TN-C-S earthing arrangement the terre and neutral the earth and neutral are combined
within the supplier authorities cable and then separate when they get into the installation
itself TNCS earthing arrangement this is how it will look in a single-phase domestic dwelling
how will it look in images shown to us for our exams the left-hand side of this drawing shows
the supply authorities cable it has a separate line conductor and a combined protective earth
neutral conductor this combined cable is the one we can see coming to the bottom of our cut out
this concentric cable has a copper earth neutral and a aluminium line conductor and will enter
the building normally underground but can come in overhead this drawing also and he shows one of the
star connected windings on the supply Authority's side as well as the center tap point to earth
the supplier authority will provide the meter and equipment and cut out fuse rated either 100 80 or
60 amps for a single-phase domestic dwelling in a TN-C-S earthing arrangement the supply authorities
provide one conductor for both protective earth and neutral this pen conduct on the
supply authority side is staked throughout it run we call this PME protective multiple earthing this
PME in the PEN conductor is very important as loss of the neutral and the supply authorities
side would mean loss of the protective conductor in the installation PME helps to prevent this
happening the neutral and earth are separate for the installation this occurs in the cutout
the PEN conductor comes in a separate neutral and separate earthing conductor come out from the same
position the exam likes to say these are connected together virus solid link the installation showed
on the right hand side can be a lot shorter in length than that for supplier authorities
cable comprising of a line neutral and circuit protective conductor the neutral in earth are only
combined on the supply authorities side are not within the installation the fuse for this circuit
will not be a hundred amps considerably less maybe 20 amps and housed within the consumer unit
the maximum external earth fault loop impedance for a TN-C-S earthing arrangement is 0.35 ohms
TN-S earthing arrangement terre neutral are separate the supply authorities provide us
with a separate protective earth and a separate neutral conductor into the installation if this
is what it looks like in a single-phase domestic dwelling how will it look in images shown to us
for our exams the left-hand side of this drawing shows the supply authorities cable with a separate
line conductor a separate neutral conductor and separate protective earth conductor coming into
our installation this is the cable we can physically see in the bottom of the cutout
this cable and often comes to installations underground but can also come overhead this
drawing also any shows one of the three star connected windings on the supplier authorities
is side as well as the center tap point to earth the supplier authorities will provide the metering
equipment and the cutout fuse sized 100 80 or 60 amps for a single-phase installation the supply
authority's protective earth will be the lead or steel the installation showed on the right
hand side could be a lot shorter in length than the supply authorities cable comprised of a line
a neutral and a CPC conductor within circuit as with the supply authority side we have a
separate line neutral and CPC in circuit the fuse for this circuit will not be a hundred amps but
considerably less maybe 20 amps and would be located within the distribution board itself
the maximum external earth for loop impedance for a TN-S earthing arrangement is 0.8 ohms
TT earthing Arrangements terre terre the left-hand side of this drawing shows a supply authority's
cable in this earthing arrangement the supply authority will only provide you with a line and
neutral conductors to a single-phase installation on the supply authority's side this drawing only
shows one of the three star connected windings as well as the center tap point to earth TT earthing
Arrangements are often given to buildings that are remote where it would be an economical for
the supplier authority to provide you a TN-S or TN-C-S earthing Arrangements the TT supply is
likely to be coming into the installation overhead it is the responsibility of the
consumer within the installation to make their own connection to earth this earth electrode
usually comes in the form of an earth stake but can come in other forms which we will discuss in
the classroom as the impedance between the consumers earth electrode and the supplier
authorities connection to earth can be very high installations must be additionally protected by
an RCD in order to clear earth faults however as always over currents and short circuits will be
dealt with by the fuse house within the consumer unit for each individual circuit the installation
waring will still be the same as previously stated in TNS TNCS earthing arrangements with a
separate line neutral and circuit protective conductor also the metering equipment will
be the same as well as the supply authorities cut out fuse being either a 100 80 or 60 amps