High Zs on SWA Armour / New Earthing Conductor

[Paul 138]

Hi all
I am after some help/Advice if possible please
A SWA sub main Distribution circuit has been given a Code 2 Rating on a recent EICR due to a high Zs of 2.8 ohms. The cable is 180 metres long and runs underground in ducting.
The Install is from a few years ago when a new Substation was put in before any of us worked here and there is a joint in the Cable run.
So the protection is provided by a Merlin Gerin MCCB (NS250 SX)with an Overload electronic trip unit(TM250 D) set at 200A for overload (Ir) and 1500A for Short time (Im).
At these settings Schneider recommend a Max Zs of 0.121 ohms to trip the unit within 5 Seconds.
The circuit begins at the Main Distribution Panel MCCB with a 120mm 4 core SWA .The 120mm runs 50m and is then jointed to a 185mm 4 core which then runs 130 metres to the Distribution point.(180 m total)
So looking to run in a separate earth conductor alongside the SWA to improve the ZS to below 0.121 ohms.
The calculation is to see if a 180m 95mm Earth conductor would be correct.

Zs = Ze+(R1+R2)
Ze = 0.03
R1 (120mm)= 0.153 m/ohm/m x 50 =7.65 milli ohms or 0.00765 ohms
+ (185mm) = 0.0991 m/ohm/mx130 = 12.883 milli ohms or 0.012883 ohms.=0.020533 ohms.
Total R1 = 0.020533 ohms.
R2 = (95mm?) = 0.193 m/ohm/m x 180 = 34 milli ohms 0r 0.03474 ohms.

Therefore 0.03(Ze)+0.020533+0.03474 (R1+R2) = 0.085273 (Zs)ohms


I would appreciate any comments if you feel this is incorrect or have any suggestions
Cheers

 

[Ian1981]

Hi all
I am after some help/Advice if possible please
A SWA sub main Distribution circuit has been given a Code 2 Rating on a recent EICR due to a high Zs of 2.8 ohms. The cable is 180 metres long and runs underground in ducting.
The Install is from a few years ago when a new Substation was put in before any of us worked here and there is a joint in the Cable run.
So the protection is provided by a Merlin Gerin MCCB (NS250 SX)with an Overload electronic trip unit(TM250 D) set at 200A for overload (Ir) and 1500A for Short time (Im).
At these settings Schneider recommend a Max Zs of 0.121 ohms to trip the unit within 5 Seconds.
The circuit begins at the Main Distribution Panel MCCB with a 120mm 4 core SWA .The 120mm runs 50m and is then jointed to a 185mm 4 core which then runs 130 metres to the Distribution point.(180 m total)
So looking to run in a separate earth conductor alongside the SWA to improve the ZS to below 0.121 ohms.
The calculation is to see if a 180m 95mm Earth conductor would be correct.

Zs = Ze+(R1+R2)
Ze = 0.03
R1 (120mm)= 0.153 m/ohm/m x 50 =7.65 milli ohms or 0.00765 ohms
+ (185mm) = 0.0991 m/ohm/mx130 = 12.883 milli ohms or 0.012883 ohms.=0.020533 ohms.
Total R1 = 0.020533 ohms.
R2 = (95mm?) = 0.193 m/ohm/m x 180 = 34 milli ohms 0r 0.03474 ohms.

Therefore 0.03(Ze)+0.020533+0.03474 (R1+R2) = 0.085273 (Zs)ohms


I would appreciate any comments if you feel this is incorrect or have any suggestions
Cheers

You would need to multiply your design R1+R2 calculation by 1.2 as to change the design temperature from 20 degrees to maximum operating temperature of 70 degrees.
If your maths is correct as I’ve not checked then your R1+R2 will be o.085 ohms x 1.2=0.10 ohms with the temperature calculated difference.
Added to your ze then zs will be 0.13ohms.
If your calculated R1+R2 is in fact 0.05 ohms then 0.05x1.2 + your ze= 0.09 ohms zs.
I’ve not checked your mv/a/m values tho.

 

[Pete999]

You would need to multiply your design R1+R2 calculation by 1.2 as to change the design temperature from 20 degrees to maximum operating temperature of 70 degrees.
If your maths is correct as I’ve not checked then your R1+R2 will be o.085 ohms x 1.2=0.10 ohms with the temperature calculated difference.
Added to your ze then zs will be 0.13ohms.
If your calculated R1+R2 is in fact 0.05 ohms then 0.05x1.2 + your ze= 0.09 ohms zs.
I’ve not checked your mv/a/m values tho.

Apart from Ian's Math answer, have you checked the connection of the SWA to the enclosures, as workmanship, (bad preparation of the enclosures) can be suspect, loose lock nuts, etc.

 

[Paul 138]

Hi Ian
Thanks for your reply .
Sorry i should have made it clear that R1+R2 = 0.020533 +0.03474 = 0.055273 ohms.
So would I then multiply this by 1.2 to equal 0.0663276 ?

Zs = 0.03 + 0.066326 = 0.0963276 ohms
Many thanks for your help
Paul

 

[Paul 138]

Hi Pete
Yes , there is no Earth ring or flying Earth lead onto the MET so this is going to be addressed also .A poor install from years ago .Still a high Zs of 2.38 ohms though

 

[buzzlightyear]

Yes , there is no Earth ring or flying Earth lead onto the MET so this is going to be addressed also .A poor install from years ago .Still a high Zs of 2.38 ohms though

now that should have been picked up.

 

[Ian1981]

Hi Ian
Thanks for your reply .
Sorry i should have made it clear that R1+R2 = 0.020533 +0.03474 = 0.055273 ohms.
So would I then multiply this by 1.2 to equal 0.0663276 ?

Zs = 0.03 + 0.066326 = 0.0963276 ohms
Many thanks for your help
Paul

Yes

 

[Toneyz]

I would start off with what Pete posted check the accessible connections especially corrosion and good mechanical glanding with earth fly leads etc not being in contact with paint of the enclosure.