Earthing was until recently, the primary safeguard against electrocution. It still plays a role but, in most issues, so-called RCD (Residual Current Detection) is now primarily effective in saving life. The units constantly monitor and compare the amount of current flowing in the active and neutral cables. Unless they are totally equal there must be some flow to earth. If that is detected, current is cut off. The above can happen, say, if Uncle Jack changes a dead light globe without realising the light switch is still on - breaks the glass and contacts a live part. Or accidently sticks his finger into the live socket. Current then flows to earth – via Uncle Jack. If polarity has been reversed (i.e., active and neutral reversed in the wiring to that switch, that globe will still be live even if the power switch is turned off). As that risk is high where people (illegally make up their own supply cables) all Australian RVs must have power switches that have so-called ‘double-pole switching’ – i.e. both active and neutral are switched.
Prior to RCDs, the probability of such electrocution was high. But an RCD detects current flows in one lead, but not the other – and cuts the current in time to avert death.
The circuit breaker’s main function is to ensure that excess current is not drawn in the event of the active and neutral leads accidentally touching, an appliance failing such that it draws
excess power, or simply the user attempting to run appliances drawing more than (in this case), 10 or 15 amps. If the supply cable’s resistance (or more correctly impedance) to the flow of current is increased (by co-joining leads etc) the circuit breaker will be slowed, or not even operate at all.
This is why supply cable specifications are now rigidly enforced. All of the cables allowed enable the system to operate in time. Longer cables are also thicker (so all, regardless of length) allow the protection to act as intended. That is why cable length must not be extended, nor cables joined together.
For RCDs to protect, the supply cable’s resistance (or more correctly impedance) to the flow of current must be within specific limits to ensure the RCD acts within 0.4 second. If that time is exceeded, the RCD may still operate - but often too late to save life.
This is why supply cable specifications are now rigidly enforced. All of the cables allowed enable the RCD to cut off the current in time. Longer cables are also thicker (so all, regardless of length) allow the RCD to act as intended. That is why cable length must not be extended, nor cables joined together.
Earthing is still required. It may carry initial fault current before the RCD and/or circuit breaker cuts off supply, but it plays no direct role in the RCD's action. It is also only partially effective against protecting against contacting live metal because, unless done 100% effectively, some part of the fault current may flow through to earth via anyone contacting such metal (i.e. by forming a parallel path).
The changed AS/NZS supply cable rules are consequent to the major reliance on RCD (Residual Current Device) and Circuit Breaker protection as well as earthing.
(The above is sourced from Standards Australia
The author has a practical and theoretical background in designing and building large electrical systems but is not a qualified electrical engineer, nor licensed electrician). As will be seen, there is more to camper trailer, caravan and motor home electrics than may at first seem. It is wise to keep yourself as informed about your vehicle as you can to know what to do in the event of a breakdown or failure but never perform any rewiring yourself as this must be done by a licensed auto-electrician.