during the build up to and the launch of the second Amendment to the 18th Edition we all
got very caught up in the storm of controversy surrounding the increased use of AFDDs and it's
possible that we overlooked some of the other key changes regarding circuit protection including
the changes that took place to the requirements for RCDs so what changes did happen with RCDs
in the latest Amendment to the wiring Regs well the first major change that we want to discuss
relates to the ongoing decline of the type AC RCD regulation 531.3.3 discusses the different
flavours of RCD protection which we're becoming more familiar with now these being types AC A F
and B the main differences between these is how they monitor and deal with DC leakage into their
electrical system to understand why DC leakage is a problem let's look in a little more depth as to
how an RCD operates now we've got a diagram here of a traditional RCD what's referred to as an AC
type RCD which is the basic RCD that we've mostly been installing for years and years
inside the RCD the line conductor wraps around this toroidal or ring-shaped piece of soft iron
it then goes off and feeds the load the neutral from the load then returns and wraps around the
other side of the toroidal core and returns to the source of supply now at the same time
wrapped around the core there's this third coil that feeds a tripping device that connects to
the switching contacts in the line of neutral connections now when the circuit is healthy the
current passes through the line and the same amount of current comes back down the neutral
as the current passes through the coil around the core it generates a magnetic field around the coil
but because the same amount of current is passing through the neutral coil at the bottom a magnetic
field is created around this coil that is equal but has an opposite direction to the magnetic
field around the line conductor the upshot of this is that the two fields cancel each other out now
if an earth fault occurs and the current flowing back through the neutral is slightly smaller this
makes the neutral magnetic field slightly weaker than the one around the line conductor so now this
slightly strong wrong the magnetic field is able to generate a small amount of current into the
tripping coil using the exact same principles as the operation of a transformer if the difference
in current in the line in neutral is big enough the current generated in the tripping coil will
be enough to trigger the tripping device and disconnect the circuit I'm sure you'll agree this
is an elegant and ingenious piece of design with the electricity running through it is a purely AC
Supply then it will operate exquisitely however modern loads that we connect to our circuits
such as washing machines dishwashers IT and entertainment equipment all leak DC back into the
AC system we can illustrate this using this lovely graphing software from Desmos here we have a
healthy AC waveform and here we can see what a DC waveform looks like just a straight flat waveform
now the problem occurs when we combine the two by leaking DC back into the AC supply because
as the DC combines with this AC waveform you can see it lifts the whole AC waveform up the y-axis
now just a word of caution for our understanding here the subsequent lower waveform down at the
bottom here does not indicate that there is less current in the neutral than the line if
this was true the device would trip when the AC waveform drops below zero into the negative
range it indicates that the current has changed Direction so now when the current is going one
way around the circuit it is slightly higher than when it reverses and goes the other way
this has the effect of slightly magnetizing the magnetic core in one direction leaving a slight
magnetic field in there all the time now if an earth fault occurs the magnetic field around the
neutral is slightly weaker but because there is a weak permanent magnetic field inside the soft iron
core there isn't enough strength in the imbalanced field to overcome that slight remaining magnetism
and generate enough current in the tripping coil to disconnect the circuit so the device simply
won't trip now for the more mature members of our audience you may remember the old Robin testers
that featured the d-lock function this allowed the tester to carry out an earth fault loop
impedance test on circuits that were protected by RCDs and it worked in the exact same way it
would inject DC current into the circuit and stun the RCD by saturating it magnetically meaning the
impedance test could then be carried out without tripping the RCD now that's all very interesting
but what are the real life implications of this well if our installations connect the
loads we discussed earlier in the video then it's likely that our old AC type RCDs won't operate on
default conditions meaning that someone could be receiving a lethal amount of electricity
and the RCD simply won't disconnect the circuit so it seems with all the DC leaky loads people
are connecting to our electrical installations the type AC RCD is a thing of the past and it
seems the Second Amendment is reinforcing this idea after defining the different types of RCD
Regulation 531.3.3 states RCD type AC shall only be used to serve fixed equipment where it is known
that the load current contains no DC components Note 1 just below adds further insight as to just
what an AC type RCD can be used on where it says examples of fixed equipment with a load current
containing no DC components can include but not be limited to electric heating appliances and or
simple filament lighting neither containing electronic components as you can imagine in
this day and age where the simplest heater may well incorporate digital controls or lighting
will be based on LEDs it seems that the type AC RCD may well be breathing its last and the base
level of RCD protection will be the type A moving forward if you'd like to know how amendment 2 has
affected the use of RCDs in split load consumer units please watch this video here and for a
full breakdown of the changes Amendment 2 made to circuit protection check out our
free training package by clicking the link in the description below thank you very much for watching