in this q and a video we'll be answering the question do i need
pen fault protection on an EV charger connected to a three-phase supply
this question relates to the recent video that Gary and Gordon made on a really innovative EV
charge point from Zaptec which cleverly juggles the charging of multiple electric
vehicles from several charge points using just one reasonably sized supply it's a great looking
and very intelligent solution to the tricky issue of charging multiple vehicles so do go and check
that video out for a full teardown and description you may have noticed in that video and others that
we did in fact install pen fault protection to our Zaptec charge point so why did we do that we've
described pen faults in some detail on our channel as part of our training videos so by now hopefully
you're familiar with the dangers involved in a PEN fault if the combined neutral and earth cable
supplying a house becomes disconnected outside the building the current inside the installation
will try to find ways back to the transformer supplying the property and if someone's really
unlucky and standing on the actual Earth we walk around on or holding onto a metallic pipe
buried in the ground it could flow through them inside the property we can resolve this problem
by connecting all the exposed conductive parts of a circuit to the extraneous conductive parts via
the main earthing terminal which means that all the metal work is at the same potential
and a person is much less likely to get a shock to the literal Earth that we walk around on
an EV charging up outside presents an extra hazard because if a PEN fault has occurred
and the bodywork of the car has become live then it's possible someone touching the car will have
a reasonable connection to the literal Earth and they will receive a potentially fatal electric
shock because of this regulation 722.411.4.1 tells us that a PME earthing facility shall not be used
as the means of earthing for the protective conductor contact of a charging point located
outdoors or that might reasonably be expected to be used to charge a vehicle located outdoors
unless one of the following methods is used so you can't connect the earth terminal on an EV charge
point to the PME terminal of a TN-C-S supply if the EV connected to it is likely to charge up
outside however you can do this if you use one of the options listed in that reg and we've discussed
these at length in other content however one of those options is no longer available to us as
you can see here it's been deleted by Amendment 2 of the 18th edition what was it and why has it
been removed well we can look back to the first Amendment to the 18th edition which was the one
that had no physical release if you remember just the digital version that you could pay extra to
print out and that first indent is present there and it states that the charging point forms part
of a three-phase installation that also supplies loads other than for electric vehicle charging
and because of the characteristics of the load of the installation the maximum voltage between
the main earthing terminal of the installation and Earth in the event of an open circuit faulting the
pen conductor of the low voltage network supplying the installation does not exceed 70 volts rms
so at first glance that seems pretty acceptable that if we're connecting an EV charge point to
an installation fed by a three-phase supply that we can connect the protective conductor
to the PME and all will be well so why was that acceptable well remember the problem occurs when
the combined neutral and earth conductor into the installation loses its connection and the current
is looking for that route out meaning that the voltage between the bodywork of the car and the
literal Earth was likely to go well above 70 volts which is the point at which the voltage
would become potentially lethal in this situation in a three-phase system though the current in
the neutral works a little differently in fact if the loads on each phase are perfectly balanced or
equal then the current in the neutral would be zero and you can actually do without the neutral
connection entirely so if a PEN fault occurred on a perfectly balanced three-phase system
then there'd be no neutral current trying to find its way back to the transformer and the voltage
between the bodywork of the car and the actual Earth we walk around on will be effectively zero
however that brings us nicely on to the end of the old indent we just read because it gives this
little caveat because of the characteristics of the load of the installation the maximum
voltage between the main earthing terminal of the installation and Earth in the event of an
open circuit fault in the pen conductor of the low voltage network supplying the installation
does not exceed 70 volts rms so why would that happen well we said that in a balanced load
the current trying to flow down the neutral and therefore the voltage to earth would be
zero however the problem occurs when the loads become unbalanced and you have different amounts
of current flowing through the three line conductors when this happens some current
will flow down the neutral and the voltage between the bodywork of the car and the earth will rise
and if it rises above 70 volts then anyone touching the car could be in serious danger
if a PEN fault then occurs so how do we know at what point that voltage would rise to 70 volts
well note one to that indent tells us that annex 722 item A722.2 gives some information relating to
one so turning to that reference we find that where triple harmonics can be neglected
condition 1 of regulation 722.411.4.1 which is the reg we're reading at the moment
may be assumed to apply where the following condition is met this formula should take account
of cmax in line with PD CLC/TR 50480 that then follows a rather intimidating looking formula Im
times Uo times cmax over IL1 plus IL2 plus IL3 is less than or equal to 70 volts the formula is
actually pretty simple it's just ohm's law and the power formula all jumbled up together on the top
line Im is the current in the neutral which can be found by applying the rather scary formula in
A72 2.1 and is rather dear to my heart as someone who loved teaching electrical science then we've
got Uo which is the nominal voltage to earth in the system so 230 volts and Cmax is just a way of
adjusting the voltage to allow for variations to the supply voltage these are multiplied together
and then the division part on the bottom row is just the three currents in the three phases
added together so all this is doing is indicating that the maths on the left should be less than or
equal to 70 volts so the annex is saying that if that formula is true and the max is less than or
equal to 70 volts when a PEN fall occurs then the installation will be safe importantly however it's
also saying that the opposite is the case and if the maths on the left is more than 70 volts
then the installation will not be safe now obviously that could be a little bit tricky
to calculate so i've valid all of that math into a graphing program and we can see what happens as
the currents change so here we've got the neutral current formula that inputs into the less than
or equal to formula here and down here we've got sliders to adjust the currents in the three phases
i've set a line here just to show when things get dangerous and a red line here indicates
what the voltage will be between the body of the car and the literal Earth as the currents change
so let's just bear in mind that an unbalanced three-phase system in itself is not great
but it's not dangerous what we're showing here is what happens when the system isn't balanced
and the PEN faut occurs so first of all we've got all three phases perfectly the same the system is
balanced and you can see the red line is down at zero and the system is safe should append
fault occur and you can see it doesn't matter what those values are as long as they're the same we've
got zero volts between the body of the car and the literal Earth and that makes sense because if all
the currents are the same in each phase then the neutral current is zero and when you multiply and
divide zero the answer is always zero however look what happens when we start to unbalance the system
let's load up the current in L1 a bit because that's what tends to happen in the real world
can you see that the voltage between the body of the car and the literal earth is now going up
until if the difference in the phases gets big enough it goes over that threshold value of 70
volts and suddenly the person touching the car and standing on the ground has 70 volts across them
and is in serious danger and actually this danger is magnified at lower current if we set them all
low down at say 5 amps it only takes a very small amount of difference for the voltage to go over
70 volts so as you can see it's actually quite tricky to keep that voltage down below 70
volts unless the phases in an installation are reasonably balanced and so unless you know the
currents in a three-phase installation can be kept at a level where they won't tip the voltage over
that value in the event of a PEN fault then you can't apply indent one of regulation 722.411.4.1
it may be possible in some installations where you know you'll have balanced three-phase loads
in place all the time but it's very difficult to guarantee and that i believe is the reason that
if we look at that same regulation in the second Amendment to the 18th Edition you'll notice that
this option in indent one has been deleted which means that from the 27th of September 2022 onwards
you'll no longer be able to rely on the charge point being connected to a three-phase system as
protection enough and you'll need to carry out one of the options from indents two to five to provide
PEN fault protection so there we go that's why i believe that indent has been removed and also
why you will need to provide some kind of PEN fault protection to an EV charge point installed
on a three-phase system with a PME terminal but as always we want to hear from you were you aware
that load balancing was critical in keeping to the 70 volt limit in a three-phase system
do you think it was the right decision to remove that option whatever your thoughts and questions
please leave them in the comments section below and as always thank you very much for watching