An Awful Feeling This

[La Poste]

Table A.3 Page 105 Guidance note 3:

"Table A.3 gives the multipliers to be applied to the value given in table A.1 for the purpose of calculating the resistance at the maximum operating temperature of the line conductors and /or CPC in order to determine compliance with the earth fault loop impedance of tables 41.2,41.3,41.4 of BS 7671."

The multipliers being 1.2 for 70 C thermoplastic insulation.

So this says to me that if you use the 1.2 multiplier then there is no need to also use the 0.8 multiplier.

You either use one or the other not both.

If you use 0.8 you do not use 1.2
If you use 1.2 there is no need to use 0.8

Any thoughts people?
I have to get this right myself also.

 

[malcolmsanford]

Your correct yes there is no need to use both methods. The 1.2 factor is normally a design criteria used for designing your circuit and so if you used the 1.2 you would not need to use the rule of thumb.

I would use the rule of thumb on an actual measured Zs test. In the 2391 often questions will be one or the other. The question that the OP posted would be more of a design question and the C & G would expect to see you use the factor.

Another question would be, especially in section B, a list of maximum Zs values taken from the tables in BS 7671-2008 and then a list of measured Zs, they would then ask if these results are acceptable and show your calculations, and that is when you use your 0.8 rule of thumb.

 

[La Poste]

One question.
When a 2391 question asks for me to calculate the ZS of a circuit and gives me the Ze and the resistivity of copper plus the circuit lengths much the same as the previous question we are talking about here would it be correct to use the 1.2 multiplier.
Do City and guilds want the 1.2 multiplier used in a general Zs calculations or do they just want the Zs measured at 20 C?

 

[malcolmsanford]

It's been a long time since I did my 2391-001 so I may be wrong, and I'm sure Widdler will correct me if I'am wrong but if they have given you Ze and the resistivity of the cable and most likely the 20c temperature then yes I would calculate it with the temperature factor of 1.2.

 

[Sparky Ninja]

It would depend on if the question is asking for it measured or at operating temperature.If you work out both and label them then you can't go wrong.

 

[gap30]

I'll make one up..

Your designing a circuit 28m in length.
Ambient temperature is 25 degrees
You are using A 10mm Line conductor and a 1.5mm cpc.
Cables will be installed in surface fixed pvc conduit.
Ze at the origin is 0.14 ohms.

Calculate Zs?

sorry been out all day with the wife and kids will answer in an hour or so when i get home

typing this on phone

Zs = Ze + m/ohm/m x mf x length/1000
not sure what the resistance of 10mm of top of my head so lets say its 4.6m/ohm/m and i think 1.5 is 12.10 (not at home at the moment)
4.6 + 12.10 = 16.70
16.70 x 1.2 x 28/1000 = 0.56
Zs = Ze + (r1 + r2)
Zs = 0.14 + 0.56 = 0.70

Zs = 0.70 ohms

please tell me this is correct

 

[brucelee]

One question.
When a 2391 question asks for me to calculate the ZS of a circuit and gives me the Ze and the resistivity of copper plus the circuit lengths much the same as the previous question we are talking about here would it be correct to use the 1.2 multiplier.
Do City and guilds want the 1.2 multiplier used in a general Zs calculations or do they just want the Zs measured at 20 C?

Hi mate the 1.2 is for cables bunched or part of a cable that contains multiple conductors for pvc thermoplastic if it was thermosetting then 1.28
the 0.8 is used to apply to the 100% figures in bs7671 tables 41.1 etc and takes into account when measuring increase in resistance due to increase in operating temp due to operating conditions they are not the exact words but near enough all in gn3 AND YOU COMPARE YOUR MEASURED VALUES TO THE 80% TABLES IN gn3 OR osg

 

[Sparky Ninja]

sorry been out all day with the wife and kids will answer in an hour or so when i get home

typing this on phone

Zs = Ze + m/ohm/m x mf x length/1000
not sure what the resistance of 10mm of top of my head so lets say its 4.6m/ohm/m and i think 1.5 is 12.10 (not at home at the moment)
4.6 + 12.10 = 16.70
16.70 x 1.2 x 28/1000 = 0.56
Zs = Ze + (r1 + r2)
Zs = 0.14 + 0.56 = 0.70

Zs = 0.70 ohms

please tell me this is correct

Where did the 4.6 ohms resistivity value come from?


Value of resistivity for 1.5mm = 12.10 milliohms per metre
Value of resistivity for 10mm = 1.83 milliohms per metre

Therefore (R1&R2) resistivity in milliohms per metre = 12.10+1.83 = 13.93 milliohms per metre
Length = 28m
Factors = 1.02 & 1.20 (from tables 9B and 9C OSG)

Therefore, R1&R2 = (milliohms per metre x Length x Factor) / 1000

= (13.93 x 28 x 1.02 x 1.20) / 1000 = 0.48 ohms.

Zs = Ze + (R1&R2)

= 0.14 + 0.48 = 0.62 ohms

 

[La Poste]

Hi mate the 1.2 is for cables bunched or part of a cable that contains multiple conductors for pvc thermoplastic if it was thermosetting then 1.28
the 0.8 is used to apply to the 100% figures in bs7671 tables 41.1 etc and takes into account when measuring increase in resistance due to increase in operating temp due to operating conditions they are not the exact words but near enough all in gn3 AND YOU COMPARE YOUR MEASURED VALUES TO THE 80% TABLES IN gn3 OR osg

The 1.2 is used to bring the measured values of R1+R2 at 20C up to the figures in BS 7671 so they are directly comparable (Zs=Ze+r1+r2)@70C
The 0.8 is used to bring the figures in BS 7671 down to the measured results of R1 + R2 at 20C so they are directly comparable.(Zs=Ze+R1+R2)@20C
You would use one or the other when comparing measured results at 20C to BS 7671 results measured at 70C.
I can not think of any scenario where you would use both the 0.8 and the 1.2 in the same calculation.
Please correct me if I am wrong.

 

[gap30]

Where did the 4.6 ohms resistivity value come from?


Value of resistivity for 1.5mm = 12.10 milliohms per metre
Value of resistivity for 10mm = 1.83 milliohms per metre

Therefore (R1&R2) resistivity in milliohms per metre = 12.10+1.83 = 13.93 milliohms per metre
Length = 28m
Factors = 1.02 & 1.20 (from tables 9B and 9C OSG)

Therefore, R1&R2 = (milliohms per metre x Length x Factor) / 1000

= (13.93 x 28 x 1.02 x 1.20) / 1000 = 0.48 ohms.

Zs = Ze + (R1&R2)

= 0.14 + 0.48 = 0.62 ohms

as i said earlier i guessed the restivity for the 10mm

as a side note i included the 1.2 multiplier but why did you include the 1.02 multiplier?

i am still out and have not got my books in front of me

 

[Sparky Ninja]

1.02 is a multiplier due to the ambient temperature.

 

[La Poste]

as a side note i included the 1.2 multiplier but why did you include the 1.02 multiplier?

The 1.02 multiplier is to correct the readings from 25C down to 20C.
Once the readings are at 20C we can use the rule of thumb multiplier of 1.2.
Actually I am thinking it is 1/1.02

 

[gap30]

1.02 is a multiplier due to the ambient temperature.

oh so i have to get it down to 20C then add the 1.2 multiplier?

 

[gap30]

Listen guys i really appreciate all the help i am getting at the moment and bruce lee has sent me some really good revision material, thanks for your help.

Am i ok to keep picking your brains.

 

[La Poste]

If the resistance at 25C is higher than at 20C because of the nature of copper and temperature change IE as the temperature increases so does the resistance then why would we multiply a value that we recorded at 25C by a factor of 1.02 to correct the reading to 20C.
The reading at 20C should be lower than the reading at 25C not higher.

 

[Sparky Ninja]

Listen guys i really appreciate all the help i am getting at the moment and bruce lee has sent me some really good revision material, thanks for your help.

Am i ok to keep picking your brains.

It's a saturday night and we are here on our own accord. Clearly there is something wrong with us but you would be a fool not to take advantage of it.

:wink5:

 

[telectrix]

Listen guys i really appreciate all the help i am getting at the moment and bruce lee has sent me some really good revision material, thanks for your help.

Am i ok to keep picking your brains.

you are welcome to pick my brain at any time, but there's no guarantee that you will find anything in there!:biggrinjester:

 

[brucelee]

the tables at 20C are for ambient temp when testing in order to know how a T/E resistance would be increased then the multiplier is used as a correction factor to 70 C for cables bunched or a core in a cable thermosetting multiplier is 1.28


If it was a single conductor not bunched then a 70 c thermoplastic is 1.04 and thermosetting 90c conductor is also 1.04 doesnt change for a single conductor not core of a cable or bunched
this is in GN3 pg105 table A3 read notes 1-4

table A2pg are correction factors for ambient temp to correct from 20c at time of test the designer will give values taking into account at 20C you can apply correction factor if different ambient temp FOR test puposes only
but not recorded you only record on the schedule of test results what you actually measured on the tester at time of test

table B5 pg 112 gn3 are ambient temp correction factors if the ambient temperature is not within the range of 10c to 20c and should be applied to the earth fault loop impedences of tables B1 to B4

 

[La Poste]

Looks like some serious reading is in order this Sunday.
It's like a finger pointing at the moon.

 

[gap30]

when showing drawings of the earth fault path for tncs, tns and tt is tt the only drawing where i should show the mass of earth?

 

[gap30]

A BS EN 60898 circuit breaker has an lcn of 10 kA and an Ics rating of 6 kA.
State
a) how the circuit breaker is likely to perform if the circuit developed a fault current of 8kA (1 mark)
b) the likely operational condition of the circuit breaker following the fault (1 mark)
c) the action that would need to be taken to ensure the circuit was protected should a similar fault occur in the future. (1 mark)

This is one topic i have not covered in depth, correct me if i am wrong here

Uo/Zs = Ia

 

[La Poste]

This is one topic i have not covered in depth, correct me if i am wrong here
Uo/Zs = Ia

Correct.

 

[IQ Electrical]

A BS EN 60898 circuit breaker has an lcn of 10 kA and an Ics rating of 6 kA.
State
a) how the circuit breaker is likely to perform if the circuit developed a fault current of 8kA (1 mark)
b) the likely operational condition of the circuit breaker following the fault (1 mark)
c) the action that would need to be taken to ensure the circuit was protected should a similar fault occur in the future. (1 mark)

This is one topic i have not covered in depth, correct me if i am wrong here

Uo/Zs = Ia

Ics is the max fault value that a device can clear and be put back into service again.

Icn is the max fault value that a device can clear but the device will not function again.

A) The device will interrupt the fault.

B) The device will not be operational.

C) The protective device should be replaced.

 

[gap30]

Ics is the max fault value that a device can clear and be put back into service again.

Icn is the max fault value that a device can clear but the device will not function again.

A) The device will interrupt the fault.

B) The device will not be operational.

C) The protective device should be replaced.

i have just paid for a copy of the regs online so i will have a bit of reference material, so the circuit breaker would be faulty, thanks

i have been looking in gn3 for information on 100mA rcds, i know they are tested at times half and at times 1 but i cant find any info on tripping times, if i remember correctly its 200 ms for the bs one and is it 300 ms for 61009

 

[tony mc]

Just remember BS EN RCD have 300ms trip time

BS numbered RCD is 200 ms..

Remember the EN as extra time to trip.

You only do the 5x test on 30mA RCD

 

[gap30]

Just remember BS EN RCD have 300ms trip time

BS numbered RCD is 200 ms..

Remember the EN as extra time to trip.

You only do the 5x test on 30mA RCD

thanks

Determine the maximum permitted value of voltage drop for each of the following circuits.

a) A 230V lighting circuit supplied directly from a public low-voltage distribution system. (1 mark)
b) A 400V three-phase motor circuit supplied from a factory transformer. (1 mark)
c) A 230V immersion heater circuit in a domestic installation.

a) 3% 6.9v
b)
c) 5% 11.5v

i dont know the answer to b, i am assuming its 5%, but why the difference between a and c? i would have thought all 230v circuits would be 3%?

there is not a lot of info on vd in gn3, i am going to have a look through amberleafs sticky

 

[La Poste]

Value of resistivity for 1.5mm = 12.10 milliohms per metre
Value of resistivity for 10mm = 1.83 milliohms per metre

Therefore (R1&R2) resistivity in milliohms per metre = 12.10+1.83 = 13.93 milliohms per metre
Length = 28m
Factors = 1.02 & 1.20 (from tables 9B and 9C OSG)

Therefore, R1&R2 = (milliohms per metre x Length x Factor) / 1000

= (13.93 x 28 x 1.02 x 1.20) / 1000 = 0.48 ohms.

Zs = Ze + (R1&R2)

= 0.14 + 0.48 = 0.62 ohms

I have been thinking about this.

Value of resistivity for 1.5mm = 12.10 milliohms per metre
Value of resistivity for 10mm = 1.83 milliohms per metre

Therefore (R1&R2) resistivity in milliohms per metre = 12.10+1.83 = 13.93 milliohms per metre
Length = 28m

I agree, these figures are taken from table A.1 and are the value at 20C
Therefore at 20C our circuit resistance would be
13.93*28/1000 = 0.39 Ohms.

We have designed the circuit at 25C so if we were to measure the R1+R2 value at 25C we would need to add a correction factor because at 25C the resistance would be slightly higher, this correction factor is 1.02.
So the realistic resistance figure at 25C for our circuit would be:
.39 Ohms * 1.02 = 0.398 Ohms

This is what the correction factors for ambient temperature are for, they are to be applied to the tables EG table A.1 to change the values in those tables measured at 20C to whatever temperature the circuit is being measured at in reality.

Now The rule of thumb (the .8 rule) is used to multiply the tabulated figures in BS7671 which are measured at 70C
The 0.8 is used to bring the figures measured at 70C down to a figure that they would be if the circuit were measured at 20C.
Conversely if we want to convert our figure of R1+R2 measured at 20C to a figure measured at 70C we would multiply it by 1.2 this figure would then become our R1 + R2 measured at 70C and we would add Ze to this. We would then be able to compare this figure directly to the tables in BS 7671.

So in this instance we need the figure for the circuit we are designing, the R1 + R2
measured at 20C.
We calculate what this figure is by using the above formula EG:
13.93*28/1000 = 0.39 Ohms.
This is the figure at 20C.
We then multiply the figure at 20C by a factor of 1.2.
0.39 * 1.2 = 0.468 Ohms.
Add this to Ze
Zs = .14 + .468 = 0.608 Ohms.
This is the figure we would use to compare to figures in BS 7671.
We would not use the ambient temperature figure in this calculation.
The only time we would use the ambient temperature correction factor of 1.02 was if we were measuring our R1+R2 at 25C and we needed to compare these measured figures with those in the book which were recorded at 20C
We would take the figures in the book measured at 20C multiply them by a factor of 1.02 and this would give the value of the table figures at 25C.
We would then be able to compare our measured reading at 25C directly to these tabulated figures.
We would not use both the factors of 1.02 and 1.2 together in the same calculation.

This is how it appears to me, shoot me down if I am wrong.

One more thing, I noticed a nice little fact today.
The resistance of copper will increase by 2% for every 5C increase in temperature.
So if in doubt use this way to measure what the resistances would be at different temperatures.
If a cable increases in temperature from 20C to 70C its resistance will increase by 20%

 

[La Poste]

thanks

Determine the maximum permitted value of voltage drop for each of the following circuits.

a) A 230V lighting circuit supplied directly from a public low-voltage distribution system. (1 mark)
b) A 400V three-phase motor circuit supplied from a factory transformer. (1 mark)
c) A 230V immersion heater circuit in a domestic installation.

a) 3% 6.9v
b)
c) 5% 11.5v

i dont know the answer to b, i am assuming its 5%, but why the difference between a and c? i would have thought all 230v circuits would be 3%?

there is not a lot of info on vd in gn3, i am going to have a look through amberleafs sticky

B is also 5% I think, it's not a lighting circuit so it is 5%

I do not know the reason for the difference in the 3% and the 5% I just know it's a fact set in stone so I don't argue with it.

 

[IQ Electrical]

thanks

Determine the maximum permitted value of voltage drop for each of the following circuits.

a) A 230V lighting circuit supplied directly from a public low-voltage distribution system. (1 mark)
b) A 400V three-phase motor circuit supplied from a factory transformer. (1 mark)
c) A 230V immersion heater circuit in a domestic installation.

a) 3% 6.9v
b)
c) 5% 11.5v

i dont know the answer to b, i am assuming its 5%, but why the difference between a and c? i would have thought all 230v circuits would be 3%?

there is not a lot of info on vd in gn3, i am going to have a look through amberleafs sticky

It's 8% from a private supply source.

The reason for the lower level for lighting is that some types of lighting cannot strike up on lower voltages so tolerances are tighter.

 

[brucelee]

Hi

for single phase 3% for lighting 6.9v
5% for power circuits and distribution 11.5v
and for the answer to b) its 20V which is 5% of 400v

no sorry i am wrong its not 20V as its there own supply not sure

 

[tony mc]

thanks

Determine the maximum permitted value of voltage drop for each of the following circuits.

a) A 230V lighting circuit supplied directly from a public low-voltage distribution system. (1 mark)
b) A 400V three-phase motor circuit supplied from a factory transformer. (1 mark)
c) A 230V immersion heater circuit in a domestic installation.

a) 3% 6.9v
b)
c) 5% 11.5v

i dont know the answer to b, i am assuming its 5%, but why the difference between a and c? i would have thought all 230v circuits would be 3%?

there is not a lot of info on vd in gn3, i am going to have a look through amberleafs sticky

Have a look at Appendix 12 in BS 7671 it covers VD only.

 

[La Poste]

Hi

for single phase 3% for lighting 6.9v
5% for power circuits and distribution 11.5v
and for the answer to b) its 20V which is 5% of 400v

no sorry i am wrong its not 20V as its there own supply not sure

I'm guessing it's 5% per phase so it's 11.5V at each phase where the equipment is installed, not measured between phases.
But I'm guessing.
Sorry it's 8% from a private supply source (This is a new one for me) so I am guessing it is 8% per phase = 18.4 V
But it's just a guess so hopefully I'll be put right by someone.

 

[tony mc]

Have a look at the question as it tells you the voltage is 400v three phase.

So its 8% of the 400v if the low voltage installation is supplied from a private low voltage supply.

Hope this helps.

 

[IQ Electrical]

Have a look at the question as it tells you the voltage is 400v three phase.

So its 8% of the 400v if the low voltage installation is supplied from a private low voltage supply.

Hope this helps.

Spot on, the percentage is actually of 400 Volts ie. 32 Volts

 

[gap30]

the loop length for circuit 1 is 50 m long the ze is is 0.11 ohms

determine showing all calculations the r1 + r2 and the expected zs

7.41 x 50/1000 = 0.37 12.10 x 50/1000 = 0.60

zs = ze + (r1 = r2)

0.11 + (0.37 + 0.60) = 1.08

have i done this correctly?

 

[IQ Electrical]

the loop length for circuit 1 is 50 m long the ze is is 0.11 ohms

determine showing all calculations the r1 + r2 and the expected zs

7.41 x 50/1000 = 0.37 12.10 x 50/1000 = 0.60

zs = ze = (r1 = r2)

0.11 + (0.37 + 0.60) = 1.08

have i done this correctly?

I can't confirm the mOhm values but the maths is correct.

You MUST use (R1+R2) when doing these calculations and remember that r1, rn and r2 are reserved for ring continuity figures, this exam is very big on correct terminology.

 

[IQ Electrical]

Hang on, is this a ring final circuit?

 

[gap30]

Hang on, is this a ring final circuit?

yes i just shortened the question the m/ohm/ for 2.5 is 7.41 and 1.5 is 12.10

i am not great at the terminology but i am getting there with the calculations

in some crazy way i am actually finding the long questions easier than the short ones atm

 

[IQ Electrical]

If it's a ring then the R1+R2 value is r1+r2/4

R1+R2=0.37+0.60/4

R1+R2=0.24 Ohms

Zs=(R1+R2)+Ze

Zs=0.35 Ohms

 

[gap30]

Have a look at the question as it tells you the voltage is 400v three phase.

So its 8% of the 400v if the low voltage installation is supplied from a private low voltage supply.

Hope this helps.

just ordered a set of regs from fleabay so they should be with me in the next couple of days - the only problem i found with the regs in the past was navigating them, i always favoured the on site guide for obvious reasons

 

[gap30]

If it's a ring then the R1+R2 value is r1+r2/4

R1+R2=0.37+0.60/4

R1+R2=0.24 Ohms

Zs=(R1+R2)+Ze

Zs=0.35 Ohms

no thats for your cross connections isnt it are you not thinking of step 2 of ring final circuit continuity? now i am really confused

your end to end resistances are your r1 and r2 values

 

[IQ Electrical]

If you are asked to calculate R1+R2 on a ring final circuit and you are given the 'loop length' then you need to divide the end to end continuity readings for r1 and r2 by 4 to arrive at a figure for R1+R2.

It's actually mentioned in step 3 of the continuity of ring final conductors test (if you were measuring R1+R2 then you would not need to divide by 4).

 

[gap30]

If you are asked to calculate R1+R2 on a ring final circuit and you are given the 'loop length' then you need to divide the end to end continuity readings for r1 and r2 by 4 to arrive at a figure for R1+R2.

what a d**k! of course, i keep getting caught on the little things so basically there was about 10 marks down the drain

LOOP LENGTH

(0.37+0.60)/4 = 0.24 + 0.11 = 0.35

 

[IQ Electrical]

what a d**k! of course, i keep getting caught on the little things so basically there was about 10 marks down the drain

LOOP LENGTH

Scan the question twice, find out EXACTLY what you've been given and EXACTLY what they're asking.

 

[gap30]

Scan the question twice, find out EXACTLY what you've been given and EXACTLY what they're asking.

these questions are all simple in theory, but a nightmare in principal lol

 

[IQ Electrical]

these questions are all simple in theory, but a nightmare in principal lol

It'll just click once you know the formats and double check the questions.

Also remember that on longer testing questions, if you fail to mention say deducting test lead resistances before a continuity test, even though you may go on to describe the rest of the process perfectly, your end result would be incorrect so answers are heavily penalised in that scenario.

 

[brucelee]

yes mate practice writing test procedures over and over then you will have it nailed

Iq is spot on as usual

you miss a important part of test procedure and will get no further marks for that question

and when doing r1,r2,rn the r2 will always be slghtly higher for T/E as the cpc is smaller so resistance will be higher approx 2.5/1.5mm = 1.67 times higher
so you R1 +R2 will be slightly higher than the R1 + RN value

and like he says read the question a couple of times until your sure you know what they are asking before answering

 

[gap30]

i have looked at that many questions in the last four days my head is fryed but thats the advice i was given - read gn3, then read it again, then read it again, and keep going over previous exam papers, i have just ordered on site guide as well

 

[brucelee]

also have a look at GS38 which is for test instruments eg finger guards 4mm probe tips preferably 2mm
also memorandum of guidance on elecy at work regs i have it in pdf if you want it as some of these come up
when you get the regs read part 6 and especially 7 zones for fountains bathrooms etc
100mA rcds sockets outlets in agricultural
300mA rcds protection aginst fire in agricultural
500mA rcds sockets on construction sites
a few questions come up out of part 7 special locations
locations containing baths or showers
rooms containing sauna heaters
swimming pools and other basins

learn IP ratings
protection against water
Ipx8 protection against continous submersion
ipx7 protection against temporary submersion eg min for zone 0 bath
ipx6 protection against powerful water jets
ipx5 protection against water jets
ipx4 protection against splashing water
ip3x protection against spraying water
ip2x protection against dripping water when tilted up to 15degrees
ipx1 protection against dfriping water

protection against objects

ip6x dust tight
ip5x dust protected
ip4x protection againstsolid objects greater than 1mm and protection against 1mm wires for horizontal top surface eg CCU tops of sockets, enclosures etc solidobjects
IPXXD is same but only protection against 1mm wires
ip3x against solidobjects greater than 2.5mm
ip2x protection against solid objects greater than 12mm eg your finger or a standard test fingr and small rocks stones etc
ip1x protection against 50mm solid objects eg hand

IpxxA protection against large part of body
ipxxb protection against contact by standard test finger only
ipxxc protection against tool eg screwdriver
ipxxd protection against contactby 1mm wire

 

[gap30]

tell you where i am struggling conductor and insulation resistance

what happens to conductor resistance if a cable length is increased etc etc

i keep getting them wrong time after time

oh and the schedules of circuit details the one you stick in the door on the db

list 5 items - ermmmmm?