charity Ze

[Tridentsystems]

8 ft 5/8 rod driven in to undisturbed agricultural land. Rocky soil, clay at four foot depth. Guesses for Ze in a postcard please ! Supply goes in tomorrow so result soon. Closest guess picks the charity and I will send it £50. Just a bit of fun. Two decimal places please !!!!

 

[Tridentsystems]

No results yet. However western power jointers have just completed and given me a PME to a steel agricultural barn. However the Ze measured by them is .51 which they have showed me an internal memo that states it acceptable if fuse is derated to 80 A. Anyone come across this ??

 

[telectrix]

i assume their thinking is that the 80A fuse needs a lower fault current to blow within specified time, thus allowing for a higher Ze. however, i'd dispute their thinking because although they may be correct as regards their fuse, it cuts your limit on R1+R2 for your circuits and could lead to you needing to install larger cables, especially if you have any C or D type OCPDs. (or am i talking bullpoo).

 

[Tridentsystems]

Apart from the technical aspects the Certs gonna look bad

 

[Fin170]

Error: Battery low

or..... 35.65

 

[Tridentsystems]

Verified by three of us each with our own meters. Readings accurate and obviously expected as they carry a memo with instructions to derate

 

[Tridentsystems]

Haha sorry fin just got what you meant. Sorry long day

 

[Engineer54]

i assume their thinking is that the 80A fuse needs a lower fault current to blow within specified time, thus allowing for a higher Ze. however, i'd dispute their thinking because although they may be correct as regards their fuse, it cuts your limit on R1+R2 for your circuits and could lead to you needing to install larger cables, especially if you have any C or D type OCPDs. (or am i talking bullpoo).

Well you could say the same for a TN-S system that's almost or on it's maximum value of 0.80 ohms. Personally i can't see a problem with a Ze of 0.51 ohm's for a TNC-S system!! You'd be pleased as punch to see that sort of value on a TT system even on a TN-S you'll be happy!! ....lol!!

 

[Engineer54]

No results yet. However western power jointers have just completed and given me a PME to a steel agricultural barn. However the Ze measured by them is .51 which they have showed me an internal memo that states it acceptable if fuse is derated to 80 A. Anyone come across this ??

Just thinking out loud, ...Why would you want a PME supply in a steel framed building, that probably has parts of that steel frame in permanent contact with the soil and or re-bar of the floor/foundations!! Did you check the frame Ra value before any PME bonding etc??

 

[Tridentsystems]

Didn't want or design for a PME and certainly didn't expect one to be provided. Assumed it would be TT hence original post. My confusion is the regs say .35 max but WP are satisfied with .5. Don't see how I can cert it as compliant as Ze is too high for the regs. If I get good Ra today then I think I will stick to my TT design

 

[Plonker 3]

So you would rather install a earthing system that will probably not be maintained for several decades, price in the cost of several RCDs/RCBOs. Than use something that you have been told is satisfactory by the DNO, who are also responsible for the maintenace of the earthing.


No the Ze in BS7671 is just a guide and as the owner of the network is satisfied with the readings and provided you with the relevant paperwork which you could attach to a copy of your cert then there si no reason why you shouldn't be satisfied with it.

 

[armbands0]

Clay soil at about that depth at my place, existing single rod 60 odd ohms, now I have multiple rods.

My guess for your Ze 60.34R

 

[Engineer54]

Didn't want or design for a PME and certainly didn't expect one to be provided. Assumed it would be TT hence original post. My confusion is the regs say .35 max but WP are satisfied with .5. Don't see how I can cert it as compliant as Ze is too high for the regs. If I get good Ra today then I think I will stick to my TT design

Well you have a PME system now, or rather should i say a TNC-S system. So why don't you still provide an 8 foot rod and connect the rod to the barns PME's MET. (As another but local PME multi N-E connection!!).


What are you talking about, that you can't ''Cert'' the installation?? Forget the damned Reg's, if the supplying DNO are happy with 0.51 ohms, what are you worrying about? Are you under the impression that all TNC-S DNO supplies are all under 0.35 ohms, if you do, then you really need to speak to a DNO Area Engineer or to those that work for a DNO company, i can assure you, they are ''NOT'' all at, or below 0.35 ohms!!!

For a start the maximum 0.35 ohms value refers only to Full PME supplies. TNC-S supplies can be any value the DNO are happy with, in many cases this will include network distribution cables that are in the process of being PME'd but don't as yet meet full PME specifications. (Eg Number of cable joints that have N-E bonding!)


Unless you can achieve a Ra of 0.51 ohms on your TT system, then it's a no brainer to hang on to the TNC-S earthing.

 

[Tridentsystems]

TT result in. 12.81 ohms. Who won ?

 

[telectrix]

unless i've missed something, des56 was closest with 11.4 something.

 

[Pete999]

78.987
Pete999