in this video we're going to take a look at catenary wire used to support cables above ground
level we can see here that it's fixed into the main building and the other end of the catenary
wire goes all the way down to a garden shed and they've supported a cable on there to feed that
garden shed we can use Appendix D of the On-Site Guide in order to look at the rules behind using
a catenary wire and we've used this before when looking at fixings for steel wire armored and twin
and CPC cables we're going to need to use Appendix D table D2 and I've blown it up to make it bigger
for you to see it on camera so if I bring in my blown up version I've also made some highlights
on it so the minimum length of span and minimum height above ground for overhead wiring between
buildings so we're going to be talking about first of all for me is that catenary wire but there
is some other ones that you'll see as you're say walking around your town or village that also have
cables above ground so the most obvious one would be bare overhead so we used to see in pylons and
we used to seeing those cables supported on those pylons there's no additional wire on those to hold
the weight for the cables it's actually the bare conductors that are spread between the poles and
then the other one that you might think of as we go through his presentation is getting out of the
way to start with would be for BT cables or phone cables that go from the wooden pole in your street
and you see them spanning across the road or spanning between buildings and all you can see
is that black cable which is the phone cable now you're thinking well there's no catenary
wire there Gary so how are they being supported because they're actually therefore hanging on the
conductors at both ends which is not good when you think there'll be wind birds sitting on them all
the rest of it and obviously stressing out those conductors those type of cables as it suggests
here with aerial cables actually incorporate a catenary wire within them so those phone cables
that we see spread across roads actually have a catenary wire built into them and that's something
that I only found out a few years ago and it makes logical sense now but we're going to be looking at
a catenary wire so in other words probably a piece of steel that's strapped between two fixed points
used to support the cables and we saw at the very start of the video this sort of arrangement here
where we had maybe this fixed to the building okay into position and then we've got this here
which is our hook which then our steel goes around here in order to support our cable and the idea of
this is is that you can tension the steel so you can wind this in and out because obviously that
steel will be quite heavy and will sag and we don't want it to say we want it to possibly be
as tight as we can so it keeps the height up between the two fixed points so that's what
that is at the very start of the video if we look here then so our cables are sheathed with
thermoplastic PVC or having an oil resistant and flame retardant or HOFR heat oil and flame
retardant sheath that was easy for me to say supported by catenary wire let's think about
cables supported on the catenary wire this is the column here we're going to need within number one
so column number one this section here is where we talk about the catenary wire that I showed at
the very start of the video and then when we come across to column number two this is the maximum
length that the span can be so as we roll down to the one with the container it says NO Limit
so you can have it as long as you want it to which makes logical sense because we're not
supporting any of the cables as a catenary it's this steel that's fixed between the buildings
that holds the weight of the cable so it can be unlimited in length next one we're going to look
at the title of this here says minimum height of the span above ground at a road crossing so
this is the minimum height so it must be at least this or greater so when we come down and look at
where there is a road crossing with a catenary wire it says it must be a minimum of 5.8 you
must take into consideration where this is so if you're on a caravan park with very high loading
lorries moving caravans around you're going to find this high increases remember this is the
minimum okay that it can be so we can certainly have it higher than this you can't go any lower
than that so that's 5.8 meters as a minimum or higher for that height where there is a road
crossing and then we've got the next one to it in positions accessible to vehicles or traffics
or other crossings in very similar wording and of course a very similar minimum height of 5.8
now it gets interesting and this is where maybe some of those multi-choice questions will come up
because in the next one which is column number five there is a different number so let's read
what the title says here in positions inaccessible to vehicles and traffic so this is maybe where
you can walk down a path and you get then for a walk underneath the catenary wire and obviously
that height is not going to be anything going to be an issue to a person because your height
is governed by yourself and I don't think many of us are going to be strapping down the road at
3.5 meters so where there is no vehicles present we have a minimum height of 3.5 again that's the
minimum we can be considerably higher than that if we want to in order to support our cables and
when you think about supporting your cables onto that catenary wire I think you also need
to take into consider iteration UV degradation in other words sunlight and is it right to use
standard twin and CPC cables if it's going to be in direct sunlight possibly not and if we think
back to the cables that we see maybe for Sky TV on the outside of our building or those phone cables
that come in on those own individual catenary's built within them they're usually black they're
usually designed to be outside and in sunlight and that makes you think about the fixings onto that
steel catenary wire as well and I don't think it's a very good idea to use PVC cable ties
and you often see this don't you where you see a cable attached to a catenary wire and these over
time have broken off and then all of a sudden the cable starts to sag away from the catenary wire so
in other words it isn't supporting as it was first intended so maybe that's an opportunity to use a
metallic cable tie in those type of situations if you want to see how we used Appendix D to work up
the maximum clipping distance for PVC PVC twin and CPC cables check out the video over there