r1/r2 and ze dont measure up?

[jam2020]

ive just tested a 16a radial socket circuit. r1+r2 was 0.14 ze was 0.2 but measured zs was 0.74. am i right in thinking this is to high? why would i get this figure?

 

[johnboy6083]

bad connection in the board?

 

[jam2020]

sound connections through out,even with bonding/parralell paths involved the reading should only be 0.16 or slighty more?

 

[Jimmy Boy]

Did you IR the cables ?

 

[jam2020]

all greater than 299 on my brand new megger. what value zs would you accept if you were testing?

 

[Jimmy Boy]

I take it you did the Zs using plug top method at last point at radial ? Check connections at board and back of socket(s) or measure it with probes on the back of socket see if the same (Don't forget your safety specs)..
0h and was this a hard test or a soft test ?

 

[jam2020]

yes measured zs with plug adaptor, what reading would you be looking for steve? also why will ze=r1+r2 never really give an accurate reading? thanks for all your help im quite new to the testing side of things and classroom learning and on the job situations are a bit different!!

 

[SimpleSimon]

Any substantial part of the cable going through any insulation or temperature change?

 

[telectrix]

reading is well below max. allowed for 32A MCB. acceptable. if you test on no trip for Zs and high current for Ze, that could be the reason. still poss poor connection. terminal screw thru insulation? try other sockets. if you get big difference between 2 sockets, that,s where to look for bad connection

 

[jam2020]

no its a 6m run clipped to joists under floor,no insulation anywhere near it, im going to go back and retest it as its playing on my mind now and i want to sleep tonight!

 

[keniff]

I wouldn't worry about this at all, u will never get a calculated answer bang on when u use your megger, especially if it's on no trip, u can be up to 2.87 ohms on a 16 amp mcb so u have a good reading, also just make sure u zero ur leads out?

 

[nickblake]

As keith says 2.87 is Max Zs , what could be causing the high reading is a worn socket outlet you may never get the perfect readings

 

[beaver74]

I have just today had this
tested ZE live side of RCD then load side and there was a differance of .40 ohms
this would explain it

 

[tomxhug]

possibly bad connection with your plug top adapter, either the leads on the Socket outlet?

 

[jam2020]

cheers lads, im planning on using it for an niceic assessment so probably being a bit over cautious,as i say megger is brand new so perhaps it is the no trip/high settings. ill have a play with it tommorow on another installation and see where i go. thanks again

 

[Jimmy Boy]

yes measured zs with plug adaptor, what reading would you be looking for steve? also why will ze=r1+r2 never really give an accurate reading? thanks for all your help im quite new to the testing side of things and classroom learning and on the job situations are a bit different!!

Your R1+R2 is a 'Dead test' isn't it, the Zs is a 'Live' test isn't it, so because it's a Live test the cpc of the circuit is connected to earth and everything bonded to earth so you will have parallel earth paths.

 

[Jimmy Boy]

I have just today had this
tested ZE live side of RCD then load side and there was a differance of .40 ohms
this would explain it

Sorry if I have wrong end of stick it's late. A Ze test is conducted with the supply to the installation OFF and preferably locked off the main Earth is disconnected from the CU and measurement is taken between Line a E. After this you reconnect the Earth and carry out your PEFC and PSSC test to gain your PFC for the cert, so why would you have the load side energised ? and measured through an RCD ? Not looking for an argument just to clarify your method.

ATB Steve

 

[Sintra]

I think beaver was testing each side of the RCD to check for a rise in impedance although this would not be a Ze test.

 

[Jimmy Boy]

Sorry mate I don't understand ? He distinctly said he was testing for Ze ?.
Can you explain the test he was doing as I am eager to learn

 

[Sintra]

I have had instances in the past were my figures were way out as in the original post and couldn't find an obvious answer apart from the additional components involved between continuity testing and earth loop testing. ie an MCB or RCD or both. If you have an MCB etc with a high resistance then your Zs figure will be high so a way to check out a MCB or RCD is to conduct a loop test at both the supply and load sides of the device. This is the reason that i would always test Zs on all circuits as this is the only test that verifies the complete circuit including MCB's, RCD's etc.

 

[malcolmsanford]

ive just tested a 16a radial socket circuit. r1+r2 was 0.14 ze was 0.2 but measured zs was 0.74. am i right in thinking this is to high? why would i get this figure?

Was this a PIR ? or a new installation. If it was an old installation you may find that you will need to putin/remove the tester plug a few times to clean the contacts as sometimes an old socket outlet can give you odd results.

Also certain type of cheap socket outlets can give you false results as the contacts are made of inferior material. Try pluging something in a few times to clean the contacts then retest. You normally find that a live Zs would be lower than a calcualted one so your results are a suprise.