very High Zs!

[Storm247]

Hi Guys,

I wounder if i could pick your brains? you all seem very helpful on this forum.

Im in the middle of C&G 2391 so please dont shoot me down if iv made a mistake.

Iv just bought a Megger tester 1552 so thought I would start getting to grips with it and test the house im renting at the moment. Everything was good until i measured the Zs which i first did with the socket connection provided, which came out high at 2.40. Now this is a TN-S system and i was testing a ring main with 30A BS3871 type 2 fuse. Iv checked the OSG page 92 table 2D and the reading should be within 0.91, so i thought i might be using the equipment wrong so also tested it at the supply and yes i did turn the supply off before disconnecting the earth and checked leads and with known source to prove safe isolation.I then connect the earth and test lead before powering up But when i try to test for Zs with the leads to the cpc and phase the megger does not start up so i take it its not making a circuit? I checked the earth from the supply which i did first before anything and its 16mm2 as it should be, could it be the clamp to the supply thats got a bad connection if any? And if so i would like to get this sorted straight away, so would i have to contacted my supplier as its there responsibility?

Like i say i could be totaly wrong in what im doing but if you dont ask

Thanks guys sorry its a bit long winded, please if im going wrong with the test shoot away

 

[pdcelec]

Has the meter been calibrated would be my first question?

 

[Storm247]

Hi Thanks for getting back so quick, the megger is brand new with calibration certificate so all good.

 

[Lectrician]

Check from L to N at the origin first to get the L N loop - then try the L E again. This will prove meter is working correctly, and you are connecting it correctly.

Also try L to E with the houses E connected - the 2.4ohm may have been due to other parallel paths, and your actuall connection to the supply is faulty. Thats why we remove parallel paths when testing Ze.

 

[mst]

I thought you had to leave the main earth in when doing a zs reading?

im learning too!

i thought that you only take the main earth out when your doing ZE measurement.

did you set the leads to 0null? (short them together to go back to 0)

 

[dingbat]

I thought you had to leave the main earth in when doing a zs reading?

im learning too!

i thought that you only take the main earth out when your doing ZE measurement.

That's correct, but think about why:

A) When measuring Ze you don't want any parallel paths.
B) You should never energise an installation without full earthing and bonding, so you need to reconnect before you measure Zs.

 

[Storm247]

Also try L to E with the houses E connected - the 2.4ohm may have been due to other parallel paths, and your actuall connection to the supply is faulty. Thats why we remove parallel paths when testing Ze.

Thanks Lectrican forgot about parallel paths

im learning too!

We all start somewhere and i would rather make mistakes so i know why and how to rectify them

 

[mst]

yeap thinking about it now ....am i missing something because you wouldnt be able to pick up parrlel paths when testing ze because you have to connect ya clip on the main earth with it out of its normal postion in the C/U???

so it only goes straight to the supplers earthing or rod dosent it?

 

[Storm247]

My Bad! was Ze not Zs

 

[Storm247]

am i missing something because you wouldnt be able to pick up parrlel paths when testing ze because you have to connect ya clip on the main earth with it out of its normal postion in the C/U???

Yeah but all the earths are still connected via the MET? or have i got that wrong?

 

[dingbat]

Ze is the external earth fault loop impedance. You are concerned only with measuring the value of the phase-to-earth impedance and verifying that your means of earthing is adequate. That is why you measure Ze via the earthing conductor only.

As your primary protective measure is EEBAD you must have all your earthing and bonding connected, thus introducing parallel paths (hence lower impedance) to earth, which is why you would expect that, at any point, the measurd Zs is likely to be less than the sum of Ze + (R1 + R2).

 

[mst]

Also i take it you have your tester set to PFC dont you?

If theres one thing i cant seem to get my head round its knowing which settings are what on my 1552, tho i am getting used to it more now.

Did you out it on one of the PFC settings or on something else?

 

[mst]

in one of my books it says that -

ZE = ZS + R1 + R2

but why does ze = zs?

can you clear that one up for me dingbat the great!

 

[Storm247]

Also i take it you have your tester set to PFC dont you?

Iv got it set to Hi Loop as there isnt and rcd, is that correct?

 

[dingbat]

in one of my books it says that -

ZE = ZS + R1 + R2

but why does ze = zs?

Er... it doesn't!

Zs is theoretically equal to Ze PLUS (R1 + R2)

In other words Z system = Z external + circuit resistance.

(We normally put R1 + R2 in a bracket as they are measured in one go.)

You need to go on a course!