This is a "what have I missed?" thread as I hope to look again this weekend.
My neighbour who I get on with very well (we do things for each other etc) has a greenhouse watering system that uses a water pump to pump water into a spray bar fed from a water butt filled by the greenhouse roof. Other than it needs main electricity - a very green solution to plant watering.
Today the pump was tripping the plug in RCD. He inspected the mains plug and found a few spiders, but nothing actually to blame. I tested the plug, lead and pump with my Megger (an old 500 volt tester) and found a shockingly low resistance between the live pin and the earth pin. So much so, I tested my tester by removing a lead to ensure it really did read infinite resistance when there was no connection at all. It did.
So I repeated the test and still got a low resistance between the earth and live pins.
The pump was then cautiously tried, and it didn't trip the RCD. From my readings, it would be forgiven if the RCD exploded - but no, it supplied power. After I left, another RCD was tried and it too worked the pump.
I plan to go back, repeat the test, and since the low resistance is in the range of my AVO, if I get the same result, try the AVO.
The Megger I believe supplies AC to test, and the AVO is of course DC. But an RCD would a waste of plastic if it didn't respond to AC faults. Of course a fault shouldn't be that specific, but if a diode was involved - perhaps.
In terms of test gear for mains, I only have the Megger and AVO but the combination has answered any mains queries before.
So, what have I missed?
Replies that say, get my neighbour to pay an electrician will be filed in the bin, I have better qualifications than most of them...
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And certainly better than mine.
However, did you try plugging the pump into another power supply, perhaps just an extension lead back to the house (with & without the RCD)?
I'm one of those that solves problems by methodically eliminating possibilities, even when they seem pointless.
Last edited by: No FM2R on Fri 3 May 13 at 23:07
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The testing was done unplugged from the mains, so if the greenhouse power supply was compromised in some way, testing of the pump would not show it.
I like how you are thinking though, and it wouldn't be a bad idea to run a known good extension lead out to the greenhouse and try the RCD and pump on that.
Last edited by: Slidingpillar on Fri 3 May 13 at 23:13
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I imagine it is possible that the factor causing the cross pin low resistance result may be pre-existing and not the fault which is now causing the RCD to trip.
Or perhaps not, but probably easiest, quickest and cheapest to narrow it down and be sure first.
Last edited by: No FM2R on Fri 3 May 13 at 23:19
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The fact all this started off with the RCD tripping, and it now doesn't must be relevant and the only thing I can think of that would do it, would be a damaged supply lead. Although I didn't inspect it, it looked ok from a cursory look.
I will be carefully looking at the lead now though.
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If you look back into the supply, is there somewhere for the earth current to go to?
Phrased in a slightly less obtuse way - is it possible that the socket which was being used to power the device had a poor earth connection?
Last edited by: Number_Cruncher on Fri 3 May 13 at 23:50
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An RCD works by detecting a imbalance in the live and neutral currents. So while the earth connection is the obvious candidate for a fault the RCD can still work on a two wire connected appliance.
So if the mains wiring from the house had a broken earth wire, it probably would not make any difference to the RCD operation.
But, testing the continuity of the earth would be sensible, and the use of an extension lead would permit this sensible test.
So far, I think I have to ask my neighbour if he has an extension lead that will reach and I need to whip a couple of earth pins out of plugs to enable me to test between the greenhouse and extension lead sockets. The Megger has a handy low ohms setting just for doing this too. Roughly a good earth is less than an ohm although I'd expect a bit greater from what is essentially two extension leads.
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You do not indicate what sort of pump?
Does it have a motor start capacitor?
Is it possible that the capacitor is leaky to earth?
or that the capacitor has now 'reformed', and you only have a marginal tripping problem?
Use the following suggestion with care: If the earth is temporarily disconnected, does the earth lead from the motor' float up' - then see if it can supply current!
If it is submersible pump and has spent the winter underwater will it dry out?
I use a pit sump pump that regularly trips after a winter of non use. A few days of sun in the greenhouse at 40C may sort it out?
PS I suspect I share your views on 'qualified electricians'
Last edited by: pmh on Sat 4 May 13 at 07:34
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Truthfully, I've not examined the pump (other than electrically) so don't know what sort of motor drives it. But if a capacitor reforming is involved, one presumes another test will give different results.
The suggestion to disconnect the earth - yes I can see where you are going, but although I will do seemingly alarming things to myself safety wise, I'm very much more cautious where other people are concerned. I may - but I'll try hard to get the answer without taking that step.
All the replies in this thread definitely give me fuel for thoughts so thanks to everyone.
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You can leak significant current to earth on a typical RCD without it tripping, so my guess would be that it's something inside the pump breaking down. Don't think it's any coincidence that the problem has reduced now that the weather is getting warmer and drier.
Have you had a look inside the pump? Come to think of it, have you tried disconnecting the earth and checking for a potential between the pump and ground?
Edit: just read the post above. Can you remove the pump and take it home for easier testing?
Last edited by: Fursty Ferret on Sat 4 May 13 at 10:27
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A typical RCD is supposed to trip on 10 to 30mA of imbalance I think. As the difference has to go somewhere, although one assumes it will go through earth, it doesn't have to, just not the live or the neutral conductors.
The 'problem' went from a tripping state, to a non tripping state the same day so although changing ground conditions could be a factor, I very much doubt if it was the case here.
Potential of a disconnected earth has already been suggested as a possible investigation, but I am doing this at somebody else's property and need both to be safe for them, and not to be seen as a trouble maker! To this end, I don't think removing the pump is an option - yet.
My neighbour is now away, so I can't investigate today.
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I suppose there's a possibility that your megger has had an effect:
(ISTR a test voltage of 250 or even 500 volts in the 1970's - certainly enough to make a telephone exchange engineer jump/swear at you, if you meggered a cable at the remote end to check insulations as he pulled the "u" links out..).
Used to check A to B, A to earth, B to earth, and be looking for a few megohms, and similar values on each measurement, for a music circuit.
During (and after) a spell of wet weather you could actually see the readings to improve briefly on a second test. I presumed it was the paper insulation on a cable down a wet ducting drying out.
I can't remember if the megger was hand cranked, or if it was only my "Tele-F" which was... all too long ago.
Last edited by: AnotherJohnH on Sat 4 May 13 at 16:07
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Have a look here:
www.richardsradios.co.uk/megger.html
Still common, not worth much but a very useful bit of kit.
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>> The 'problem' went from a tripping state, to a non tripping state the same day
>> so although changing ground conditions could be a factor, I very much doubt if it
>> was the case here.
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I had that with my garden wiring a while back.
Turned out that the problem was that ants had built a nest inside the tube supporting one of the garden lights. Add heavy rain and capilliary action draws damp up the ants' nest to the wiring, tripping the breaker. A day's sunshine warms the exposed tube and dries everything out, resuming normal service.
Took a while to work that one out as, courtesy of sod's law, the affected light was the last one I dismantled while searching for the issue. Solution was to remove the ants' nest, then remake the connections, setting the wiring junction in hot melt glue to prevent any further damp ingress. Finally reassemble the light with a healthy amount of ant killer inside it, belt and braces style.
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Tale of RCDs and a pump - No FM2R
