Earth rods to reduce Ze

[Marxman99]

Can anyone help.
The electrician has measured the Zs of a new submains cable installation and it is too high.
The set up is this:
We have a new external cabinet housing a 200A 3phase TN-S intake. From the intake 1 new 150metres 95mm sq 4core swa + sep 35mm sq earth cable has been run to a main switch in a switchroom. In this main switch we wanted to install 200A BS88 fuses but the Zs fails. The recorded Zs is 0.29ohms (IEE regs allows 0.19ohms max for 200A BS88 fuse). The Ze at the intake position is 0.22ohms. We do not want to use an RCD on any point of the network. How can earth rods be implemented?
This is the link to the photos:
https://dl.dropboxusercontent.com/u/19108813/1_external intake cabinet.JPG
https://dl.dropboxusercontent.com/u/19108813/2_external intake cabinet.JPG
https://dl.dropboxusercontent.com/u/19108813/3_external intake cabinet.JPG
https://dl.dropboxusercontent.com/u/19108813/4_external intake cabinet.JPG
https://dl.dropboxusercontent.com/u/19108813/5_external intake cabinet.JPG
https://dl.dropboxusercontent.com/u/19108813/6_main switchroom.JPG
https://dl.dropboxusercontent.com/u/19108813/7_main switchroom.JPG
https://dl.dropboxusercontent.com/u/19108813/8_main switchroom.JPG
Regards

 

[Plonker 3]

How come it was installed before any calculations were carried out?

 

[telectrix]

what's the design load of the installation allowing for diversity ( if any)? maybe you could drop the fuse In to 125A.

 

[D Skelton]

I can tell you now without doing any calculations that the 95mm cable has been incorrectly selected, especially for volt drop! Regardless of that, Zs compliance will be achieved by increasing the size of those conductors, or by lowering the size of the fuse.

This smells to me to be nothing short of a monumental balls up by the electrician, who obviously has no design knowledge whasoever! I'd seriously consider getting someone in who knows what they're doing!

 

[shanky887614]

I can tell you now without doing any calculations that the 95mm cable has been incorrectly selected, especially for volt drop! Regardless of that, Zs compliance will be achieved by increasing the size of those conductors, or by lowering the size of the fuse.

This smells to me to be nothing short of a monumental balls up by the electrician, who obviously has no design knowledge whasoever! I'd seriously consider getting someone in who knows what they're doing!

it does sound a bit small even without volt drop.

4 core 95mm swa gives about 165amps in direct in ground/ducting.

in free air/perforated tray it gives 250amps.

obviously without any derating factors or voltdrop

 

[Darkwood]

The Zs required is stated wrong too the tables show 0.19ohms for a 200amp BS88 but have you applied 0.8 factor which will require a 0.15ohms to comply.

From your forum history on here - 3 yrs ago you were doing an Electrical Engineering course and asking about designing a supply system for a backed-up server system, this implies your courses were heavy with installation and cable calcs of supply systems .... with all due respect then regarding your history I would have thought you would be the one helping others as oppose to asking such a question.

When you say 'The Electrician' measured.... What was his role in this job, did he design, install the supply and may I asked what was your role in the job and why are you getting involved if others have made big errors?

And as above its sounds a monumental cock-up in design ...like a calculation has never been applied to the cable for derating factors!

The 200amp fuses overate the CCC of the cable by a large marging
Volts drop does not comply

When the cable set on fire through overload and the front end fuses only operate on the cables destruction then who's name is on the design of the installation???

and you thought your biggest issue was Zs here!!!!!

 

[Engineer54]

How can earth rods be implemented?

Seriously, with great difficulty!! You're trying to reduce an already relatively low Ze value of 0.22 ohms with earth rods!! So you're going to need a good few of them. ...lol!! Do you know how close you are to the supplying DNO distribution Transformer?? Also a good chance that DNO TN-S Cable is probably derived from an old TN-S network cable that's been PME'd....

 

[stevethesparks]

Apart from all other issues of Vd, ccc and Zs, out of interest what is the Ze value

 

[Carlosfandango]

0.22!!

 

[Plonker 3]

Apart from all other issues of Vd, ccc and Zs, out of interest what is the Ze value

Try reading the whole post by the OP

). The Ze at the intake position is 0.22ohms.

 

[stevethesparks]

Thanks for assisting my selective reading ;-)

 

[Kamikaze]

I think he’s talking about replacing the 125A fuses in the switch fuse in picture 8 to 200A. It looks to me as if the original installation was designed and installed for 100A. The old supply was on 100A BS1361 fuses.

 

[Plonker 3]

I think he’s talking about replacing the 125A fuses in the switch fuse in picture 8 to 200A. It looks to me as if the original installation was designed and installed for 100A. The old supply was on 100A BS1361 fuses.

More selective reading

Can anyone help.
The electrician has measured the Zs of a new submains cable installation and it is too high.

 

[Kamikaze]

From the intake 1 new 150metres 95mm sq 4core swa + sep 35mm sq earth cable has been run to a main switch in a switchroom. In this main switch we wanted to install 200A BS88 fuses but the Zs fails.

Perhaps…lol. He’s screwed whichever way you look at it.

 

[Marxman99]

Telectrix - a lot of money was spent upgrading the previous ukpn (utility operator) 100a supply to 200a.
D skelton - I am aware of that the 200a fuse doesn't comply with the ccc of the cable, i'll correct this.
Darkwood - the engineer that designed the project has now retired. I have been asked to look into this.
Engineer54 - i don't know if the intake position is near the DNO transformer, how would that help?
Kamikaze - yes we want to replace the 125a fuse to a 200a.
Can an RCD on the incomer help?
Thanks to all.

 

[davesparks]

No an RCD can't help you, despite the rumours about them being a magical cure-all device.

 

[Darkwood]

your opening post is very misleading it does not suggest you are trying to upgrade a 125 to 200 amp supply ... ignore the zs here the cable is maxed out already with cable calcs so not an option.

 

[Marxman99]

your opening post is very misleading it does not suggest you are trying to upgrade a 125 to 200 amp supply ... ignore the zs here the cable is maxed out already with cable calcs so not an option.

Davesparks, The 125a was only put into the main switch (in switchroom) because the 200a did not comply. The background is: Original the building was run from a 100a supply, it has now been upgraded to a 200a supply. The 200a is in the new external intake cabinet. Read my first post.

 

[Marxman99]

I forgot to add; although it is new, the cable will be changed, so the fact that the cable is maxed out will soon not be a problem.

 

[not clever]

Somebody has a costly rectification job then.