- In this Q&A video, we're going to answer the question,
what is a clean earth?
Now, just before we explain the answer to this question,
please be aware that this video
is one of a series that we've made
on a subject of underfloor to desk wiring systems.
They can be viewed individually
or you can click the link in the description below
to view them as part of a free online training package
to help you with your CPD
and receive a certificate to prove
you've completed the course.
You may have noticed in other videos in this series
that the power track that sits at the core
of this system from Marshall Tufflex
has a couple of variants.
One of them has gray colored tap off points
and the other has these red ones.
That's because the red version is a clean earth system.
First of all, let's outline the problem
that we need to solve with this clean earth system.
A lot of electrical equipment produces
electromagnetic contamination or EMC.
This basically consists of
magnetic fields that cut across conductors,
and when this happens, it generates
or induces current inside the conductor.
The induced currents are transient in nature
and will come and go depending on
how the equipment generating them is behaving.
They're also referred to as noise in the system.
In main conductors that are already carrying current,
this isn't generally a huge deal.
However, installations should be designed to try
and reduce EMC in the first place.
The bigger potential problem occurs
when this current is generated into the conductors
that form the earthing arrangement
and then get conducted around the system.
Many forms of IT equipment rely
on a good connection to earth in order to function properly
and if they're connected to circuit protective conductors
that have transient current spiking in them,
it can seriously disrupt the way they work
and even, in extreme cases, damage them.
So in a large installation, it may become necessary
to supply critical IT equipment with a connection to earth
that has this electromagnetic contamination reduced.
Theoretically, this could be done
by installing a completely separate earthing arrangement
to the installation that had no connection
to the sources of EMC
and only connected to the IT equipment
that needed protecting.
While this wouldn't remove the problem entirely,
it would certainly reduce it.
However, we can't do that because BS 7671
pretty much forbids having
two separate earthing arrangements in a single installation,
which makes perfect sense because a rising voltage
on one of the earthing systems
wouldn't show the same rise on the other
and you could very easily get a shock from one to the other.
So there's a kind of halfway house approach
using what's probably more accurately referred to as
a low noise earth, or as we know it better,
a clean earth system.
This is where a separate conductor is installed
alongside this EPC and connects to the earth terminal
of the IT equipment you're seeking to protect.
This separate CPC will still be connected
to the normal earthing arrangement,
but not at the point of use,
much like a regular earthing arrangement where
you have a main earthing terminal,
then distribution conductors to other earthing terminals
and final circuit connections.
Depending on the size of the installation,
there will usually be a clean main earth terminal
from which clean distribution earthing conductors
will be run out to other clean earthing terminals
and onto final circuit clean earthing conductors.
This main clean earth terminal will be located
near the main earth terminal
and these will be connected together here and here only.
There will be some conduction of the EMC currents
onto the clean earth system
and some induction on there as well
but it's very much reduced.
For the final circuits to the power tracks,
a separate clean earth conductor
is run out to the end feed unit
and then connected to the center terminal
of the connection block.
You can see quite clearly here
that the terminal for the clean earth
is marked a little differently to the normal earth terminal.
On the normal terminal,
the earthing symbol is enclosed in a circle,
on the clean earth terminal,
it's got a half circle identifying it.
So the clean earth conductor will connect
to that center terminal
and the regular CPC connects to one of the outer terminals.
Keep in mind though that there's absolutely
no electrical connection between these two terminals,
except back at the main earthing terminal.
I should say at this point that there are other ways
of achieving a clean earth where you can be
a little less rabid about this separation
but this is the basic principle.
From there, the tap off points
have an extra connection inside them.
The general earth connection, which is picked up
from the metal bodywork of the track,
and the clean earth has its own separate buzz bar
on the inside and the tap off points
and connectors feature an extra terminal and pin
to carry the clean earth connection through separately.
You can see the extra conductor
poking out of the end of this flexible conduit,
colored black with a bit of green and yellow sleeving
and a clear label stating this is a clean earth.
That's a really neat solution to make it clear
which earth conduct is which
while still complying with the requirements
for identification in BS 7671.
Now, obviously there's no point in having
the two separate CPCs all the way
from the main earthing terminal,
all throughout the entire system
only to have them join together at the load.
And this can be particularly challenging at a socket outlet.
If we take a look at the front of these two sockets,
they look very similar apart from two key details.
One is this red dot, which is how Marshall Tufflex
identify their clean earth components,
and the other is that these screws
holding the socket outlet to the face plate
are metal on the standard socket
and plastic on the clean earth variant.
So why is that significant?
Well, flipping them over, you can see the red sticker
on the clean earth socket was sitting
over the fixing of this earth terminal on the front plate
which is missing from the standard socket.
Again, this is about maintaining that separation
between the clean earth and the regular system.
The metal bodywork of the socket
and the floor box it sits in are exposed conductive parts
and so need connecting to the general earthing system,
and that's what the connection on the front plate is for.
But the earth terminals in the front of the socket
need connecting to the clean earth system
so the terminals on the socket itself
are used for this purpose.
However, pretty much all sockets have
this metal strip across the back,
which in general installations, earth the back box
via the fixing screws.
However, in our clean earth system,
this would simply connect the clean earth
to the general earth, and that's why the clean earth socket
has these plastic screws so there's no electrical connection
between the clean earth in the socket terminals
and the general earth on the front plate.
Clever stuff.
So there we go.
That's what we mean by a clean earth.
Now, this power track system is great
where you've got a floor cavity,
however where there's no raised floor,
we can use an alternative In-Screed trunking system.
To find out how to calculate the number
of conductors we can install in them
check out this video right here
or click the link in the description below
to watch it as part of our free training package
to help you with your CPD and receive a certificate as well.
All that remains in this video is to say
thank you very much for watching.