- This video is one of a series
that we've made on circuit protection.
The videos 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 that you've completed the course.
If you're already watching this as part
of the CPD package, we'll crack on.
So, let's answer the question, what is an MCB?
Well, first of all, it stands for Miniature Circuit Breaker.
You may be wondering what makes them miniature
as they don't look very small,
but the M for Miniature there is to show this is different
to an MCCB, or Molded Case Circuit Breaker,
which differ from MCBs in many ways,
because MCCBs are large in size by comparison
and have adjustable protection settings.
Of course, the granddaddy of all circuit protection
is the fuse, and this is still to be found
doing sterling work in all sorts of different places.
However, it's highly unlikely that you'd install
a consumer unit featuring fuses into a new installation
or as an upgrade, so we're not going to dwell on them here.
So, what does an MCB do,
and what will it protect a circuit against?
Well, when we connect a load up to an electrical supply,
it draws a certain amount of current,
which is completely normal,
and the load needs that current to operate.
However, it's a fundamental fact of electrical science
that when current passes through a conductor,
it heats the conductor up.
The more current flowing in the conductor,
the hotter the conductor will get.
If this heat makes the conductor get too hot,
it can start to have some really bad effects,
such as melting the insulation from the cable
and leaving the live conductors exposed
and open to being touched.
It can also get so hot that,
depending on how it's installed,
it can actually start a fire.
In fact, it was for this reason
that the wiring regulations came into being.
Insurance companies were so sick of forking out huge amounts
of money to rebuild properties after fires caused
by shoddy wiring burnt them down that they started pushing
for regulation of electrical installations to stop amateurs
from carrying out dangerous work at the lowest cost.
Thank goodness that 140 years later,
we're not still dealing with that problem.
Anyway, it's clear that we need to protect
the electrical installation, the property,
and the lives of the people using the property
from the effects of too much current
flowing in our conductors.
We refer to this problem with the expression "overcurrent",
and one defense we have against it is the MCB.
There's two ways that overcurrent
can be caused in a conductor.
The first is if too many loads are connected to the circuit.
This is referred to as an overload.
Now, for most circuits, this is relatively unlikely.
Let's take an immersion heater as an example.
When it's switched on, it draws a fixed amount of current,
depending on its power rating, and without some excessive,
and most likely sketchy, electrical work,
you can't just connect an additional load to this circuit.
There's nowhere to plug it in or hook it up.
So, this is a fixed load,
and it is unlikely to ever draw more current
than the circuit was designed for.
The circuit that's a real culprit for drawing
too much current is one that's feeding socket outlets.
In theory, it's possible to very quickly connect
too many loads to this kind of circuit,
especially if there's a cold snap and someone has access
to a whole heap of electric heaters.
So, to prevent circuits from being overloaded,
the MCB has a device inside it called a bimetallic strip.
This is made up of two different types
of metal that are welded together,
and as current passes through this strip,
it starts to heat up, and because the two different types
of metal expand at different rates,
the strip starts to bend.
Once this strip bends to a certain point,
it will operate a tripping mechanism
inside the circuit breaker and disconnect the circuit,
stopping the current from flowing
and allowing the conductors to cool down.
Now, it's important to understand
that there isn't a tipping point,
where a little bit too much current flowing
causes an instantaneous fire.
So, for example, if a circuit has a 32 amp MCB
protecting it and 33 amps flows,
the property won't spontaneously burst into flames.
For this reason, the bimetallic strip is designed
to trip more quickly the more current flows through it.
It would be really irritating if the circuit was to trip
if just slightly overloaded for a short period of time
because this wouldn't present a huge amount of danger.
However, there is a second way that too much current
can flow through a conductor, and that is when
two conductors in a circuit touch each other.
In a single-phase domestic property,
this means when a line conductor touches a neutral,
the so-called short circuit.
It's called that because the current has a shorter path
to take rather than going through the load.
It can also happen when the line conductor touches
a circuit protective conductor, or an earthing conductor.
This is what the regulations refer to
as an earth fault of negligible impedance.
You can kind of just take this
to mean very, very low resistance.
When this happens, because the resistance of the circuit
is so low, the current that flows
can be absolutely enormous,
often in the range of thousands of amps.
When this happens, the heat in the conductor
can build up really quickly, making this situation
very dangerous and needing a quicker response
than the bimetallic strip can offer.
For this reason, the MCB has another device inside it
to monitor for large amounts of overcurrent.
This takes the form of an electromagnetic coil.
One of the other effects that current has
as it passes through a conductor is it creates
a magnetic field around the conductor.
This magnetic field will get stronger
as a higher value of current flows.
So, in the event of a short circuit,
or earth fault of negligible impedance,
the current becomes instantly huge,
causing the magnetic field to be very strong.
This is intensified by the electromagnetic coil in the MCB,
which then draws a plunger made of soft iron into itself,
operating the circuit breaker and disconnecting the circuit.
So, the MCB combines two different forms
of protection to keep circuits, properties,
and people safe from the effects of overheating conductors.
However, you may be wondering what happens
if a person comes into contact with a live conductor.
Will the MCB keep someone safe in that situation?
Well, the answer to that question is no.
To offer that kind of protection,
you'll need a residual current device, or RCD,
which you can watch a video on by clicking right here
if you're on our YouTube channel
or by clicking the next button below
if you're taking part in our free CPD.
All that remains to say is thank you very much for watching.