it's fair to say that the increased use of AFDD is an amendment 2 to the 18th edition of BS7671
was one of the more controversial and hotly debated subjects of the last few years certainly
in electrical circles to try and cut through the opinion and speculation and help us to understand
what an AFDD is does and doesn't do we created a free training package to help you with your CPD
this is the video from that training package that we've decided to release to our wider audience
we really hope that it helps your understanding of this subject and if you'd like to complete
the course and receive an accredited certificate to count towards your annual CPD
then please click the link in the description to take you to the relevant web page we'll be
considering the following subjects what does an AFDD protect against how does an AFDD work what's
inside an AFDD and what does BS7671 say about AFDDs so watch this video and then successfully
complete the multiple choice questions and you'll automatically be sent a CPD UK certificate
to verify that you've updated your knowledge so what does an AFDD do well to answer that we
need to understand circuit protection and the types of faults that can occur within circuits
in general so let's have a very brief side quest into the history of circuit protection
there are two big hazards associated with electricity one is it can hurt and even
kill people and the other is that it causes heat which can lead to fires in fact this second hazard
is what really led to the wiring regulations coming into existence they were brought about
at the insistence of insurance companies who were sick of paying out for building fires caused by
unregulated and dodgy installations the problem with excessive heat occurs when too much current
passes through a conductor this causes the conductor to get so hot that it can start a fire
what causes too much current to flow it can be one of two things the first is when too many loads are
connected to the circuit causing it to literally overload which is the name we give to this kind
of fault the excess current in this case is likely to be relatively small but can still cause damage
the other reason too much current could flow through the circuit is if a line conductor comes
into direct contact with another conductor either another line conductor if it's a three-phase
circuit or the neutral or the CPC this is referred to as a fault of negligible impedance which kind
of means very very low resistance and because it has a very low value of resistance ohm's law shows
us the current flow in this circumstance would be huge again leading to overheating and possibly
starting a fire to prevent this from happening we provide over current protection this used to
be in the form of a fuse which was like inserting a weak link into a chain in that a thin filament
of copper inside the fuse would melt when too much current flowed through it thus disconnecting
the circuit over time this was superseded by the circuit breaker which had a mechanically operated
disconnection system and had the advantage of being quick and easy to reset after it had tripped
the other hazard we want to mitigate against is people coming into contact with the electricity
and receiving a shock the simple way we do this is by covering the conductors in insulation
to stop people touching them those are the ways we do this as well however sometimes
these preventative measures can fail and someone can receive a shock most of the time this occurs
to earth when this happens a fuse or a circuit breaker won't even notice it this is because the
path the electricity takes through a person's body has quite a high value of resistance which means a
relatively small amount of current nowhere near enough to trip a circuit breaker or a fuse but
more than enough to kill so to prevent this from happening we install a residual current device
or RCD this monitors the current flowing through the line and back down the neutral and if there's
a difference in the amount of current between the two conductors the device assumes that current is
flowing through something it shouldn't either the CPC or a person and disconnects potentially saving
the individual however there is one more kind of fault that has become increasingly common and that
is the arc fault these are generally caused when connections work loose over time and the
connection between the conductors becomes tenuous if the current flowing into the circuit is high
enough it can jump this small gap in the form of an arc this situation can then be intensified by
the arc carbonizing the PVC insulation around the conductor which can then start to generate some
serious heat in tests at the Lewden factory the carbon rod used to simulate the PVC insulation
reached 200 degrees Celsius and it's not unheard of for arc faults to generate temperatures of 6000
degrees Celsius as you can imagine this could very easily lead us to a situation
where this could start a fire leading to very dangerous consequences but surely our fuse circuit
breaker or RCD will detect and disconnect this well unfortunately not the fuse or circuit breaker
will operate when too much current flows but in the event of an arc fault there isn't an excessive
amount of current flowing it can be a relatively small amount but the danger now arises from the
arc causing damage not heat caused by the current in the conductor what about the RCD surely that
will see the fault and trip well again no when an arc fault occurs the current causing the arc
is still passing down the line and back through the neutral and so the RCD won't see a difference
in the currents through those conductors and therefore it won't trip so in order to disconnect
an arc fault we need a new form of protection enter the arc fault detection device or AFDD
we'll explain in a bit more detail how they work a little later but let's stick with the subject of
faults for now as there are two different types of arc fault these are series and parallel arc faults
the series arc is the one we've already discussed where conductors don't make a brilliant connection
with each other when they should so let's say the line conductor in a circuit feeding
an electric shower isn't brilliantly terminated into the isolator the current may continue to
pass through the connection and jump the small gap caused by the loose wire then continue
down the outgoing conductor to the load because the current continues down the same conductor
this is what's known as a series arc or sometimes you may hear it referred to as a
serial arc much like a series connected load the current has to pass through the fault
to reach the load now a parallel arc is a little bit different it's a bit like a short circuit
in that the current passes from the line to the neutral without going through the load and it can
happen when the insulation surrounding a conductor has broken down or been damaged perhaps by mice
or rats chewing on it now this may sound exactly like a short circuit but it's a little different
because the connection between the two conductors in this case is quite tenuous and intermittent
so the fault won't generate enough current to operate the MCB or fuse but because it's tenuous
and intermittent it will create arcs which can then cause an excess of heat and start a fire
so an AFDD will protect against both theories and parallel arc fault however there is one exception
to this which is very relevant considering what BS7671 says about the use of AFDDs as we'll see
this exception for protection relates to ring final circuits when a circuit is wired as a ring
an AFDD will still be able to detect and protect against a parallel fault but it won't be able to
detect a series fault this is because in a ring final circuit the current has two paths that
it can travel down so if a situation occurs that could cause an arc fault such as a high resistance
joint then the current that could potentially cause the arc and cause the device to operate
will simply take the alternate low resistance path back to the board and not cause the device to trip
of course in that circumstance you've still got the problems inherent with an incomplete ring
final circuit maybe one more sign that they're on the way out however it's really important to
note that in this event an AFDD connected to a ring final circuit will continue to provide the
other forms of protection that it offers including over current over voltage and parallel arc faults
more on these in a moment as well as this the AFDD will also detect a serial arc in
the lead of a load that's been plugged into the socket outlet of a ring final circuit
whether that ring is complete or incomplete so it's not just the fixed wiring of the circuit
that's protected against dark faults but also the leads and loads connected to the circuit as well
so if an arc fault won't draw excessive current or leak it to earth then how exactly does the AFDD
spot these faults well it's all down to monitoring the shape of the current waveform that the
arc creates while it's occurring we're used to thinking of a healthy AC supply of current looking
something like this with the current rising and falling predictably and evenly however in reality
it's actually a lot messier even in a relatively healthy circuit and in a circuit experiencing an
arc fault it's likely to look more like this and that is how an AFDD detects an arc fault
occurring it's programmed to monitor the shape of the current waveform supplying the circuit
and then trigger the disconnecting mechanism when it sees characteristics like this
now if we take a look inside an AFDD we can see the components that allow this to happen
it takes protective devices to the next level by incorporating a small PCB that's been programmed
to monitor and recognize those waveforms we just discussed this then features a triggering
circuit that operates the tripping mechanism and disconnects the circuit however as electricians
we know that electricity can create sparks even in healthy equipment operating totally normally
for example common household items that use motors such as drills or hair dryers will often produce
sparks during normal operation it's simply caused by the connection between the carbon brushes and
the contacts on the motor when the motor spins it can bounce the carbon brushes back a bit on their
springs so there's some moments of disconnection and reconnection so is there a risk that these
sparks could fall the AFDD into thinking there's a fault and disconnecting a healthy circuit
well no because again this normal operation of a motor will produce its own distinctive waveform
of current and the PCB in the AFDD is able to recognize that as a normal healthy function
and not cause it to trip the way you think of it is that those sparks you see in the back
of a motor are generally just that a spark not an arc so you can imagine that this is slightly more
technologically advanced than a simple fuse MCB or AFDD protection however it is very reassuring
when you look inside the AFDD and see that all those components still remain you can see the
bi-metallic strip here and the electromagnetic coil that takes care of the overcurrent protection
and then the current transformer that helps when using an RCD for additional protection
so that helps us to understand that when installing a single module AFDD it's not just
providing protection against arc faults it's also providing overcurrent and RCD protection however
even with all of this there is also one more type of protection provided by an AFDD and that is
over voltage if the AFDD sees a voltage over 275 volts it will again disconnect the circuit so
this leads some to the natural question does that mean if the AFDD provides over voltage protection
i don't need to install a surge protection device well a surge protection device or SPD is designed
to deal with intense but incredibly brief spikes in the supply voltage these can be caused by many
things depending on many different factors these spikes can happen relatively often or hardly ever
but a surge protection device is designed to kind of cushion the blow of these spikes while keeping
the electrical system active and operating safely you'll probably have noticed on an SPD there's
no reset button because it doesn't trip the device just works away until it eventually needs
replacing when the AFDD sees a greater than 275 volts fault it will actually trip and disconnect
the circuit in order to protect the delicate electronics inside itself not the attached
circuit they also differ in another important way which is that prolonged over voltage on an SPD
will reduce its working life a typical spec for an SPD is to withstand 335 volts for 5 seconds
and after that the SPD will need replacing AFDDs disconnect the supply so the potential damage
only lasts a short time and they are designed to have a slight delay so they won't respond
in under 3 seconds but no later than 10 seconds and as the over voltage gets higher the time
taken to operate gets quicker so for example at a spike of 400 volts the device will trip in
between 0.1 and 0.2 seconds so AFDDs and SPDs are looking for different forms of over voltage fault
and protecting different things therefore using an AFDD and an SPD in conjunction with each
other offers the correct form of protection in line with the regs for certain circuits and the
final clever thing that an AFDD does is it will test itself periodically to make sure that it's
working properly when it's initially energized it will perform a series of internal checks to make
sure everything's operating correctly and then it will continue to self-test every hour thereafter
to confirm that it's still working if it finds something wrong with its own internal processes
it will then disconnect itself from the circuit obviously with so much going on internally
there are a number of reasons why an AFDD may operate and switch off the circuit it's supplying
and to help the homeowner and electrician know what's going on inside they feature
an LED indicator to show what's going on now this may vary slightly from manufacturer and
it's common for them to include a sticker or label to show what the different LED states mean we're
going to use this Lewden AFDD as an example of how the LED communicates what its internal status is
if the led is constantly lit then it means the AFDD is healthy and doing its job if the
AFDD has operated and is then re-energized it will flash a different sequence to indicate what caused
it to trip one flash per second for 10 seconds indicates that either a series or parallel arc
fault has occurred if it's flashing twice a second for 10 seconds then that means that the device has
experienced an overvoltage of more than 275 volts so having learned about AFDDs and what they will
and won't protect against we need to consider what BS76711 says about AFDD use in installations
421.1.7 arc fault detection devices conforming to BESN62606 shall be provided for single-phase
AC final circuit supplying socket outlets with a rated current not exceeding 32 amps in higher
risk residential buildings houses of multiple occupation purpose-built student accommodation
care homes so notice there the word shall was used so in all these places mentioned there you have to
install AFDDs on circuits supplying sockets with a rating of up to and including 32 amps there's no
exceptions mentioned here at all and just in case you're wondering what a higher risk residential
building is there's a definition there in note 1 which reads higher risk residential buildings are
assumed to be residential buildings over 18 metres in height or in excess of six stories whichever
is met first it is anticipated that in many areas higher risk residential buildings will be defined
in legislation which can be subject to change over time as well as in risk management procedures
adopted by fire and rescue services current legislation should be applied
as that points out the definition of HRRBs is laid out in legislation and can change over time
it's worth noting that residential in this context doesn't just refer to where people live or sleep
but also includes areas where people may reside during the day for work so office blocks could be
included under these circumstances as well local planning offices should be consulted if in doubt
so what about other installations that fall outside the four bullet points there
well the regulation continues for all of the premises the use of AFDDs conforming to BSEN62606
is recommended for single phase AC final circuits supplying socket outlets not exceeding 32 amps so
similar circuits their use is recommended for socket circuits up to and including 32 amps in
rating but the key to understanding this part of the regulation is in that word recommended what
does that mean does it mean it'd be nice to do it but it's up to you if you install them or not
well BS7671 states that recommended as used in the regs means should or more specifically it's an
expression in the content of a document conveying that among several possibilities
one is recommended as particularly suitable without mentioning or excluding others so
this indicates that if you want to comply with BS7671 then a recommendation isn't something
that you can just ignore you have to take some kind of measure to address the situation
it doesn't have to be the one in the regulation but that is the one the
authors of the regs have selected as the best one so you don't necessarily have to install
AFDDs to protect against arc faults but you have to do something about them you can't do
nothing so there we go we've considered what an AFDD protects against how they work and what's
inside of them as well as what BS7671 says about AFDDs and where they need to be installed we hope
that information was helpful to you and once again if you'd now like to complete the training package
by answering a few multiple choice questions then please click the link in the description
below and do please check out the rest of our free training packages on the know how page of
our website efixx.co.uk all that remains in this video is to say thank you very much for watching