elcb and borehole pump

Sparkingchip

I know it's the pump or cable. It is controlled by a pressure switch and I eliminated that as the cause before worrying about the pump. Plus the RCD trips only on startup, which suggests a transient current effect and this is more likely to be caused by the pump than anything else. I'm looking for someone who has encountered this problem before or with sufficient detailed knowledge of RCDs to speculate on the cause.

There have been reports of RCDs tripping on large startup currents (often largish motors or welders). The problem seems to be due to RCDs only being accurate within a certain range of currents - as I understand it the effect is down to things like manufacturing tolerances so that the L and N coils don't precisely cancel even when exactly the same current is flowing in and out. Say the two current carrying coils on the torroid are only accurate within 0.01% of each other, so with say 63A flowing the residual current might be out by 6.3mA but as that would still be below the RCDs tripping threshold it still wouldn't trip. But if the load current was say four times that, even if only for a short time, the error might climb above 25mA and be enough to trip the RCD.


If would imagine that a borehole pump might indeed  have a difficult time starting, if it needs to get a 20m column of water moving before things start turning normally (I've no idea if they incorporate any kind of soft starter these days).


Because the difference between tripping and holding in might be so slight, simply replacing one RCD with another apparently identical one might make the difference between it tripping or not (or similarly replacing the pump), but if the  RCD has a relatively low current carrying rating (25A, 40A, 63A say) replacing it with one with a higher rating (80A or 100A) is perhaps more likely to help. Similarly if an instantaneous 30mA isn't actually required (if say it's hard wired rather than plugged into a socket and cables aren't concealed in walls etc) than a higher residual rating and/or time delay version would reduce the risk of nuisance tripping - or perhaps no RCD at all if it's a TN installation and Zs is within tolerance.


Or of course it might be a simple N-PE fault anywhere downstream of the RCD (not necessarily on the pump circuit) that's leaking an extra bit of current when the pump startup current increases voltage drop in the supply and N drifts a little further away from Earth than normal.


   - Andy.

Hello Mike

I came on here to ask a different borehole pump related question and found your post.

The only similar fault I have encountered is a motor that tripped the RCD when it stopped. That turned out to be an earth leakage fault on the cable going to a limit switch. It was only when the limit switch was activated to turn the motor off that the fault occurred. Is there anything equivilent in the borehole pump,ie. something that is only in the circuit on start up?

Is your objective is to find the reason for the problem or just to get it working? If the latter,

• does the house wiring allow you to try a RCD with a higher rating or time-delayed?

• are there other loads connected to the RCD that may be "leaky"?

• I've found with PV inverters that some brands of RCD are more tolerant to the transient effects on start up than others. Wylex RCD's seem to work when brands with the same rating don't.

Dave

I can speculate, but I cannot see what I am speculating about, a picture of the consumer unit arrangement would help.


If the RCD is in the tails it can apparently make a difference as to which way around it is installed, feed in at the bottom rather than the top.


The starting current raises the neutral voltage more than the running current, so if there is a neutral insulation fault to earth it would be more prone to trip on starting rather than when running. But the fault could be in the installation rather than the pump or it’s flex, however it’s not a particularly powerful pump.


Are you testing with testers or just bang testing to see what trips the device?


Andy Betteridge

There is not enough information,  just a bit of speculation. Something simple needs to be ruled out first.  You have used two separate points of use. Two different RCDs but we have no idea how the RCDs are configured with the rest of the installation.  Try both points of use again, but this time, isolate, as much as possible, all existing loads 1st. So unplug everything from any socket. Switch off  all MCBs, excepting that part feeding pump socket. Try again and see what happens.

My money would be on the leakage current from the pump and flex being within acceptable limits, but adding to existing leakage elsewhere in installation and appliances, possibly also a neutral fault somewhere along the circuit.


I am not surprised the installer isn’t rushing back to change the pump. Given it is connected with a plug and socket I would disconnect the pump and plug it into a Portable Appliance Tester to do an insulation test and a leakage test, though the insulation test can be done with any insulation tester. Then if it gives good test results move on and test the installation with an installation tester.


Given that if this was a job I was being paid to do I would turn up onsite with over a thousand quids worth of testers and a selection on different RCDs of varying specifications from different manufacturers plus other installation materials, I would be doing a bit more than speculating.


Post a picture of the consumer unit arrangement and the device that’s tripping, we can speculate a bit more, but only that.  


Andy Betteridge

If the pump is modern, with some built-in soft-starting (or running) electronics, it may have a filter with capacitance L-E & N-E as well as L-N, so a brief current can flow L-E when turned on.  I've seen this trip at least older RCDs from e.g. a washing-machine filter, just at the time of turning on or off the socket. However, like several other speculations before it, this doesn't very satisfyingly explain why the RCD would show no sign of trying to trip when it, instead of the pump control, turns the pump on.  The obvious ideal is to view the start-up residual current, but I assume you don't have some fancy clamp and plotting-device. Given that RCDs are easier to change than pumps down deep holes, the earlier reply about a brand recognised as being more resistant to brief leakage might be worth pursuing, in spite of the cost.  The idea of N-E faults (for the rest of the installation too, with normally-connected appliances, not just the pump) that was already mentioned sounds worth checking with a simple insulation test, if the motor takes high starting current in spite of its low running current. A diagram of the setup (including pressure switch) and the tests you've already done would be helpful.

mikelee:

The old pump is no longer on site and the new one is 20m down the borehole. The elcb is a standard domestic consumer unit bog standard elcb by Proteus.

weirdbeard:

Allow us a few moments to fire up our crystal balls, but in the mean time could you post a few clear pics of the relevant parts, such as the elcb ,the old pump, the new pump Etc?


:)

Hi mike, thanks for the reply, do you have any idea what the problem was with the original pump? What were the symptoms of failure, and how long was it between the original suspected problem and the new pump being fitted, and was 


Cheers :)

Of course with a correctly installed borehole pump there is probably now a highly effective earth rod that may have been missing up to now.


Andy B

just because something has been working for years does not always mean it is fault free.

Just because something is brand  new, does not always mean it is fault free either.


If it was me, I'd be trying to separate effects due to the pump from effects due to the rest of the wiring.


1) I presume it is actually an RCD - not an earth leakage circuit breaker (an RCD looks at the difference of neutral and live current, and tripe if the mis-match is too  great.  An earth leakage circuit  breaker measures current in the earth wire, and trips if it is too great.)

Since about 1980 RCDs have been preferred ,as they can also detect current leaking to earth via paths other than the green and  yellow wire, perhaps through people and wet feet to ground)  If it is an ELCB, then it should be replaced. I bet it is not.


And be sure it is an RCD that is tripping, and not an MCB (that would be an overload matter)



2) So, is there a problem with the rest of the wiring , or the new pump ?

How is the pump controller wired and how is it earthed. Also, what do the maker's instructions recommend. I imagine it will say to supply it via a normal 30mA RCD is OK, but it would be good to confirm that point, and they do not recommend something else.

Can you run the pump, and just the pump, via the offending trip? You need to be able to completely isolate every thing else on the same supply to do this test.


How is the main earth organised (TT, TNS, TNC-s ) , and if TT, where is the electrode in relation to the borehole?


If it runs OK in isolation, then we need to look at the rest of the installation. If not we need to look harder at the pump and it's wiring.


Can you verify there is no neutral to earth fault in the rest of the installation (and if there is an N-E fault, note that flicking off single pole breakers is not enough t for this test as it does not disconnect either N or E - they only break the live. )


Can you measure the residual current (LN imbalance) and is this high enough for concern ? About 30% is the threshold - so any more  than 10mA L-N difference on a circuit protected by a 30mA RCD would be cause for some sucking of air between the teeth.