ring final circuits whether you love them as an example of post-war ingenuity
you hate them for their extra testing and seeming double standards in the regs
or like Gary you're just driven incandescent with rage when people call
them ring mains they are very much a part of the
landscape for electricians even if we all agreed to stop installing
them today there must be millions of them already
in the wild and therefore they will be around for years probably
decades to come as part of our ongoing Q&A feature we
answer the question relating to the use of a 16 Amp
junction box on a ring final circuit and I explained
why this wouldn't be acceptable now a viewer by the name of
Daddy B I'm assuming that's not what's written on his birth certificate
asked if we could produce a video demonstrating the principles of how
current flows in a ring final circuit using a real-life experiment
so we have i've constructed a ring circuit here using these double sockets
and surface mounted patresses sent to us by BG and they really were
lovely to fit plenty of room in the color-coded terminals in the back
and the great styling on the front right down to the little
screw cap covers here that you can see now you'll notice that between each
socket there's a couple of power resistors one connected to the line
conductor and one connected to the neutral these
are just here to mimic the resistance in the cable
that we'd find in a real life circuit now even this won't give a perfect
representation of a real ring final circuit as
each one of these is the same value of 0.22
ohms which means that in the real world this would represent a ring final
circuit where each piece of cable would be exactly the same length between
sockets however it's going to demonstrate the point nicely for us
the legs of the ring are wired back to this BG consumer unit
and connected into this RCBO here now i haven't done the same with the
cpcs to the sockets they aren't necessary for this demonstration and are
only there for safety now i'm going to plug in a two kilowatt
load in different positions in the circuit and look at how the current
splits so we'll expect the current drone to be
two kilowatts so 2000 watts divided by 230 volts which is about 8.7 amps
so we're going to connect our load up here first of all this socket is the mid
point of the circuit electrically speaking so that means that
the resistance of the conductors here in this path
to the consumer unit is the same as the resistance of the conductors
in the other path let's take a couple of current readings we're going to use this
rather lovely metrel md9235 to measure the current
it's actually capable of measuring power in unbalanced and
balanced three-phase systems but we're going to use one of its more basic
features here and just measure the current flowing in
each leg of the ring so we'll turn that on
so at the bottom here when we power this up we are measuring let's have a look at
that we're measuring 4.27 amperes
i'll just bring in the meter cam here and we'll have a look at that
you can see here that we are measuring somewhere in the region of about 4.3
amperes and then if we go up here you can see
that we're measuring again somewhere in that region we've got
4.2 amperes there so we've got just about the same amount
of current in both of those legs and add them together
and we've got about the current that we were expecting now it makes sense that
both currents are the same because from the midpoint of the circuit
to the consumer unit the resistance of the conductors is the
same on each path back so the current is split evenly between the two legs
but watch what happens when we plug this load into a different socket so we're
going to plug it in down here instead now
and then we're going to measure the current and watch what happens now
if we measure the current in the bottom first we can see there if we connect
that up and again we'll just bring in meter cam now to have a look at this you
can see that the current has increased here we're now in the region of 5.4 5.5
amps somewhere around there and if we go and measure the current in
the top leg of the ring here so we'll connect that up there and measure that
you can see now that we've got 2.8 amperes so we've got less current
flowing in the top path now so that's quite
interesting we can see that the current is not splitting evenly
between the two legs of the ring so you can see we've
still got pretty much the same total current but we've got
more in this leg and less in this one why is that happening
well look at it from the current's point of view from here
to the consumer unit there's only 0.44 ohms of resistance in the cables whereas
if they go this way they've got 0.88 ohms now it always
makes me a little frustrated when people say that electricity follows the path of
least resistance it doesn't it is split in inverse
proportion to the resistances what that means is that most of the
current will flow this way where there's less resistance
and less will flow in that direction let's do it again this time we're going
to put it in the socket that's right next to the consumer unit so we're going
to put it there what's going to happen now well
let's find out we'll power up the load we'll measure the current flowing
through the bottom leg first and again if we bring in
meter cam and have a look at that you can see now we've got
about seven amps there 7.1 amps flowing through
the bottom leg of the ring and if we look at the top
once again you can see we've got about 1.4 1.5 amps flowing
through that one there so still roughly the same total current as we had before
however now the vast majority of the current is running through this leg
of the ring back to the consumer unit so now we've connected this
closer to the board the resistance offered by the cable is a mere
0.22 ohms and the other way it's 1.1 ohms
so the vast majority of the current is going in this direction here
and therefore the cable has to carry the most amount of current
so in appendix 15 of bs7671 where it talks about locating socket outlets to
provide reasonable sharing of the load around
the ring what we sometimes call balancing the legs of a ring this is
what is meant trying to keep the current in each leg
roughly the same so that one side doesn't have to carry
significantly more current than the other now at this point we can
look at this and think well that amount of current is not such a big
deal but let's load it up a bit more hashtag more power and see what happens
so now we've got about five kilowatts connected to the circuit and let's
measure the current flow we'll pop that there
bring in meter cam and have a look and you can see there we've got
over 16 amps is running through this leg so as you can see
the vast majority of the current is running through here not a crazy amount
of load all i've got plugged in here is a kettle and a toaster but clearly
more than 16 amps flowing and that's the reason we can't use a 16 amp junction
box to extend a ring final circuit there's only so
much we can do to be sure that the legs remain balanced we can't
assume the current will split evenly between the two legs of the ring all
that remains in this video is to say thank you very much for watching