in a previous video in this series we looked at how current behaves in a typical ring final
socket circuit and we saw that when the circuit was loaded up at different points
different amounts of current flowed in each leg of the ring now in this video we'll be
looking at how current behaves in a ring final circuit when it's wired in dado trunking so what
i've done here is i've wired up two separate ring final circuits through this dado trunking system
now this is the Elite Compact range from Atkore and it's got some absolutely great features
that i'll explain in a future video so keep an eye out for that the first one i've wired in the
following manner i've taken the first leg out from this rather lovely new consumer unit from Premspec
from here out to the first socket and then the second leg goes from the consumer unit out to
the far socket and then each socket in between is connected together to complete the ring now before
we explain how the second circuit's been wired let's load up the ring and take some measurements
so i'm going to connect a four kilowatt load up to the circuit and measure the current that
flows i'll plug into the nearest socket first and then we're going to measure that the sockets that
feature in this video by the way are from the Verso range from Premspec and i'm sure you'll
agree that they're great looking and really good quality as well so let's get measuring
the current that flows in each leg so when i was installing this i slipped a little bit and
accidentally brought the cables out through the lid here now i'm not for a moment suggesting that
this is the way to install these cables it just gives me a neat and convenient way to measure
the current flowing in the conductors now this conductor here goes to the far socket and this
one goes to the closest socket we've got the load connected now i'll measure the current flowing
using this metrel md9235 clamp meter and as you can see we've got going out to the far socket
just bringing meter cam here so you can see there we've got about 2.8988 amps there and on this one
you can see we've got about 12.67 so as perhaps you may expect if you've seen the previous video
in this series we've got the bulk of the current flowing through this leg to the nearest socket
but let's look at what happens as we move the load further along the ring so what we'll do is we'll
move the load over to here and then we're going to measure the current again that's flowing in each
leg of the ring so again in the far leg you can see if i bring in meter cam again so you can see
there we've got 4.16 amperes and then on this one we're getting somewhere around 11.69 thereabouts
and that makes sense we're increasing the resistance in this section of the ring and
reducing it slightly in this section of the ring so the current splits accordingly but let's jump
along to the final socket now in a previous video we saw that as we connected the load to
the socket fed by the other leg of the ring the majority of the current flowed through that leg
as it had the lower resistance going back to the board so let's power the loads up so we've got
those switched on and let's measure the current in the legs so the leg that comes straight from
the socket to the consumer unit which is that one you can see there now that we've got 7.94 amperes
and in the leg that comes back via all the other sockets you can see we've got 7.99 so almost
exactly the same amount of current in each leg there so even though we're at the far socket we've
got the current fairly evenly split why is that well it's because we're actually roughly at the
midpoint of the ring in that socket electrically speaking the conductor from there straight back
to the consumer unit is roughly the same length as the conductor that goes from there to each socket
and then back to the consumer unit which means that the conductor resistance is roughly the same
from here to the consumer unit along both those return paths so what does this show is well it
shows us that it doesn't matter where we connect our load along here we're almost always going to
be drawing the vast amount of current down this leg in other words it's very hard to balance the
legs in dado trunking when the ring final circuit is wired like this in fact the only time that's
really going to happen is when we only connect one load to the end socket there is that a massive
problem well appendix 15 of bs7671 provides some interesting information on the subject where it
outlines the conditions for a reliable and safe ring final circuit the very first point suggests
that socket outlets need to be located to provide reasonable sharing of the load around the ring so
how can we achieve this for a ring final circuit in dado trunking well one of the key points is to
try and keep the legs of the ring the same length so if we bring the first leg of the ring to this
socket then we bring the other leg of the ring to this socket and that makes them roughly the same
length then we use the hit and miss method we take a conductor from this socket over to this one and
then we take a conductor from this socket to this one from here to here hitting a missing every time
and then we just join up the last two sockets on the circuit now at this point let's make it very
clear this method of wiring up the ring circuit here won't perfectly balance the legs of the ring
all the time however it turns the ring from being pretty much guaranteed not to have the same amount
of current in each leg to giving you a fighting chance to do so let's demonstrate so i'll plug
into the first socket and measure the current so we'll do that now so i'll bring this over here
plug those in there and measure the current in each leg of the ring so in this case this leg goes
to this socket and this leg goes to this socket so let's measure those currents and see what we're
getting so you can see there that in this leg we are getting via meter cam 12.25 amperes there
and in the other leg we've got 3.64 amps now if we move the load to our second socket on our ring
final circuit if we did that on the circuit above we'd expect the current just to even
out a little bit wouldn't we even though we saw that we'd never get the other leg using its full
capacity we won't even come close to that but what happens in this circuit well let's measure it up
so once again you can see now that in the leg that goes to this socket here we're getting 4.34 amps
and in this one that goes to the second socket there we've got 11.65 so the current is now
being drawn mainly through the other leg of the ring so what does that allow us to do
well if i plug one load in here and one load in here like this
and then we'll measure the current in the two legs again so let's connect up there and measure that
leg you can see there we're getting 8.41 8.4 amps and in this one we're getting 7.72 amperes so you
can see from that we're actually in a position where we can plug loads into different sockets
and stand the chance of actually balancing the current in each leg of the ring now in reality
of course we can't always tell what's going to be plugged in where and there's never a guarantee
that the legs of the ring will be perfectly balanced and indeed the the very nature of
a ring final circuit means that it doesn't have to be perfectly balanced all the time
but by wiring your circuits in dodo trunking in a fashion like this you've given the legs a fighting
chance of being balanced and actually you've shown your commitment to complying with bs7671
as much as possible but of course as always we want to hear from you is this how you connect ring
final circuits in dado trunking or do you do it another way do you have any other hints tricks or
best practices for installing rin final circuits in dalo trunking or do you think that maybe
this is just another sign that the ring final circuit has had its day and it's time to retire
it's the history of electrical curiosities along with lead sheath cables and wooden fuse boards
whatever your thoughts please leave them in the comments below and be sure to check out our other
videos on dado trunking all that remains in this video is to say thank you very much for watching