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Topic Title: Earthing Conductor Installation / High Zs
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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 12:20 pm
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

It's all a bit bigger than I'm used to, but first thoughts...

At those sort of sizes (>16mm2) I would have thought you need to consider the reactance of the cables as well, not just their resistances (your per metre figures might include that, I'm not sure).

What happens if a fault occurs between a line conductor and the armour part-way along the cable? As the armour is effectively insulated from your additional conductor along the length, I suspect that Zs at the fault could be significantly higher than at the far end.

2.8 Ohms seems rather high - given a Ze of only 0.03 Ohms that would imply the existing cable has an impedance of around 2.77 Ohms. A quick google suggests that 120mm2 4-core SWA should have one core + armour impedance of about 0.8 or 0.9 milliOhms/metre - so 180m shouldn't be more than about 0.16 Ohms (even ignoring the benefit of the 185mm2). I do wonder if there's something else wrong - like an open c.p.c. connection somewhere and the earth loop is completing only via some random parallel path? If so, finding and fixing that problem might reduce or even eliminate the Zs problem you're seeing.

- Andy.
 21 January 2019 01:56 pm
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gkenyon

Posts: 5354
Joined: 06 May 2002

Originally posted by: AJJewsbury
2.8 Ohms seems rather high - given a Ze of only 0.03 Ohms that would imply the existing cable has an impedance of around 2.77 Ohms. A quick google suggests that 120mm2 4-core SWA should have one core + armour impedance of about 0.8 or 0.9 milliOhms/metre - so 180m shouldn't be more than about 0.16 Ohms (even ignoring the benefit of the 185mm2). I do wonder if there's something else wrong - like an open c.p.c. connection somewhere and the earth loop is completing only via some random parallel path? If so, finding and fixing that problem might reduce or even eliminate the Zs problem you're seeing.
Agreed - worth looking into this, BS 5467 Table F2, for example, is only 0.69 milliOhms / m for round conductor, and 0.76 milliOhms / m for shaped conductors.

By my reckoning, this still gives R2 of 0.137 Ohms, which is itself above the Max Zs for the MCCB stated in the OP, so reinforcement of the earthing is still required.

BS 7671 would require Adiabatic to be considered in addition to the points Andy raised.

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 21 January 2019 01:59 pm
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chrispearson

Posts: 1095
Joined: 15 February 2018

Presumably the Zs is the same on all three phases, so it must be the earth part of the loop which is below spec.

According to Prysmian's data, the resistance of the armour (at 20 deg C) is 0.04 Ohms for the 95 sqmm cable and 0.08 Ohms for the 185 sqmm, so it is difficult to see how a target of 0.121 Ohms could ever have been met.

Can you test Zs at the joint? That would at least narrow down the section of cable which needs to be fixed, or even the joint itself.
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