- In this video, we will be looking at the materials
which go into making some of the safest cables in industry,
the operating temperatures of cables,
and the current carrying capacity as well.
And don't forget, this video is one in a series
that we've made in association with Doncaster Cables.
They can be viewed individually, or you can watch them
as part of a free training package
to help you with your CPD.
And you get a certificate as well.
Any manufacturing process always amazes me.
You start with a raw material,
like this big spool of copper and these pellets,
and at the end, you get this single core cable
with an insulation wrapped around it.
What amazes me more is how much thought
has gone into the process.
I'm not talking about the tooling
of the equipment or the machines,
but the composition of the insulation itself,
and the high standards required.
You may remember from video one me talking
about the tragedy of the King's Cross fire in 1987.
The thick black smoke engulfed the ticket hole
and although other factors like the paint contributed
to the smoke and acrid fumes given off,
it was the identification of cable
and its materials which was found
to have contributed further to the tragedy.
It was a turning point in
cable manufacturing and installations.
Large projects and organisations began
by only allowing low smoke
and zero-halogen cables on their builds.
You may know this cable as low smoke and fume cable, or LSF,
or low smoke, zero halogen, or LSZH, but there is more.
In fact, you may come across this many acronyms
when looking into low smoke zero halogen cable:
PVC PVC, polyvinyl chloride;
LSF, low smoke and fume; LSHF, low smoke, halogen free;
LS0H, low smoke zero halogen;
LSZH, low smoke zero halogen;
0HLS, zero halogen low smoke;
ZHLS, zero halogen low smoke;
LSNH, low smoke no halogen.
A very common misconception as well is
that LSF cable is the same as LSZH cable,
but this is not the case
as we're gonna be looking at later in the video.
We're going to explore how these cables react
when in a fire, starting with PVC.
Polyvinyl chloride-insulated wire releases a thick cloud
of black smoke in addition to a lot of hazardous substances.
These elements can obstruct vision
and result in suffocation.
The cable gives off roughly 28% of hydrogen chloride gas,
which not only carries an unpleasant odour,
but also transforms into hydrochloric acid
on exposure to water.
The smoking gases emerging
during ignition can pose a greater threat
to individuals than the fire incident itself.
PVC cables are best installed in structures
with a low hazard potential like
in the floor space in a domestic property,
and are not typically recommended
for large commercial properties or public buildings.
Composed of a specialised PVC compound,
LSF cables have a slightly lower emission
of hydrogen chloride gas and smoke upon combustion
compared to typical PVC.
However, with the emissions ranging
from 15% to 22% based on quality,
dense black smoke is still a byproduct due
to the PVC in the compound.
It is important to note that these cables do contain halogen
and can be mistaken for low smoke halogen free cables.
The PVC content in LSF cables varies across manufacturers,
introducing an element of uncertainty
in their use for public installations.
Low smoke halogen free cable serves
as an effective cable in fires due
to not admitting smoke and fumes.
One of the cable's fundamental features is the emissions
of less than 0.5% hydrogen chloride gas,
offering a safer environment during a fire.
Finally, the low smoke production,
which helps preserve the visibility
of evacuation pathways and signs during a fire.
It may come as a surprise to learn that
low smoke halogen free cable - Low smoke fume cables and
low smoke zero halogen cables- - Serve as an effective
fire-resistant cable- - Are not the same.
In fact, they're not even close
when it comes to performance in a fire.
But all these cables, including PVC, need to be subjected
to fire testing to get their relevant standard numbers.
Since 2017, cables are required
to meet the construction products regulations, or CPR,
or BS EN 50575 standard.
And this is for all cables in homes,
commercial installations, and business facilities.
This standard deals with the control
of heat emission and fire propagation,
but not the harmfulness of released gases or smoke.
This is a different standard.
So what does the wiring regulations say?
It acknowledges the necessity for cables
to adhere to CPR regulation
and possess a Euroclass for fire resilience.
Oh, and Euro classification is another test
these cables have to be subjected to.
It's critical to note, however,
that BS 7671 doesn't specify any Euroclass
for particular environments or applications.
Also, BS 7671 doesn't stop us using cables
that emit hazardous fumes and smoke.
It's the designers and installers who need to ensure
that their chosen cables match the level
of fire danger present in the building or application.
So let's have a look at operating temperatures
of different cables and their compositions.
For cables to work at their best, we have a number
of factors that we look at: the type of cable,
what it is supplying, how it's been installed,
and the temperature we expect it to work at.
The safe functioning of electrical cable
over a certain period of time heavily relies
on the operating temperature.
This is the span between the lowest and highest temperatures
in which the cable can efficiently operate.
The material that either insulates
or sheaths the cable determines this crucial factor.
Each specific material has a temperature scope,
both upper and lower bounds
within which it remains fit for its purpose.
These temperatures significantly vary dependent
on the type of material and if the cable needs
to flex at these temperatures.
Materials tend to soften with increasing temperatures
and stiffen as the temperature falls.
So they may not be suitable for applications
that need flexing at either extreme of the temperature.
For instance, a typical insulation material
like PVC has a temperature range
that lies between minus 15 degrees C
and 70 degrees C for applications.
Silicone rubber is used in applications
that have a temperature span
of minus 60 degrees C to 180 degrees C.
These different operating temperatures
also impact the amount of current each cable can carry,
something we call the current carrying capacity
of the cable, and we can find everything we need
in Appendix 4 of BS 7671.
So let's start with a 70 degree Celsius flat PVC cable.
Just a quick advisory though, we are looking
at current carrying capacity, not cable calculations.
Table 4D5 tells us how much current cables
of different cross-sectional areas can carry
in certain installation environments.
If it's installed in a 10-millimeter squared cable
and clipped directly to where it's needed,
it could carry 64 amps.
But the same 10 millimetre squared cable,
when installed on a plasterboard
with thermal insulation not exceeding 100 millimetres,
can only carry 45 amps.
So why can the cable clipped directly carry 90 more amps
than the one laid on plasterboard
with insulation laid on it?
It comes down to temperature.
With a cable laid under insulation,
we can safely assume that the heat will take longer
to dissipate from the cable.
And no matter what we do,
if we put current in cable, it will heat up.
So the less current in the cable,
the less heat it will emit.
So the cable can lay under some insulation.
We only need to look at the next column,
which is for cables laid under more than 100 millimetres
and the maximum current in our 10 mil is 36 amps.
A cable clipped direct tends to be in free air,
so can dissipate its heat freely away.
But what about cables with different
operating temperatures for example?
A 70-degree Celsius PVC armoured cable compared
with a 90-degree Celsius cross-linked polyethylene cable,
well, let's take a look and compare.
The 70-degree C PVC armoured current carrying capacity
can be found on table 4D4A
and for the sake of consistency,
and my love for the number 10,
we will use 10 mil cable clipped direct.
We can see that a 70 degree C
10 mil squared cable clipped direct can carry 67 amps.
Flicking through the table to 4E4A
for 90-degree Celsius armoured cable,
a 10 mil can carry 85 amps.
Cross-linked polyethylene cable
can carry more current due to its compound.
It's significantly more stable with longer molecular chains,
meaning it can hold higher temperatures
and allow for high currents to be carried.
So why don't we just instal
cross-link polyethylene everywhere?
Issues do arise when we consider the accessories
the cable is going to be connected to.
For example, if I wire this armoured cable
to an outdoor socket, I can be satisfied
that the cross-linked polyethylene can cope
with 90 degrees Celsius,
but the socket insulation may only be rated
to the lower 70 degrees Celsius,
meaning I have to account my current carrying capacity
to the lowest temperature.
So there we have it, that's all the fuss
of why we should not judge a cable by its insulation.
In the next video, we'll be looking at cable segregation
and how innovative cables,
like the EV-Ultra from Doncaster Cables,
can combine different band cables
to an industry-leading solution.
And remember, this video is made
in association with Doncaster Cables
and it's one of a series that we've made on cables.
You can watch them individually
or 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.
But until next time, it's bye-bye for now.