Earth rods to reduce Ze

Marxman99 · May 25, 2014

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

davesparks · May 25, 2014

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

Darkwood · May 25, 2014

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 · May 25, 2014

darkwood said:
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 · May 25, 2014

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 · May 25, 2014

Somebody has a costly rectification job then.

davesparks · May 25, 2014

So let me get this straight, what you actually want to know is what to do about the fact you have a 200Amp service with a Ze of 0.22 when you need to achieve a Zs on a submain of 0.15?

I have to say it is an interesting question as based on the info supplied it isn't possible.

Off the top of my head I can only come up with splitting the installation into 2 and running a second 125A submain alongside the one you have.

stevethesparks · May 25, 2014

You may be in a position to double up rather than replace, if the calcs agree it could be a far cheaper option.

davesparks · May 25, 2014

stevethesparks said:
You may be in a position to double up rather than replace, if the calcs agree it could be a far cheaper option.

Double up what exactly?

He has a Ze of 0.22 and needs a Zs of 0.15 for the submain!

D Skelton · May 25, 2014

The cables

davesparks · May 25, 2014

But the Ze is higher than the required Zs, no matter how big the cable R1+R2 ain't going to be -0.07!

D Skelton · May 26, 2014

Ah, I missed that bit. Yeah, I'd go with two 100A supplies then. The DNO isn't gonna be able to help.

davesparks · May 26, 2014

More accurately the DNO won't want to help

Darkwood · May 26, 2014

Marxman99 said:
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.

So this has gone from a long winded discussion about the impression you gave with your opening post to a real question of having the cable upgraded etc but as the incomming is already higher than the required Zs it can't be met ...

Run - 2 x 100amp sub-supplys from the 200amp incomer and split the distribution room into 2 supplies ..Zs complies you can re-use one existing cable and just add another......simples

Engineer54 · May 26, 2014

Shame no-one actually bothered to conduct a Ze test after the new 200A supply was installed. Perhaps then the question could have been asked to the installing DNO, how their 200A service fuses will comply or operate effectively with a Ze of 0.22?? Or do their 200A fuses work on a completely different electrical science/theory?? ...lol!!

Gavin A · May 26, 2014

Engineer54 said:
Shame no-one actually bothered to conduct a Ze test after the new 200A supply was installed. Perhaps then the question could have been asked to the installing DNO, how their 200A service fuses will comply or operate effectively with a Ze of 0.22?? Or do their 200A fuses work on a completely different electrical science/theory?? ...lol!!

does sound like a DNO issue if they've been paid to supply a 200A per phase supply, and haven't supplied the earthing required.