Head scratcher!! Strange Zs readings

[Lagentium]

Evening all, bit of advice needed please. First time posting in this section because I don't really seem to come up against things that baffle me like this.
The job is an EICR on a production plant. Main supply to the plant doesn't have a neutral so someone has installed a 3 phase to the phase transformer which has a neutral connection on the secondary side to supply the lighting and small power, that is basically the premise of the job. The strange thing is that when I am testing I am getting a really high reading on the Ze and subsequent Zs readings. All insulation resistance and R1 and R2 values are where they should be. Due to me cross checking with the old test sheets as I go. The old test sheets, which are 7 years old. Have most test results around the 25 ohm mark. Now when I am testing I am getting around the 300 ohm mark? This is when the tester is set up in Zs RCD mode, when I press it and change the setup to Zs Loop I get readings around the 10 ohm mark. Tester is a Metrel 3125. Can someone please educate me please. I have a feeling it's to do with the TX regardless, but if someone can explain to me what's occurring I'd be chuffed. I even got one of the other lads to come down with his tester to check the results. I re checked my tester on another part of the site I had tested the other day and got the correct result in Zs RCD mode.
Baffled......thanks in advance

 

[westward10]

Is this a DNO transformer.

 

[Lagentium]

No Westy, it's a transformer installed by the customer, never seen it done before!!

 

[Ian1981]

No Westy, it's a transformer installed by the customer, never seen it done before!!

It's not a power factor correction unit is it?

 

[Lagentium]

Evening Ian, no mate it's not, definitely not. I remember this transformer getting changed years ago, so know of its use and function. Cheers though.

 

[Ian1981]

Maybes get the manufacturers details of it and give them a call perhaps?

 

[Wilko]

Can I ask a dumb question or 3? (just learning)
I assume DNO supply is 3 wire only, going into the primary of a local on site transformer ? (in delta config). The secondary is 4 wire with neutral? The neutral is earthed on site at the transformer? The DB is supplied as TNS (or TNCS) from the transformer ? If all these things are true, then shouldn't the Ze of the DB be much lower than you are seeing, in the usual sub 1 Ohm range as you are measuring the impedance of the winding and a bit of thick cable? Or perhaps the installation DB is actually TT'd and you are reading a change over time in the site Ra?

 

[westward10]

No Westy, it's a transformer installed by the customer, never seen it done before!!

I assume this is only supplying small lighting and power so have they installed if because of over voltage damaging equipment.

 

[Deleted member 26818]

I would assume they've installed it because they have no neutral.

 

[westward10]

I would assume they've installed it because they have no neutral.

I have seen no neutral supplies in existing 1950s factories but that is internal sub circuits, it is highly unusual for the mains not to incorporate a neutral for ancillary lighting and power.

 

[Lagentium]

Ian - I gave the manufacturers a buzz today but they were struggling to find details because it's 7 years old, I got a feeling I was ----ing in the wind after talking to the lad on the phone.
Spin - correct!
Westy - extremely unusual I'd say mate, I've never seen the arrangement before, but sadly that's the setup I'm faced with.
I just can't understand why the tested reads so high on Zs RCD, but so much lower on Zs Loop, there is obviously an earth present because the tester is getting a result.
Still baffled........

 

[davesparks]

By Zs RCD and Zs loop do you mean high current and low current tests?
If so then the difference you get is the reason that a high current test should always be used unless circumstances make this impractical, the low current test just doesn't give accurate readings.
I would imagine in this situation that having the transformer in the loop which is being tested is introducing the extra impedance measured by the low current test.

 

[static zap]

I would assume they've installed it because they have no neutral.

If they are creating a neutral , the balancing of the phases will probably play a part .
(what type of rating has the transformer got - vs -Load= is it just some lighting ?)
... A lost phase will send neutral way off ?

 

[martin mcguinness]

wilko it looks like we are both going down the same route
lagentium did you get any pics of the transformer or data plates is the transformer HV to lv or lv to lv
as normally the hv side has no neutral normally delta (3 wire ) and the lv side star with the starpoint earthed and neutral taken from the star point
the high ze and zs would indicate a problem with the earthing at the star point
i normally work with 11kv as an HV /LV ap at a hospital with 7 substations 5 of which are dual packaged so 11 transformers
normally you would get a very low ze or zs
do you know how the star point is earthed as this could be the issue also if the star point isnt earthed correctly it may cause issues with voltage reff point
if you got any photos can you upload