Ze/Zs differ (wildly) between no-trip and 'regular' settings on Fluke

[Noddy67]

Hi,

A puzzling issue today.

When I test for Ze and Zs on my Fluke I tend to go for the 'no trip' setting out of habit. I believe whether there's a RCD or not this test might take a little longer to get the reading (as it saturates the RCD with a DC current) but it's the same result as the regular Ze/Zs test in the end.

But I just tested a house circuit (Ze and Zs) and got a c150 ohms reading! tested it again and same. But when fiddling with the meter I tested the socket again on the regular setting (ie Not No-Trip) it was an acceptable 1.1 ohms.

Any ideas as to why this happens? Shouldnt' they be the same?

thanks

Richard

 

[Sparky Ninja]

This was recently discussed regarding a 'megger' here maybe it will help?

 

[Noddy67]

I did see a thread started in Mid December by 'David Amp' but looking through that it looked like marginal differences (+/- 10-20%). Mine problem seems more fundamental (+ 15,000%) if my maths is right.

 

[Sparky Ninja]

Have you read the one I linked to?

 

[Noddy67]

Thanks - I didn't see the hyperlink on my screen.

Again, that thread showed fairly subtle differences compared to my concern. Any my circuit only has fuse wire at the board, no MCB's or RCB to consider in terms of impendance.

 

[Guest123]

I'm a bit confused, as Ze & Zs are totally different tests.

Are you saying you have a Ze of 150ohms??? if so is it a TT system???

Can you clarify a bit please.

 

[Noddy67]

As far as the test meter is concerned Ze and Zs are exactly the same test, but whether is 'e' (external) or 's' (sum) depends on where you conduct the test (Ze at board, Zs at furthest point).

It's a TNS arrangement.

 

[sparks1234]

I think what he means is that if you were to test say the Ze on trip and no trip you get a different reading. the machines sample the results on no trip and is not that accurate.
the other thing I have found is that the RCBO's I use in a Hager board generally have an inbuilt resistance of around 0.68 ohms
I

 

[Noddy67]

oh, and thanks for the responses so far.

I should also say I tested this a few times at different times and points and got similar results. If it were a one off I'd happily have just put it down as 'one of those things'.

Rgds

 

[Noddy67]

Thanks Sparks 1234 as well, but no RCCB's, no MCB's... just a very old fuse wired board (the owners just got PV fitted, that coulnd't be an issue could it? I know, I know, he spent £9k on PV last month but won't spring for a modern consumer unit)

"No trip" may not as accurate, but these figures might have meant me calling in the supply company if i hadn't fiddle around with the settings.

 

[kmcgold]

No trip are not has accurate only use on rcd's
my meter is a dai log 17th tester it will give different results every time in the same socket
you might get two the almost the same after a few goes .
sent meter back twice told it was ok.

 

[telectrix]

suggest testing on another installation to see if you get the same difference. or it could be a fault with the test meter.

 

[topquark]

Is the PV disconnected when testing? I would typically have the CU off and the main earth removed from terminal bar and connected to the meter (and hence there would be no reason to use "No Trip" settings for Ze.

Coupled with R1+R2 for the circuit that should give a good indication of what to expect for the Zs (if you test at the point with the highest R1+R2 reading).

If the PV installation were faulty and allowing DC to flow then that would most definately affect the readings.

 

[Noddy67]

thanks Teletrisc - i tested the meter when i got home as it was a bit of a head-f***. And you sometime know when you're testing something that just not right.

Topquark. Tested the Ze as you describe and got the same silly figure... maybe it is the PV somehow but to be honest I don't really understand how 'DC saturates an RCD for testing' and equally why a faulty PV would have the same affect but it does sound right if the PV is feeding into the meter and the test includes some element of the PV.

Oh well. I'm going to have to put this down to experience or perhaps post a PV led question on that part of the forum.

But thanks again - I've had three questions on the forum today and all have got results. Brilliant.