UK Earthing Systems And RF Earthing
In the UK there are Currently 3 Main form of domestic earthing employed
1 .TT or Terra- Terra
The Electrical Supply Company (DNO) doesn’t not provided an earth
terminal (MET), Instead your earth is derived from an earth stake, usually
copper or copper coated. Your supply must be fed from a RCD
2. TN-S or Terra, Neutral, Separate
The Electrical Supply Company provide a Earth terminal, your supply is
derived from a cable containing Live (phase),Neutral and Earth, The earth
will either be the Outer of the cable or a Split concentric cable, with half of
the concentric being insulated(neutral) and half non-insulated (earth)
This used to be the Main means of supply in this country,
(However due to network renewals your connection may be partially PNB
more on this later
3. TN-C-S or Terra, Neutral, Combined, Separate
The Electrical Supply Company provide an Earth terminal, your supply is
derived from a cable containing Live (phase) and a Combined Neutral/Earth.
Your supply with either come from a new PME service or an Old Service
converted to PNB, However for Safety these should be treated as one and
the same.
PME- Protective Multiple Earthing
Your cable come from your local Substation at Various points along the way
extra earth rods are connected to the combined earth/neutral cable, Hence
the term Multiple earthing
PNB- Protective Neutral Bonding
Your Cable come from an Old TN-S Cable with separate Earthing, at the
Branch joint the existing neutral and earth are combined onto a Combined
Neutral earth Cable. This Cable is your supply
(ALL NETWORK REPAIRS ARE CARRIED OUT TO PNB, Hence if you
have a TN-S Service to your house at some point the supply could be PNB)
In a TN-C-S supplied Property any RF Earth must also be bonded to the
MET, As it’s classed as an extraneous conductive part.
What is Bonding, In a House all the services, i.e. Gas, water, Oil if you have
it, must be bonded back to the MET, It used to be the case that all exposed
metal work in a house with TN-C-S be bonded, but with the recent changes
if you have a supply totally fed from RCD’s or RCBO’s this requirement no
longer exists.
TN-C-S and The Amateur
In a TN-C-S situation if the Network has a Neutral/Earth Failure on your
supply, Your RF Earth could become the Local Neutral/earth point, if this
happens your RF Earth could need to carry many amps. If poorly designed
or inadequately installed this could become dangerous or cause a fire. It’s
rare for a Neutral fault to happen, but it can. And if it does we need to take
steps to minimise the risk
How to minimise this
Firstly possibly not use the TN-C-S Earth, If your shack is in an outbuilding
or similar have a TT Supply Installed, with its Own Earth
Convert your whole house to TT, However this is only possible if you have a
detached house, If you have a semi-detached or Terraced house, the DNO
may not let you convert your house due to shared services with your
neighbors, As what you do with your house earth can affect your neighbors
earth
Convert a room to TT, However if you do this all forms of earthing in that
room, must be separate from the house earth, this is includes, radiators,
pipes, structural steel. Not often possibly in a domestic situation
All the above require Input from an approved Electrician as they are
reportable works under Part P, they would require Inspection, testing and
Certification.
The other way is to design an earth system that would carry the neutral load
in a fault situation.
This would work on either a TN-C-S Installation or a TN-S Installation
This would Involve your Rf Earth being a suitable Earth Rod to Bs7671,
Bonded back to the MET with suitable sized earth cable to Bs7671, IET
regulations 544.1.1 and Table 54.8, give the sizes, Usually when the Supply
Conductor is 35mm or less a 10mm Earth cable will suffice
This rod should have an approved termination cover and not left to the
elements.
Your shack earth Bar or terminal should then go to this earth rod also with
10mm cable. Under Pme/pnb rules the individual equipment earths should
also be 10mm, earth braid of a suitable csa will more likely be easier to use
than 10mm earth cable.
The connections to the earth stake should be made with approved
connections and crimped ring terminal used on the ends of the earth cable
The earth stake should be marked “Safety Earth Terminal Do Not Remove”
Again this requires the input of a registered Electrician, Additional earth
stakes should be recorded and there resistance measured under Part P
guidelines