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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?

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    Thanks Andy. So if I understand you correctly, even if I were to run a 24V DC motor drawing 100W on say a 120W transformer, then although there would be maximum potential 5A current available at 24V travelling through the water (assuming no fault in the insulation between windings within the transformer), then the voltage would be around 12v between the positive and negative wire (about halved), the current would be substantially reduced by the resistance of the water, even if you managed to get your finger inside the pump housing to be close to the wires.

    I suspect not quite. There's no 5A limit as such - that'll just be the nominal rating of the supply - most types of supply will be able to supply rather more current if pressed - usually until the transformer gets far too hot and some thermal trip intervenes. Generally its the resistance of the overall circuit (intended or due to a fault) that regulates the current flow.

    Don't forget that as far as electric shock is concerned, it's the voltage difference across the victim that counts. We only (sort of) think in terms of absolute voltages for things like mains supplies because they're deliberately referenced to Earth - in that case you can get a shock between some live or faulty part and anything that's at all Earthy. The big advantage of separated systems (mains or ELV) is that they're NOT referenced to earth - so even when there's a fault (or you just choose to put a finger on an exposed terminal) there's no way to complete the circuit that passes a shock current through the victim - as there's nothing to drive a voltage difference across the victim's body. With separated circuits you need to expose the victim to two different breakages in the insulation simultaneously - which should be a pretty rare even of itself and generally mitigated by occasional inspection for visible damage. Make it a SELV circuit and they you get the advantages of the much safer voltage in addition to separation - so unlikely to come to serious harm even if the odd tingle can be felt.

    The 12V a.c./30V d.c. limit comes from BS 7671's limits for SELV systems for baths & swimming pools.

       - Andy.

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  • 0

    Thanks Andy. So if I understand you correctly, even if I were to run a 24V DC motor drawing 100W on say a 120W transformer, then although there would be maximum potential 5A current available at 24V travelling through the water (assuming no fault in the insulation between windings within the transformer), then the voltage would be around 12v between the positive and negative wire (about halved), the current would be substantially reduced by the resistance of the water, even if you managed to get your finger inside the pump housing to be close to the wires.

    I suspect not quite. There's no 5A limit as such - that'll just be the nominal rating of the supply - most types of supply will be able to supply rather more current if pressed - usually until the transformer gets far too hot and some thermal trip intervenes. Generally its the resistance of the overall circuit (intended or due to a fault) that regulates the current flow.

    Don't forget that as far as electric shock is concerned, it's the voltage difference across the victim that counts. We only (sort of) think in terms of absolute voltages for things like mains supplies because they're deliberately referenced to Earth - in that case you can get a shock between some live or faulty part and anything that's at all Earthy. The big advantage of separated systems (mains or ELV) is that they're NOT referenced to earth - so even when there's a fault (or you just choose to put a finger on an exposed terminal) there's no way to complete the circuit that passes a shock current through the victim - as there's nothing to drive a voltage difference across the victim's body. With separated circuits you need to expose the victim to two different breakages in the insulation simultaneously - which should be a pretty rare even of itself and generally mitigated by occasional inspection for visible damage. Make it a SELV circuit and they you get the advantages of the much safer voltage in addition to separation - so unlikely to come to serious harm even if the odd tingle can be felt.

    The 12V a.c./30V d.c. limit comes from BS 7671's limits for SELV systems for baths & swimming pools.

       - Andy.

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