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Topic Title: Earthing Conductor Installation / High Zs
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Created On: 21 January 2019 08:53 AM
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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
 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.
 21 January 2019 02:35 PM
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OMS

Posts: 22864
Joined: 23 March 2004

It would be worth looking at the potential to replace the standard thermal/magnetic trip unit with a micrologic electronic unit (they are usually interchangeable on the particular MCCB

There is certainly something adrift in the measured Zs (unless the Ze is particularly high) - and it should be perfectly reasonable to adjust a 250A MCCB with electronic tripping to have a much lower earth fault pick up threshold to accommodate a circa 180m run of 120mm2 SWA (you might need to pick a 200A unit to go in the 250A breaker frame

I suspect the original install would probably have never achieved 5s disconnection under earth fault condition

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 21 January 2019 03:56 PM
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PWatts

Posts: 5
Joined: 05 June 2015

Many thanks for all of your replies.
It may well be that this Installation would never have achieved a 5s disconnection time , it was installed way before I began working here and was only recently picked up on an EICR .Any past Certificates cannot be found unfortunately so we are where we are and we need to rectify this installation as soon as we can .
The Ze at 0.03 ohms is low as the Main switch room sits adjacent to the main Incoming Transformer.
We have asked a local Contractor to assist us and are awaiting his findings. The underground joint which is a 120mm to a 185 mm is suspect and maybe breaking down with the Earth link across the Armouring.
I have contacted Schneider Technical a few times and they have assured me that a Zs reading of between 0.121 and 0.146 is required (depending on the short time setting of either 5 or 6).Yes this is very low so we have thought about fitting a Micrologic electronic Trip Unit if we can get one to fit in the enclosure as the existing MCCB is a Merlin Gerin Compact NS250 SX..
As such a low Zs is required I don't know if replacing the joint would help solve this and it would be difficult to cut it out and remake it anyway.
There are no Earthing Rings/Banjos or Earth fly leads at either end either but I am not sure that by fitting these (which we will be) will bring the Zs down to 0.121 ohms.
Just heard from the contractor and he is proposing 200 m of 120mm Earthing conductor .
 21 January 2019 04:21 PM
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Alcomax

Posts: 450
Joined: 12 November 2009

Apologies if stating the obvious....but are we assuming the measured Zs , from an EICR, is credible?
Has another measurement, with a known reliable instrument, been taken?
 21 January 2019 04:24 PM
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PWatts

Posts: 5
Joined: 05 June 2015

Hi
Yes it has been measured a few times now using different instruments

Many thanks
 21 January 2019 05:37 PM
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Alcomax

Posts: 450
Joined: 12 November 2009

I don't know if replacing the joint would help solve this and it would be difficult to cut it out and remake it anyway.


So the position of the joint is known. Is the difficulty due to this being outside? I ask, as one solution that is being considered is installing an additional earthing conductor with the existing SWAs. This suggests any new conductor would also go through the ducts?

Taking a guess, was the 185mm the original cabling and the 120mm was added on later with the substation move?

There may be a compelling reason for proceeding with the "earth" fix and not actually attending to what has already been admitted as a likely "dodgy joint" , but is it likely that sooner or later someone is going to have to excavate the joint and repair?

Things could be easier for a solution if the position of the joint was under cover in a building or a small hut could be constructed above it. 120mm 4 core armour is fine as a cpc for 5 secs disconnection , BS6346/5467/6724.

185mm would also be fine IF it was the old bog standard BS6346 , but not the 5467/6724.

The above are rules of thumb / deem to comply. As others have said the adiabatic will determine. But I do not like the sound of the dodgy joint.

In an ideal world you would like to excavate, repair and reform in a new brick hut with an appropriate switch fuse disconnector.

But nothing is ideal in electrical installation land.
 22 January 2019 09:48 AM
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chrispearson

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Just a thought: would FI have been more appropriate than C2?
 22 January 2019 11:01 AM
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AJJewsbury

Posts: 17795
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Just a thought: would FI have been more appropriate than C2?

I suspect the logic is that the inspector has sufficient information to conclude that the Zs/protective device combination was inadequate without needing to pin-point the exact cause.

A careful inspector might have noticed that the Zs readings didn't tally with the length & size of cable - so suggesting a dodgy/deteriorating connection somewhere - which perhaps warrants an FI on its own (and would so do even if Zs happened to to be OK for the protective device).

So I might suggest an FI in addition to the C2, rather than instead of (in an ideal world at least perhaps).

- Andy.
 22 January 2019 11:06 AM
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mapj1

Posts: 12039
Joined: 22 July 2004

IF the resistance of the earth path is over 2 ohms, what is the resistance of the cores and the IR results looking like ? If water has got into a joint, then you could be sitting on a bigger problem to come, where at the most inconvenient moment possible it goes snap crackle and pop.. As others have said, the equivlalent of an earth fault relay to provide RCD-like protection will get you back within the regs, or as you propose an external CPC. However, neither is cheap, and both are something of a sticking plaster.
IF you wanted to identify the losses, then a low voltage supply and a few tens of amps will make the loss appear as a hot spot.
If you are dismantling to add banjos or pirahnas to the glands, then while you have it in bits, note that the most likely place for resistance in an undamaged cable is where the steel wires meet the brass glands, if they have not been properly combed out and seated flat before tightening, then only one or two strands are really pinched and connected to the gland, and the rest may look ok but are free to flap about and not make contact.
If you have access in the dry, and can 'vampire' a meter contact to the armour (and then cover the damage with self amalgamating tape) you can verify the steel armour to brass resistance directly. It might be all at one end.

Edit and of course while doing that verify the resistance from the brass gland to the rest of the metal it is supposed to connect to as well.

-------------------------
regards Mike


Edited: 22 January 2019 at 11:14 AM by mapj1
 22 January 2019 11:06 AM
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John Peckham

Posts: 9097
Joined: 23 April 2005

I have just run that through my Amtech. I used 180m of 120mm. The maximum Zs is calculated at 0.1306 ohms and the calculated Zs is 0.1306 ohms, which just squeaks in.

I would suggest there is a fault. Has the paint been removed under the SWA glands, is the banjo present and bolted through, if either end has a removable gland plate is there a flying earth from the plate to the earth bar? Can you examine the joint or is it encapsulated? Can you measure R1 + R2?

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 22 January 2019 04:21 PM
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PWatts

Posts: 5
Joined: 05 June 2015

Hi Many thanks again for all of your replies. To Recap the SWA cable run starts at 120mm ad after around 50m it is jointed with torpedo type joint underground in a large pit /Manhole. The joint is totally encapsulated .It would not be possible to rejoint the cable as there is not enough slack and it would cost a lot of money to then put in a new piece of SWA (120mm or 185mm) back to the supply MCCB.The cable is jointed to a 185mm 4 core then travels around 140m to a pump house where it is glanded off into a spreader box below a switch Fuse which then feeds a bus bar which various Supplies then branch off of . There is not a Banjo present or Flying earth lead at either end which we are obviously going to rectify. Having a high Zs of 2.38 ohms of course I am hoping fitting Banjos and flying leads will bring the Earth Loop down but I cannot see it going down to 0.121.I phoned Schneider today and even fitting a Micro Logic electronic trip unit would not bring down the Maximum permitted Zs that much .They explained that setting the Short time trip to 6 would only give a Max Zs of 0.17 at 1200A.Is a right pain but at the end of the day the Earth Loop is too high for a 5 second disconnection time so we are looking at the option of running a separate earth conductor which a contractor has calculated at 120mm.Thats a large Earth to run under the ground for over 180 metres. My calculations were coming out at 95mm but as you can see they were not calculated using a computer program. It might be an idea to fit the banjos , clean up the metal plates and fit flying leads and then take another reading Zs reading before committing to a big spend for the 120mm .Thanks again guys
 22 January 2019 04:27 PM
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mapj1

Posts: 12039
Joined: 22 July 2004

If in future it does fail, depending on the arrangements, it may be cheaper to dig a bigger hole, and install 2 new joints a metre two apart, and a few metres of new cable, assuming the bulk of the cable is OK, and just the ends are a problem.
A bit of scratting and a low ohm range meter test between the brass and the steel at each end will tell you how much you will gain before you bother to dismantle to fit glands etc.

-------------------------
regards Mike
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