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?

 

[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