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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?
 
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?
 
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
 
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!!
 
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?
 
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?
 
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
 
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.
 
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.
 
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?
 
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:p
 
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.
 
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
 

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