Why does some eqpt leak to earth?

[benstoneham]

Reading up on 17th, the guidebook I have (ECA Guide to the wiring regulations 17th Edition, which I have fouind quite good by the way) notes that "Most modern individual pieces of equipment on a circuit creates some amount of earth leakage current and these currents add in the cirtuit's protective earth conductor'.

The go on to suggest that the circuit should therefor be designed so that the circuit runs with a standing leakage of < 25% of the rated operating current (i.e. 7.5mA I think).

2 questions are then, firstly why does the equpt do this? A result of induction/ eddy currents of some kind (perhaps switching in/out inductive loads?)

And secondly, how would you calculate/ allow for this (or would it just be based on experience)?

 

[ricicle]

Mains filters have capacitors in them that are connected between Phase and Earth, and between Neutral and Earth to quench any overvoltage in the supply

 

[benstoneham]

Doh! turned 2 pages at once... the next page gave guidelines (althought not the reasons behind it).

Likely culprits:

Switched mode PSUs (Tvs, PCs, Photocopiers etc) allow 1mA per piece of eqpt.

Heating elements (allow 3mA <= 3kW and 6mA upto 6kW)

 

[bernardgreen]

Another likely culprit.

Long runs of steel wire armoured cable which has significant capacity between Live conductor and the earthed armouring.


And concentric would probably have similar capacity related "leakage"