in a previous video on insulation resistance we looked at the conductors that need to be present
and connected in order to carry out that test correctly it threw up some questions in
the comments and i'm trying to answer them in this video we're going to answer what happens
to insulation resistance when conductors get longer as well as why would we linking
together live conductors and test them to the protective conductor are not individually testing
line to CPC and neutral to CPC and we'll see if that test where you link them together is
more onerous than doing it individually so let's consider what's happening to insulation resistance
test when the circuit gets longer so first of all i've got my conductors exposed here as if i was
carrying out the test i've got my line and neutral and we know that the copper conductors are covered
in thermoplastic PVC PVC stands for polyvinyl chloride and we know they're a great insulator
they're not absolutely perfect but they are a fantastic insulator of the conductors so if we
come over to here and we're just looking at those two conductors we've got a line and we've got a
neutral insulated throughout the cable and there is a massive amount of resistance between these
in our first test that we carry up the continuity of CPC we're looking for a circuit a complete
circuit in the insulation resistance test you're looking for lack of continuity in other words
we wouldn't expect with all loads removed the line connection to be connected to the neutral
connection because if it was connected together when you turned it on it would obviously go
bang so we're looking for lack of continuity here so in other words because these are insulated
there must be a massive resistance between these two conductors and we're measuring our insulation
resistance and often we see on the machine greater than the machine can read because the resistive
value is so high so if we put that resistor in so if i bring it in i'll put it as say 1 billion
ohms of resistance so that resistor between there is 1 billion ohms our machine wouldn't
read that high but it isn't perfect okay the insulating material around these is not perfect
but it's going to be absolutely fantastic so that's our first cable there we measure between
live conductors and we get a reading off the machine but if we could read it let's say it was
1 billion ohms of resistance fantastically high value now that conductor is of this length if
we add the same amount of conductor in again so if i bring this one down here so i've added the
same amount of conductor and again so i bring that into circuit i bring it in like so and
i've now doubled its length so i've doubled the length of that cable the first part of the cable
when we measured its insulation resistance say was a billion ohms i've brought exactly the same
cable in of exactly the same length so what value of resistance would you expect this to be here
okay i'm sure you're thinking hopefully like i did another billion ohms of resistance and
that is correct and they are connected in parallel so it's going to link back to our
electrical science and principles as well so let's bring that resistive value in between here
and here we've got another resistor at 1 billion ohms and they're connected in
parallel so one billion ohms of resistance there and one billion here so they're connected
in parallel they're both of the same value so if we were to draw that circuit
we would have one resistor connected to another resistor of the same value
okay and let's just put them down as one billion they're connected in parallel
there's two resistors of the same value connected in parallel hopefully from our electrical science
and principles or the videos i've produced on these we know when they're both the same
and they're connected in parallel it's half the value of one resistor so actually the overall
insulation resistance now would be point five of a billion ohms of resistance so let's go back here
we've got this one connected in parallel with this one of the same value we've doubled the length but
what's happened to the insulation resistance we've actually halved the insulation resistance we're
down to half a billion ohms of resistance it's a massive number so we can say a couple of things we
can say that when we do our insulation resistance test we're looking for a lack of continuity so
conductors aren't connected together we expect the value of resistance to be super high even
though we know PVC is not a perfect insulator and we know that if we double the length of the cable
we actually halve its resistance so longer circuits have lower insulation resistance than
shorter circuits because of the opportunities for the insulated material which is fantastic
but not perfect to actually leak across so we got our insulation resistance test for longer
conductors will be lower than shorter conductors because of that and they're really key points as
you move through your training so we go back and think about that next test insulation
resistance where i suggested it should be live conductors together to the protective conductor
we know that the protective conductor must be connected to the earth terminal and all
our protective bonding the earthing conductor and the source earth from the installation need to be
in place when doing this test so i couldn't actually carry out an insulation resistance
test with it off the board like this this CPC here would need to be connected to the earth bar and we
covered that in a previous video but let's think about it if we did just the test between line
and CPC and then we did a separate test between the neutral and CPC we can see from
our previous imagine the resistance in here was one billion ohms when we tested between those two
and then when we tested between these two we also got one billion ohms okay so that
would be our insulation resistance test but it asks you to link together your live conductors
so in other words we should have our liner neutral link together and test them linked
together to our protective conductor so all of a sudden then the resistance that's in here
and the resistance in here are in play at the same time if we said again that they were 1 billion
ohms for both of them of the resistors we can now see that we've got this one and this one
connected at the same time you carry out the test it won't be one billion for the overall insulation
resistance because you've got the two connections together exactly the same as we did before
they're connected together at this end as we do our insulation resistance test so you get the two
effectively in parallel so by connecting live conductors together and test them to
the protective conductor it's a more onerous test because this conductor here is tested to
the protective conductor and this conductor here is tested protected conductor at the same time
so when we do individually test between say line and CPC record our reading between neutral and CPC
separately we're only doing part of the test but when we bring the two together all of this
insulation material around the two live conductors neutral and line are tested at the same time to
the CPC more conductors in circuit mean more opportunity for the insulated material to leak
across to the protective conductor and we know that obviously the length has doubled because
we've got now the neutral and the line connection together to the protective conductor than we did
when we did it individually so therefore by testing the live conductors together to the
protective conductor it's a more onerous test on the installation and has more conductor in circuit
more conductor means the overall insulation resistance test would be lower we also suggested
that this CPC needs to be connected into the earth terminal within the consumer unit where
the protective bonding is where the earthing conductor is and the earthing conductor needs
to connect into the source earth so if this installation resistance test on this cable here
and there was a pipe adjacent to it what you've got to remember is that these would be connected
together inside of the connection within the consumer unit so the earth network would be
connected to the earth bar of the consumer unit where the CPC is connected so now all of a sudden
when we carry our insulation resistance test we're actually carrying it out to any metal work as well
so this part here also gets tested so in other words as you bring the cable in we're testing it
between the conductors to the protective conductor and any earth metal work it could be this cable
is under a floor very close to a metal pipe the metal pipe is connected to the protective bonding
back to the earth terminal back to the source earth you'd actually be testing the insulation
between the live connections in here as well to the earth metal work as well as the CPC
so that's why we must make sure our CPC is connected to our earth bar because our earth
bar then connects to all our metal work within the installation and the insulation resistance test
would test the live conductors to the earth of the system okay in other words this metal pipe and the
CPC would be tested at the same time might take a little bit of thinking on that one in order to get
it round so you understand what's going on there especially in your early training if we didn't
and we just checked between our CPC disconnected from our board and our line connection we can see
how that's a a lot easier test for the cable to put up with than when we link together our live
conductors to our CPC and bring in all our bonded metal work and test between those live connections
to everything that's connected to earth so hopefully that's added to our understanding
a little bit more about the insulation resistance test we can hopefully now clearly see the longer
the cable the more opportunity when carrying out the test for it to leak maybe between live
conductors and as we add more and more cable actually we'll lower the insulation resistance
whether we see it or not on our instrument and hopefully we now understand that when we link
together our live conductors and test them to our protective conductor we've got more cable in
circuit more conductors more conductors mean that that is a lower insulation resistance test than
carrying out individually i know on site often we do say line CPC neutral CPC as our insulation
resistance test but if we put the two together we've obviously doubled the length of live
conductors therefore we will lower the insulation resistance and also we've seen the fact that any
earth metal work adjacent to the wiring system will be tested during that insulation resistance
test as long as our CPC is connected into our earth bar where all our bonding's in place to
our gas water and oil as well as the earthen conductor that goes to the source earth as well
is in place we'll check and test on those as well but as always i hope this video has been some help