I am looking for a more detailed reason.....just trying to 'really'
understand the concept here.
Thanks.
Simple really.
1. If you don't connect exposed metalwork to anything, then leakage currents
will flow to it, and potentially through you to ground. So if an insulator
breaks down, or leaks badly, it can kill you.
2. If you connect exposed metalwork to neutral then it won't be at ground
potential, it will be a couple of volts up because of the voltage drop in
the neutral. This won't kill you but it will cause corrosion if wet and it
could under some circumstances cause fire, it can certainly light a torch
bulb because a low resistance connection from neutral to ground will carry
the full circuit current albeit at low voltage. What can kill you is the
current surge when a fuse or breaker trips. You get a momentary very large
current before the trip, in which the whole supply voltage to be dropped
half each in live and neutral leads. This short pulse can stop hearts,
rarely, although with modern overcurrent protection the risk is not as
serious as it once was.
3. Finally, when the breaker does go out there is an inductive pulse, which
is what makes it arc. How high and how long depends on lots of things, but
that can jump fuses, jump breakers, raise your equipment up to kilovolts for
milliseconds and generally create havoc. It also rings at high frequency so
can be coupled capacitatively. This pulse is probably more likely to be
responsible for deaths. The worst case is probably when there is no load on
the supply transformer, a colleague of mine put accidentally put an ohmmeter
across a 240V 15A spur under these conditions. His test meter exploded
despite all the fuses and breakers in the circuit!
If instead you return the exposed metalwork through a non-current-carrying
conductor right back to the electromotive source, usually a transformer, to
a point which is electrically connected to the groundwater, then you only
have the leakage current to create voltage drop, not the fault current, and
all the mayhem stays safely inside its Faraday cage, and you can't get
killed by conduction from exposed metalwork through you to the groundwater
and back to the source.
Tim Jackson