Big linear bench PSU trips RCD at turn on or off

[Chris Wilson]

I got immense help from here fixing up my old Farnell 60V @ 50A variable bench linear supply and it's keeping my shack warm nightly powering a 1kW amp TX'ing on LF

It has one annoying trait though. Quite often at power on or power off from the dual pole PSU panel mains switch it trips the mains distribution panel 30mA RCD. I suspect it may be something to do with the filter caps and huge choke on the mains input to the supply. Is there a fix, it's a PITA both running up and downstairs to reset the RCD, and for the earache I get from the wife as her "whatever TV trash of the moment" is interrupted? I attach the relevant part of the schematic below, the input caps values are to the right, thanks. Supply is on UK mains 230V at 50 Hz.

A full size version of the schematic is at http://www.gatesgarth.com/H60001.jpg

[Station X]

I'd be inclined to get the unit P.A.T tested, or do your own test using a Megger to make sure there no problem with C21 and C22 leaking. These should be Class Y caps.

[Ambientnoise]

It will have earth leakage by design as the two input caps from live and neutral will produce it (they will not cancel out as live is much further from earth than neutral of course). The only thing I can suggest is to reduce the value of these input caps, at the expense of reduced rf suppression. "Low leakage/medical grade kit" usually omits these caps but take considerable other measures of course. It is worth checking the caps for value and leakage if you can, in case they are damaged.

Ken

[Boater Sam]

A slow start circuit using a series ballast resistor and relay over ride will solve the problem if the PSU will cope with a slow start.
This is a problem with large transformers such as welders and isolating transformers.
In fact there was a recent thread on this subject with such a transformer. But it was MCB tripping.

https://www.vintage-radio.net/forum/...d.php?t=127334

Is your RCD actually an RCBO?
RCDs do nuisance trip with transformer loads, my welder does it and there is nothing wrong with it.

[paulsherwin]

Originally Posted by Station X

I'd be inclined to get the unit P.A.T tested, or do your own test using a Megger to make sure there no problem with C21 and C22 leaking. These should be Class Y caps.

Yes, the first thing to do is to change those caps. You can simply disconnect them temporarily to confirm they're responsible for the tripping.

You could also use a plug in RCD adapter, which will hopefully prevent the consumer unit RCD tripping and keep the missus happy.

[Nickthedentist]

Originally Posted by Boater Sam

A slow start circuit using a series ballast resistor and relay over ride will solve the problem if the PSU will cope with a slow start.
This is a problem with large transformers such as welders and isolating transformers.
In fact there was a recent thread on this subject with such a transformer.

But wouldn't that trip the MCB (due to high inrush current), Sam, as opposed to the RCD (which detects differences in the L/N currents)?

[Nickthedentist]

Originally Posted by paulsherwin

You could also use a plug in RCD adapter, which will hopefully prevent the consumer unit RCD tripping and keep the missus happy.

From what I've read here, that trick doesn't usually work, unless you plug the RCD adaptor into a non-RCD-protected socket, of course.

[Station X]

At least with this thread we know exactly what's tripping ie the RCD. Often we read "the mains trips" without any indication as to whether it's the RCD or MCB.

[Refugee]

The circuit diagram is showing a soft start circuit on the mains side of the big transformer.
Some big power supplies just have filter caps that are simply too big for the RCCD.

[Station X]

Originally Posted by Nickthedentist

From what I've read here, that trick doesn't usually work, unless you plug the RCD adaptor into a non-RCD-protected socket, of course.

Multiple RCD's are a pin in the you know what. Regardless of their tripping current, you can never be sure which will trip first.

[MrVa1ve]

If you fancy a little DIY, then Boater Sam's suggestion should work nicely. If you need more detail then I highly recommend this (excellent) youtube video: https://www.youtube.com/watch?v=_pEmpvcNmXg

It shows the issue you're having and a practical inrush current limiting circuit.

Regards. D.

[Station X]

The problem here is not in-rush current. As has been pointed out, the PSU has a soft start circuit built in.

[broadgage]

Several options.

Run a non RCD supply to the point of use and add a local RCD at the point of use, this will be just as liable to trip but may be reset with minimal trouble.
If the incoming supply is TT, making an RCD at the supply end mandatory, then use a time delayed 100ma RCD at the mains incomer and a quick acting 30ma RCD at the point of use, where it can be readily reset.

Some RCDs are liable to trip on large surge or inrush currents even without any earth leakage. Test for this by connecting the power supply to the mains via several long extension leads as a test.
If the extra resistance thus introduced cures the RCD tripping, then some form of inrush current limiter is called for.

As has already been suggested, temporarily remove the mains input filter caps to see if this cures the problem.

[Station X]

Originally Posted by broadgage

As has already been suggested, temporarily remove the mains input filter caps to see if this cures the problem.

Indeed. Find out what the problem is before prescribing a cure.

[Boater Sam]

Originally Posted by Boater Sam

Is your RCD actually an RCBO?

This is a need to know.
An RCD is a trip that simply works if the live and neutral are not in balance. An RCBO is the same BUT it also is an over current trip like an MCB in the same item.

RCDs and RCBOs in series can trip in whatever order they feel like, a real pain. If possible keep to one on a circuit only.

I have never worked out why my welder trips the RCD on power up sometimes and not others, it is not faulty, just a big inductive load until the transformer saturates.

[Chris Wilson]

It's a plain vanilla RCD, but why as often as not, does this thing trip the RCD on turn *OFF* as well? Thanks for the replies so far. Oh, and it is already on a fairly long extension lead, but it's a very hefty one with 16 Amp plugs and sockets. I will remove the earth connection from the two caps and see what it does. I may just fit a higher mA RCD for the supply to the "shack", and give it its own panel.

[ms660]

Y caps are a path to earth, both switch poles won't open or close at exactly the same time.

Just thinkin' here.

Lawrence.

[Chris Wilson]

Actually, you may well have a good point there Lawrence, I used to think it did this less if I flicked the switch with some gusto...

[Refugee]

The test for that would be to swap L and N at both sides of the power switch in the PSU.

[Station X]

I can't see why that would help. An RCD detects a difference in currents in the L and N wires. It does not have an earth connection, so cannot directly detect current flowing to earth.