so today we're going to look at one of our live tests that live test being our external fault loop
impedance now this is more of a one to one video for my learners okay so they're going through
moving towards the endpoint assessment i'm going to stage and logically do the live test for them
so before i start this test you'll notice that i've got the insulation in the off position now
i've switched it off by the external double pole switch and it's also off in the double pole switch
in the consumer unit i'm gonna have to remove any parallel paths to earth so what i'm gonna do is
i'm going to take out my bonding conductors and then later on i'll remove the earthing conductor
from the MET inside the consumer now i need to remove these paths just for the fact that i'm
actually checking that my earthing conductor is connected to the greater mass of earth
through the DNO supply okay so if i had these connected what it could actually mean is when i
do my test i'm actually getting a reading off of a metallic gas or water pipe later that could be
changed for plastic and then all of a sudden we don't have a connection to the greater mass of
earth in our installation so what we're really saying there is that the incoming water or gas
actually is metallic and therefore connected to the greater mass of earth so we're removing that
parallel earth path we are yes however often now they're plastic so in that case would you really
need to remove them if they came in insulated i i still would as a matter of caution it could be
that they're being inadvertently earth through something else i would still remove them i like
that as best practice so we're going to take those out we've now only got the external earth path
provided by the supplier authorities connection to earth yes yeah okay okay then Marcus can you walk
me through what's going on inside the consumer unit and surrounding area yeah so as you can see
i've now removed my earthing conductor so i can do this test now you would have realized earlier
that we have an external MET very close to the consumer unit i could have actually disconnected
my earthing conductor there and left my bonding in place and done the test there however we know
often in the real world the actual earth bar within the consumer unit is effectively the met
yeah and a lot of times we're taking out this in this case 16 millimetre conductor from this
point so that's what we're doing on this test now the installation itself is it is it live because
you just whip the earth out no so it's actually still in the off position and what i'll do is i'll
switch on my external double pole switch just before i carry out the test and i can also see
that the circuit breakers and RCCB's are also in the off position what's the thinking behind that
so good question Gaz so because we have removed our earthing conductor if i had these circuits
livened up and there was a fault to earth okay we wouldn't have that a fault loop path and then
we wouldn't have ADS in place good thinking so okay for safety reasons we're completely off so
where we're going to turn the installation on from in order to carry out this test so i'm
not going to turn it on inside the consumer unit i'm actually going to turn it on at the external
double pole switch okay so now the top of this double pole switch the incoming tails are actually
live now and that's the only part of consumer unit that is live it is yes and we're now required to
set up our test instrument we've got our leads in the appropriate positions in the top we've shown
many times yeah what scale you're going to set it to so i'm going to set it to my earth fault
loop impedance tester and i'm going between line and protective earth now you'll notice with the
Megger testers it automatically starts you on a three lead low that's generally because rcds are
in circuit now because i don't have an rcd at the intake all i'm going to do is i'm going to change
this to a two lead high okay now we've got to be thinking about the earthing arrangement we had in
place and etcetera etcetera however this training rig here is not the origin of the installation
we might find this reading here is higher than the one set by the earthing arrangement
we might have said to the students presume the earthing arrangements TN-S and they have
to suggest whether it's in or out or TN-C-S it's actually mocked up to be a TN-C-S degree yes it
is yeah are we expecting this reading to fall below the maximum value allowed no we're not
no so we've seen this many times where this reading here will be greater than 0.35 of an ohm
yeah the reason being this is not the origin of the installation that we're mimicking so obviously
that reading will be slightly higher it's how we carry out the test that's really important
our learners understand that this reading will be too high potentially for TN-C-S it might be okay
for a TN-S so let's let's see what this reading is going to be so i'm going to clamp onto my earthing
conductor okay and then i'm going to go on to my incoming line and you'll see a voltage will appear
and then it will give us our reading of 0.45 ohms okay so we know if this was the origin of
the installation that is greater than the 0.35 it is accepted as a maximum value let's start
panicking now no we don't need to panic Gaz so in the event of that we've got to remember that
it is not at our discretion to change anything with the intake okay so we would contact the local
DNO and we would seek their advice now my advice would be when you're testing the installation if
you do come up with this scenario is you seek advice they will give you a reference number
on the comments on the existing installation part of the electrical installation certificate or any
relevant certification then we would write that reference number down and a quick explanation of
the scenario okay so there is almost a get out of jail card it is yes however
we told the learners it was a TN-S what's the maximum for TN-S a TN-S 0.8 ohms so
in that case that would be acceptable value okay so is that the test completed it is yes
okay so um simple test how would we describe this test often they call it the Ze test in
the real world what what test is it Marcus the external earth fault loop impedance test
you