IET logo
 
IET
Decrease font size
Increase font size
Topic Title: EICR on a PME Yesterday
Topic Summary:
Created On: 11 November 2012 11:02 AM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 11 November 2012 11:02 AM
User is offline View Users Profile Print this message



earthing

Posts: 56
Joined: 17 December 2008

Hi,

Yesterday I carried out an EICR for a customer. I am very inexperienced with test and inspection and wanted to communicate my experience yesterday and ask more experienced members for some guidance.

On an MI cable I found that the voltage between live and neutral was 230 and between live and earth was 110v and neutral and earth 110v. I didn't have enough time to fully investigate this and the cable is being replaced anyway but just wondering what may have caused this?

It was PME and I concluded the Insulation has broken down, I didn't conduct an I.R test.

Also, originally when I first arrived, I concluded that the installation was TNS. When I conducted my dead tests I pulled the main fuse but at some point turned the main switch on. Then after the second neutral to earth test and getting exactly the same reading 0.16, I realised my error.

I'm feeling a little unconfident about this though as its the first time I have been in this situation.

To clarify that a system is PME and not TNS I tested the continuity between the MET and the earth bond on the incoming supply cable which was 0.16

Is this ok? I am hoping to carry out an I.R on the M.I cable on Tuesday when I go back just out of interest.

Any help greatly appreciated, am going out for a few hours now but will respond when I get back,

Many thanks,

Daniel
 11 November 2012 11:26 AM
User is offline View Users Profile Print this message


Avatar for JZN.
JZN

Posts: 555
Joined: 16 November 2006

You probably got 110V because the earth is not connected properly or at all.

As for the PME, you need to measure Ze for the installation. Measuring continuity between the PME connection and your main earth terminal only gets you so far. It's a dead test and only tells you that your side of things is intact and OK. A loop test between your main earth terminal (with bonding conductors disconnected) and the phase will give you Ze. That's the important measurement here as it tells you that things are OK on the PME as well as your main earthing conductor. This result is generally deemed OK for a PME if it's 0.35 Ohms or less.

Also, the main way of telling PME from TNS is that for a TNS you will normally see an earthing conductor welded to the cable sheath. The PME connection is normally a small metal terminal block on the service head into which you fasten your main earthing conductor.

John
 11 November 2012 01:51 PM
User is offline View Users Profile Print this message


Avatar for Legh.
Legh

Posts: 3472
Joined: 17 December 2004

I'm feeling a little unconfident about this though as its the first time I have been in this situation.
Have you considered getting somebody with some electrical experience to help you with this venture?

To clarify that a system is PME and not TNS I tested the continuity between the MET and the earth bond on the incoming supply cable which was 0.16

Its not uncommon to find that some TN-S systems having a lower Ze than their PME counterparts.

As stated already TN-S has a separate earthing conductor usually connected to the supply cable sheath. PME, on the other hand has its earthing connection to the neutral (PEN conductor) of the supply cable and mostly accompanied with a Green plastic label stating 'PME connection'



Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 11 November 2012 03:27 PM
User is offline View Users Profile Print this message



daveparry1

Posts: 6113
Joined: 04 July 2007

earthing, sorry to say this but I really don't think you should be doing EICR's yet. You do admit to not having much experience but you appear not to have much understanding of electrical systems anyway, that reading of 110 volts between N-E & L-E should have told you that the earth wasn't actually connected to earth, especially as you had 230v L-N. I know we all have to start somewhere (before I get shot down for saying this) but inspection and testing - EICR's does require an above average understanding and level of experience. Btw, one way of testing for pme is to do a continuity test between the MET and neutral, take a voltage measurement first though for your safety and that of your meter! Best to work alongside someone experienced for a while, you could cause yourself lots of problems otherwise, especially conducting EICR's,

Dave.

Edited: 11 November 2012 at 03:44 PM by daveparry1
 11 November 2012 05:23 PM
User is offline View Users Profile Print this message



earthing

Posts: 56
Joined: 17 December 2008

Originally posted by: JZN

You probably got 110V because the earth is not connected properly or at all.



As for the PME, you need to measure Ze for the installation. Measuring continuity between the PME connection and your main earth terminal only gets you so far. It's a dead test and only tells you that your side of things is intact and OK. A loop test between your main earth terminal (with bonding conductors disconnected) and the phase will give you Ze. That's the important measurement here as it tells you that things are OK on the PME as well as your main earthing conductor. This result is generally deemed OK for a PME if it's 0.35 Ohms or less.



Also, the main way of telling PME from TNS is that for a TNS you will normally see an earthing conductor welded to the cable sheath. The PME connection is normally a small metal terminal block on the service head into which you fasten your main earthing conductor.



John


Hi John thanks for the reply,

I am just wondering how it is possible to get a reading of 110 volts between Line and Earth if earth is not connected?

If the earth isn't connected then wouldn't you get the same result by simply waving the other test probe in free air? Just trying to understand this pme system!!

I can understand how you might get a reduced voltage via a loose or high resistance connection.

Yes the supply cable head looked like a TNS. I am going to take a picture tomorrow and upload it for some views and carry out some further testing on the MI cable.

The Ze was 0.19

Thanks,

Daniel
 11 November 2012 05:38 PM
User is offline View Users Profile Print this message



earthing

Posts: 56
Joined: 17 December 2008

Originally posted by: daveparry1

earthing, sorry to say this but I really don't think you should be doing EICR's yet. You do admit to not having much experience but you appear not to have much understanding of electrical systems anyway, that reading of 110 volts between N-E & L-E should have told you that the earth wasn't actually connected to earth, especially as you had 230v L-N. I know we all have to start somewhere (before I get shot down for saying this) but inspection and testing - EICR's does require an above average understanding and level of experience. Btw, one way of testing for pme is to do a continuity test between the MET and neutral, take a voltage measurement first though for your safety and that of your meter! Best to work alongside someone experienced for a while, you could cause yourself lots of problems otherwise, especially conducting EICR's,



Dave.


Hi Dave thanks for the help,

Well I'll just study harder and make more effort to understand and hopefully I won't kill myself in the process!


If the earth isn't connected at all, and say the outer sheath of the MI cable had a resistance to earth of greater than 2000 ohms, surely you would not expect to see any voltage when measuring between line and earth?

In a healthy pme circuit where the earth connection is sound, what would you expect to measure in voltage across line-neutral, line-earth and neutral-earth?

My understanding is 230 volts line to earth, (which in pme is the same as neutral) 230 volts line to neutral and 0volts between neutral and earth?

Thanks,

Daniel
 11 November 2012 05:41 PM
User is offline View Users Profile Print this message



earthing

Posts: 56
Joined: 17 December 2008

Hi Legh thanks,

Will take a picture of the supply tomorrow and post here along with further testing of the M.I cable.

Thanks,

Daniel
 11 November 2012 06:13 PM
User is offline View Users Profile Print this message



daveparry1

Posts: 6113
Joined: 04 July 2007

Hi Daniel,

If the earth isn't connected to earth but left floating you will normally get some induced voltage which can show up some voltage on a sensitive digital meter although the CURRENT will be extremely small. You could also get some voltage on the earth conductor from connected equipment.
if the earth was connected to a good earth your meter would indicate zero, even if there was some poor insulation in the cable provided the leakage wasn't sufficient to trip the breaker/fuse/rcd etc.
Your last sentence is correct.

Dave.
 11 November 2012 10:38 PM
User is offline View Users Profile Print this message



ectophile

Posts: 540
Joined: 17 September 2001

The 110V readings between live and earth, and neutral and earth are nothing to do with PME. It's a sign that the "earth" wire (or sheath in the case of MI) isn't connected to anything and is just floating.

Two parallel conductors separated by insulation forms a capacitor. If the "earth" wire is just floating, then you effectively have two capacitors in series - one between the live and "earth" and one between the "earth" and neutral. You are just measuring the voltage at the centre point of the two capacitors, which will be half the supply voltage.

-------------------------
S P Barker BSc PhD MIET
 11 November 2012 11:04 PM
User is offline View Users Profile Print this message



peteTLM

Posts: 3123
Joined: 31 March 2005

The 110v on the earth is also common when its pulled up by noise suppression on washing machines and the like and dodgy switch start ballasts. Only becomes apparent when the earth is floating as mentioned. When connected you will never notice it.

-------------------------
----------------------------------------
Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 12 November 2012 09:41 AM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11299
Joined: 13 August 2003

Also, originally when I first arrived, I concluded that the installation was TNS. When I conducted my dead tests I pulled the main fuse but at some point turned the main switch on. Then after the second neutral to earth test and getting exactly the same reading 0.16, I realised my error.

Just pulling a single pole fuse is never going to be adequate - in all systems TN-C-S, TN-S and TT, the supply N is permanently connected to earth - it's just the position (and resistance) that varies. For TN-C-S/PME as you know, the N-PE link is in the cut-out, for TN-S it's at the substation (usually not too far away and connected with reasonably thick cables) and for TT the N-PE link at the substation is connected to an electrode with the consumer's installation connected to another electrode - the resistance between the two might be several dozen Ohms, but it's still going to muck up your insulation tests.
- Andy.
 12 November 2012 08:31 PM
User is offline View Users Profile Print this message


Avatar for Robotman.
Robotman

Posts: 2
Joined: 07 November 2012

Hi Daniel

I'm sure you got the job sorted, I ET&I Radio tower instalations and believe me Earthing requirements can get a bit confusing. The cable fault and your earthing system I believe are seperate issues. If the ZE reading at the mains is lower than 0.35 then TN-C-S, TN-S is irelevent in this case If the reading had been higher than 0.35 then further investigation in to the system would be required if it were a TN-C-S. The cable needs to be fully insulation resistence tested then the continuety from the earthed sheath to the MET needs to be measured. If this is ok there should not be 110V.

Keep me posted and good hunting
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.