also as we move through the learning on our course we'll also see how the earthing
arrangement in this case TN-C-S has a maximum external earth fault loop impedance of naught
point three five 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 impedances no greater than the maximum allowed for the 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 16 mil earthing conductor so it must be disconnected from winning
the consumer unit and where he 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 link to main switch or double pole switch
I'm gonna need to isolate that turn 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 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's as
best a power down those items before you turn off the electricity within the distribution
board itself turn off each circuit breaker in turn before I turn off the main switch itself
I can now turn off the main switch and they're not to 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 now I'm 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 two conductors underneath so distribution board cover is off
and we can see that even the live parts on the top of the double pole switch is shrouded from
contact and the bottom of the double pole switch going through he's all been isolated therefore
I can disconnect my 16 mil earthing conductor in order to perform the test with the 16 mil earthing
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 installation 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 on this side into the green zone against Z Z for impedance we're measuring the
external earth fault and loop impedances Z is representing impedance I've set my instrument
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 we know I am going to probe on to
the least dangerous conductor first which in this case is the earthing conductor and then on to the
line to do the test and when I come back off I'll pull off the line conductor first and then
off the earthing conductor second my probes themselves conformed to GS38 with no greater
than four million exposure tip considerably less than the four four so I'm going to probe
on to the earthing conductor first and then I'm going to probe on to the line conductor
and I've got a reading of 0.25 of an ohm they pull off the line conductor and will 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 replaced we can now turn on the main switch an individual circuit breakers