in this Q&A video made in association with Atkore we're going to answer the question
do i need to bond or earth metal cable tray or basket
now before we start the video proper it's important to point out that this is not
a subject that can be given a simple yes or no answer cue the first comments below
like so many areas of electrical installation no two situations are exactly the same
and it's important for the electrician to use their own engineering judgment
meeting requirements of the regulations but also thinking beyond them in terms of risk and
impact having said that let's think about this situation so you've installed some cable tray
or some basket to carry mains cables from a to b the question you need to ask yourself at this
point is is this metalwork an exposed conductive part or an extraneous conductive part does it
form the cpc for a circuit or is it none of the above let's have a quick look at some definitions
according to bs7671 an exposed conductive part is a conductive part of equipment which can be
touched and which is not normally live but which can become live under fault conditions
so is the tray likely to become live under fault conditions well if it's likely that the cable can
experience a fault that causes the tray to become live then that's probably an indication that the
cable or the containment or the combination of the two is not fit for purpose in that area
if the tray or basket is not likely to become live in the event of a fault then it doesn't need
connecting to earth guidance note 8 published by the iet suggests this is the case and even goes so
far as pointing out that earthing the tray could serve to distribute touch voltage in the event
of a fault elsewhere to places that it wouldn't have existed before so can it be classed as an
extraneous conductive part well again let's look at the definition of this in bs7671
an extraneous conductive part is a conductive part liable to introduce a
potential generally earth potential and not forming part of the electrical installation
so let's just overlook the last part of that sentence and focus on the first part a conductive
part liable to introduce a potential generally earth potential so an extraneous conductive
part is one that is liable to bring an earth potential into the building in other words it
needs to have a pretty good physical connection to the literal earth that we walk around on we bond
structural steel because it's often buried in the ground making good contact with the literal earth
we used to bond incoming gas and water pipes because they were metal and buried into the
ground making good contact with the literal earth we still bond them because we kind of just got
into the habit of doing it we used to bond metal kitchen sinks because we misunderstood bonding
anyway the point is that a piece of metal work that isn't an exposed conductive part in
a building generally only needs bonding if it is introducing earth potential into the installation
so if the tray is not making good contact with the literal earth that we walk around on it probably
won't need bonding if you're using the containment as the cpc of a circuit then it is absolutely
critical that continuity of the containment is provided between joints and different sections
if you determine that your cable tray is not an exposed conductive part is not an
extraneous conductive part or does not form the cpc for the circuits on it then it will
not need connecting to earth simple but as always we want to hear from you on this issue
could you just not sleep at night if you didn't bond a tray or are you pretty comfortable
working the problem and not bonding it as always please leave your comments and questions below
all that remains in this video is to say thank you very much for watching
you