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?