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Which is safer/safest pump style in a garden 'splasher style' swimming pool?

GaryD

I'm really struggling to opt for the safer option on pump. This has come up because instructions on the pumps supplied state "do not use while in the water", but, as was inevitable eventually, someone forgot and swam without switching it off once....

I was provided with a 240v pump, externally mounted, but instructions state "do not use while in water". This design pump blew up (2x) - likely overheating as it was the resin seal that failed both times.

I bought a different one (designed for pools still), but this I discovered on arrival is submerged within a sump in the filter - it hasn't over heated - water cooled, but again "do not use while in water" label attached.

So... I know a little bit about electricity, but not a lot. So I figured that I could purchase a 24V DC pump, externally mount it, and this would be powered by an SELV transformer. The pump is 100W.

Would this DC pump running with the SELV transformer be safer?

I'm concerned because SELV means that there will not be anything  that causes the electricity to cut out if there is a fault on the DC side.

The RCD and AC 240v option however, has an earth at the motor, so theoretically, a failure will cause an earth fault and trip the RCD, but, if for some design reason, the earth isn't affected by the mechanical fault and water gets to the live, the pool is then electrified to 240v and then RCD does not trip. Typically, this is accounted for by bonding the pool water to the earth, though it's not as I understand it so trivial, since if you do that, you expose the water to potential fluctuations in the earth voltage (TNCS supply). To avoid that, you'd then convert the supply to TT setup.

In short, my understanding is that there is a risk that without bonding the pool water, especially with a submerged pump, there is a risk of electrocution even with a modern RCD fitted, regardless of type.

Hopefully you can see my confusion, and knowing a bit more than me, be able to put my mind at ease as to the safest option. Ideally without converting the supply to TT and installing earth bonding for the (temporary!) pool water.

What would you install if you owned a splasher style temporary pool?

Parents
  • 0

    note that in a full size swimming pool the pump is not submersible, but connected to the larger pool by a considerable length of pipe. The resistance of the water and the length to cross-section ratio means that in effect the water enters and leaves the pool in a way that there is no dangerous potential difference in the water. Similarly pool lights are firstly ELV and secondly enclosed in such a way that the light enters the water through a window from a sealed box with a light in it. Again, even if that box fills with water, and it is not really supposed to, the ability to induce a significant difference in voltage between any two points in the pool is very much limited.

    Standards aside, a system where the voltage is in the water is lower, is likely to be safer, unless the construction quality in the lower voltage model is poorer as a consequence. However, as Graham notes the wet human  is a lot more vulnerable than a  dry one, and apparently innocent things like metal steps introducing a near earth voltage can become a hazard  during a fault if current paths have not carefully considered and eliminated. (simple measures like decking or plastic steps can raise the impedance of the fault loop to external ground)

    However if the automatic disconnection in the form or RCD etc is working as intended, the risk is moderate, as you have to be in the water when the pump is on,  and in a place in the water where there is a significant voltage gradient, at the very moment  when a fault occurs. I'm sure plenty of folk ignore or forget the advice to unplug and live to swim another day, but the risk is not truly zero.

    Mike.

    edit PS
    you might find this discussion of hand held electrode type baby bath heaters interesting - some of the thoughts are the same.

    electrode type baby bath heater

    It is amazing the difference between CE and safety mark parts of the world, and the rest.

Reply
  • 0

    note that in a full size swimming pool the pump is not submersible, but connected to the larger pool by a considerable length of pipe. The resistance of the water and the length to cross-section ratio means that in effect the water enters and leaves the pool in a way that there is no dangerous potential difference in the water. Similarly pool lights are firstly ELV and secondly enclosed in such a way that the light enters the water through a window from a sealed box with a light in it. Again, even if that box fills with water, and it is not really supposed to, the ability to induce a significant difference in voltage between any two points in the pool is very much limited.

    Standards aside, a system where the voltage is in the water is lower, is likely to be safer, unless the construction quality in the lower voltage model is poorer as a consequence. However, as Graham notes the wet human  is a lot more vulnerable than a  dry one, and apparently innocent things like metal steps introducing a near earth voltage can become a hazard  during a fault if current paths have not carefully considered and eliminated. (simple measures like decking or plastic steps can raise the impedance of the fault loop to external ground)

    However if the automatic disconnection in the form or RCD etc is working as intended, the risk is moderate, as you have to be in the water when the pump is on,  and in a place in the water where there is a significant voltage gradient, at the very moment  when a fault occurs. I'm sure plenty of folk ignore or forget the advice to unplug and live to swim another day, but the risk is not truly zero.

    Mike.

    edit PS
    you might find this discussion of hand held electrode type baby bath heaters interesting - some of the thoughts are the same.

    electrode type baby bath heater

    It is amazing the difference between CE and safety mark parts of the world, and the rest.

Children
  • 0 in reply to mapj1

    Thanks. It is interesting you raise the point about potential for cheaper construction quality in a lower voltage model - this has been playing on my mind a lot - to the point while considering purchasing a DC pump, I was thinking perhaps I would need to buy a high watt rated LED SELV transformer because then I'd "know". The issue is you cannot see inside the transformer so you've no idea if there are any faults in the double insulation etc (or in fact if its a cheap knock off with key parts missing).

    With the current filter pump (which is 'essentially' a submersible sitting in a sealed 2litre jug of water with two pipes attached), I did consider that if I were to earth the water inside by putting an electrode through the side, this might be a neat solution - but does rely on faith in the RCD and switching to TT to avoid spurious earth currents from PME.

  • 0 in reply to mapj1

    I love the link to the baby bath heater. Thanks for that. He makes me nervous with that jug of water with a wire pulling to the side with his exposed electrical socket about 30cm away though! I keep waiting anxiously for the wire to tug on the jug enough to pull it over on top of the socket.

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