Looking to understand a ramp test result

Mike Thanks for the input. phase angle had crossed my mind. I was thought for cable inductance had more of an influence than capacitance, just means phase lag rather than lead.

It will be interesting to see what happens over the next few days, there isn't much in the way of electronics, but it's a fairly big home, actually two on a single RCD, one unoccupied. At some point the owner is going to have to give in and change the CU, otherwise one day the rcd will trip and I won't be easily available across the road to help out.

Between  two rambling houses I suppose you may have half a km of cable in total and that could be 50nF ==  50,000pF, and at 50Hz that's about 60 kohms. On its own that accounts for perhaps 5mA  of leakage, but of course there could be  things like the capacitace from windings to core of a door bell transformer that might add a spare nanofarad. and there probably is some electronics. Circa 10nF of capacitance per device seems to be common. 

If you want some numbers to conjure with, typical 2.5mm twin and earth measures between 60 and 100 picofarads from L or N to E with the unused core floating  and rather less L to N with E floating.

Inductance per unit length is a measure of the energy stored in magnetic fields around moving charges (currents) and for that typical cable again is about 1 micro Henry per metre if only one core is carrying current; but if the flow and return currents in the cable are balanced so current in  L = minus current in N the round loop inductance is quite a lot less than the sum of the inductances of the two cores, as the magnetic fields oppose and overlap and partly  cancel - you may see as little as a few tens of nano-henries for a line pair.

Capacitance is only a measure of the energy stored in the electric fields around and between conductors at differing voltages - there does not have to be anything that actually  looks like capacitor plates - that is just a good  shape for getting maximum C in minimum volume. (As a none standard capacitor shape, a plump human on a step ladder has about 500 - 1000 pF to the rest of the planet, and may feel a small tingle from a live wire despite being insulated to DC.)

As per above, all these effects in wire alone can normally be safely ignored at 50Hz unless you have very long lengths or are interested in effects that are a small fraction of  the  currents and voltages in a typical load.

Capacitance in parallel is more significant when the thing you are trying to measure is a low current, and the effect of a series inductance is more significant when it is carrying a large current. 

Generally in the earth path the currents are small, so the fraction of the 230V across the capacitance will dominate.

Mike.

mapj1 said:

Between  two rambling houses I suppose you may have half a km of cable in total

That seems a lot. 10 circuits at 50 m each?

mapj1 said:

Between  two rambling houses I suppose you may have half a km of cable in total

That seems a lot. 10 circuits at 50 m each?