Commercial Earth rods to reduce Ze

[Marxman99]

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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.