Mains powered equipment tripping RCD. Diagnosis techniques?

[Chris Wilson]

I have a semi home built, semi commercial valved RF amplifier and low voltage supply for it that on initial turn on will often trip the house main distribution panel ring main RCD. After it's been on for a bit (say a few minutes, maybe less, not really tried that yet), it will turn off and on again OK for maybe the rest of the day. The HV supply is a serious bit of kit, but not involved at all as it's in a separate cabinet and not plugged into the mains or the RF section. My "shack" is upstairs and the wife is getting Royally fed up with me tripping the RCD and the TV  etcetera going off whilst I try and diagnose. When I reset the main RCD all the outside security lights turn on, the cooker clock needs re-setting, blah blah. A real PITA!

The equipment has a soft start for the (big) heater transformer, and a 240 to 110 volt dropper transformer for a cabinet fan, plus a 240V centrifugal blower. It's pretty simple really, but I am unable, so far to determine why it does this. Once powered up it run for hours. I have tried an MOV across live and neutral. Our UK mains is similar to that in Oz, but not like the USA. We have live and neutral, and earth. Earth and neutral are bonded together at the local sub stations I believe. The supply is 50Hz.

Can I try and find the cause with it isolated form the mains? How? Is there a  way to feed it from the mains without tripping the RCD? I have a 1 to 1 isolation transformer, but not sure it will handle the current draw very happily. The fact it powers up OK once it has initially tripped the RCD a couple of times suggests a capacitor issue to me?

Thanks.

[HackedFridgeMagnet]

The RCD will detect a difference in the currents between Active and Neutral. It will trip when this goes above the mA threshold. A trip is normally caused by excessive current passing through the Active to the Earth, thus bypassing the Neutral.
As you know by now a MOV between Active and Neutral wont fix your tripping problem.
Can you check the RCD trips correctly at it's threshold without your RF amp?

These days in most houses in Australia we put RCDs on each sub circuit, it saves a lot of headaches. They make them in a standard circuit breaker size now, so it might be feasible for a sparky to just put one on your RF amps sub circuit.

If it is your RF amp actually causing the problem, it may be capacitance to earth. Maybe something High voltage.
Or maybe it is wired wrongly so be careful.
If you disconnect the equipment from the mains the Neutral and the Active should probably both be open circuit from the Earth at DC.
Can you disconnect the sub sections of the amp to determine which one is causing the trip?
The Heater transformer shouldn't cause this fault if it is isolated from the secondary.

Hope this helps.

[Chris Wilson]

Thanks for the reply. I may have one clue. If I take my basic multi meter and after having shorted the 13 amp mains plug neutral to earth pins, I take a resistance measurement between neutral pin and earth pin it starts at about 1 megohm and very slowly the resistance rises to just under 4 megohms. If I test again without shorting them a few minutes later the resistance has dropped a tad, but is still higher than after shorting them.

The RCD never trips for no reason, it's otherwise trouble free. I am having a 16mm T&E cable and its own distribution panel taken from the main house "in" feed to my shack. Maybe then I can have my own RCD and give the wife some peace Am I safe using a hand cranked "Megger" on the equipment, between neutral and earth of the plug, with the equipments main switch on? I suspect from the resistance readings it will fail though. The only bypass caps are in the RF deck itself, and disconnecting that from the rest of the gear doesn't alter the above resistance readings. There's a big motor start cap on the centrifugal blower though...

[atw60444]

My initial reaction is...you set the cooker clock??? 
 Your best bet would be to buy or borrow a "megger" type insulation tester. Something that can produce about 500 volts. And check the transformer / associated components insulation to chassis. Stay below capacitor ratings obviously.

Jon

[Chris Wilson]

I have a hand cranked "Megger" that'll reach 500V. I bought it at a ham fest the other week. I also checked capacitance on a good Fluke PM6303A and it's 49nF across the neutral and earth pins of the mains plug. That seems al lot, albeit down a long mains cable. I have tried disconnecting the primaries of the heater transformer and the 240 to 110V dropper transformer, but the neutral to earth resistance leak is still there. Thanks.


EDIT:

1.5 megohms between neutral and earth on the mains plug using the megger. Cheers.

[HackedFridgeMagnet]

Am I safe using a hand cranked "Megger" on the equipment, between neutral and earth of the plug, with the equipments main switch on? I suspect from the resistance readings it will fail though.

No don't use the Megger with the equipment plugged in.
Also watch the voltage generated by the Megger, it could damage your equipment. I would use a normal multimeter instead.
Have you tried with your isolation transformer?
Ideally it would have a suitable fuse in to protect its windings, but I would check this first.
If it has then compare the current ratings of both bits of equipment. If it seems ok or even close, then give it a go. Just watch the windings for over temperature. They should be happy for at least some seconds if the current ratings are similar.

As a short term work around you might be able to use the isolation transformer to get started and then switch to normal supply once the leakage has stopped.
Still if I was going to use the equipment I would want to get to the bottom of what is causing the leakage.

[atw60444]

Am I safe using a hand cranked "Megger" on the equipment, between neutral and earth of the plug, with the equipments main switch on? I suspect from the resistance readings it will fail though.

No don't use the Megger with the equipment plugged in.
Also watch the voltage generated by the Megger, it could damage your equipment. I would use a normal multimeter instead.
Have you tried with your isolation transformer?
Ideally it would have a suitable fuse in to protect its windings, but I would check this first.
If it has then compare the current ratings of both bits of equipment. If it seems ok or even close, then give it a go. Just watch the windings for over temperature. They should be happy for at least some seconds if the current ratings are similar.

As a short term work around you might be able to use the isolation transformer to get started and then switch to normal supply once the leakage has stopped.
Still if I was going to use the equipment I would want to get to the bottom of what is causing the leakage.

I presumed you were savvy enough to isolate the power supply from anything likely to get damaged. You said it was a separate unit to the amplifier.
 1.5M should be ok. Can you disconnect the capacitor and see if it improves? Also sounds as if the blower would be worth investigation.

Jon

[atw60444]

http://youtu.be/FAcmaG9ekSo?t=8m

[Chris Wilson]

Thanks for the replies and the link. I think I may have it sussed. But I have thought this once before... Anyway, on the mains input to the unit there's a 15 amp breaker, huge thing, originally three phase, and I am using just two of the phases as a live and neutral isolator. From the cold side of the breaker (when off) there's an original pair of fuse holders with dropper resistors built in, and neons that light to show a fuse has blown. One in each in the switched sides of live and neutral. I think one was internally faulty and not making a good contact. So sometimes when powered on there was a much better, or maybe ONLY a connection to one side of the mains input. Maybe this imbalance with what leakage there is was enough to trip the RCD? I have replaced both fuse holders and fuses with new conventional holders. The leakage is a lot less, a Megger test now shows a much, much healthier resistance to ground, and it is powering on fine. My only doubts are a small and very quickly disappearing leak to ground when the RF section is connected. Such was the leakage before this was not noticeable. I guess this is the bypass caps charging. It's fleeting and doesn't seem to bother anything. The circuit breaker switch, with the two outputs used, (of the three phases), isolated  shows a slight leakage when on, compared to nil when off. It is tiny though. Fingers crossed, I'll power it on tomorrow after being shut down all night. It was after a longish period of inactivity that it played up. Thanks for the help and advice again!

[atw60444]

Sounds pretty promising then Chris. Well done

[Chris Wilson]

Damn, it tripped again when I put it on a few minutes ago. BUT, there's definitely something "funny" about the circuit breaker, I have measure a 3 or so megohms between the terminals and the actual PLASTIC of the case. When I removed the whole breaker box from the metal panel I get infinite resistance to ground. I think I am on its track finally

[AlfBaz]

It may well be a fault but I have found heavily inductive loads can cause protection devices, including RCD's to trip.

I'm not very good with AC fundamentals so maybe someone more knowledgeable can shoot this down or explain it properly/better.

Some iron cores can hold a residual magnetism better than others. It's not until the core has been magnetised that the inductance reaches it's designed level thereby limiting the current. This peak inrush current may cause a steep dv/dt in the resistive parts of the circuit and any capacitance to ground will see this and shunt some current to ground as I = C x dv/dt

Also keep in mind that as little as 0.4uF at 240VAC 50Hz will give you 30mA which will appear as an open cct with w multimeter or "megga"

[Monkeh]

Damn, it tripped again when I put it on a few minutes ago. BUT, there's definitely something "funny" about the circuit breaker, I have measure a 3 or so megohms between the terminals and the actual PLASTIC of the case. When I removed the whole breaker box from the metal panel I get infinite resistance to ground. I think I am on its track finally

You should probably get an electrician to test that RCD. And, while he's at it, unbreak your installation so a single device isn't protecting all circuits.

[Rerouter]

It may have 2 small filtering capacitors to ground right after the bridge rectifier section, these can bleed a few uA, but if they have gotten hot can warp and start leaking current, this equaly will not show up on a standard resistance check,