- Hello and welcome to our latest video
where we're looking at circuit breakers
and the specifications usually printed on them.
We're going to answer the question
what's the difference between a 6kA and a 10kA MCB?
Now, I'm sure you always look to fit the correct type
and current rating of MCB.
For example, B6, C20, B32
or whichever is correct for your circuit.
But what's the other figure printed on there?
On this circuit breaker from Schneider Electric,
you can see the letters and numbers ICN 10,000 amps.
On some you will find the figure in a rectangle.
So, what exactly does this mean?
Well, BS 7671 does not include a definition to explain this
but table 2.16 of guidance note three
and table 7.2.71 of the onsite guide both have details
regarding what ICN is all about.
As stated in both of these publications
we're looking at the prospective fault current ratings
of our circuit breakers.
regulation 643.7.3.201 says we need to look at the current
that may flow in the event of a short circuit
at every relevant point of the installation,
which means every point where a protective device
is required to operate under fault conditions
and includes the origin of the installation.
And we need to ensure that should such a fault occur,
our protective devices are able to withstand
the large current that would flow.
If we fit a device in a setting
where the fault current exceeds the stated ICN,
we could be in danger of the device being damaged
or in worst-case scenario, catching fire or exploding.
Appendix 14 of BS 7671 gives further guidance
on these requirements and the measurements
that should be taken.
The regulation states the prospective fault current
may be measured, calculated
or determined by another method.
If we look at these options further,
the first option is measurement.
As this is a live test, suitable precautions
and test equipment should clearly be used.
On a single phase supply such as most domestic installations
both the prospective earth fault current aligned
to earth fault and the prospective short circuit current
aligned to neutral fault should be measured
and the highest value recorded and compared
to our circuit breakers to ensure it's lower than the rating
of the protective device you are installing.
When you're working with a three-phase supply,
the worst-case scenario is a simultaneous fault
between all line conductors.
An approximation of this can be found by measuring
between the line and neutral conductors
and doubling the highest value obtained.
Again, this should be compared with the rating
of your protective device.
Guidance note three includes the recommendation
that alternative means to measurement are used
to determine prospective fault current wherever possible.
The second option given is calculation.
If we go back to our basic principles
and apply a bit of Ohm's Law,
we know that the current that will flow is directly related
to the resistance, or in this case the impedance
of the fault path.
If we have measured the earth fault loop impedance
or Ze and we know the nominal voltage is 230 volts.
We can calculate the expected current that will flow.
So, while a nice low Ze is great for meeting
the earth fault loop impedance values
and therefore the disconnection times needed
the lower the Ze, the higher the prospective fault current
which in some circumstances may create other issues.
The final option given is another method
which would be inquiry
and asking your local distribution network operator
for the value.
Guidance note three tells us that except in London
and some other major city centers,
the maximum fault current for a 230-volt single-phase supply
up to 100 amps is unlikely to exceed 16kA.
So, if we're working with a declared fault current of 16kA,
shouldn't we have devices rated at 16kA or above?
Fortunately, this isn't the case,
as guidance note three goes on to give further guidance
where it tells us that as long as the consumer unit
is a type tested coordinated assembly, so no mixing
and matching of brands with a double pole main switch
meeting the requirements of BS 61439-3,
our circuit breakers can be considered to be rated at 16kA
which is suitable for our domestic installations
as it matches the value quoted by our DNO.
There are a few other criteria that must also be met
regarding the supply which are the incoming supply
must be provided in accordance with the requirements
of electricity supply quality and continuity regulations
with a nominal voltage below 250 volts
and the cutout fuse is either a BS 1361 Type II
or BS 88-3, rated at 100 amps or less.
Obviously, these are things we would ensure
but are really the DNOs responsibility.
Why then would we need devices rated higher than the 6kA
most circuit breakers are rated up?
Well, as I mentioned earlier, guidance note three
talks about single phase supplies up to 100 amps
and also talks about not being in a city center.
In either of these cases, other factors must be considered.
If we look at industrial and commercial installations,
many premises have an 11 kV or 33 kV supplied
to a transformer right outside.
The earth fault loop impedance in these cases
will be extremely low as there is so little cable
between the transformer and the consumer's intake position
that very high currents may flow when a fault occurs.
In many cases, even a 10kA circuit breaker
may not be suitable at the first distribution board
and an alternative such as a BS 88 fuse
with prospective fault current ratings of up to 80kA
may be required.
By the time you have distribution circuits
feeding consumer units within the customer's installation,
the 10kA or even 6kA devices will probably be suitable.
Of course, this is one of the relevant points
referred to earlier where we would measure
the prospective fault current to check.
In city centers, the presence
of multiple parallel earth fault paths
and the proximity of transformers will again
create very low impedances,
leading to the very high currents when the faults occur.
If when you go to install your over current
protective devices, you notice there of the higher rating,
then there's no need to worry.
I liken this to tires fitting on my car.
If they're rated at 100 miles per hour
and I drive at 70 miles per hour,
then they're gonna be just fine.
So, no speeding as it's unsafe and illegal in this example.
So, in conclusion, for domestic installations,
circuit breakers rated at 6kA will usually suffice.
However, if you're looking at industrial
or commercial installations,
you may need to fit the higher rated 10kA devices instead.
If you found this interesting
and you'd like to take a closer look
at Schneider Circuit Protection and Consumer Units,
check out these videos right here
and thank you very much for watching.