cable found equal Low insulation reading L-E and N-E.

Not low enough to take out Overcurrent Protective Device.

 

[kme]

In my experience, its usually low ohmic N-E faults - cause the RCD to have an issue determining faults. If you short the N-E busbars, the RCD should trip. If it doesn`t, the circuit with the fault is interfering with correct operation of the RCD.

KME

 

[loonytoon]

I sometimes find that circuts with a N-E fault only trip the RCD when under load.

I tend to use this principle when fault finding. If it bangs out under load, N-E fault unless you can pin it down to and appliance.