Following problems with un-explained tripping of an RCB I looked at the possibility that stray leakage currents were a significant part of the earth leakage un-balance. I looked at the inter conductor capacitance in cable and the earth leakage current that creates.
Test cable was a coil of 40 metres of 1.5 mm triple and earth on the test bench.
The cable conductor lay is Grey CPC Black Brown
An accurate milli-amp meter was connected between the CPC of the cable and the mains earth.
The current in the CPC was measured with different combinations of live and neutral on one or two of the conductors. Other conductors were left open circuit.
There was no current flow so the earth leakeage current induced by magnetic coupling was negligable.
brown wire live 2.6 mA
grey wire live 3 mA
grey wire live
brown wire neutral 0.9 mA
grey wire live
black wire neutral 0.1 mA
grey wire live
black wire live 3.4 mA
With a live and neutral either side of the CPC the leakage is very low. This is the condition in most cables in normal use.
With live both sides of the CPC the leakage is significant. This is the condition in a cable to a switch in the ON position.
The results suggest to me that the leakage on long switch cables will contribute significantly to the tripping of the RCB.
It would be interesting to find the capacitive leakage current from the live conductor into an earthed metal conduit.
The results suggest that 400 metres of switch cable would trip a 30mA RCB when all switches were ON ( assuming Live looped at ceiling roses )
Test cable was a coil of 40 metres of 1.5 mm triple and earth on the test bench.
The cable conductor lay is Grey CPC Black Brown
An accurate milli-amp meter was connected between the CPC of the cable and the mains earth.
The current in the CPC was measured with different combinations of live and neutral on one or two of the conductors. Other conductors were left open circuit.
There was no current flow so the earth leakeage current induced by magnetic coupling was negligable.
brown wire live 2.6 mA
grey wire live 3 mA
grey wire live
brown wire neutral 0.9 mA
grey wire live
black wire neutral 0.1 mA
grey wire live
black wire live 3.4 mA
With a live and neutral either side of the CPC the leakage is very low. This is the condition in most cables in normal use.
With live both sides of the CPC the leakage is significant. This is the condition in a cable to a switch in the ON position.
The results suggest to me that the leakage on long switch cables will contribute significantly to the tripping of the RCB.
It would be interesting to find the capacitive leakage current from the live conductor into an earthed metal conduit.
The results suggest that 400 metres of switch cable would trip a 30mA RCB when all switches were ON ( assuming Live looped at ceiling roses )