Poor Zs

[stuart569]

Getting around 0.15ohms on a 400amp supply. It's a 4-core 240 and the run is about 120m. The spark at the job says he thinks the cable has been jointed twice due to being dug up and damaged. Its hrc cartridge link fuses. Had a squint around and reckon the max Zs is 0.09? Is this correct?

 

[Taylortwocities]

Zs of 0.09 ohms after a 120metre run? I don't think so. Dont forget how you get to a Zs figure.....

What is the Ze at the origin?

 

[johnboy6083]

Zs of 0.09 ohms after a 120metre run? I don't think so. Dont forget how you get to a Zs figure.....

What is the Ze at the origin?

on a cable of that size, you nwill havfe very very low readings. Also Ze at the origin of a site with an integral tx will be 0.01 or there abouts

 

[stuart569]

Ze I am told is 0.02.

 

[Taylortwocities]

Ah 240mm². I missed that bit. Sorry:blush2:

 

[Engineer54]

Your problems will almost certainly lay at the two joints that you say are part of this 120m cable run.

Now what's the betting, no-one marked the position of these two joints?? lol!!

 

[stuart569]

Haha most likely not! That was my suspicion but it's always good to have someone more experienced confirm. Now to fix this issue is it possible to just run a new earth wire rather than replace the cable?

 

[Engineer54]

It's possible yes, ...but it'll still need protection, if following the route of the original cable!!

 

[stuart569]

It wouldnt be following original route. Will have to go round conveyors etc. a longer run but less hassle. What size would you recommend? Just go the whole hog and run in a 120 earth?

 

[Knobhead]

How much longer will this make the earth run compared to the cable?
Can’t say I like the idea.

 

[stuart569]

Now you say it I see your point with the shorter fault path back through the armour rather than the new earth? Thought about changing to A 400amp MCCB but max Zs values are still no good for the current situation. Also thought about trying to tt it but don't know if this would be suitable

 

[stuart569]

It would be an extra 10 metres

 

[Marvo]

How much longer will this make the earth run compared to the cable?
Can’t say I like the idea.

Me neither. I don't know what the UK regs say but where I am it's not allowed, an exported earth that's external to the supply cable must follow the same route and be strapped to the cable. Even if your local regs don't specifically exclude this, I would still suggest it's not good practice.

 

[Knobhead]

To be honest I’d be looking for the joints and replacing them. To my mind the modern cast joints don’t have sufficient cross bonding for the armours. I’ve always put more copper braids in than the manufactures recommend, also a lap of braid around the armours before the constant pressure spring.
I don’t know if you can still get them but one company made joints with a support ring that slid under the armours and the braids were clamped with a jubilee clip. A sound conection.

 

[Knobhead]

I'm sure they were Scotchcast. But it's about 15 years back.

 

[stuart569]

O dear. No other option? help! lol

 

[Marvo]

To be honest I would start digging, expose the cable and the joints and then make a decision on the way forward. If you don't want to remake the joints you can install a separate earth from scratch. As far as I know the minimum csa would be dictated by the adiabatic equation in the UK regs.

I don't suppose TT'ing the installation is an option?

 

[stuart569]

That was a thought but its a guessing game as to how many rods would need pounding in. Hard clay to contend with. Thanks for your help guys. Any more is greatly appreciated

 

[Marvo]

Maybe dig a hole 2 or 3 feet deep and whack a 2.3 meter one in as a test rod and see what readings it gives you. It might be an hour and the cost of the rod well spent. If you get a half decent impedance reading just attach another one or two rods to make it deeper and get the necessary readings. A TT might save you a lot of time and money especially with the cable being 240mm².

Thinking out loud, I'm wondering why the original installer didn't parallel up two smaller csa cables. I'm also wondering what the earth impedance would be ideally for a 240mm² SWA cable when using the armour. I don't have tables to hand but it might be worth looking it up, over 100m the reading your getting might not be far off the mark in which case no amount of remaking the joints is going to give the result you need.

 

[stuart569]

Yep, I dare say a test rod or two is the next logical step. The company that originally installed it has since went bust(no wonder) but there rotten work lives on. Was PIR'd 5 years ago and passed by them. Getting the test sheets in the next couple of days so will be interesting to see what nonsense has been written down. The maintenance guy that looked after the mill previously that did the joints wan't up to much either by the sounds of it

 

[steveberry11]

Don't go down the TT route as you say the supply is at 400A I am guessing that it is some sort of factory with lots of machinery and associated equipment.
If an RCD trips you can import lots of hazards.
If the ZS value is high work out the R1 + R2 values from manufactures data add them to the ZE to determin if you do have a fault. if there is a fault repair it. If it is a case of a faulty joint on the SWA is that is due to poor workmanship. Check the resistance of all conductors from manufactures data. You can also check all conductors have the same resistance. Any odd readings may suggest that the line conductor joints may fail. You may have to remake the joints
If you are happy with the line conductors your customer may have to take the hit and excavate the cable and lay a new earthing conductor.

Asyou say the PIR 5 years ago was OK this may be a developing fault that you have found and not a cover up by the first contractor.

 

[Engineer54]

Yep, I dare say a test rod or two is the next logical step. The company that originally installed it has since went bust(no wonder) but there rotten work lives on. Was PIR'd 5 years ago and passed by them. Getting the test sheets in the next couple of days so will be interesting to see what nonsense has been written down. The maintenance guy that looked after the mill previously that did the joints wan't up to much either by the sounds of it

Think your going to need more than just a couple of rods to get anywhere near the Ra value you'll be looking for. You'll be lucky with two or three rod positions, each with at least a couple of 2.4m 5/8'' rods....

You really need to do a cost assessment as to what is going to be your best way forward. Personally i would go for running in a separate 150mm CPC, though that would still mean excavation of the original trench to a suitable depth....

 

[stuart569]

I am fully aware a couple of rods would not suffice thats why i said test rods to determine if it would be worthwile going down that road. Will most likely bite the bullet and run in new earth. Just tryin to explain to mill manager that 0.05 ohms = well over £1500 isnt good lol

 

[stuart569]

Don't go down the TT route as you say the supply is at 400A I am guessing that it is some sort of factory with lots of machinery and associated equipment.
If an RCD trips you can import lots of hazards.
If the ZS value is high work out the R1 + R2 values from manufactures data add them to the ZE to determin if you do have a fault. if there is a fault repair it. If it is a case of a faulty joint on the SWA is that is due to poor workmanship. Check the resistance of all conductors from manufactures data. You can also check all conductors have the same resistance. Any odd readings may suggest that the line conductor joints may fail. You may have to remake the joints
If you are happy with the line conductors your customer may have to take the hit and excavate the cable and lay a new earthing conductor.

Asyou say the PIR 5 yso was OK this may be a developing fault that you have found and not a cover up by the first contractor.

Fair point about cover up but from what I have heard there testing from elswhere on the site has been done by a young guy witg not much of a clue. Getting a look at sheets in next couple of days and doing some tests shortly too so will know the situation soon enough

 

[steveberry11]

I am fully aware a couple of rods would not suffice thats why i said test rods to determine if it would be worthwile going down that road. Will most likely bite the bullet and run in new earth. Just tryin to explain to mill manager that 0.05 ohms = well over £1500 isnt good lol

What is the purpose of the supply you are testing. I guess it is industrial, can you give a little more info.

 

[stuart569]

Supply comes off of main board for sawmill. Right next to tranny. This feeds 3 boards in 3 buildings on a daisy chain. As in one board feeding another, feeding another. The Zs's are no good on the other boards aswell but obviously have to sort out first in line before i can do anything about next ones

 

[Knobhead]

#25

Try reading the thread.

 

[steveberry11]

Supply comes off of main board for sawmill. Right next to tranny. This feeds 3 boards in 3 buildings on a daisy chain. As in one board feeding another, feeding another. The Zs's are no good on the other boards aswell but obviously have to sort out first in line before i can do anything about next ones

If the first leg was an at an acceptable ZS would the other legs be acceptable. Taking into account the drop of resistance in the first subtraced this from the other legs.

 

[steveberry11]

#25Try reading the thread.

??????

 

[stuart569]

Not sure will find out soon enough

 

[IQ Electrical]

You are quite within your rights to use manufacturers data for maximum Zs values rather than the often much lower values tabulated in BS7671.

Also remember that with a site transformer in very close proximity, your Zs readings will be affected by the very low impedance and inductive reactance and that most earth fault loop impedance testers are only designed for circuits rated at around 50 Amps.

 

[steveberry11]

Not sure will find out soon enough

The reason I ask is that if it won't your efforts may be a waste of time if the installation was of a poor design and a EIC cover up.
Not sure if I would want a saw mill on an RCD
if the repairs will not address the loops on the other distribution boards one answer may be to re supply them from the main supply point. Not cheap but is one answer to the problem.