Circulating currents

What looks like TN-S is often in fact TN-CS with earth and neutral being combined at cable joints in the DNO network.


This combined earth/neutral conductor will carry significant current, and the voltage drop in this conductor means it will be a few volts away from true earth. Any short length of TN-S service cable will therefore have this voltage imposed on the metallic sheath.

A bonding connection between the cable sheath and a metallic water pipe will therefore carry current.

Yes, there will be some.

When there are loads applied at each property, the live voltage droops, and the neutral voltages rise relative to terra-firma.


If there is an N-E bond, or more than one at different points in the network then current will flow to try and even up these potential differences. If in effect you put a water pipe in parallel with the street main neutral, or at least part of it, then the current will share between the paths in inverse proportion to the resistances.

If all is well there will not be much voltage to drive this, so the current will be low, but note that the impedances can be very low as well - there is a  lot of  metal in a water main, so the currents can be surprising, a few amps is not unknown - and they become very high indeed if for any reason there is a high impedance neutral in the street wiring somewhere.

Mike.

PS

skip to about 1 minute into this video of what looks like a lost neutral problem and a properly bonded water pipe. Note that turning off the power to the house may not actually help much in this sort of case, if it is someone else's neutral current.
video as the embedded code does not work properly    

Thankyou broadgage I will try to put a clamp meter round my main earth wire see how nany mA are on it just out of cuiosity

I owe mapj1 an apology I didn't ignore your reply I think we both posted at the same time   certainly within a a few cycles if each other so thankyou to you too