Issues with immersion circuit

[jimbo5050]

Hi everyone,

I have a MEM consumer box. There is a 16 amp MEM breaker that supplies the immersion heater. I have had solar panels installed for the last two years and I have been having issues when I tried to install an Immersun device in place of the immersion heater timer switch, in that a fault develops. I am now onto my fourth unit. Suspecting (no particular reason) that it may be the old wiring from the consumer box I have replaced it just to rule that out. Whilst the end feeds are exposed before I fit the replacement, I thought I would check the resistance between the Live, Negative and Earth of the two respective cables (the one from the consumer unit and the one to the immersion heater. With the main isolating switch off, there is no measured resistance between the live, negative and earth cables of the feed, which is what I would expect. I am getting 19 ohms between the Live and negative for the immersion cable, which is also what I would expect. However, with the consumer box switched back on but with the immersion breaker off I am getting a measured resistance of about 17 M ohms between the Live and Negative of the feed from the box, about 18 M ohms between the Live and the Earth and about 1 to 2 ohms between the negative and the Earth. I am also getting a PD of about 8 volts AC across the Live and Negative. This seems very odd to me as surely there should be no measured resistance across any of the cables? I am not wanting to risk installing another unit as I am concerned that the consumer box may be at fault in some way. Any advice would be greatly appreciated.

 

[Taylortwocities]

and about 1 to 2 ohms between the negative and the Earth

If you are getting that, then there is a fault either with the wiring, or the immersion element. The resistance between earth and the line or the neutral should be more than 2,000,000Ω (2MΩ)

It should be reasonably easy to track down, As you have a meter, and an identified fault.

 

[winston1]

There is no negative on AC, is is called neutral.

One or two ohms between neutral and earth once the main breaker is on is not necessarily a fault. If, say, your are on a PME system the neutral and earth will be connected together where it comes into the house. I'm surprised TW does not realise this.

 

[jimbo5050]

Thank you for the replies. That sounds like everything is okay then, so I will go ahead and fit the unit and keep my fingers crossed.

 

[ban-all-sheds]

The resistance between earth and the line or the neutral should be more than 2,000,000Ω (2MΩ)

 

[terryplumb]

I like that bas, wish I knew how you did it. regards terry

 

[Taylortwocities]

Mea Culpa. I didn't read the post properly.
Thought he was testing towards the immersion. Must be the heat…
Of course, as Winston says, you will see a low impedance between neutral and earth.

 

[ban-all-sheds]

I like that bas, wish I knew how you did it. regards terry

Find an image on the interwebby that you want to post, and copy the URL.

In your post, click the image button

and insert the URL.

 

[Risteard]

There is no negative on AC, is is called neutral.

It is also called phase 50% of the time.

 

[flameport]

I am also getting a PD of about 8 volts AC across the Live and Negative

If measured with a typical multimeter, that is due to capacitive coupling between the immersion cable and other adjacent cables.
It's also probably why the 17Mohms appears, as even a tiny voltage on the cable will cause defective readings - multimeters typically apply a couple of volts to the cable when testing for resistance, so any voltage from other sources will give totally misleading results.