Why doesn't this trip an RCD?

[TicklyT]

Just curious really:-

I had a look inside a little water heater and found the construction is something like this:-

The 5 KW uninsulated spiral wire heater element is directly immersed in the water to be heated.

The pipe tails are earthed

Water is supposed to be a conductor.

The 30 mA RCD does work.

So why doesn't it trip the RCD every time it's switched on?

 

[Lectrician]

Copper element is most likely coated

Over time, as it breaks down, I guess it will operate the RCD

Never seen an immersion element quite like that one.

 

[davelx]

"pure" tap water is not that good a conductor. cf. no requirement to supplementary bond metal taps fed by plastic pipe.

Just out of interest, I did a 1000V IR test on a wineglass of water with about 6cm between the probes - 20K, which is 12mA at 240V.

 

[TicklyT]

No, the element is just plain wire with a bit of green gunge (copper oxide?) deposited on it by the water flow.
Even the brass ferrules crimped to the ends of the element are immersed in water.

 

[TicklyT]

"pure" tap water is not that good a conductor. cf. no requirement to supplementary bond metal taps fed by plastic pipe.

Just out of interest, I did a 1000V IR test on a wineglass of water with about 6cm between the probes - 20K, which is 12mA at 240V.

I suspect that is where the answer lays. It looks like the manufacturer has made the distance through the plastic body from the pipe tails to the element longer than they need to be otherwise.

How pure is 'pure' tap water? How much does chlorination and limescale increase it's conductivity?

 

[ColJack]

tap water is far from pure.. it's quite a good conductor depending on where you live with all sorts of minerals disolved into it.

pure water is a very poor conductor..

the sales people of water purifiers use a precipitator to show this by immersing 2 rods in a glass of tap water and passing a current through it making the rods corode and colouring the water a rusty brown.. they repeat the process with water that's been through their filter and it stays clear because the current doesn't flow as well or at all..

 

[davelx]

tap water is far from pure.. it's quite a good conductor depending on where you live with all sorts of minerals disolved into it.

.

That's why I put the "pure" in quotes. But the fact is that it is NOT a very good conductor. Once it comes into contact with salt on the surface of a person's skin, that is another matter. From Wiring Matters, Autumn 1996:

"The ERA tests confirm that tap water in a plastic
pipe is a poor conductor of electricity. One metre
of 15 mm diameter plastic pipe filled with tap
water from Leatherhead where the ERA are based,
has a resistance of 100,000 Ω. This one metre of
pipe will restrict currents to less than fatal values
and of course in practice, there would be many
metres of pipe between metal items of plumbing
equipment and earth"

Full article here:

Plastic Pipes

 

[empip]

554-05 ??