in this q a video we're going to be answering the question why do you
interconnect and divide by four when you're testing a ring final circuit
this video relates to the just brilliant ring final circuit adapter from metrel
this is a product that i wish had been around for decades because it just makes life so much
easier when you're testing i've made a full demo video on this so please do go and check that
out after watching this video it's awesome the product that is not the video the video is just
me being average and nerdy now there's been quite a bit of debate recently on social
media about ring final circuits and how relevant they are to modern electrical installations but
whether you think they're an enduring testament to ingenuity or an outdated nightmare the fact is
that even if they were outlawed tomorrow they'd still be a big part of our lives in existing
installations so it's important to understand the processes involved when we test them
so of course we know that the first thing we do is we test for continuity between the two line
conductors giving us our so-called little r1 value then we do the same for the two neutral conductors
and the two circuit protective conductors and that gives us the little rn and the little r2 values
respectively and would record those results on the test sheet now the purpose of this part of the
test is pretty clear we're just trying to confirm that the individual conductors go out of the
consumer unit and come back again giving us a true ring circuit however this test in itself is not
enough to prove that the ring circuit is compliant for example if someone got a bit confused during
the installation of a new ring or if an existing ring had been added to by amateurs over the years
we could end up in a position where the circuit has been wired incorrectly with a smaller loop
than was previously thought and a long leg going off with spurs off spurs now this is obviously
a problem if left because those sockets that were spurs of spurs are being protected by a 32 amp mcb
and the cable feeding them can only really handle 26 amps at the very most so how can we tell after
installation that this hasn't happened well that's where the interconnection comes in i've got here
a representation of a ring circuit with the line neutral and cpc shown going out to each socket and
then back to the consumer unit each little mark of tape on the conductors is where it enters a socket
so they represent the socket outlets now what we normally do is at the consumer unit we normally
take the neutral from one leg of the ring and connect it to the line conductor on the other leg
so here you can see we've got the neutral from one leg of the ring and we're going to connect that
onto the line conductor from the other leg of the ring as you can see here and then we do exactly
the same but vice versa so we connect the neutral from the other leg onto the line of the first leg
so we get those connected to each other like so then we figure out what value we expect to
get at each socket when we test between the line and neutral and we do that by adding the values
of resistance that we measured for the line in neutral together and dividing that value by four
why do we divide by four well that will become clear in just a minute now we go to each socket
outlet on the circuit and test between line and neutral if the circuit has been wired correctly
it doesn't matter which socket we go to we should get roughly the same value every time why is that
well let's say we go to this socket here is represented by the blue tape
if we pick this up off here and imagine that we're testing between the line and neutral conductors of
this ring so i'll just remove that from there so imagine that my fingers are the probes what
happens is that when we test across there you can see we're actually testing across exactly one half
of the ring circuit we're testing from one side of a circular loop of conductor exactly to the other
side so we'll pop that back and then we'll pick up another socket and do the same thing so again we
can do the same thing we can test from any point on this ring final circuit so let's test that the
green socket outlet this time the green and yellow one and again if we remove that from the
board you'll soon see that again imagining that my fingers are the test probes that we're going to be
doing the measuring with you can see that when we open this out we very simply test across one big
loop of the ring from one side to the other and it doesn't matter where we test we should get the
same reading every single time if we get a high reading then it's possibly an indication that our
measurement includes an extra length of conductor such as you may find if you've got a spur off the
ring final circuit and if you get a series of rising values then it's possible you've got a
spur off a spur off a spur and so on so that's the reason we do the interconnection before testing at
the sockets it's because it allows you to check that a circuit that you can't see the root of
has been wired correctly but why do we need to do that divide by four bit to get your expected value
well let's look at the maths involved we're connected across the two opposite sides of this
loop of conductor as we demonstrated a moment ago which means that we're measuring the resistance of
this conductor here and this conductor here each section of this conductor is half of the big loop
and the resistance of the big loop is the combined value of the line conductor and the neutral so the
resistance of this conductor is half the combined resistance of the line in neutral so to find that
we divide little r1 and little rn added together by two then we need to bear in mind that we're
effectively measuring the value of two resistors of equal value connected in parallel so what's the
total resistance well do you remember the trick that your college lecturer tried to hammer into
your brain just before your science and principles exam half the value of one of the resistors so
we've divided the resistance of this big loop by two to find the resistance of one side of the
loop and then we've halved it again to account for two resistances of equal value in parallel
and when you divide a number by two and then divide it by two again it's exactly the same
as dividing by four clever stuff so there we go that's why we interconnect and divide by four when
we're carrying out ring final circuit testing but of course we want to hear from you what
are your opinions on ring final circuits and the extra testing that they require do you think that
they've got their place or do you think they're destined for the museum please leave your thoughts
and questions in the comments section below and as always thank you very much for watching