Discuss Ze readings >2000Mohms in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

Reaction score
0
Fluke MultiFunction Tester 1662

TNS Earthing system.

Switchboard - 415v - 3 Phase + Neutral + Earth.
MCB 1000A
3 individual Earths (185mm) connected to the Switchboards Earth Busbar via a main earth bar which is then connected to the sites Earthing Grid that is bonded with exothermic welding.

Performed Ze reading on main incoming supply to Switchboard with Earth disconnected, without any issues.

Ze - L1 - 0.13 PEFC - 1.9 kA
Ze - L2- 0.13 PEFC - 1.9 kA
Ze - L3 - 0.13 PEFC - 1.9 kA

The issue is only apparent on some of the outgoing supplies.

Lighting board A with earth disconnected on incoming supply - Ze readings -

Ze L1 - 0.27 PEFC - 891A
Ze L2 - >2000Mohm
Ze L3 - >2000Mohm

Lighting Board B with earth disconnected on incoming supply - Ze Readings -

Ze L1 - >2000Mohm
Ze L2 - >2000Mohm
Ze L3 - >2000Mohm

Lighting Board C with earth disconnected on incoming supply - Ze readings -

Ze L1 - 0.25 PEFC 960A
Ze L2 - >2000Mohm
Ze L3 - >2000Mohm

HVAC Switchboard with earth disconnected on incoming supply Ze Readings -

Ze L1 - 0.13 PEFC 1.9kA
Ze L2 - 0.13 PEFC 1.9kA
Ze L3 - 0.13 PEFC 1.9kA

For the life of me, I can't understand why I can get Ze readings on some outgoing supplies, and not the others.
And in some cases only getting a Ze reading on L1, but not on L2 + L3.

I've tried various other Multifunction Test equipment and issue remains.
I've had an outage on the board, inspected the incoming supply, internal busbars and torqued everything.
Performed a DUCTOR test on the board, without any issues.

Any help would be appreciated

Kind Regards
 
That's some interesting readings!

A Ze of 0.13 is way too high for a 1000A supply and a PFC of 1.9kA is way too low. How far away (in terms of conductor length) is the supply transformer? The inductance of the transformer may be screwing with your test results.

It would help if we you could clarify exactly what you are testing and what is being disconnected for these tests?
Ze is only measured at the incoming supply position with the installation isolated and the means of earthing disconnected.
All other earth fault loop impedance tests are Zs and, as they require the installation to be live must be conducted with all earthing and bonding connected. Plus they must only be carried out after dead testing has proved the continuity of the earthing up to that point.
 
That's some interesting readings!

A Ze of 0.13 is way too high for a 1000A supply and a PFC of 1.9kA is way too low. How far away (in terms of conductor length) is the supply transformer? The inductance of the transformer may be screwing with your test results.

It would help if we you could clarify exactly what you are testing and what is being disconnected for these tests?
Ze is only measured at the incoming supply position with the installation isolated and the means of earthing disconnected.
All other earth fault loop impedance tests are Zs and, as they require the installation to be live must be conducted with all earthing and bonding connected. Plus they must only be carried out after dead testing has proved the continuity of the earthing up to that point.

My mistake the MCB for the board is 500A.

The Distribution board itself is downstream from another distribution board (800A MCB), which ties into a transformer via busduct.

2 single core 300mm cable for each phase + neutral at a length of 32M.
1 single core 300mm cable for Earth at a length of 32M.

We are required to provide Ze readings for each LV Distribution board on site.
Incoming supply Earths are always disconnected, and Main Circuit Breakers / rotary isolators are always switched off + isolated before taking a Ze reading.

This specific is task is to capture Ze reading on the distribution (500A MCB) and then capture Ze readings for all downstream LV distribution boards - 3 Lighting DBs and 2 HVAC DB's.

Yesterday, We revisited a lighting distribution board (which is an outgoing supply of the board in question) that we previously had acceptable Ze readings for. The circumstance of the Distribution board upstream of the Lighting Board had 0 load, acceptable Ze readings were obtained. Tested again yesterday with load on the upstream board of 200Amps and the Ze readings were all >2000Mohms.
 
@Julie. will be able to give a much better description than I can
however, in brief when you are that close to the transformer the readings from a multi tester are useless because they can only measure the resistance element of the circuit and not the inductance.
 
The description of what you are doing does not make much sense to me, Are you trying to measure the Ra of the sites Earthing Grid? Having that is the region of 0.13 ohms is sort of sensible. But that is not Ze which is relative to the TN-S supply's earth point and for a 1000A system it ought to be far less and your PFC far bigger. Probably above what a MFT can accurately resolve!

As for the odd differences when testing in different places, have you tried new test leads?
 
My mistake the MCB for the board is 500A.

0.13 still aounds too high for the Zs
The Distribution board itself is downstream from another distribution board (800A MCB), which ties into a transformer via busduct.

2 single core 300mm cable for each phase + neutral at a length of 32M.
1 single core 300mm cable for Earth at a length of 32M.

We are required to provide Ze readings for each LV Distribution board on site.

Again, that is Zs and not Ze, Ze is the external earth fault loop impedance.

Incoming supply Earths are always disconnected, and Main Circuit Breakers / rotary isolators are always switched off + isolated before taking a Ze reading.

How are you actually disconnecting the incoming earth to these DBs? This isn't normally possible.

Where exactly are you connecting the test leads when taking these measurements?
 
0.13 still aounds too high for the Zs


Again, that is Zs and not Ze, Ze is the external earth fault loop impedance.



How are you actually disconnecting the incoming earth to these DBs? This isn't normally possible.

Where exactly are you connecting the test leads when taking these measurements?

It's not a Zs reading. Why would we be disconnecting the main earth of the supply cable to the distribution board to record a Zs?

We've captured Ze readings on over 100+ DB's on this PowerStation thus far.

This isn't an issue of where the test leads are being connected.

I think I'm starting to agree with the distance of the Transformer / length of the conductors.
 
Next thing I think I'm going to do is have no load on the Main DB, as there's roughly around 200A being pulled at present. then try and record Ze readings for each of the downstream DB's, see if that changes anything. Maybe the running of one of the HVAC boards is interfering with the readings.
 
It's not a Zs reading. Why would we be disconnecting the main earth of the supply cable to the distribution board to record a Zs?

Ze is the earth fault loop impedance external to the installation, you are measuring earth fault loop impedance within the installation so normally that is Zs, hence why I have bee saying Zs. But now you've revealed this information:
We've captured Ze readings on over 100+ DB's on this PowerStation thus far.

It's a PowerStation, that's a whole different kettle of fish and things can be very different.

Is there not an engineer there who could shed some light on the issue?

This isn't an issue of where the test leads are being connected.

OK, but it is always worth checking the simple things first as we all make that kind of mistake sometimes. I've spent ages in the past looking for a similar issue until I realised I'd got the tester set to a 2 wire low current test instead of a normal high current test.

I think I'm starting to agree with the distance of the Transformer / length of the conductors.

The distances you've mentioned above are normally plenty to get far enough from your average substation transformer to get sensible readings.
 
is always worth checking the simple things first as we all make that kind of mistake sometimes
Totally agree, first thing I did was change the leads / meter and check the settings. Same outcome.

It's a PowerStation, that's a whole different kettle of fish and things can be very different

Yes a huge difference, and is quite difficult to explain the exact installation with words / without uploading a one liner. Multiple 415v switchboards feeding multiple downstream 415v switchboards, which then feed multiple downstream distribution boards e.g lighting DBā€™s.

Itā€™s the engineer that has requested Ze readings on all switchboards / distribution boards, excluding the switchboards connected to the step down transformers via bus duct. The previous commissioning team failed to take any results on first energisation.
 

Reply to Ze readings >2000Mohms in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

Similar Threads

On an EICR I found a 200 amp 3ph supply, TNS earthing (which looks original) and Ze of 19 ohms. No rods. No RCDs. L1 -> N was about 0.5 ohms...
Replies
11
Views
1K
Hi All New to this forum, have read the posts on here from google but only recently signed up. I'm having some issues and some input would be...
Replies
13
Views
2K
Hi, I've had an unusual result from taking a Ze at a clients house and wondered what everyones take on it would be: The property is TN-C-S A Ze...
Replies
9
Views
4K
Hi everyone, I have a particular job visited this morning, that on all circuits, at each point, i got a reading off around 25ohms for a earth...
Replies
14
Views
2K
Doing a lot of EICRs at the moment and have came across what I'm sure is a common enough problem. In an off grid rural cottage I have a TT system...
Replies
20
Views
3K

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc

YOUR Unread Posts

This website was designed, optimised and is hosted by untold.media Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock