Hi all,

The UK Code of Practice for vehicle installations is FCS 1362
http://www.fcs.org.uk/my%20files/fcs_pdfs/codesofpractice/fcs1362_2010.pdf

It is a change, in December 2010, from MPT 1362.

The old MPT version said both leads to battery, both fused.
The negative lead did provide an alternative connection from the vehicle 
chassis to the battery negative and even with the engine/starter earth strap 
intact the current through the rig was non-zero.  With a duff strap it could be 
over 100A.

FCS 1362 avoids that risk by saying positive fused to battery, negative to 
nearest chassis bonding point, no fuse.

That does introduce a common negative impedance for the rig and the vehicle 
electrics, providing a route for import and egress of interference.
The FCS took a value/hazard judgement on the relative magnitudes of the problem 
and the probability of occurrence.

There is no solution for avoiding both problems but with a substantial chassis 
and a close or common bonding point for the battery negative to chassis and 
engine strap to chassis that aspect is minimised.  Outstanding then is the 
vehicle management electronics earthing.

Regards  73  Alan  G0HiQ
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