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?

 

[buzzlightyear]

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.

put it down to bad 1 croc clip &1probe.

 

[davesparks]

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.

Why are you doing it at the sockets? The pfc reading needs to be at the DB not the ends of the circuit.

The variations at the origin are likely to be down to the inaccuracy of the meter and possible poor test leads or contact pressure.

 

[Gavin John Hyde]

@davesparks i did do pfc at the board but its habit of always flicking dial round to test pfc on individual circuits too, something i was taught in college as an extra check, whether correctly or not and have done it ever since. i do think with the kewtech testers though it might be acceptable to do at sockets, might be wrong.... it was demonstrated at one of them tradefairs once with kt63 doing pfc at sockets to check individual circuits.... was couple years back mind. the guy did a zs at the socket then pressed a button to get psc/pfc...

 

[freddo]

The 64DL is limited compared to the KT65DL which can give readings down to 0.001, but switching off ATT will likely provide a more accurate Ze reading.

 

[Deleted member 72666]

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!

My thought too!

 

[The Ghost]

switching off ATT

That is specifically required in the manual when reading Ze

 

[stevels4]

Could I just ask why you are taking a Ze reading, I assume this was made by removing the red link between the cut out and meter?
If so this comes under Mocopa or DNO rules and should not be done unless you have the required authorisation to remove the red link, anything on the load side of the meter feel free to test.