Bonding to pipes

[Jay Sparks]

The strange thing was that when I took out the probes and dried them, then put them back in again, the reading continued from when I took them out.....I would of thought that it would of started from the 18 kohms again.

I might try it on my pee next time but holding the probes and trying to pee will be hard.lol

 

[Silly Sausage]

Jay...
Just guessing here, I wonder if that's got to do with electrolysis?
The longer you do it, the less ions there will be in water, so conductivity falls.
You've probably slightly plated your negative probe!


edit: obvious mistake rectified!!!

 

[telectrix]

the water itself is non-conductive, it's the impurities in it that cause conductivity.

 

[davethsparky]

Can't remember which course I was doing but on one of them we were told that if a plastic 22mm pipe is over 600mm long then the water inside will be of high enough resistance to not require bonding.
Seemed a bit fishy to me as water in different areas will have different resistivity.

 

[Guitarist]

the water itself is non-conductive, it's the impurities in it that cause conductivity.

And if the property has a water softener, then the conductivity will be relatively high, due to the salt content in the water.

 

[hoppy]

Thanks for a great educating post, I have learned a thing or two.

Can I confirm I have got my head around this subject correctly.

To test if something is extraneous, IR test @ 500V between main earth and the part in question, if below 45k ohms then considered extraneous (why 45k ohms)?
Then continuity check between part and MET value must be below 50v / Ia or the part in question will require supp bonding regardless of RCD protected or not as it is considered to not be effectively connected to earth?

Also, you should continuity test between all extraneous and exposed conductive parts in a bathroom and the resistances between these parts should again be below 50V / Ia? Again if not then sup bonding is required regardless of RCD Protection?

The lack of sup bonding if calculated and measured to be required is therefore always a code 2 regardless of rcd protection?

Thanks

Craig

 

[D Skelton]

Up to 5mA across the body, although perceivable, will not cause any ill effects ie. pain or involuntary convulsion of the muscle. Anything above 5mA and there is a chance of damage caused directly or indirectly as a result of the shock. Many people will accept anything over 10mA to be the dangerous level of current where involuntary muscle contractions will definitely happen. I and many others consider good practice to be preventing the chance of anything higher than 5mA to flow through the body in the event of a fault. GN3 I believe (although I could be wrong here) also uses 45kohms as the figure to determine whether or not something is extraneous or not. It's all about safe leves of current at the end of the day, in all honesty it's a point we could all argue until the cows come home as we will each have our own views as to what 'safe' current levels are as there isn't really anything written down in statute. The reason for 45kohms is simple; 230/5mA = 46kohms. 1000ohms is accepted to be the lowest possible value of bodily resistance, so this is then taken away from the 46kohms leaving 45kohms. In other words, if you were isolated from earth and connected to 230V nothing would happen, if you were to then touch something measuring 45kohms to true earth then adding your 1000ohms to the equation would mean that no more than 5mA would flow through your body. It would be a perceivable amount of current but under normal circumstances you would be under no danger.

In answer to your other questions; yes, although some points could be debatable (see previous posts)

 

[Guitarist]

10mA is considered the "let-go" threshold, and as such GN8 uses 22k as the figure (the reason hospital RCD's were always 10mA). That said, it does acknowledge that in certain circumstances the designer may consider 10mA too high.