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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

    The conductivity of a municipal swimming pool water is monitored and reported in units of 'total dissolved solids' although the exact choice of "action limit" when the water needs changing or filter need cleaning depends on things like chlorination levels. 

    Typically a TDS level of a few thousand PPM for a swimming pool. (some hundreds of ohms on a cm cube ) Compared to a few hundred for tap water (some k ohms per cm cube.).

    Converting to a single % 'TDS' total dissolved solids from measuring conductivity is a bit of a con, as the different ions do not have quite the same mobility in a given electric field, but close enough for some generalizations to within a factor of 2. That may sound terrible, but as the salt concentration varies by factors of ten and is about as uniform as wind-speed in places where water flows and mixes, it is often good enough.

    (1uS/cm is 1 megohm.cm and 100uS/cm is 10k.cm 1000uS.cm 1k ohm.cm)

    These are American figures, and their idea of clean tap water is not the same as ours, but it is quite similar.

    In the table  below "442 solution" contain the following salts diluted in pure water: 40% sodium bicarbonate, 40% sodium sulphate and 20% sodium chloride. These represent the main conductive ions that are in typical surface and ground water. A purely sodium chloride solution is probably more representative of brackish salt marshes or sea water.

    Note the dramatic effect of seawater -  less than 20 ohms across a 1cm cube compared to k ohms.

    Mike

    PS

    At the other extreme I have worked on an RF power  system where cooling water was pumped over an anode at 10kV DC off ground, and 28kV p-p of RF superimposed, isolated by a few feet of what was in effect rather expensive garden hose. That rather scary system also monitored conductivity and shut itself off at 1 megohm cm or so, and had an ion exchange resin filtering a percentage of the main flow, so that the low conductivity was maintained. (well you had to run the pumps for 15 mins until it would even allow you to switch the HV on ;-)  )

Reply
  • 0

    The conductivity of a municipal swimming pool water is monitored and reported in units of 'total dissolved solids' although the exact choice of "action limit" when the water needs changing or filter need cleaning depends on things like chlorination levels. 

    Typically a TDS level of a few thousand PPM for a swimming pool. (some hundreds of ohms on a cm cube ) Compared to a few hundred for tap water (some k ohms per cm cube.).

    Converting to a single % 'TDS' total dissolved solids from measuring conductivity is a bit of a con, as the different ions do not have quite the same mobility in a given electric field, but close enough for some generalizations to within a factor of 2. That may sound terrible, but as the salt concentration varies by factors of ten and is about as uniform as wind-speed in places where water flows and mixes, it is often good enough.

    (1uS/cm is 1 megohm.cm and 100uS/cm is 10k.cm 1000uS.cm 1k ohm.cm)

    These are American figures, and their idea of clean tap water is not the same as ours, but it is quite similar.

    In the table  below "442 solution" contain the following salts diluted in pure water: 40% sodium bicarbonate, 40% sodium sulphate and 20% sodium chloride. These represent the main conductive ions that are in typical surface and ground water. A purely sodium chloride solution is probably more representative of brackish salt marshes or sea water.

    Note the dramatic effect of seawater -  less than 20 ohms across a 1cm cube compared to k ohms.

    Mike

    PS

    At the other extreme I have worked on an RF power  system where cooling water was pumped over an anode at 10kV DC off ground, and 28kV p-p of RF superimposed, isolated by a few feet of what was in effect rather expensive garden hose. That rather scary system also monitored conductivity and shut itself off at 1 megohm cm or so, and had an ion exchange resin filtering a percentage of the main flow, so that the low conductivity was maintained. (well you had to run the pumps for 15 mins until it would even allow you to switch the HV on ;-)  )

Children
  • 0 in reply to mapj1

    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.

  • 0 in reply to GaryD

    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

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