so when we turn up an installation possibly the first thing we're going to do is look at the
mains end to determine the earthing arrangement the size of the incoming fuse and the size of
the tails and earthing conductor so let's have a look at several different earthing arrangements
and see if we can spot the clues to tell us whether they're a TN-S earthing arrangement
or a TN-C-S earthing arrangement so as we look at the incoming cable from the supply authority
into the bottom of the cutout arrangement here we can see there is a connection or a
clamp around the outside of this cable which then takes a conductive material into the MET
this earthing arrangement is TNS to remind you that T for terre the french word for earth
N for neutral and S for separate so in this case the earth and neutral are separate
from the supply authorities cable so it's brought you in a separate protective conductor
and a separate neutral conductor into the installation making the earthing arrangement TN-S
the cutout fuse in this installation is 60 amps but could have been 80 amps or 100 amps the double
insulated meter tails are 25 millimetres squared and the earthing conductor from the MET into the
distribution board is size 16 millimetres squared all this information needs to be inputted into our
test paperwork along with either the measured reading for external earth fault loop impedance
or the maximum the maximum external earth fault loop impedance for a TN-S earthing arrangement
is 0.8 ohms so our second installation now looking for the earthing arrangement
we can see the supply authorities cable once again comes into the bottom of the cut out the cutout
fuse this time is sized 100 amps but could have been 80 or 60 amps this time we can see that the
earthing conductor which again is 16 millimetre squared this time is connected into the side of
the cutout if we could see beyond the plate on the front there you would see that the neutral is also
connected into the same termination making this TN-C-S terre neutral combined and then separate TN
terre neutral combined in the supply authorities cable so it's using a pen conductor coming in with
a protective earth neutral and we separate it in the installation so we have a separate earthing
conductor coming out here shown in 16 mm and a separate neutral conductor from the meter going
out into the installation in 25 millimetre squared cable you would either need to record the maximum
external earth fault loop impedance for this earthing arrangement being 0.35 ohms or measure
the external earth fault loop impedance for your test paperwork the final one in this presentation
plays on the skills of the previous two parts of the presentation from the point of view when
you go to the mains end in this part of the installation you see instantly the incoming
supply authorities cable has a conductor off it which goes into an MET so straight away we're off
the sheath of the supply authority's cable and we're thinking the earthing arrangement was TNS
however if we look from the MET up we see a green and yellow conductor earthing conductor
being connected into the cut out and that would be connected into the neutral within the cutout
so this system started off life as TN-S however the supply authorities have upgraded
it to a TN-C-S earthing arrangement by adding a connection from the neutral within the cut out
back down to the MET and then the supply authorities have also had the 16mm earth and
conductor and double insulated meter tails then go through into the installation itself so this
earthing arrangement started off live as TNS and was converted to TNCS it's worth having a look at
this one because these are the sort of things can catch you out when inputting the details on your
test paperwork of the type of earthing arrangement the supply authorities have given the installation
you