so in today's video we're going to look at calculating our total earth fault loop impedance
okay effectively our Zs now we know the formula for that is Zs equals Ze plus R1 + R2 okay now
usually what we would do is we'd measure R1 and R2 add it to our Ze which would give us our Zs now
we're going to look at it slightly different now from a designer's point of view when we measure
R1 and R2 let's say in an electrical workshop the ambient temperature probably is around 20
degrees okay so quite a low ambient temperature now when we're actually in an installation there's
going to be a lot of factors that will change that temperature of the conductors in the cable
okay so what we're able to do as designers is we can apply a factor okay from the On-Site Guide
which we can calculate our R1 and R2 at its maximum operating temperature of the cable so
for thermoplastic will be 70 degrees thermosetting will be 90 degrees and what that actually allows
us to do is when we're looking at the tables in BS7671 with our maximum Zs values okay we use
to finding the 80 value but when we apply this temperature factor okay to allow for the fact that
it could be operating at a higher temperature we can actually use the hundred percent values that
we have in BS7671 so we're going to go on and look at the calculation where we can find these factors
and how we produce that calculation to allow us to prove that as a designer okay we're going to
look at how we can calculate Zs remember Zs is the earth fault loop impedance for the entire system
okay and part of the design process is normally we would maybe measure that wouldn't we but part of
the design process is we need to confirm that our circuits and the cables we've selected okay still
comply with the Zs values in BS7671 now i know normally we would take 80% of them values to make
sure they comply however for the design purposes we use the values in BS7671 not the On-Site Guide
okay so we need to be able to confirm that once the system is installed it's going to comply
with the requirements of BS7671 so that ADS is in place etc so we're just going to look at how we
can calculate Zs if you remember Zs so our earth fault loop impedance system equals Ze external our
fault loop impedance plus R1 plus R2 it might be for design assignments etc you know the Ze because
it might be that you've obtained that from the DNO so you might be as a designer you know that
there's going to be a TN-C-S earth arrangement in there so they give you a Ze of 0.35
being the maximum they might be able to give you a different value to that normally give you the
maximum that would mean if we have the Ze we need to know how to calculate our R1 and R2
of our circuits okay so we're going to look at how we can calculate that and that is by this
equation here okay so R1 and R2 okay remember R1 being in relation to the line conductor
R2 being in relation to the CPC the circuit protective conductor equals million per meter
and we're going to look where we can get them from in the On-Site Guide multiplied by the length
multiplied by the temperature factor okay and we'll look at different ambient temperatures that
we need to apply certain factors to to get our R1 and R2 and then obviously because we have milliohms
here we need to divide it by a thousand so what we're going to do is we're going to move forward
and we're going to look at exactly how we can calculate our R1 and R2 so that we can incorporate
into this equation here so we're going to use this equation to get this and then once we have this
and we have this here so once we have the R1 R2 and the Ze then we'll be able to use that to
calculate our Zs okay so look we're going to look into this equation here okay so R1 plus R2 line
plus CPC the resistance of them equals milliohms per meter times length times multiplied by the
temperature factor and then we divide that by a thousand and now we're going to look at how we can
do this and we're going to use the On-Site Guide to this so you're going to need the On-Site Guide
now which has been given some simple stages here the circuit is wired in 2.5 millimeter squared
PVC PVC twin and CPC cable and it is 25 meters in length so we're going to calculate the R1
R2 based on this information we've been given for this circuit in particular now first of all we're
going to concentrate on the milliohm per meter so this is the resistance the amount of resistance
per meter in a copper conductor and we're going to use the On-Site Guide for this if we come to
our On-Site Guide and we're in appendix i okay table i1 which is on a page 196 as you can see
now as you can see this gives us the values of resistance per meter for copper and aluminium
conductors at 20 degrees Celsius let's go back to our our scenario so we have 2.5 millimeter squared
PVC twin and CPC cable so when i come to this table okay an appendix i I obviously have a twin
CPC cable therefore in that cable i have line neutral and CPC okay so we both have the line
conductor and the protective conductor so we know that 2.5 millimeter square twin CPC cable has a
1.5 CPC so what we're going to do is we're going to find the line conductor first we know it's 2.5
and they're here then we're going to match that to the corresponding protective conductor
where we know we have a 1.5 millimeter squared CPC so as you can see here i've got 2.5
and 1.5 that gives me a milliohm per meter value of 19.51 enter that into here so we know we have 19.51
now the length has the length being given to us yes as is 25 meters okay
19.51 multiplied by 25 then we have to look at the temperature factor in and we're going to do this
look at the factors in the On-Site Guide now if we go back to appendix i and we go over to 197
we'll see here that we've got ambient temperature multipliers okay for table i1 so for verification
purposes the designer will need to give the values of the line and circuit conductor resistances at
the ambient temperature expected during the test now what i want to point you toward here table i3
gives the multipliers to be applied to the values given in table i1 the purpose of calculating the
resistance at maximum operating temperature of the line conductors and or circuit effective
conductors in order to determine compliance with as applicable the earth fault loop impedance of
table 41.2 table 41.3 table 41.4 BS7671 now you know we spoke about the 80% values before in BS7671
we said we would have to find the 80% values that's because then values in BS7671 are taken
at 100% maximum operating temperature therefore when we're calculating our Zs or our R1 R2 for
design purposes we need to do in accordance with them tables therefore we're gonna have to
apply the factors from table i3 okay which is on page 198 so i have a twin and CPC cable
is that going to be not incorporated in a cable or not punched or is it going to be incorporated in
a cable or bunch okay so if you think about it the conductors they are incorporated in
a cable aren't they okay now we know that twin and CPC cable is 70 degrees C okay or we're
we're going to assume that it's thermoplastic which means we need to apply temperature factor
of 1.20 because it is thermoplastic PVC and it is incorporated in a cable so our conductors
are incorporated in the cable together so then we're going to add in our temperature factor
1.20 and then we're gonna calculate that equation that puts into our calculator so we have 19.51
multiply some of that's the milliohm per meter that's the value of resistance
given to us from the On-Site Guide for a copper conductor we have 2.5
line conductor and a 1.5 protective conductor we're going to multiply that by the length 25
and then we're going to multiply that by a temperature factor remember because we
in the design process and we're using the On-Site Guide in accordance with BS7671 the values that
are stated in BS7671 a hundred percent values therefore we use table i3 okay multiply by 1.20
gives us 585.3 and then remember because this is milliohms we must then divide it
by a thousand get back to our base value but gives us a value of 0.58 okay and if we want
to round it up we could go 0.59 so in fact if we come back to here and we do this calculation
0.59 ohms oh if we go back to here now we have our R1 and R2 okay let's just say
for argument's sake that my Ze is given to me as 0.76 okay it's TN-S earth arrangement 0.76
so what i can actually do is now do this calculation so i can do
take my Ze which was given to me 0.76 plus my R1 and R2 okay which was 0.59 and that will
actually give me the Zs for my circuit so what i can do okay is go to my calculator
0.76
add 0.59
give me a total Zs for that circuit of 1.35 ohms
one
we've calculated our Zs okay and it come out at 1.35 ohms let's say the circuit we had
designed already okay we had chose it was a an A3 radial circuit so it was a 20 amp
circuit breaker BSEN60898 we would then come to BS7671 okay and we'd come to table 41.3 and as
you can see this this has our maximum fault loop impedance and it's on page 62. we obviously have
other tables which are for different overcurrent protection devices so we've got fuses to BS88-2
etc okay so we've got a type B 20 amp 60898 circuit breaker we've come to this table here
and as you can see here we have type B circuit breakers to BSEN60898 and the over current
characteristics of our RCBO's is to be a BSEN61009 so we'd come here we'd pick the rating that we
chosen for our over current protection device which we would have done in previous calculations and we
said it was a 20 amp okay and then we looked at our maximum permittable Zs value okay to achieve
disconnection connection times 2.19 and as you can see we've got our calculated Zs is 1.35 ohms
and our maximum permits will be in 2.19 therefore is an acceptable value we
we can say that in the event of a fault okay under ADS conditions our overcurrent protection device
complies with the relevant circuit details of the maximum permittable fault loop imprdance