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Alright folks,
First time poster but im at my wits end! Got a mobile welfare unit which has a small 3kw hand wash in it. The water to the hand wash is fed from a small pump which comes on when the sensor for the hand wash is activated. There is a flying earth lead from the pump which is now getting between 60 and 80 volts to neutral and the same to earth. With this lead disconnected everything works as it should but obviously when i connect it it just blows fuse straight away.
Initial thought was obviously a faulty pump so changed the pump but new one is doing exactly the same. Could this be down to bad neutral or something somewhere? Im completely lost. All other cables and circuits appear to be testing out fine unless im missing something stupid.

Should also mention its all fed from an on board generator.
Hope someone can point me in right direction,
Thanks
 
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Pete999

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I would advise you get an Electrician local to you to come and sort the problem for you, advising from a remote Forum, for something like this is ill advised, nothing like someone with eyes on the problem.
 
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  • #3
Afternoon Pete,
Thanks for the response. I am an electrician, with 12 years experience but have never came across a fault like this before. Was just hoping someone else might have seen something similar. Appreciate your response tho, ill maybe phone a friend ☺
 

Pete999

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Afternoon Pete,
Thanks for the response. I am an electrician, with 12 years experience but have never came across a fault like this before. Was just hoping someone else might have seen something similar. Appreciate your response tho, ill maybe phone a friend ☺
Ah right sorry
 
I think you need to isolate the power, connect the flylead and do some insulation resistance tests. You may well have a fault to a piece of floating exposed metalwork and by connecting the flylead you are completing the fault path. That is one possible scenario.
 
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  • #7
I think you need to isolate the power, connect the flylead and do some insulation resistance tests. You may well have a fault to a piece of floating exposed metalwork and by connecting the flylead you are completing the fault path. That is one possible scenario.
Thats interesting and quite possible. The fly lead wasnt connected to the main cpcs, it was onto a bolt on the metal work. Ive had enough for today but will try that tomorrow and report back.20190717_171534.jpg
 
So the green/yellow wire is the flylead, connected into the pump? When connected to your arrowed earth stud the fuse goes?
 
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  • #9
So the green/yellow wire is the flylead, connected into the pump? When connected to your arrowed earth stud the fuse goes?
Yeah thats correct. And when its not connected to the earth stud and i put my meter across the green/yellow fly lead and my main earth or neutral im getting between 60 and 70 volts
 
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  • #11
No sorry the main earth bar at the consumer unit. That is a metal casing which houses the pump and hand wash unit
 
B

Bobster

So the metal frame is connected to the MET at the CU correct?

What are the IR test results?
 

Lucien Nunes

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What device exactly blows or trips?

If the cable is the only earth connection to the pump, when it is floating (or connected to the outside world only via the meter) the voltage is not very meaningful and tells you as much about the input resistance of your meter as it does about the pump. Reading a voltage does not in itself indicate that anything should trip, as it could be the result of a tiny and normal amount of leakage that can only find its way to earth through the meter, as the lead is disconnected from its rightful place on the earth terminal.

Note that the OP states that power comes from an on-board generator. We expect it to be TN-S but have to keep an open mind about the neutral-earth linkage situation until proven.
 
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  • #18
Morning guys, firstly thank you for all the help, appreciate it. Quick update this morning. Have done an IR test on the pump and thats all clear. When i do an IR test between the earth stud on the metal work and the live feed from the DB to the hand wash im getting a reading of about 0.70 so assuming the metal enclosure is actually the issue not the pump. Going to.dig a bit deeper and see what i come up with.
 
All sounds mysterious. If the fly lead earth is the source of earth for the pump I can't see how connecting to that stud would create a fault issue. Keep us posted.
 
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  • #20
Im struggling with that as well because when the fly lead is not connected everything works fine (except for the fact im getting a voltage on the lead ) but as soon as you connect to that stud, or any earth i imagine, it blows the inline fuse and trips rcd. Even when pump live and neutral are connected im still getting clear IR readings to the flying lead. Gonna be some serious head scratching!

Should also mention the neutral earth is linked at generator. But as nothing else is having issues im convinced its nothing to do with that.
 

marconi

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Please confirm that it is a fuse (what size) that blows/ruptures and it is the one which protects the pump or is it for the water heater as well?

Does the van have a neutral-earth link in the consumer unit when supplied by the generator?

Has the problem with the pump just started - it was previously satisfactory? Or is this a new van being wired now?

Is the van's metalwork and metal pipework bonded to the consumer unit earth terminal?

Have you IR tested the final circuits to the main earth terminal?

Are you powering up from the generator or mains when fault happens?
 
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Right heres where we stand now. The neutral earth link is at the rcd fitted to the on board generator. I have IR tested right back to it and discovered that when i disconnect the link the fault clears. Im not overly clued up on generators my co-worker normally does that side but unfortunately he is on holiday.

In response to marconi
Its a 16A inline fuse which protects the handwash and the pump. All IR readings previously taken were clear but this was prob due to rcd at db being off. Its an old welfare trailer and think there has been a combination of works done including replacing some parts on generator and a faulty handwash unit which was replaced and assumed to be the issue.
 

Lucien Nunes

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By removing the N-E link you are simply breaking the earth circuit in a different place and stopping it doing anything. I would expect that to stop the tripping, so that makes sense, put the link back and leave it in otherwise there is a serious shock hazard that the RCD won't protect against.

The fact a 16A fuse is blowing indicates a fault of very low resistance. This does not tally with the floating earth tail from the pump hovering around 70V, you would expect it to go up to full mains w.r.t. earth. It's unlikely that both the old and the new pump would have such a fault.

Please could you confirm exactly what the 16A fuse is feeding and how. Is there a water heater? Could its element chamber be in contact with the pump? If so, I'm more inclined to think the fault is in the heater, and some nefarious arrangement of metalwork is making the pump's earth pigtail its only earth connection too. The element L & N may be disconnected from the supply until the water flow / pressure switch closes, so an IR test with the pump not pumping might not show the problem up.
 
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  • #25
Its a bit of a crazy setup but I'll explain it best i can. The 16A fuse is in line on the live to a 3kw instant hot water handwash. The pump is fed from the solanoid valve on the handwash unit. The handwash is activated by a sensor which then sends power to the pump via the solanoid valve. The handwash unit is essentially just a small water heater with a tap coming off it. The pump and the handwash are seperate but just housed together in this metal casing
 

marconi

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I think that:

1. There is an N-E conductor link at the CU from N in to the CU MET. Thus all van wiring N and cpc are connected together.

2. There is no bond between the CU MET and van metalwork. The generator earth is connected to van metalwork someway. gernator earth is connected to MET.

3. Electric appliances are earthed back to MET but isolated from van metalwork. So cpcs are floating wrt metalwork. This changes when fly lead to pump motor connected. Keep reading...

4. If L and N incomer has been connected correctly to L and N terminals of the Main Switch/RCD of CU, then check next L and N connected correctly to CEEFORM plug at the generator (if there is one). If not, then check connected correctly to L and N as marked on generator.

5. With Ohmmeter check continuity between generator socket earth and generator casing. If there is continuity then next check between L and then N and to earth at socket. if socket Earth is not connected to generator casing check between L and N and generator casing. In either case it should only be N connected if at all.

6. If there has been a crossover of L and N at gennie, between gennie and CU or at CU, and there is a N-E link at generator but now effectively applied to L CU incomer, and the van metalwork is floating with respect to MET because of no bond to it, then when the earth fly lead is connected a short circuit via the fuse is put across the generator.

Or something like this. Easier to see if one draws it out.
 
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Should also add when i disconnect the generator from the DB then everything tests out fine so i thought must be in the generator so I tried running a temp cable from the dist board to an alternative supply but then the short between live and earth comes back even with generator isolated
 
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  • #29
But what your saying makes sense there must be something reversed somewhere. Ive visually inspected all connections and everything appears to be connected correctly but havent put the meter on. Will try that now
 
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  • #31
Traced it back to a short between live and the motor casing in the generator. Disconnected motor to test it but couldnt find anything. Have since reconnected motor and fault has now cleared. No idea how and still none the wiser to the fault path it was taking to affect the pump etc. Thank you everyone for the help and hopefully i can repay the favour someday
 

Lucien Nunes

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the motor casing in the generator
Alternator casing?

Then, as Marconi points out, there must be a disconnection somewhere in the CPC / bonding arrangements. i.e. this was a multiple fault scenario. Otherwise the generator would have been permanently short-circuited as follows: Genny Line > machine frame > van structure > MET > N-E link > Genny Neutral. Something metal that the pump is in contact with, is not connected to that stud on the casing, e.g. the water pipe might be connected to the generator frame via the chassis, but not to the MET. You should find and fix this too, otherwise the next time a similar fault occurs, the fault path might include a human.
 

marconi

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Panda07 - please jot us a brief ditty to confirm you have done what Lucien Nunes has said otherwise at least two EF folk will not sleep tonight.

:)
 
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  • #34
The pump is not really in contact with anything. The pipework is all flexible hose and its attached to the metal housing with rubber washers on a plastic base. The earth lead is the only thing connecting the pump with the rest of the metal work. Im going to keep hunting though to get to the bottom of it!
 

UNG

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I might be missing something here but exactly how is the earthing in this mobile unit referenced because generators as far as I'm aware don't generate an earth
 

Lucien Nunes

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Nothing generates an earth, you just connect one side of your supply to it if you want to use its nice big handy equipotential (and then call that side the neutral). Within the van, if the supply is generated and used only on board, the only equipotential zone of interest is the van body, so that serves as the local 'earth', linked to neutral in the DB. If power generated on board runs outside, the whole shebang should be connected to a rod so that the ADS works outside too.
 
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