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

Thank you, that's a really informative response. I wasn't aware of the figures for RCDs and ADS though I understand that RCD performance varies between seemingly identical devices; due to manufacturing tolerances I guess.

With regards to your mention of the length of the pipes and csa impacting the RCD trip times. I'm interested because if I were to bond the pool, and I don't want to, I'd bond it (in this scenario) with either a short length of earth wire trailing through the return pipe, or perhaps a ring of copper sheet snuggly inside the end of the pipe, entering immediately after the pump, that way resistance between pump and the earth wire is minimal. This way my logic would be that the rcd should trip before the current gets to the main body of pool water. Being really really reluctant to bond the water and convert to a TT supply with grounding and the associated testing etc, I've shyed away from this idea, not investigating just how long the pipework would need to be before and after the pump in order that an RCD would certainly stop the current before it moved into the pool.

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.

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.

Thanks, that's really interesting. I have read about TDS while trying to understand how to use the chlorine and other chemicals correctly, but it hadn't occurred to me that this electrical reason is likely a key reason that public pools etc want to know it.

I accept your point about no voltage being safe - having scared myself with the sparks from a car battery and indeed electric fences, the idea even of attaching electrodes from 12v let alone a 24v power running through the water and certainly much less 240v doesn't thrill me - which means I'm nervous where this might be done for me at some random point by an electical fault.

SELV (especially 12V a.c. or 30V d.c. or below) has a lot to recommend it. In normal operation it's completely isolated. Under single fault conditions (as it's separated - SELV rather than PELV) no shock current can flow as the fault merely references the system to the local (earth) potential at the point of the fault. Even if you had two different faults on opposite poles, they'd have to be widely spaced to be able to put a current through a human body, and even then the currents involved (due to the low voltage and resistance of the water) are likely to be tiny.

Nothing's 100%, but it's a pretty good bet.

   - Andy.

I think there are multiple reasons - they want to know about the salt load and chlorine and things like sweat, urine and other yuckies in the water as this tells them filtration is working, and if the water change-out rate is right.  This is almost proportional to the no of users. In the summer, our local replaces about 30 -50 litres per customer visit, so several tonnes a day in summer season.

Being sure of the electrical isolation from the pumps is a bonus....

Mike

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. At a distance of circa 2m of plastic 32mm pipe, the main body of pool water would be at a substantially lower max current and voltage, even though with no RCD on the SELV side it would be uninterrupted, this shouldn't be dangerous so as to cause tingle even (guessing now, I've not done the sums based on water resistance yet) - which I guess then would make me ponder - would it be possible for there to be an undetected fault in the SELV pump motor, submerged or otherwise that might permit the motor still to run seemingly normally while electrifying the water?

This scenario is far outside the physics I studied at school, but I think I have a grasp of the concept at least. I've expanded my knowledge by buying the "Requirements for electrical installations" and other similar texts, but, my formal education in this area is to school A-Level physics, which focussed more on functionality of electrical windings and such like than safety to this level.

I'm looking to ensure for certain that one cannot get close to remotely like 30 milli amps of shock (RCD rating), even at 24v, and this will be a factor of 10 times less dangerous (ok, electical danger isn't linear like that), than a 240v with earth bonding etc.

Would PELV in a TT system be even safer than SELV though on the basis that an RCD would potentially cut the circuit removing any current in the event of fault, however small - this feels true, though PELV is considered less safe than SELV because of the link to the supply side (presumably because that's easier to say than "SELV is safest with any circuit other than TT").  Am I missing something obvious here, or am I correct that the electrical wiring regs have been written with simplicity in mind in this scenario?

Would PELV in a TT system be even safer than SELV though on the basis that an RCD would potentially cut the circuit removing any current in the event of fault, however small - this feels true, though PELV is considered less safe than SELV because of the link to the supply side (presumably because that's easier to say than "SELV is safest with any circuit other than TT").  Am I missing something obvious here, or am I correct that the electrical wiring regs have been written with simplicity in mind in this scenario?

It rather depends what earth reference you use for the PELV, and if it is at the same voltage as anything else in or near the water or another not quite earth some volts offset. This is not as easy to be sure of as ensuring  ' fully isolated' - shavers sockets have the same problem, so SELV is often safer.

The key to shock reduction is clean non salty water, and low enough voltages not to get dangerous currents through the essential parts of the human. Swimming in HV transformer oil is probably off limits for other reasons, but electrically would be one solution.

And yup,  the regs have to say something better than 'think it out case by case' but the general guidance is just that - general - and there will always be some corner cases where the regs book answer is not the best.

Mike.

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.