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
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