Lower Ze reading than expected

[Gavin John Hyde]

On a job today and when testing got a Ze of 0.03 ohms on a tncs. Using a Kewtech 64DL.

Installation has main fuse in hallway outside the 2 bedflat then a switched fuse feeding a submain cable to the actual CU. I did Ze at both immediately before switched fuse and in CU with earth disconnected each time. Both times got 0.03 ohms even with the submain cable running 6 metres.. given the MFT;s have a margin of error this is a pain, I have checked the tester on a calibration box and it measures 1 ohm fine. it looses half an ohm at 100 ohm reading on loop test according to calibration box.

Zs@DB is 0.12 ohms... only utility feeding the property is a district heating system with metal pipes running through building into ground and heating building next door, these pipes are 4 inch diameter into building and around floors into water re heater boiler by flat entrance. the water is fed via plastic pipes. so would provide a very good parallel earth

The building is just over 2 years old and is just up road from the transformer.

Due to low Ze i am also getting funny and varying PFC PSCC readings which would if readings are correct then i have a very high lpf..... Have attached some images to explain better... Am wondering if leads might be playing up? MFT is only a month old out of the box... might use the R2 lead to add some extra resistance then take it off values when i go back tomorrow.
Any thoughts?

 

[Wilko]

Keep in mind I've now started pain killing rituals here, but I'm lost again. I got 0.03 as Ze measuring L-E with the earth dis from the board, but why does it rise to 0.12 when the other bonding is included?

In work we have nearly 20 MFTs so could justify buying the Megger test box, cracking bit of kit that gives us the option to test loop impedance. I have to say all the Meggers and Metrels we have are always within range. The Flukes can be a little more temperamental.

I don't think we'll be buying Kewtech in the near future...

Mine works perfect, always has, always will . Hang on, just gotta go sacrifice a fatted lamb or summat.

 

[Strima]

Keep in mind I've now started pain killing rituals here, but I'm lost again. I got 0.03 as Ze measuring L-E with the earth dis from the board, but why does it rise to 0.12 when the other bonding is included?

Mine works perfect, always has, always will . Hang on, just gotta go sacrifice a fatted lamb or summat.

I thought the 0.12 was the Zdb not the Ze???

 

[Wilko]

I was harf way thru that post when this joyous IT device posted it. B thing has a mind of its own, maybe that's a good thing ..

 

[suffolkspark]

are your leads okay? might sound stupid but I got in a right muddle with my KT63 last week! was testing sockets in a house and getting 0.73 zs on one socket, then on others on the same ring stupidly low readings like 0.03 and massive kA!!!! I would have thought dodgy leads should have increased the reading but for some reason it didn't! cracked out some fresh leads and everything stabilised and all sockets tested near enough the same at 0.73....

with the substation so close though its entirely possible that it isn't reading too far off!

 

[Gavin John Hyde]

okay 0.03 ohms is the Ze i get whether i test Ze at CU or the switched fuse as its technically origin... the 0.12 ohms is ZSdb which given the flat is serviced by huge metal pipes for the district heating which are bonded its got a parallel path of some substance! the heating system has a boiler type thing which is fed by the pipes and if i run a continuity test on the boiler pipes i can connect to one next door in their cupboard and get a beep.
If it helps anybody understand i have had copies emailed to me from the original install certificates by the owner. back then they only got a Ze of a 0.11 and the Zs values for circuits are pretty much what i got today, maybe a fraction out in some cases. if you look at the Zs values they are all quite low. so i suspect the flat has never had much of a Ze to begin with.if i do a PFC or PSCC pluged in at the sockets i get more sensible readings but at the CU they are all over.
i have blocked out the cert number to stop cowboys copying for fakes...

View attachment cert1.jpeg

View attachment cert2.jpeg

 

[Gavin John Hyde]

are your leads okay?

I am thinking that might be the case, so might buy some new ones to be sure... at very least it gives a comparison for the current ones. interestingly if i use the plug lead to test my readings are more stable

 

[buzzlightyear]

i have blocked out the cert number to stop cowboys copying for fakes...

now who would do a thing like that.

 

[Gavin John Hyde]

now who would do a thing like that.

cowboys lurk everywhere... homebase or screwfix carparks, on checkatrade and trustedtrader and even down your local pub... they are out there hiding ready to rip off somebody at a moments notice with a freshly ebay sourced scam logo on their van...

 

[Wilko]

I've missed that the switch fuse is 0.03 and further away in the flat you get Zdb 0.12 (?). Anyway, with 0.12 all your protective devices are within their breaking capacity so that's ok? If the impedance reading is suspicious (with very low numbers), perhaps try adding a known resistance cable in series?

 

[buzzlightyear]

you missed one home on the range .

 

[Gavin John Hyde]

@Wilko , yes thats correct Ze at Switched fuse of 0.03 and a Zs @ DB of 0.12. I am going to get a 2nd set of leads and compare and then do what you suggest, but when i look at the original EIC (I uploaded them as images) the original readings were also very low. mine are too far out the norm compared to these assuming the original firm tested correctly.

 

[Ian1981]

High resistance readings through the switch fuse contacts?
What was the R1+R2 reading on the submain?

A lot of my work colleagues use a kewtech and this problem of accuracy at low resistance testing was highlighted at a large installation on student accommodation we installed.
When carrying out a R1+R2 test on a 120mm submain to a block when using the kewtech on 2 separate kewtech meters the result was 0.00ohms.
When tested with my own megger 1552 mft I got a reading of 0.04 ohms.
We also tested other blocks some 150mm submains and one was 240mm.
Kewtech constantly read 0.00 whole the megger had readings of around 0.03 and 0.02 respectively. Submains varied in lengths but the kewtech couldn't read the low resistance continuity tests unlike the megger.
Just an example

 

[Gavin John Hyde]

@Ian1981 I haven't as yet done a R1+R2 on submain, which maybe is my poor terminology as its feeding the Cu from the switched fuse as tails would be too long otherwise, I wasn't expecting to have the results i have had so far in all honesty. i was only adding a circuit in for a heated towel rail! They aren't too far from original install tests looking at the cert from owner.
I get 0.03 ohms even testing Ze before the switch fuse with the earth disconnected from the bolt in switched fuse enclosure and testing on incoming line/ neutral. you can see what i mean in the pics in my original post on thread. i took earth out from the 16mm bolt for the Ze.

 

[davesparks]

0.03 ohms is below 0.35 so complies but my lpf is over the breakers at 6ka are no good if we are going by the regs...

But you said the Zdb is 0.12 ohms, so the prospective fault current at the DB is going to be around 2kA, well within the capability of a 6kA mcb.
The switched fuse at the origin will have a fuse with a much higher breaking capacity so that will almost certainly be fine too.

 

[Gavin John Hyde]

But you said the Zdb is 0.12 ohms, so the prospective fault current at the DB is going to be around 2kA, well within the capability of a 6kA mcb.
The switched fuse at the origin will have a fuse with a much higher breaking capacity so that will almost certainly be fine too.

okay, maybe i haven't explained things too well. I appreciate the replies... my confusion come from doing pfc and psc and getting varying results as shown in the pictures. when doign a pfc plugged into the sockets on rfc's i was getting 2.4kA and 2.65kA. both well within limits i cant explain the variation in readings the kt64 was giving out at CU.