- This video is one of a series that we've made
on circuit protection in association
with Schneider Electric.
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.
If you're already watching this as part of the CPD package,
we'll crack on.
So what is an RCD and what kind of protection does it offer?
Well, an RCD offers two main forms of protection.
The one that's probably the most common nowadays is found
in regulation four 415.1.1:
"The use of RCDs with a rated residual operating current
not exceeding 30 milliamps is recognized in AC systems
as additional protection in the event of failure
of the provision for basic protection and/or the provision
for fault protection or carelessness by users."
So let's try and unravel this.
The RCD is viewed as additional protection.
In addition to what?
Well, the regulation tells us it's in addition
to basic protection, which are the methods we use
to stop people coming into direct contact with bits
that are meant to be live,
as in, we wrap conductors in insulation,
or put live parts in enclosures and behind barriers
and so on.
It's also in addition to fault protection,
which is what we use to limit the contact people can have
with live parts when stuff goes wrong.
Like the examples we looked at for the video about the MCB.
So the RCD offers protection in addition to these,
but the key point is that it is additional protection,
so it has to be used as well as basic and fault protection.
It doesn't replace them.
This is then bourne out in the next regulation.
415.1.2 states that, "The use of RCDs is not recognized
as a sole means of protection and does not obviate the need
to apply one of the protective measures specified
in sections 411 to 414."
So, we still need to use insulation overcurrent protection
and so on, as well as using RCDs for additional protection.
We can't just rely on the RCD alone.
But what is this additional protection that it offers?
Well, when we were discussing MCBs,
we mentioned what happens when a line conductor comes
into contact with a CPC, or an earthing conductor.
The resistance of the circuit becomes very low.
Current flow, therefore, becomes very high
and the MCB disconnects quickly.
But does the same thing happen when a person comes
into contact with a line conductor?
Well, when someone touches a line conductor,
they generally will receive a shock to earth.
Now, by that we mean the literal earth
that we walk around on.
So the current passes through the person
and completes the circuit by traveling back
through the literal earth to the supply transformer,
which has an electrode rammed into the ground
and connected to what is effectively the neutral
of the supply.
So in this circumstance, would an MCB disconnect the circuit
and save the person's life?
Well, the human body and the literal earth we walk
around on doesn't conduct electricity spectacularly well,
which means the resistance of the path
that the electricity takes, is quite high.
This means that when someone gets a shock to earth,
the current that flows is actually pretty low.
So low, in fact, that the MCB probably won't even notice it
and so it will not offer any kind of protection
to someone getting a shock.
But an RCD will offer a measure of protection
in this situation.
How?
Well, an RCD works by monitoring the amount
of current flowing into the circuit down the line conductor
and then back out of the circuit in the neutral conductor.
If these two currents are the same,
the RCD knows that the circuit is healthy.
If, however, it sees a difference between the current
in the line and the current in the neutral,
it knows that somewhere current is flowing
to places that it shouldn't,
either down the circuit protective conductor
or through a person to earth, and it will disconnect.
An RCD can detect absolutely minute differences in current.
For a device offering additional protection,
this current will be as small as 30 milliamps
or 30 thousandths of an amp.
That's a microscopic amount of current.
It's usually considered that it takes 50 milliamps
of current flowing across the heart to kill a person.
So an RCD won't allow this amount to flow
through a person to earth and could therefore save the life
of someone receiving a shock.
It's worth noting, though, that if someone receives a shock
from line to neutral, there is not a device available
that will disconnect them from the circuit.
So while RCDs will protect against the vast majority
of electric shocks, it isn't a cure all
that will keep people safe in all circumstances.
In domestic situations, because cables are
either feeding specific circuits
that require additional protection, such as lighting
or socket outlets, or are likely to be non-armored,
buried less than 50 millimeters deep,
they will require additional protection
in the form of a 30 milliamp RCD.
So that's the major application of RCDs
in electrical installations.
But we said there's two.
So what's the other one?
Well, we can use it for fault protection as well.
In some installations, the people
who provide the electrical supply to a property
won't bring you an earth connection.
This is usually in rural areas where the building
is a long way from the supply transformer.
It may also happen in other very specific installations,
such as ones which have swimming pools, for example.
When this happens, the electrician must provide the property
with an earth connection instead.
This is usually by means of an electrode, such as a rod
or a plate buried into the literal earth we walk around on.
This, again, provides a path through the planet,
back to the earth supply transformer.
This type of earthing arrangement is referred to as TT,
which stands for Terre Terre.
Terre is the Latin word for Earth.
The designation TT just indicates that the supply provider
and the installation have their own connection
to the literal earth connecting them together.
Again, we find ourselves in a position where,
in the event that a fault occurs
and the line conductor comes into contact with, say,
the metal casing of a load,
the path the current takes through the literal earth
has a really high value and it may not be enough
to cause an MCB to trip, leaving us with a live metal part
that someone could touch and get a shock from.
So when we're working on a TT installation,
every circuit will need to be protected by an RCD.
But in this application, its primary function is to act
as protection against earth faults,
and for some circuits we may also enjoy the happy accident
of getting additional protection thrown in as well.
Again, it works in the same way, by looking
at the currents in the line and neutral conductor
and disconnecting if it sees a difference in their values.
In this instance we may use RCDs that have different ratings
than the 30 milliamps required for fault protection.
Designing circuit protection for a TT system
is a whole training package in itself.
So we'll just mention a couple of key points here.
It's worth noting regulation 411.5.2,
which states for a TT system, "One or more
of the following types of protective device shall be used,
the former being preferred. Number one, an RCD,
Number two, an overcurrent protective device."
So according to indent two, you can get away
with using an MCB, as long as you meet the requirement
in Note one, which says,
"An appropriate overcurrent protective device may be used
for fault protection, provided a suitably low value
of Z-S is permanently and reliably assured."
In practice it's very hard in a TT arrangement
to get a low enough value of Z-S to let you use an MCB.
And so RCDs are usually the way to go,
especially as they're likely to be required
for additional protection as well.
Note two is also worth a look.
That says, "Where an RCD is used for fault protection,
the circuit should also incorporate
an overcurrent protective device in accordance
with Chapter 43."
So that makes sense, as an RCD by itself won't monitor
for overcurrent, it could have thousands of amps flowing
through the line conductor and as long as it's
exactly the same amount flowing back down the neutral,
it won't care and it won't trip,
even if the building is burning down around it.
To overcome this, you can either have an RCD
and an MCB wired in series with each other,
or have them combined into a single device
that offers residual current and overcurrent protection,
which is what we call a residual current breaker
with overcurrent, or RCBO for short.
Other places that specifically require RCD protection
for reasons other than additional
or fault protection, relate to section 422.3,
which is concerned with locations with risks
of fire due to the nature of processed or stored materials.
According to note one there,
this may include barns, due to the accumulation
of dust and fibers, woodworking facilities, paper mills,
and textile factories, due to the storage
and processing of combustible materials.
In areas like these and similar, regulation 422.3.9 states
that in these locations,
"Wiring systems, other than mineral insulated cables,
busbar trunking systems or powertrack systems,
shall be protected against insulation faults
in a TN or TT system,
by an RCD having a rated residual operating current
not exceeding 300 milliamps,
according with regulation 531.3.2
and to relevant product standards.
Where a resistive fault may cause a fire,
e.g. for overhead heating with heating film elements,
the rated residual operating current
shall not exceed 30 milliamps."
So it may be, again, that one of these circuits
has a 30 milliamp RCD,
offering additional protection already,
but if not, then you may need to provide a 300 million RCD
to offer protection against faults in the wiring insulation
or 30 milliamps in the example given of a heating system
that's using film elements overhead.
And in addition to all of this information,
the 18th edition of the Regs brought in references
to different types of RCD in regulation 531.3.3.
These types of RCD had been around for some time,
but with the increase in DC leakage
within electrical systems, the standard type AC RCD
is becoming less and less appropriate
for additional protection, as they can be blinded
by DC current injected into the system
by electronic loads and fail to operate as desired.
Now the type A is becoming the standard RCD,
as it can tolerate up to six milliamps of DC current flowing
into it, before it becomes blinded.
Moving up the scale, there's also a type F and a type B,
which can cope with increasing amounts and types
of DC current.
The Type B may well be necessary
on EV ChargePoint installations in certain circumstances.
There's also part seven of the regulations,
which relates to special locations,
and pretty much every one of these areas mentioned
in this list will require some form
of RCD protection on various types of circuit,
but there's far too many to dive into here.
So if you find yourself working in any one
of those environments, please make sure
that you read the relevant section of BS 7671,
section seven, carefully.
The next video in this series is on surge protection devices
and can be found by clicking the link right here
or by clicking the "next" button below,
if you're taking part in our free CPD.
All that remains to say is thanks very much for watching.