Fountain tripping RCD

[NeilBell]

Hi all,   As I’ve got some free time at the moment I have been trying to resolve a tricky electrical problem on a garden fountain and hoped somebody might be able to point me in the right direction?

The fountain trips the RCD in the main consumer unit but I am struggling to establish any logic as to when and why it occurs.   It is fed from the consumer unit to a small distribution board in the shed and from there via an underground armoured cable to a junction box near the fountain. The submersible pump is then connected into the junction box with a 10m long 3 core flex cable.    The fountain will trip the RCD within a few seconds when it is switched on.

To test the pump I disconnected it from the junction box and connected it via an extension lead to a 13A Skt in the greenhouse.  This is fed via a separate armoured cable from the distribution board in the shed.  Initially this seemed to solve the problem but then discovered that when the flex cable between the junction box and pump is wet it will trip intermittently (but far less than originally).

I have carefully inspected the flex but could find no visible damage.

I then tried submerging sections of the flex into a bucket of water (similar to checking for a puncture on a bike tyre) but the RCD did not trip.  I then submerged the whole flex in the bucket and again it did not trip, however when I put my hand in the bucket to remove the flex received a shock.  Probably not full 240v, but enough for me to let go rather quickly!  This still did not trip the RCD.

I have tried two different extension leads between the greenhouse and the flex and the intermittent tripping problem is worse with one than the other.

At one point the fountain had been running for a considerable time, then tripped when I stood on the flex. At other times I cannot get it to trip by standing on or moving the flex.

I wondered if there could be several issues which are combining to give this odd behaviour but wanted to get your thoughts on how best to proceed.

Sorry for the lengthy post and thanks in advance for any advice. 

Neil

 

[ProDave]

Clearly the pump or it's cable has a problem.  They are usually not servicable, so do yourself a favour and throw it away and get a new one.

To prevent a skip diver going through the same experience, cut the flex off as close to the pump as you can and dispose of that separately.

 

[Geoff1946]

You're a braver man than me!  Dunking a live suspect flex in water is not a recognised fault finding procedure, but by getting a shock from it you have proved where the fault lies. Whether or not you can see the fault the insulation is defective.

Is your pump the type with a sterilising lamp in it?  If so these are prone to water getting into the lamp housing. It is possible to dry those out and fit new O rings. Any other type consign to the bin.

 

[Sharpend]

I can’t help but wonder what you were expecting to see leaking from the cable? Oh wait it must have been floating currents! 
 

seriously, have you actually removed the pump and checked that the filter isn’t blocked? 

 

[Geoff1946]

I can’t help but wonder what you were expecting to see leaking from the cable? Oh wait it must have been floating currents! 

Actually the idea made me think of the chap who earthed his mobile generator with a spike in a bucket of soil.

 

[poni]

By putting it in a (plastic) bucket you have removed the fault path to earth that is why you got a shock ,you were standing on the grass  (Earth)you would have done better to throw the water over the cable in place ,with it off of course then retire to a safe place , i take its a twin cable so no earth cable, throw it away with it cut up into little bits .

 

[kerching]

You have two main options

1) bin it. Cut it into pieces and buy a new one. Job done

2) see option 1)

 

[NeilBell]

Thanks for all your replies.  I did think when writing the post that I might get some stick over my bucket 'test procedure'!!  The reason for doing it was that the pump flex is about 10m long but only about 1m is needed to connect into the junction box, therefore I thought that if I could pinpoint the earth leakage point I may be able to cut the flex shorter and reconnect.  I assumed it was reasonably safe as the flex has an earth conductor, but clearly my reasoning was flawed.  I can only assume the live wire insulation has broken down inside the flex but the earth has not - does that sound feasible?

The bit that still confuses me is that the pump tripped the RCD all the time with the original armoured cable, occasionally with extension lead 1 and rarely with extension lead 2.  The only explanation I can think of is accumulated earth leakage, partly due to the cable and partly due to the pump flex.  Any thought on this would be appreciated as I would like to avoid spending £120 on a new pump and then find there is still an issue. I also would really like to fully understand whats going on for my own peace of mind.

Cheers  Neil

 

[ProDave]

Well all you can try is shorten the flex to the minimum you need, re connect it to the junction box and try it.  You might get lucky and find you have removed the troublesome bit of it's long flex.

If not then it probably is a new pump as these are not normally built to be dismantled.

 

[SPECIAL LOCATION]

Any thought on this would be appreciated as I would like to avoid spending £120 on a new pump and then find there is still an issue. I also would really like to fully understand whats going on for my own peace of mind.

Cheers  Neil

Without approved test equipment that can measure Continuity, Insulation Resistance, and test the RCD's operating characteristics..

And then fully test the whole supply path from house to shed to pond & pump, you are pretty much guessing with your "visual" testing methods...

As..

1/ Electricity is invisible...

2/ It is odourless..

3/ At domestic voltages it is pretty much silent...

4/ It is not wise to touch it as it can kill a healthy adult in less than half a second...

Your problem could be due to a multitude of issues...

and/or 

It may be just due to natural leakage across several circuits with a few aging appliances/accessories and a poor design of installation that is only relying upon a single RCD..

{Unclear from your description if the RCD that trips is supplying multiple circuits in the house as well or just the shed} 

If the arrangement was designed with an RCD in the shed providing discrimination from the house fuse box, then this would have eliminate some possibilities straight off.

But bottom line is....

like the majority of fault finding investigations can be carried out on dead circuits with correct test gear..

There is no need to risk endangering anyone with a serious or fatal electric shock by trying to testing live!!

:coffee