[TR] Lucas Master Battery Switch - SSB103 - and other grounding questions

[dave1massey at cs.com]

I have an 'earth' side master switch I want to install on my project for a
combination safety and theft prevention.  see:
http://www.autoelectricsupplies.co.uk/product/144

My plan was to place it on the underside of the dash support bar (steering
column side) by cutting a hole in upper right (driver's) side of the battery
box, using a grommet (probably the same one as the wiper) for insulation
purposes.   Also, I was going to put some heat-shrink insulation on it from
just before the hole to the connection on the switch.    To make it look
original (and to hide the fact that there is a switch) - I am going to somehow
connect a fake end to the firewall above the battery, feeding it from the heat
shrink portion of the ground cable.

I need to locate a good spot to attach to ground after the switch that won't
create havoc to instruments, etc under the dash.   Any suggestions?  Pedal
'box' mounting bolts?

Also, a side affect I hadn't planned on with having both the body tub &
chassis powder coated - any suggestions on how to insure a good solid ground.
Would attaching a  cable (similar to the engine to chassis ground) from the
tub to the chassis  in a couple places suffice?  Perhaps one at the firewall
and the other at the back somewhere.  I'd need to grind to metal for the
connection.

Finally, one of those inquiring mind kind of questions.   The concept of
electrical current is that there is a circuit coming from the battery to the
load and back through the 'ground' to the battery to complete the circle. If a
person touches both sides in a house circuit - you definitely feel it.
However, if the car metal (chassis/body) is the grounding side - why isn't
there an issue?  Or am I thinking it through wrong.


 I would make the "ground" connection directly to the engine using a battery
cable-sized cable.  The worst case current is starter current so design around
that.  Everything else is trivial.

Powder coating can be an effective insulator.  To ensure a good bonding to
chassis and body use star washers or grind away some of the coating under the
attachment point.  Use dielectric grease to ward off corrosion.  Grounding the
frame is not critical since there are no electrical items grounding to it but
a good ground strap between body and engine is a good idea.

The main difference between auto electrics and your hose wiring is, mainly,
voltage.  Your house is wired for 120 Vac (or 220, depending on where you
live) whereas your car electical system is noly 12 volts.  The impeadance of
the human body is such that 21 volts will not force enough current into the
body to be life threatening.  Not so with your house wiring.  It can kill
you.

This is not necissarilly so with the new hybrid and the comming electric cars.
Some of those have battery systems that run in the hundreds of volts.  The
wiring on those systems will not be so casual as they are on 12 volt systems.
The high voltage wiring will be protected and sequestered away from human
contact as it is in house wiring.  Probably more so.

Cheers,

Dave