okay and we're going to look and see which earthing arrangement we have this time
could be the same as our last video our last video we identified a TN-C-S earthing arrangement we're
going to have a look and see what we've got this time so as before we've got a kilowatt hour meter
we've got our cut out section here including our cutout fuse which this time is rated at 60 amps
remember it can either be rated at 60 80 or 100 amps for a single phase domestic dwelling we've
got our tails our tails to our consumer unit here as before are sized at 25 millimeters squared and
our earth in conductor is sized at 16 millimeters squared please note in this installation the
supply authority side of the meter the tails are 16 millimeter squared and not 25 millimeters
squared this installation unlike the previous one we looked at for the TN-C-S earthing arrangement
has no double pole switch inserted in the tails so in these tails here we cannot isolate the consumer
unit itself isolation for the distribution board has to occur within the distribution board itself
so in the event of you required to change the distribution board you would need to pull out
the in this case the 60 amp fuse which is sealed by the supply authorities and you would require
permission before breaking those seals so to workout the earthing arrangement we have this
time we'll look at the supply authority's cable in coming into the bottom of the cutout which is
here we can see it's considerably larger in size physical size that is than the one we had on our
previous earthing arrangement and we can quickly identify here this braided section of the sheath
of the incoming supply cable then terminates into our MET our main earthing terminal and goes off in
the 16 mil earthing conductor to our consumer unit so we've got a slightly different arrangement here
we haven't got it coming out of our cut out as we did on our TN-C-S earthing arrangement where
the earth and conductor and neutral were together within the cutout we've actually got it separated
off the sheath of the incoming supply cable we see the connection from the sheath of the incoming
supply cable has been labeled with safety electrical connection do not remove if this
was to be disconnected from the installation the whole installation would lose its incoming earth
so we're going to look at the supply authorities cable incoming to work out
the earthing arrangement we've got we can see this time we have this steel outer sheath being taken
from the outside of the supply authority's cable into the MET the main earthing terminal and not in
our previous earthing arrangement TN-C-S coming out the same termination point as the neutral
so therefore this earthing arrangement must be different than the one we previously looked at
in our video so we're now looking at a earthing arrangement where it comes off the outer sheath
of the supply authorities cable so we're looking at a TN a terror neutral and then S for separate
in other words the supply authorities will bring in a line conductor a neutral conductor and a
separate earthing conductor into the installation the two common materials used for this separate
earthing conductor coming into this installation are either lead of the sheath of the cable or
steel of the sheath of the cable in this instance we're coming off with a connection to a steel
sheath cable if it was a lead sheath cable care must be taken to check the clamp around
the lead is correct sometimes they try and use bonding clamps which works really like holding
putty or play-doh the more you squeeze it the more the lead will move away and be less tight
you normally have to sweat onto a lead sheath cable in order to connect your earthing conductor
so to recap then the supply authorities have brought in a cable containing three conductors
line and neutral conductors terminated within our cutout separated for a neutral and the line
going through in this case the 60 amp incoming fuse we're using the outside sheath of the cable
to provide us with our earth connection okay so it goes into our MET and then goes out into
our 16 mil earthing conductor the incoming supply authorities cable often have lead or steel sheath
for that protective conductor in this case we're connected to one which is made of steel we will
see later on in our learning at college that the TN-S the terra neutral the earth neutral are
separate TN-S earthing arrangement has a maximum external earth fault loop impedance of 0.8 ohms
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 whether they're a TN-S earthing arrangement or 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 TN-S to remind you that T
for terror 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 millimeters squared and the earthing conductor from the MET
into the distribution board is size 16 millimeters 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 the left hand side of this drawing shows the supply authorities cable
with a separate line conductor a separate neutral conductor and a separate protective
earth conductor coming into our installation this is the cable we can physically see in
the bottom of the cutout this cable often comes to installations underground but can also come
overhead this drawing also only shows one of the three star connected windings on the supply
authority's side as well as a centre tap point to earth the supply 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 either lead or steel
the installation showed on the right hand side could be a lot shorter in length than the supply
authorities cable comprising 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 100 amps considerably less maybe 20 amps and would be located within the distribution
board itself the maximum external earth fault loop impedance for a TN-S earthing arrangement is 0.8
ohms the earth fault loop impedance path of an installation connected to a TN-S earthing
arrangement in the event of there being a fault between the line conductor and the installation
and an exposed conductive part the fault path or earth fault path would be as followed
the exposed conductive part would become live this is connected to the circuit protective
conductor the CPC this would become live this goes back to the earth terminal situated within
the consumer unit which would become live this is connected to the incoming earth in conductor
which comes in with the double insulated meter tails this earthing conductor becomes live the
earthing conductor is connected to the lead or steel sheath of the supply authorities cable
which becomes live this is connected to the earth neutral point within the transformer
which becomes live background the line conductor completing the circuit the fault current will pass
through the main cutout fuse rated at either 100 80 or 60 amps but will not cause operation
the fuse that will operate is the one closest to the fault in this case the fuse located within
the consumer unit will operate clearing the fault in circuit this is the full earth fault
loop path and needs to be a very low impedance in order to generate a very high fault current
to cause rapid operation of the over current protection device we looked at two maximum
times for disconnection either a maximum time of five seconds or a maximum time of 0.4
i have described to you the earth fault loop path for an installation connected to a TN-S earthing
arrangement total earth fault loop impedance Zs is measured at the furthest point of a radial circuit
and comprises of the impedance offered by the line conductor R1 the impedance offered by the circuit
protective conductor R2 and the impedance offered by the supply authority's cable the maximum
impedance of the supply authorities cable depends on the earthing arrangement the drawing shown here
is a TN-S earthing arrangement the supply authority's maximum impedance of that cable
Ze is 0.8 of an ohm the total earth fault loop impedance of a circuit
measured in ohms is found by Zs being the total earth fault loop impedance equals
Ze the supply authority's cable's impedance added to R1 the line conductor's impedance
added to R2 the circuit protective conductor's impedance the formula is Zs = Ze + R1 + R2