NTC in Variac Circuit

Simple. Remove the earth fault before energising.

Very helpful.. NOT.

RCD's, GFI's as we know them here in the USA, are well known for being fussy about working in series with highly inductive loads with high inrush currents.

So as with PWSlacks advice I too say get the device out of the circuit.

If you insist that the load you are running off of the variac has be RCD protected it should be on the output side not the input side.

Hi tcmtech, thanks for your reply.

The variac doesn't have anything attached to it when the wall RCD trips, It's purely the inrush to the variac that's causing this. I actually want an RCD in the wall outlet too as this also serves (hopefully) as some protection for other equipment attached to the same socket. As mentioned I do also have another RCD on the variac's output.

Since asking my question though.. I have gone ahead & fitted a 12A NTC in series on the variacs mains input, this seems to have cured the tripping issue. I'm just monitoring HTC temps now with loads on the variac to make sure all is safe.

If it was the overcurrent protection (MCB) that was tripping, then the addition of NTCs in series with the primary side would probably have nailed it. NTCs will run quite hot (~250 deg C) if left in circuit, so a bypass contactor/relay seems apptropriate.

Since, however, it is the RCD that is tripping, it is not likely that NTCs will do much to eliminate the problem. I'd say that if you can't get rid of the RCD, you are probably going to have to put up with the nuisance trips.

Hi,

The RCD only trips infrequently really. I am sure that this is only when I switch the variac on (no load) at the exact point at which the variacs initial current surge coincides with the AC's peak signal?

At present (only a few hours I admit) I cannot recreate the tripping problem, however I'm still testing & monitoring the HTC's temperatures just to see whether or not I may need to perhaps heat sink the HTC or perhaps try another value.. Perhaps a 20A, 1Ω HTC. There are no problems yet though with regards to heat. However I'll continue to test to be doubly sure.

Thank you

One can only hope that the cable between the wall outlet and the equipment is correctly sized for the higher current. Errors tend to make one unpopular with fire insurance companies.

Mains cable is correctly sized.. 4.0mm² 32A Arctic Yellow.

Instead of NTC, better use a resistor in series and a power on delay timer to bypass the resistor. Will never misbehave.

Gajanan Phadte

Ah ok.. I shall look into this also.

Thanks for the suggestion.