Zs reading

[Robojin]

Finishing off a refurb today and had one of those unexpected results (well for me that is)

All circuits have nice low R1+R2, IR's are all high as you would expect

Check Ze and get 0.11ohms

First RCBO 16A R1+R2 was 0.18ohms and Zs of 0.28ohms so as expected

Next RCBO 6A R1+R2 was 1.07ohms with Zs of 1.15ohms (did not check both 2way position yet)

Get to the first RCD circuit bank, Ring 32A R2+R1 0.21ohms, and Zs of 1.56ohms - er ?:|

so pick a circuit on the 2nd RCD bank Ring 32A R1+R2 0.23ohms and Zs of 1.68ohms -

So picked another at random, and similar result X(

As it was getting late and the house is unoccupied, decided not to headbang but retest each circuit tomorrow

Am I correct that a Zs reading from meters are calculated?, I know that I can add Ze to R1+R2 to get Zs, but can't say I've ever had this before, am I being daft, what have I overlooked?

The board is a contactum, have one here at home without issue, so this has thrown me a bit

 

[Jono Pashley]

May sound daft but you have checked the connections to the RCD and mcbs in the board arent loose or damaged? Connections to the earth and neutral bars would also be a rea to look at, other than that i dont know :S

 

[Robojin]

May sound daft but you have checked the connections to the RCD and mcbs in the board arent loose or damaged? Connections to the earth and neutral bars would also be a rea to look at, other than that i dont know :S

That was a thought as I was driving home, to check N continuity on the DB when I'm back there on Thursday

Before I left the house I did check the Buss bar connections just to ensure one was not loose

 

[green-hornet]

I would suspect a loose neutral, seen it many times where "I" have clamped the neutral to the wrong side of the rcd clamp.

I know its a stupid mistake but if you test and check everything I am sure I never left a property like that :^O

However if your getting it on both RCD banks then I would check all the factory connections in the cu.

 

[Robojin]

I would suspect a loose neutral, seen it many times where "I" have clamped the neutral to the wrong side of the rcd clamp.I know its a stupid mistake but if you test and check everything I am sure I never left a property like that :^O

However if your getting it on both RCD banks then I would check all the factory connections in the cu.

I'll check these on Thurs, thanks both for concluding what I thought it likely to be

 

[NozSpark]

As above, but why not do your RCD tests first... it could have a dirty contact? :|

 

[Robojin]

As above, but why not do your RCD tests first... it could have a dirty contact? :|

I did do an RCD test on each before I left

RCD 1 = 46ms/22ms (although failed on 1x the first time)

RCD 2 = 30ms/28ms

and the first RCBO 19ms/18ms

The problem has be something simple that I've overlooked, after all a CU is not exactly complex

Again thanks for the input Noz

 

[GHE]

Ive had a similar fault and it was found to be the resistance of the RCD through the live side contact probably dirty as Nozspark said I had a resistance of 1 ohm which obviously pushed the reading up higher than expected. Sounds funny to have 2 RCDs dirty though!

 

[binky]

Sounds funny to have 2 RCDs dirty though!

Possible manufacturing default - same batch if both 32A RCBOs have same problem. Try operating RCBOs a few times and see if that improves, otherwise suggest loose terminal, or you clamped on insulation rather than copper wire -easily done with RCBOs because of the deep recess cable fits into.

 

[Robojin]

Possible manufacturing default - same batch if both 32A RCBOs have same problem. Try operating RCBOs a few times and see if that improves, otherwise suggest loose terminal, or you clamped on insulation rather than copper wire -easily done with RCBOs because of the deep recess cable fits into.

No problem with the first two circuits on RCBO's

Checked with the wholesaler as I was in there, they recalled that someone had reported an issue like this recently

So will check it all tomorrow

 

[Robojin]

OK here was the result

When measuring Ze on the incomming side of both RCD's Ze of 0.16ohms (0.15ohms at the main incomer)

With the RCD open measuring the outgoing RCD L N Ze I get 1.6ohms, ?:|

Spoke to the manufacturer's technical office, they had a few calls about this but never got to the bottom of it as the callers never called back

Even tracked down my old college lecturer to see if he could fathom it (it was his suggestion to measure loop either side of the RCD L N)

In the end they concluded that it made no sense and may just be the combination of meter + RCD peculiarity, and provided that the RCD's tripped correctly and that my R1+R2+Ze were compliant that I should put it down as a comment on the EIC including discussed with manufacturer

But still a frustrating day headbang , particularly as the customer was around and picked up on it, so took a while to reassure them that it was all ok :|

I have the same secondary board at home I installed last year, so I'll retest that tomorrow

Still I'd like to get to bottom of this

 

[NozSpark]

I did do an RCD test on each before I left RCD 1 = 46ms/22ms (although failed on 1x the first time)

RCD 2 = 30ms/28ms

and the first RCBO 19ms/18ms

The problem has be something simple that I've overlooked, after all a CU is not exactly complex

Again thanks for the input Noz

Thats a fail then... :O

Could you rely on this RCD to save your life???

If it doesn't work the first time how do you know wether it'll work the second, third, fourth or tenth time???

 

[Robojin]

Thats a fail then... :OCould you rely on this RCD to save your life???

If it doesn't work the first time how do you know wether it'll work the second, third, fourth or tenth time???

OK have now got to the bottom of this

Firstly the RCD's trip as they should they are working as designed

Having spoken to Kewtech they are aware that the Zs test on Kewtech/Megga and possibly Fluke may give a false reading on the Zs test on some makes of RCD, fundementally it's down to the way meters create the loop fault at three frequencies and derive the result from an average

There is no specific as to which brands of RCD highlight this, I can only confirm that Contactum RCD's do affect the Zs result, however RCBO's are not affected (possibly due the electronics in the RCBO's)

The solution if you believe the Zs reading you're getting on your meter via an RCD circuit is at odds with your Ze+R1+R2, then measure the Ze on the incoming side of the RCD, and then Ze on the outgoing side if they are the same look elsewhere if however they differ deduct the value from all of your Zs Meter readings, as many will point out if you have Ze+R1+R2 you have Zs (albeit without parallel paths factored in)

Just for the record on site I have 1.30ohms error value, and on testing a board I have here I get 0.5ohms

There's no point having a 'my meter is better' thread, I just thought it might save others half a day of headbang that I had

 

[Andy™]

Some useful info there

 

[rugbymacc]

i get high Zs readings (no trip) on my fluke 1653b model (3month old)

yet my mate can do the same test using his megger multi and gets a reading much lower than mine.

say 1 ohm for the fluke ... meggar 0.75 ohm (t/e 2.5)

but in the calibration checkbox we get the same !!!!!

?????????????

 

[Robojin]

i get high Zs readings (no trip) on my fluke 1653b model (3month old)yet my mate can do the same test using his megger multi and gets a reading much lower than mine.

say 1 ohm for the fluke ... meggar 0.75 ohm (t/e 2.5)

but in the calibration checkbox we get the same !!!!!

?????????????

This could be explained by the method each manufacturer use to calculate Zs from the test the meter does on a live circuit

In your case you need to look at the differences between your real circuit compared to the check box (control method), you may be seeing this RCD issue on the live circuit, after all on one CU it was only 0.5ohms out for me, it could be the 0.25ohms for you

It might be worth you both doing the Ze on the RCD as I describe above, if you find 0.25ohms you have your answer

 

[green-hornet]

I am now even more curious.

To be fair I have only had high readings once or twice and found the problem to be the RCD, however I did not know the different readings you could get from one tester to another of different makes, and the effect in readings you can get.

Now I am aware of this I may buy another model of tester to double check.

 

[Nicky Tesla]

That was a thought as I was driving home, to check N continuity on the DB when I'm back there on ThursdayBefore I left the house I did check the Buss bar connections just to ensure one was not loose

Zs is live to earth, nothing to do with the neutral

 

[kme]

Zs is live to earth, nothing to do with the neutral

And the meter uses the neutral as a reference marker, and to control the pulse limits so as not to trip the RCD ( except on the newer "flash 2 lead no trip" testers).

Loose N can have an effect mate.