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.