so we've completed our continuity of CPC test at the same time we've included polarity and now
we're moving on to our insulation resistance test our tails are connected to the supply Authority's
meter and equipment and importantly it's Earth however the supply for the installation is still
isolated if at this stage you didn't know what it was isolated you'd need to perform the safe
isolation to prove the consumer unit was dead as well as proving the consumer unit is isolated you
must remove all loads before carry on that test we can see the lamp is fitted in the lighting point
behind me and that would need to be removed that is connected across the line and neutral and will
cause us issues during the test more importantly sensitive electronic items need to be removed such
as smoke detectors digital time clocks TVs etc because the test voltage for this is 500 volts
DC and we wouldn't want to pass through sensitive electronic items that test voltage because it can
damage them equally we wouldn't want to pass 500 volts DC through our RCCB which also contains
electronic items and we must ensure this is turned off during the tests almost my megger MFT in order
to perform the insulation resistance test to do this I'll need to rotate around into the red
scale here which is accompanied by the large M and the Omega symbol for mega ohms mega standing for
million the circuit I've installed is a 230 volt single-phase lighting circuit if I don't installed
a three-phase 400 volt circuit the test voltage for insulation resistance is exactly the same the
test voltage is 500 volts I need to rotate round to 500 volts and that's 500 volts DC going to be
passed through the circuit under test as before I need to insert my leads I'm going to be using
the red and green sections as I did before for this test again it doesn't matter if I use the
blue lead but I must go in the red and green holes provided so we'll use red and green leads
and they're inserted like so I'm going to make sure that my leads haven't broken often they can
break or become damaged down this end because if my leads didn't have continuity it would give me
a false reading during the test a false reading in this case could be one that's very good so
I'm going to link together my crocodile clips and I'm going to test that circuit which is
just the lead and hopefully I'll have zero in other words are dead short I have if my leads
were broken so let's just disconnect them we'd be measuring the insulation resistance between
here which is a fantastic huge resistance between my leads so you'll see now my reading is infinity
or greater than the scale of the machines are greater than nine nine nine mega ohms so a broken
lead can often lead to a reading that we think is fantastic during insulation resistance so care
must be taken to prove that the leads themselves have continuity by doing that test first so I
shorted them together and got a reading of zero mega ohms I'm now ready to carry out the test
so we're gonna perform our insulation resistance test so while we performing that test what we're
trying to do is prove that the cables in circuit so the cables in this case feeding my light and
switch have not deteriorated over time or become damaged during the installation process in this
case because it's a brand new installation we're actually trying to prove the lack of continuity so
we're trying to prove the line and neutral and not connected together and the live conductors
are not connected to in this case of circuit protective conductor so the insulation resistance
test should give a fantastically high reading that these circuits are not connected together
the smallest acceptable value of insulation resistance is 1 mega ohm or 1 million ohms
however BS7671 says that when the reading falls to below 2 mega ohms further investigation is
required this is a brand new installation where I believe the conductor insulation material should
not be damaged or deteriorated so we're probably expecting a reading in the hundreds of mega ohms
let's see what reading we get so we said before starting the test all loads need to be removed so
I'll do that at this stage the lamp in this case needs to be removed which is naturally
connected across line and neutral we also said there's some sensitive electronic components in
our RCCB so that will need to be turned off we're going to carry out our insulation resistance test
from the top of the circuit breaker however the test doesn't realize it's got to go out and do
the circuit it actually goes both ways therefore if we left it on it would pass the 500 volts back
through the circuit breaker and back across the RCCB with the electronic components in it so it
equally that needs to be turned off before we can do the test there is no correct sequence for doing
this test so I don't want which order in which we do it we'll be doing between line and neutral line
and CPC and CPC and neutral but it doesn't matter which order in which we do it in so I see that
test so it doesn't matter which order in which I'm going to do this test I'm gonna do from the top of
the circuit breaker picking up the line conductor anywhere on the neutral bar and anywhere on the
earth bar there's solid brass bars therefore it doesn't matter where I connect my probe first
of all i am going to connect across the top of the circuit breaker to pick up my line connection
I will try and keep my fingers at the way as best I can and then on to the neutral bar press and
hold my test button and my reading has gone off the scale of the machine so it's greater than nine
nine nine mega ohms the smallest acceptable value of insulation resistance is 1 mega ohm we've got a
brand new installation I expect more reading to be a huge reading I don't expect the cable insulation
materials to have started to break down over time or been damaged during the installation process
next i am test between the line and CPC back in the top of the breaker anywhere on the earth bar
press and hold again greater than nine nine nine mega ohms and in finally between the neutral bar
and the CPC greater than nine nine nine mega ohms the smallest acceptable insulation resistance is 1
mega ohm further investigation when it falls below 2 a brand new installation we expect it to be a
huge resistance the cable hasn't deteriorated or broken down over time hope this has been some help