Equipment earthing conductor. This provides an electrical connection between non-current-carrying metallic parts of equipment and the earth. The reason for doing this according to the US U.S.
National Electrical Code (NEC), is to limit the voltage imposed by lightning, line surges, and contact with higher voltage lines. Note that
equipment earthing does not provide protection from equipment ground faults, unless it is a grounded system (see below) and the voltage is over one thousand volts (typically). This is because
the earth is generally a very poor conductor—it takes a large voltage to push enough current through it back to the electrical system's source to operate a circuit breaker or fuse. The equipment earthing conductor is usually also used as the equipment bonding conductor (see below).
Equipment bonding conductor. The purpose of the equipment bonding conductor is to provide a low impedance path between non-current-carrying metallic parts of equipment and one of the conductors of that electrical system's source so that should these parts become energized for any reason, such as a frayed or damaged conductor, a short circuit will occur and thus cause an overcurrent protection device such as a circuit breaker or fuse to activate and disconnect the faulted circuit.
Note that the earth itself has no role in this fault-clearing process since current must return to its source, not the earth as is sometimes believed. By bonding (interconnecting) all exposed non-current carrying metal objects together, they should remain near the same potential thus reducing the chance of a shock. This is especially important in bathrooms where one may be in contact with several different metallic systems such as supply and drain pipes and appliance frames. The equipment bonding conductor is usually also used as the equipment earthing conductor (see above).
Grounding electrode conductor. is a conductor which connects one leg of an electrical system to one or more earth electrodes. This is called "system grounding" and most but not all systems are required to be grounded. The U.S. NEC and the UK's
BS 7671 list systems that are required to be grounded. The grounding electrode conductor is usually but not always connected to the leg of the electrical system that is the "
neutral wire". The grounding electrode conductor is also usually bonded to pipework and structural steel in larger structures. According to the NEC, the purpose of earthing an electrical system in this manner is to limit the voltage to earth imposed by lightning events and contact with higher voltage lines, and also to stabilize the voltage to earth during normal operation. In the past,
water supply pipes were often used as ground electrodes, but this was banned in some countries when plastic pipe such as
PVC became popular. This type of ground applies to radio antennas and to lightning protection systems.
OK, I will restate - the EARTH has NOTHING to do with protection. This is a HORRIBLE
myth and one that probably will never be understood by the majority. The earth is not a magical sink for current....
The reason that neutral is bonded to earth is to prevent lightning strikes ON THE POWER lines themselves conducting down the wires INTO buildings and frying people, and to provide a reasonable reference for neutral.
PHYSICS says a current will flow only when the circuit is COMPLETE. In the case of mains power that is from the "hot" conductor out of the transformer, through the wires that feed the house, into the fuse box, down the internal circuit hot wires, back along the neutral wires to the fuse box, and BACK up to the transformer neutral. The current does NOT flow to earth.
The same is true ALL OVER THE WORLD. Physics is global.
If you want to prove this - try this experiment:
Breakers and Ground Wires If you don't understand this experiment p/m me and I will explain.
Apologies for being pain in the rear over this, but this is SO OFTEN misunderstood it drives me nuts!
- GY