BEMI - Bättre Elmiljö
BEMI - Better Electromagnetic Enviroment

Stray currents and magnetic fields

It is common for electric current to wander away onto metallic pipes whenever given the possibility. Here in Sweden the electric system is constructed in such a way that the protective earth wire is in contact with both the neutral return wire as well as metallic enclosures of pumps etc. This gives the return current two pathways to choose between--the return wire or the water pipe. Since the pipe contains a lot more of metal it has a lot less electric resistance and thus the current will choose the pipe. This is called stray current and gives rise to magnetic fields both around the pipe as well as the cables (which lack the return current).

Since the stray current on pipes gives a magnetic field that isn't counteracted by a nearby current of the opposite direction, the magnetic fields will be quite high despite the "low" currents (1-50 Amperes).

Galvanic isolation

By inserting something non-conductive between two parts of the pipe one can break up the current pathway along the pipe. It is important the the bolts and everything also are isolated, there should be no possible way of metallic contact between the parts.