Lower Ze reading than expected

[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.

 

[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.