IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Ze too high
Topic Summary: 200A sub
Created On: 10 January 2017 08:54 AM
Status: Read Only
Linear : Threading : Single : Branch
<< 1 2 Previous Last unread
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 11 January 2017 01:04 AM
User is offline View Users Profile Print this message



SKElectrical

Posts: 1115
Joined: 01 February 2009

Originally posted by: John Peckham
if you want really good and much more accurate loop tester Megger do one for circa £5000.
.


I could just lose a few digits off my readings as I put pen to paper and save £5K!!
 11 January 2017 02:31 AM
User is offline View Users Profile Print this message



SKElectrical

Posts: 1115
Joined: 01 February 2009

Originally posted by: OMS
A separate earth 25mm also enters building with Supply cable and bonded to the metal enclosure.


It exits to an electrode somewhere along the service cable run (probably just outside the door) - this is required usually because of the ESQCR requirements - your building has a particular disposition on the distributor out on the road


Are you sure? I come across a lot of TNS supplies with separate earths run in adjacent to service cable. What they are externally connected to I do not know, see below (the black cable in henley block comes out of ground)

 11 January 2017 09:00 AM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 15811
Joined: 13 August 2003

Are you sure? I come across a lot of TNS supplies with separate earths run in adjacent to service cable.

That might even be a TN-C-S supply - many DNOs now have a policy of placing the N-PE link back into the street where there are situations like multiple supplies to a steel framed building (so effectively there's one N-PE link per building rather than per consumer - all to reduce diverted N currents flowing through the structure and avoid associated problems with EMI and overloading the CNE on the shortest feeder cable). Nowadays they specify split con or some other 5-conductor (SNE) cable, but I can imagine them using a separate conductor in earlier times or where the supply cable is already existing. X-ray vision needed indeed.

See section 6.13 here - http://library.ukpowernetworks...on+Earthing+Design.pdf

- Andy.
 11 January 2017 09:57 AM
User is offline View Users Profile Print this message



OMS

Posts: 22359
Joined: 23 March 2004

As Andy highlights, you don't know it's not TN-C-S, only that it's not PME

It's quite probable you have a 3 core concentric service cable from a PILC distributor - the earth has been provided from either a SNE connection or the CNE back at the outside joint - there may or may not be an electrode at that position - the DNO wouldn't declare it as PME as the distributor main probably doesn't meet the ESQCR requirements

In addition to neutral currents, they may also be eliminating N-E voltage drift (so a type of PNB but just outside the premises).

Your photo isn't necessarily anything like your OP description either

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 11 January 2017 10:09 AM
User is offline View Users Profile Print this message



mapj1

Posts: 9346
Joined: 22 July 2004

well, that's a 3 phase 60 A 'presents as TN-S' supply and nothing to do with the OP.

Now I cannot tell how many NE bonds or company earth electrodes there are outside the building, only that L-E and L-N loop impedances may well differ. There may be still scope for diverted neutral current to come back up the earth and then go down the plumbing, it depends if the original outdoor installation team could count only to 4, or to 5.

-------------------------
regards Mike
 11 January 2017 10:46 AM
User is offline View Users Profile Print this message



SKElectrical

Posts: 1115
Joined: 01 February 2009

Sorry i didn't have a photo of OP, but this seemed relevant to recent messages.
Are we not to tick a TNCS box unless there is a PME sticker present?

I will look to change main fuses to 100 as regards the OP and any increase becomes a problem for future designers!

It's all a bit deflating really... even my meter is considered inadequate.
A correct EICR should read something like this "LIM LIM LIM LIM LIM LIM LIM...."
And chuck in a few Zs's -which are probably just "fruit machine readings (C.)".
 11 January 2017 11:06 AM
User is offline View Users Profile Print this message



OMS

Posts: 22359
Joined: 23 March 2004

A correct EICR should report on the installation condition in terms of continued service and be supported by such test measurements that are necessary to support the assertion that it is safe, based on an understanding of the test instrument accuracy and resolution

Inspectors should posses above average knowledge and be familiar with the installation type being inspected

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 11 January 2017 11:28 AM
User is offline View Users Profile Print this message



leckie

Posts: 4316
Joined: 21 November 2008

I have a little enclosure with a 1 ohm resistor in it terminated to jack plugs. If, as often happens my EFLI test reads zero, or very low if I am near the transformer when doing Ze/ZsDB, I plug the meter into the 1 ohm box and try testing again.

Say I initially measured 0.05 ohms, you would think the +1ohm reading would be 1.05 ohms- but often it is not. Often it will read something like 1.15 ohms. I tend to think that the true EFLI is nearer to 0.15 than 0.05.
 16 January 2017 09:43 PM
User is offline View Users Profile Print this message



SKElectrical

Posts: 1115
Joined: 01 February 2009

Originally posted by: OMS

Originally posted by: SKElectrical


A separate earth 25mm also enters building with Supply cable and bonded to the metal enclosure.



It exits to an electrode somewhere along the service cable run (probably just outside the door) - this is required usually because of the ESQCR requirements - your building has a particular disposition on the distributor out on the road


Regards
OMS


Yeah, you were right.
 16 January 2017 10:52 PM
User is offline View Users Profile Print this message



OMS

Posts: 22359
Joined: 23 March 2004

Ta Dah

OMS

-------------------------
Let the wind blow you, across a big floor.
 18 January 2017 12:12 PM
User is offline View Users Profile Print this message



leckie

Posts: 4316
Joined: 21 November 2008

I have just dug this back up to mention earth loop testers. I have read the specification on the LT 300 and it looks very good with a test current of 12A as described by JP.

Then I looked up a MFT 1700 series, I couldn't find the test current but the accuracy was less that the LT 300.

Now I have a couple of old Megger MFT 1520's that I use, but high current setting says 25A for EFLI. I looked in the data sheet and it also says 25A. So that's more than the LT 300 or presumably the 1700 series meters that are much newer

Then I checked the accuracy of the 1520, it claims accuracy as good as the LT 300 and better than the all singing all dancing 1700 series.

I better keep my antique meters going a bit longer! Along with my Elliot Lucas pliers of course.
IET » Wiring and the regulations » Ze too high

<< 1 2 Previous Last unread
Topic Tools Topic Tools
Statistics

New here?


See Also:



FuseTalk Standard Edition v3.2 - © 1999-2017 FuseTalk Inc. All rights reserved.

..