Leakage current through a synchro motor -- inductance?

Hi everyone I have a fun question (ok , depends what you do for fun)

I had some issues with my main rcd that I resolved, but it gave me cause to take a closer look at sources of apparent leakage around the property. At first I thought the boiler (pump etc) might be leaking to earth but ... it seems it's simply the two motorised valve heads leaking to the pipe work / alternate path to ground such that it shows up on a clamp meter. 

One is leaking a 1-2mA, the other less than 0.5mA. So I figured I would IR the one that shows the most just in case it's on the way out. I found nothing of note ... it shows > 10 Mohms at 500v. I couldn't find anything else in the case (relay etc) with suspect insulation resistance. With the motor head dangling off the pipe work, the clamp reads 0 which is right, any leakage back up the cpc would cancel out as I haven't split the cable. All back together and on the pipes, the last reading was 1.75 mA leakage & it drifts around.

I am wondering if the leakage is just induction between the windings and the nice aluminium case that it all lives in. The one that shows the most leakage is a Corgi valve head, the one with the least is a Honeywell one. The former is newer. I'm thinking of just swapping the synchro motor in the Corgi one for a fresh one.

Just for general interest at this point tbh, there doesn't seem to be any critical problem to fix, just tinkering etc. Wondered if anyone has seen this before.

Personally with such a low leakage current if it were me I'd be off down the pub with the cash instead of buying a new actuator head, I mean, if you're going to go around replacing things with a leakage current of 1.75mA then where is it going to end? A new PC perhaps? Washing machine maybe? Taken to the 8th degree you could be replacing all kinds of gear, plus how accurate is your clamp meter? I have several and unless you are using one designed for low current measurements and you know it's accuracy then you could be getting inaccurate readings,

I take it you are fairly new to the game?, most experienced guy's wouldn't bother chasing a leakage in the order of a couple of milliamps, there's an old saying, "if it ain't broke don't fix it". There are loads of things that give leakage currents, certain power supplies are well known for it, whilst it is good to be able to track something down it is also good to know what is an actual fault and what isn't. If you are really interested, have you tried doing a ramp test on the RCD? it may surprise you, it will probably trip at anything between 22and 26mA.

Nothing here is intended as a criticism, just an attempt to give you an insight into the way a more experienced spark would probably handle it.

best wishes

Phil

Yes pub sounds good  Haven't had any false trips for ages now. This is in the category of pure scientific interest.

For reference, the rcd ramp test trips at 26mA (a brand new MK one trips at 21mA). There's enough leakage across all the protected circuits that with a brand new one installed it was twitchy. The boiler always seemed to be the last straw (and the only thing that could be the source) it's possible the 2mA is instantaneously much higher when it comes on. Clamp meter reads down to 10uA and seems reasonable, the results tally up with all the ramp tests etc.

I should add, approx 11-12 mA goes missing through that rcd ( so its just the wrong side of a no trip test with everything on ). I don't think there's anything at all to be concerned about across the other circuits, nothing is broken and no one device accounts for anything substantial just a nice household collection of reactive loads including some (hmmm .... jury is out in these) usb integrated sockets etc.

I can live with one or two more false trips in the future so I might wait till I get one and - if so - swap the motor to scratch the itch. Don't think I need to swap the whole actuator head, the motor is 15 quid in a popular over the counter diy outlet beginning with s.

On reflection my IR test of the suspect device was at room temperature, whereas it normally operates at 50 degrees or higher, could be relevant. That said, it shows 4x the leakage of its twin brother right from cold.

There's another saying I like --- " if there are bear droppings around your tent, it is likely there are bears in the area ". One more cheeky rcd trip that tallies up nicely  with the hot water coming on, and this will most definitely flip back from being scientific curiosity to active mod. I have a long list house things to fix in the meantime, like most people I am sure! 

Thanks for the response, and the not criticising bit   

Firstly I wouldn't put the heating on an RCD. If it did have to be on an RCD make it an RCBO.

Your solution to the non-existent problem is the worst of both worlds, not only is it an unnecessary fix it is going to cost money, and you've already wasted more than enough time on the issue.

Thanks, yes it'd be nice if the heating was on a dedicated circuit wouldn't it !! And I think I may have said that out loud recently 

Not parting with my 15 quid unless I see another trip that correlates, just intrigued atm.

You're still spending money on the wrong solution to a non-existent problem though.

Not sure I understand-- if it trips again, and the time of day is when the HW comes on..? A chain of events already led me to look closely at this motor. Or or are you saying, bite the bullet, drop a new circuit in? 

If nothing happens, nothing to do, all good.

Arthurdent said:

Not sure I understand-- if it trips again, and the time of day is when the HW comes on..? A chain of events already led me to look closely at this motor.

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But it isn't faulty. There are a collection of circumstances leading to your "problem", but the issue isn't that a single synchronous motor in a 2 port valve leaks 1mA to earth.

Arthurdent said:

Or or are you saying, bite the bullet, drop a new circuit in? 

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You could, that would be more of a solution than changing the perfectly working and non-faulty valve head.

If the problem is that the RCD keeps tripping then you need to keep looking for the cause.

Yes ok, and quite likely the problem is no longer a problem. I agree the 2mA leakage would not have tripped even the 21mA rcd (with 9mA of margin left) it would have to be an associated power-on spike of some form. 

Arthurdent said:

Yes ok, and quite likely the problem is no longer a problem. I agree the 2mA leakage would not have tripped even the 21mA rcd (with 9mA of margin left) it would have to be an associated power-on spike of some form. 

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Remember that a 30mA RCD can legitimately trip at anything above 15mA, so it can legitimately trip @ 16mA and be considered acceptable for use by BS7671.

Also it can trip @ 30mA and also be considered acceptable.

A bit of a discrepancy perhaps, but, welcome to the world of BS7671.

Yeah I like that range. If for some unknown reason you have to swap out an rcd in a situation like mine with a lot of big circuits, and the new one trips at a lower threshold, resulting in false trips, what do you do? Spend ££ splitting the CU, keep trying different compatible makes until you get lucky, trade notes on rcds that are compatible but are known on the forums to have higher trip thresholds based on experience of recent purchases?

If it's an MK board you "must" fit an MK RCD,

If it's a Wylex board you "must" fit a Wylex RCD , etc

Hmm... Tell that to whoever rammed a wylex mcb into my MK CU, and no it doesn't fit the bar properly (the clamp doesn't align) and so makes the cover hard to fit. One of a list of things, this being amongst the more trivial.

In this world of supercomputer smart phones being used mostly for sharing photos of cats, I wonder should installation certificates include filing a photo of the inside of the CU after job completion 

Arthurdent said:

Yeah I like that range. If for some unknown reason you have to swap out an rcd in a situation like mine with a lot of big circuits, and the new one trips at a lower threshold, resulting in false trips, what do you do? Spend ££ splitting the CU, keep trying different compatible makes until you get lucky, trade notes on rcds that are compatible but are known on the forums to have higher trip thresholds based on experience of recent purchases?

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Depends on the exact circumstances of the problem. If there are a lot of leaky circuits and a replacement RCD causes excessive nuisance tripping because there are 20 circuits leaking 0.75A then I would say the general arrangement of the installation/initial design is wrong and running round trying to knock the odd mA or 2 off the cumulative total is not the first/correct course of action.

Arthurdent said:

In this world of supercomputer smart phones being used mostly for sharing photos of cats, I wonder should installation certificates include filing a photo of the inside of the CU after job completion 

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Some companies do this, I have seen people install things incorrectly and then take a picture anyway. Basically as with everything else, no-one cares until someone wants to blame someone for doing something wrong, which doesn't happen often enough, if at all.

swapping the rcd was pretty effective

Arthurdent said:

swapping the rcd was pretty effective

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Meant to say " .. touch wood " .. the edit button vanished tho