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Topic Title: Earthing Conductor Installation / High Zs
Topic Summary:
Created On: 21 January 2019 08:53 am
Status: Read Only
Linear : Threading : Single : Branch
 Earthing Conductor Installation / High Zs   - PWatts - 21 January 2019 08:53 am  
 Earthing Conductor Installation / High Zs   - AJJewsbury - 21 January 2019 12:20 pm  
 Earthing Conductor Installation / High Zs   - gkenyon - 21 January 2019 01:56 pm  
 Earthing Conductor Installation / High Zs   - chrispearson - 21 January 2019 01:59 pm  
 Earthing Conductor Installation / High Zs   - OMS - 21 January 2019 02:35 pm  
 Earthing Conductor Installation / High Zs   - PWatts - 21 January 2019 03:56 pm  
 Earthing Conductor Installation / High Zs   - Alcomax - 21 January 2019 04:21 pm  
 Earthing Conductor Installation / High Zs   - PWatts - 21 January 2019 04:24 pm  
 Earthing Conductor Installation / High Zs   - Alcomax - 21 January 2019 05:37 pm  
 Earthing Conductor Installation / High Zs   - chrispearson - 22 January 2019 09:48 am  
 Earthing Conductor Installation / High Zs   - AJJewsbury - 22 January 2019 11:01 am  
 Earthing Conductor Installation / High Zs   - mapj1 - 22 January 2019 11:06 am  
 Earthing Conductor Installation / High Zs   - John Peckham - 22 January 2019 11:06 am  
 Earthing Conductor Installation / High Zs   - PWatts - 22 January 2019 04:21 pm  
 Earthing Conductor Installation / High Zs   - mapj1 - 22 January 2019 04:27 pm  
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 21 January 2019 08:53 am
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PWatts

Posts: 5
Joined: 05 June 2015

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 I am 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 95mm Earth conductor 180m long would be correct.

Zs = Ze+ (R1+R2)
Ze = 0.03

R1 (120mm) = 0.153 m/ohm/m x 50 = 0.00765 ohms
+ (185mm) = 0.0991 m/ohm/mx130 = 0.012883 ohms =0.020533 ohms.
Total R1 = 0.020533 ohms.

R2 = (95mm?) = 0.193 m/ohm/m x 180 = 0.03474 ohms.

R1+R2 x 1.2 (design temperature from 20 degrees to maximum operating temperature of 70 degrees) = 0.020533 +0.03474 = 0.055273. x 1.2 = 0.0663276 ohms.

Therefore 0.03(Ze) +0.020533+0.0663276 (R1+R2) = 0.0963276 (Zs) ohms
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