so let's see if we can work out which of the earthing arrangements we've got here
in this installation kilowatt hour meter we're fortunate to get a linked main switch or a double
pole switch inserted in the tails which allows us to isolate the supply without the need to remove
the fuse here okay so that's handy so if i wanted to work in my distribution board perhaps he will
replace it in this installation i can turn it off from out here prove the installation is dead
and then change it without the requirements to get the supply authority's permissions in order to cut
these metal seals here and here and remove the fuse itself so we've got double insulated meter
tails in brown and blue coming in even though we look as if they're single insulated actually
double there's two layers of brown and two layers of blue on those tails and going out we've got our
neutral and line conductor through our double pole or linked main switch meaning that when we
isolate it we turn off both live conductors both line and neutral are isolated in the installation
and as we look where the the earth in conductor in this case is a 16 mil earthing conductor
compared to the 25 millimeter squared tails it is connected into the side of the cut out it's
been the cut out section here and the cut out fuse and it means that it is actually connected to the
neutral which is from the supply cable coming in it separates with the two conductors
this is a concentric cable one that will be shown in the classroom one that has an
aluminium center line conductor and a combined protective earth neutral which is copper
which actually looks like almost like an armouring of the cable but you'll see that in the classroom
which comes out and becomes both our neutral and uh earthing conductor so in here is a solid block
which contains both what is the terminations for the neutral and for the earthing conductor so the
earthing arrangement we're looking at here then is one that has a combined neutral and
earth so the earthing arrangement being TN for terra neutral the earth neutral C combined
combined in the incoming cable and then separated in the installation so we have a separate neutral
and a separate protective conductor entering the installation making this earthing arrangement
TN-C-S other key points of interest in here as well are the sticker here the green sticker and
i say to my learners don't go looking just for the green sticker could have fallen off but if
the green strip is there and present it says words like earthing terminal and PME protective multiple
earthing which is what the supply authorities do to the combined neutral and earth throughout
its run so therefore making this the TN-C-S earthing arrangements there's another good clue
to which earthing arrangement we're looking at we've also got the cutout fuse itself in this
case it's rated anywhere between 100 and 60 amps are in this installation is rated at 80 amps but
make note of the one in the installation you're looking at whether it be 60 80 or 100 amps also
as we move through the learning on a course we'll also see how the earthing arrangement in this case
TN-C-S has a maximum external earth fault loop impedance of 0.35 TN-C-S earthing arrangement
the terror and neutral the earth and neutral are combined within the supply authorities cable and
then separate when they get into the installation itself TN-C-S 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 cutout this concentric cable has a copper
earth neutral and a aluminium line conductor and we'll enter the building normally underground
but can come in overhead this drawing also only shows one of the star connected
windings on the supply authorities side as well as the center tap point to earth
the supply authorities will provide the metering 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 support
authorities provide one conductor for both protective earth and neutral this PEN conduct on
the supply authority side is staked throughout its run we call this PME protective multiple earthing
this PME in the PEN conductor is very important as loss of the neutral on the supply authority's 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 cut out 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 via a solid link the installation
showed on the right hand side can be a lot shorter in length than that for supply authorities cable
comprising of a line neutral and circuit protective conductor the neutral and earth
are only combined on the supply authority side and not within the installation the fuse for this
circuit will not be 100 amps considerably less maybe 20 amps and housed within the consumer unit
this drawing shows a TN-C-S earthing arrangement and the supplier authorities cable now
has a maximum external impedance Ze of 0.35 of an ohm total circuit impedance Zs = Ze + R1 + R2
we've identified at the origin of the installation the earthing arrangement is TN-C-S we're now
going to carry out the external earth fault loop impedance to confirm our external earth fault loop
impedance is no greater than the maximum allowed for this installation which is 0.35 of an ohm
in order to measure the external earth fault loop impedance in this installation i'm going to need
to take off the cover of the distribution board i have no separate MET where i could disconnect
my 16mm earthing conductor so it must be disconnected from within the consumer unit
and where it is terminated in the main earth bar in order to do this i'm going to need to
isolate the consumer unit at the main switch the linked main switch or double pole switch i'm going
to isolate that turning off both line and neutral before doing so it would be advisable to turn off
individual in this case circuit breakers to reduce the amount of load that i would be switching off
it is advisable to tell the customer before turning off the electricity
not just for inconvenience sake but if they have sensitive electronic equipment that's in
on and being used it would not be advisable just to turn the power off to those type of equipment
as best they power down those items before you turn off the electricity within the distribution
board itself i'll turn off each circuit breaker in turn before i turn off the main switch itself
i can now turn off the main switch and then i can remove the cover i still need
to be careful because the top of the double pole switch will still be live
the rest of the installation will be isolated once the main switch has been turned off
so i'm now going to remove the distribution board cover before doing so
just going to move this out of the way and take off the cover
exposing the conductors underneath so distribution board cover is off
and we can see that even the live parts on the top of the double pole which is shrouded from contact
and the bottom of the double pole switch going through has all been isolated therefore i can
disconnect my 16 mil earthing conductor in order to perform the test with the 16mm earthen
conductor removed from the main earth terminal the parallel earth paths created by things such
as gas water and oil have been removed giving you a true external earth fault loop impedance
of the install in order to carry out this test i need to set my MFT to the appropriate range
in this case i need to go into the green section here which is L for line and protective earth
and i need to move it on this side into the green zone against Z Z for impedance we're measuring
the external earth fault loop impedance and Z is representing impedance i've set my instrument on
to two lead high for this test i've got the left hand one on line protective earth and
my right hand one on impedance so i'm going to carry out the test now to see what reading i get
i'm going to probe onto the least dangerous conductor first which in this case is the earthing
conductor and then onto the line to do the test and when i come back off i'll pull off the line
conductor first and then off the earth and conduct a second my probes themselves conform to GS38
with no greater than four mil exposure tip this is considered to be less than four so i'm going
to probe onto the earthing conductor first and then i'm going to probe onto the line conductor
and i've got a reading of 0.25 of an ohm we'll pull off the line conductor and we'll
pull off the earthing conductor there we go the external fault loop impedance has been measured
it's less than the 0.35 ohm of the maximum being 0.25 therefore we've proved the external earth
fault loop impedance meets the requirements the earthing conductor reconnected and the
distribution board cover replace we can now turn on the main switch and individual circuit breakers