http://www.iee.org/Publish/WireRegs/...s_answered.pdf
copy and pasted
Exported earth..
here goes; basics first..
Why do we use "earth at all"?
There is protective & functional earthing.
protective earthing comprises source & electrical equipment earthing.
[functional earthing is for stuff like IT equipment with high leakage currents...but we will ignore them for today!]
quote from NICEIC..
"The purposes of source earthing are:
• To preserve the security of the supply network by limiting the potential of the live conductors (with respect to that of Earth) to a value consistent with their insulation.
• In the case of a TT system, to provide a path for earth fault current and protective conductor current to return to the source of energy via Earth. Without this path, devices for protection against electric shock and fault current to earth will not operate. "
equipment earthing is joinin all of our metal bits etc.. together! {exposed conductive parts joined to MET} create our EEBADS zone.
In the event of a fault a path must be present for all of them nasty electrics to make there way back home to their source.. through a circuit (i.e. the earth fault loop path) and provide a means to cut off the supply!
The earth fault loop path (Zs) must be a low enough resistance to cause the circuit breaker (MCB or fuse) to trip within the required safe operating time. {5sec 0.4sec?}
With a longer circuit, e.g. remote garage bottom of the garden, the Zs may be to high. unless you put in some bigger CSA cables.. simple solution can be to make garage a TT installation .
You are permitted in the wiring regs to join the TT rod & MET together.. BUT if you do.. "All of the CPC's must be capable of carrying the max PFC for any part of the installation" because for example.. a fault in the shower on the house.. could make a path down to the earth rod in the garage! So unless you go back to stickin in mega big CPC cables... the way to stop house fault currents going through the garage is to keep both earth arrangements separate! (542.1.8) The earth of the SWA is protected back to the house.. but no further forward to the garage.
Keepin the above bits in mind.. we need to also consider potential difference..
If you have 4 equal resistors across a supply.. they acts as a potential divider splitting the supply voltage into quarters if you put a meter across each resistor!
A long earth path is a potential divider to any earth fault voltage..
nearest the source it is closer to zero.. further along the wire with reference to the source the voltage increases..
If you have simultaneously accessible "near earth potential" conductors, during a fault, you would need to be absolutely sure no fatal voltage induced current could flow through somebody grabbing hold of both earths.
With our garage we have a local earth rod..
and the house MET potential could be 'exported' out to the vicinity of the garage via the SWA protection...
The SWA must be protected... but this protection must NOT be accessible to Joe Public leaning up his metal clad garage fitting..
He should ONLY have contact with the garage earth rod potential..
Unless of course you have baco-foiled the whole of the garden to ensure the mass of earth around the house is all at the same potential???
This is taken from else were , but covers the basics quite well
HTH
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