Pumps and RCDs

I've got a strange problem with a new borehole pump - it trips the RCD on
the circuit whever it's turned on. However, after about 3 attempts in
succession (resetting the RCD each time), it starts OK.
According to the manual, you are able to use and RCD with it, although they
recommend use of a RCD that can sense pulsed DC faults, which it seems
standard RCDs might not. At the moment the pump has a standard RCD, but my
impression is that a pulsed DC type RCD will be even more sensitive.
The pump has a soft-start - to check this I put a clamp meter on the pump -
there is a 2 second delay after switch-on before the motor starts and there
was no initial kick in the current at all - the current rose smoothly up to
8.5A over about 5 seconds and then settled back to 8A.
The circuit is brand new, and the pump is the only thing on it. Next time
I'm there I'll try and check to see if there's anything between active and
earth and neutral and earth, in case there's some threshold current.
I suspect it might be due to the nature of the pump's current draw - as far
as I can tell the pump (Grundfos) uses some kind of frequency converter
drivng a permanent-magnet motor.
Anyone have any experience with this kind of pump?
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I have a submersible Grundfos bore pump and have no problems with the RCD. Sounds like either a problem in the wiring to the pump or the pump itself. Who wired the pump up, if you did it yourself, better get it checked.
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Is it one of the SQ or SQE series? The wiring is very simple - the pump installers put the 6mm cable onto the pump when it was installed - it's just 2-core plus earth - no start capacitor or control box. The electrician put a 3-pin plug on the end and installed an outdoor GPO next to the bore head. Perhaps there's some water ingress down the hole somewhere - I hope not because pulling the pump up might be a bit of a task - it's 114m down.
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This pump only has 3 wires going to it - active, neutral and earth - that's it. The single to 3 phase thing sounds similar though - from the little I understand, I'm guessing there is a AC to DC converter, then a DC to variable-frequency 3-phase drive for the motor. You can get another version of the pump that has a pressure sensor, control box and extra wire going down to it that varies the pump speed depending on the system pressure.
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It could be the depth - you will get some capacitive current from a lead that long (expecially if a good portion of it is submersed in water). Usually its not recommended to have long leads on with RCDs as they can be prone to nuisance tripping. There could possibly be some filters in the soft starter which may have some leakage current as well.
You should check to see if its nuisance tripping or a real fault - megger the pump and lead, but be sure to follow the manufacturers instructions, the megger can damage some components. How did they get the 6mm cable to fit in a standard 3 pin plug? even 2.5 is a struggle....... You could theoretically take the RCD out of the circuit or get one with a higher earth fault trip current, but you would need to hard wire the pump for that to be legal in a residential installation.
Most bores Ive seen are three phase and generally dont seem to have RCD's
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The guys who installed the pump removed the insulation from the 6mm cable, put a standard plug on it and wrapped it up with tape, but it was clear that was temporary. We ran the pump with a 30m extention lead, and that did trip the RCD on the associated circuit, but only if the pump was re-started within about 5 minutes of previous operation.
The final installation was 30m of 6mm cable from the switchbox to an industrial-type socket (normal 3-pin plug but with a screw-on ring and larger cable entry on the plug) next to the bore. The sparky spent about half an hour trying to fit the 6mm cable to the plug, then gave up and used a bit of ordinary cable - only from the pressure switch to the plug.
The manual advises against putting a megger on the pump, but the submerged cable capacitance could be the cause. Having to dump the GPO will be a pain though - and expensive - I doubt the sparky will give me credit for the industrial socket and plug.
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I've seen a similar problem but it was on an incorrectly wired single to three phase converter/speed control on a 5 HP motor. The motor had a few configurations and the electrician mixed up two different diagrams without smoking anything but the RCD intermittently tripped on switch on.
Once it was wired up ok it worked fine with no trips at all
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Mark Harriss
I agree.
Most submersible pumps I have seen have the wiring run directly from the distribution CB to the borehead cover box where it is permanently joined to the pump cable. In my experience (as installed on my 3 ph submersible) the pump protection is achieved with a combination current overload and thermal CB in the distribution box, but no RCD. It is not as though general members of the public are likely to come in contact with the wiring at any time during normal operation and when repairs are necessary a qualified electrician should be doing the work, so RCD's are not really required.
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Ross Herbert
All very true - we have a bore pump on another property which is directly wired, partially because it has a start capacitor and associated control box.
In this case, since the pump was only 2kW and required no control box, it seemed appropriate to simply install an external GPO and plug it in. Well, it seemed like a good idea in hindsight :)
I haven't had a chance to measure for any conduction to earth, but speaking to the installer they have had one other instance with the same pump tripping the RCD and was traced to a fault in the cable entry on the pump.
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