IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Earth loop impedance tables for swa.
Topic Summary:
Created On: 23 April 2014 10:12 PM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 23 April 2014 10:12 PM
User is offline View Users Profile Print this message



hertzal123

Posts: 330
Joined: 26 August 2007

In my copy of Trevor Marks book"Handbook on the regulations",a table for swa cable gives design value eli in ohms/1000m.At the bottom of the page it states"impedance is based on final temp the armour can be expected to reach with fault current flowing"
For 16mm 4 core the figure is 4.4ohms/1000m,so for 100m run,would I simply write 4.4x100/1000,or must other multipliers be used for ambient temperature etc.
Thanx for any advice,
Hz
 24 April 2014 09:12 AM
User is offline View Users Profile Print this message



lyledunn

Posts: 651
Joined: 13 August 2003

Hz,
Is this a slightly dated book? No problem using the figures as they are probably assessed on 160 degrees C and thus would be a wee bit more on the onerous side.

-------------------------
Regards,

Lyle Dunn
 24 April 2014 11:36 AM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

What cable and what protctive device is it based on - there is a significant difference between 70C PVC and 90C XLPE insulated SWA cables.

Using any sensible approach to conductor temperature and armour temperature, for 16mm2 4core I can't actually determine what the 4.4 ohms/km value is derived from,

I doubt the armour has been corrected to 160C as the volumetric heat capacity of steel is significantly different to that of copper.

Typically, you would correct the line conductor data given at 20C for either the actual operating temperature, the limiting permitted operating temperature or the final limiting temperature.

However, you would usually correct the armour data given at 20C for either 60C for PVC insulated or 80C for XLPE insulated and not assume that the 160C (for PVC) conductor limit also raises the armour to that temperature (the fault isn't usually present for long enough).

For reference, 20C data for XLPE insulated 16mm2 4c armoured, for R1 and R2 would be 4.35ohms/km - which suggests that some correction has been undertaken to get to the 4.4 ohms/km you quote, but I can't determine what that is.


To answer the question about ambient temperature, then usually you would have used a factor, Ca, applied to Ib or In as required for cable sizing prior to determining EFLI - if however you think the effect of ambient temp is significant then you can calculate the impedance for that ambient temperature and use the condctor actual operating temperature instead of the normal permitted maximum operating temperature - but not usually done when the protective conductor is the sheath or armour of a cable as it will be conductor temperature rather than ambient temperature that sets the parameters of the armour - ie it will tend towards conductor temp regardless of ambient temp.

Regards

OMS

-------------------------
Failure is always an option
 24 April 2014 01:52 PM
User is offline View Users Profile Print this message



Thripster

Posts: 653
Joined: 22 August 2006

I have a copy of the 6th Edition and the nearest I can find to the OP's data is from page 470 and Table ELI 1 - which shows a maximum resistance of armour of 4.37 ohms/km (EFLI = 6.21 ohms/km), which is for XPLE cooper conductor armoured cable, 4 core. At the bottom of the table, alongside what the OP has stated ("impedance is based on final temp the armour can be expected to reach with fault current flowing") is the comment " Earth fault loop impedance includes the impedance of one phase conductor and the armour impedance"

Another table on page 463 and Table R XLPE 1, entitled 'Resistance, reactance and impedance of XLPE insulated copper conductors for single core cable or two, three and four cored cables. For 16mm² the following figures apply (per 1000m):-

Temp 20ºC
R=1.15
Z=1.15

Temp 90ºC
R=1.472
Z=1.472

Temp 140ºC
R=1.702
Z=1.702

Temp 170ºC
R=1.840
Z=1.840

(Have left Z in as a check and knowing that R + X = Z)


Notes to the bottom of this table. " The temperature 140ºC=(30ºC+250ºC)/2 is used when the conductor is used as CPC, or earthing conductor, and the ambient temperature is 30ºC and the conductor is not installed with other live conductors. The temperature 170ºC = (90ºC+250ºC)/2 is used when the conductor is carrying its full rated current, or the conductors temperature is 90ºC."


Hopefully, this will help us derive the figures used? I trust I haven't raised any copyright issues. Sadly, I think Trevor Marks may have passed away as in the front pages are dates saying 1931-1998. The 6th Edition was published in 2002. It is a shame to think that there might not be updates to reflect the latest amendments.

Regards

Edited: 24 April 2014 at 02:01 PM by Thripster
 24 April 2014 02:03 PM
User is offline View Users Profile Print this message



Thripster

Posts: 653
Joined: 22 August 2006

Therefore, it looks like he has used a 170ºC figure for single core resistance, plus 4.37 ohms armour to get the total 6.21 ohms EFLI. But we do not know what temperature assumptions were made for the armour.

Regards
 24 April 2014 02:24 PM
User is offline View Users Profile Print this message



Thripster

Posts: 653
Joined: 22 August 2006

Oh.......perhaps, as Lyle says, he has a different edition and therefore different adjustments have been made to reflect different regulations/dates........and therefore my post may be complete garbage......................forgive me if so. (GICO)


Regards
 24 April 2014 02:51 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

OK - I read the OP as 4.4 ohms was R1 and R2 rather than just R2

as I said, it would be usual to only correct the armour resistance to 80C for XLPE cable

So (in ohms/km)

Armour resistance at 20C = 3.2

Correction for 80C = 1.27

Armour resistance at 80C = 3.2 x 1.27 = 4.064

Adding for one core of 16mm2 copper which has resistance at 20C of 1.15 and correcting to the average of limiting and permitted temps using a factor of 1.6 = 1.84

Adding 4.064 + 1.84 = 5.9 Ohms

So I'm still confused as to where the data is originating from and what corrections are being employed t which part of R1 and R2

Regards

OMS

-------------------------
Failure is always an option
 24 April 2014 09:20 PM
User is offline View Users Profile Print this message



hertzal123

Posts: 330
Joined: 26 August 2007

My copy of the handbook states revised 3rd edition,published 1992,and at the top of the table states"Eli for pvc and xlpe copper swa to bs6346,5476 and 6742.
The figure for 4c 16mm pvc is 4.3/1000m and for xlpe is 6.21 ohms/1000m.Does this clarify anything?Thanks for the replies,
Hz.
 24 April 2014 10:16 PM
User is offline View Users Profile Print this message



Thripster

Posts: 653
Joined: 22 August 2006

Thank you Hertz - then I was looking at XLPE instead of PVC to BS6346 as 4.37 ohms for the armour resistance was the nearest I could find to your quoted figure of 4.4 ohms. There were no EFLi figures close to the seemingly precise 4.4 ohms and so I had wondered whether there had been a mistake.

The required figure for PVC is 2.71 ohms for armour resistance and 4.30 ohms for EFLi (both per 1000m) as per your edition. Corresponding figures for PVC conductors taken from page 461 Table RC 1 are:-

Temp 20ºC
R=1.15


Temp 70ºC
R=1.38


Temp 95ºC
R=1.495

Temp 115ºC
R=1.587

Notes to the bottom of this table. " The temperature 95ºC=(30ºC+160ºC)/2 is used when the conductor is used as CPC, or earthing conductor, and the ambient temperature is 30ºC and the conductor is not installed with other live conductors. The temperature 115ºC = (70ºC+160ºC)/2 is used when the conductor is carrying its full rated current, or the conductors temperature is 70ºC."

Hopefully, this will make more sense now.

Regards
 24 April 2014 10:36 PM
User is offline View Users Profile Print this message


Avatar for sparkingchip.
sparkingchip

Posts: 6335
Joined: 18 January 2003

Fancy doing a calculation for 21 kilometres of buried swa running at 66,000 volts? Feckenham to Evesham
 25 April 2014 09:44 AM
User is offline View Users Profile Print this message



Thripster

Posts: 653
Joined: 22 August 2006

Cannot get the link to work Sparking..........


Regards
 25 April 2014 10:03 AM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

OK - to be absolutely correct, you cannot lump 70C PVC and 90C XLPE armoured cables together. The different types of insulation and bedding change the overall cable diameter depending on what insulation is used - and the insulation itself has different limiting and operating temperatures

That change in diameter results in a change in the cross sectional area of the armouring

My copy of the handbook states revised 3rd edition,published 1992,and at the top of the table states"Eli for pvc and xlpe copper swa to bs6346,5476 and 6742


That's where the error is - but is probably safe in practice.

As an example, calculating 16mm2 XLPE 4C with armour at 80C and conductor at an initial 90C and ending at the average of final limiting and max operating temperatures (ie 90C and 250C - In practice we would look to limit the armour temp to say 200C however) would give R1 and R2 of 5.9 ohms/km

The book value is 6.2 ohms/km which has some correction applied - but how much and where.

Perhaps I should also say, that the data of the book suggests to me that the author is still looking at older regulations (but current then) that drew a distinction between protective devices listed in BS 7671 and protective devices not listed ie we used slightly more conservative values if the device wasn't listed in Appendix 3 (typically MCCB's and ACB's)

Regards

OMS

-------------------------
Failure is always an option
 25 April 2014 10:29 AM
User is offline View Users Profile Print this message



Thripster

Posts: 653
Joined: 22 August 2006

Just to be clear, Table ELI 1, page 470 of the 6th Edition, has values which agree with the third edition. Although there is one table heading, figures for PVC and XLPE are separated to give the following values for 4 core, 16mm².

PVC to BS 6346
Armour resistance/1km= 2.71 ohms
EFLI/km = 4.3 ohms (agrees with OP's revised posting)
Gross area of armour = 72 mm²

XLPE to BS 5467 & BS 6742
Armour resistance/1km= 4.37 ohms (this was the figure I originally quoted as was nearest to 4.4 from OP's original post)
EFLI/km = 6.21 ohms
Gross area of armour = 49 mm²

Regards
 25 April 2014 10:56 AM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

OK - with you now.

Based on that, the armour resistance is clearly being corrected for temperature - and I suspect to that of the line conductor as a conservative approach rather than to the usually adopted temperatures for armour of 60C and 80C respectively

Regards

OMS

-------------------------
Failure is always an option
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.