Zs higher than calculated

[brucelee]

Hi all

I have a megger 1552 and when carrying out a Zs on socket outlets using the 13A plug in adapter my measured Zs was higher than my calculated using the No trip setting because of RCDs
i was expecting lower due to resistances in parallel due to bonding

when using this adapter should the reistance be noted of the lead or auto nulled usin say a 13A single socket of a extexsion lead which has been linked out inside

or is this just because the no trip setting isnt as accurate as the high zs setting??

 

[topquark]

You can get oxidisation build up on the sockets plug terminals that can lead to a higher reading, try inserting removing the lead a few times and see if it drops. I think the kettle leads resistance is already built in to the unit (as it's attached to a different port on the mft).

 

[brucelee]

Hi mate

cheers for reply it was new sockets that i tested and did try a few times on each side of every double socket all higher than Ze + (R1 +R2)

I wasnt ever told to null this lead but my head was ticking wether it should be nulled or not as i was lead to believe from all courses and books it would be lower due to parallel paths

cheers topquark

 

[johnboy6083]

what reading do you get from the load terminals of the MCB, and what do you get on the supply terminals? im trying to see if your MFTispicking up any impedance from the MCB's. Try the same with the RCD too.

 

[topquark]

Yes mate you're right, you'd normally expect it to be lower. The no trip setting doesn't always like measuring through the RCD/RCBO. If the CU's not completely devoid of any loads (on other circuits) that can also affect the readings (but you'll often see the noise symbol on the mft then).

 

[telectrix]

and you don't null a 1552 on ZS test, only forr continuity. probably the higher readings are due to 1. no trip setting and 2. internal workings of MCBs/RCDs. these can push you readings up. all you need to worry about is that the reading is within the max. allowed for whatever MCB is on the circuit.

 

[brucelee]

Hi cheers to all

so its probably due to the no trip setting and measurig through the RCDs ad Cbs

thanks topquark, johnboy and of course Tel

as usual you have saved me some head scratching

all Zs readings comply with max measured Zs of GN3

thaks guys

 

[HandySparks]

I don't know about Megger, but with the Fluke 165X series you DO have to zero the lead and any adaptor before doing loop impedance testing (and low ohms). May be worth checking your instruction book.

 

[topquark]

It's not mentioned in the megger manual for loop impedance testing, only for continuity.

 

[brucelee]

yes the fluke i think has to be nulled as it uses the same skt ports and leads as the continuity testing but with a megger the 13A adapter lead for zs on skts has its own port on megger this is what was confusing me and hadnt ever been told to null it on the megger I only ever used a fluke for 2391 practical assessment 4

had already checked the booklet and this site for similar as well as google

cheers and thanks for replies