Newbie help with Zs test result please

[pencilpusher]

Guitarist - Have you had any issues with your no trip readings as per the "known" problems with the 1700 series?

 

[Guitarist]

Guitarist - Have you had any issues with your no trip readings as per the "known" problems with the 1700 series?

Basically, the 1700 series does not give accurate readings on the 2-wire no-trip setting, although on 3-wire I have found it to be fairly accurate. Lots of people make the mistake of carrying out a Ze reading on 2-wire no-trip and then can't understand why the Ze is so high. I have spoken to Megger about making this more obvious in the literature and they have said it will be looked into.

To summarise-

Ze readings - 2-wire high current setting - Very accurate

Zs readings (where no RCD is present) - 2 or 3-wire high current setting - Very accurate

Zs readings (with RCD in circuit) - 3-wire no-trip setting - Fairly accurate


Personally, I never use the 2-wire no-trip setting now, as without the reference neutral the reading is not accurate at all. Megger have told me that with an RCD in circuit, an inaccurate reading isn't a problem as we have a few hundred ohms to play with.

The above comments are purely my opinion after my own experience, testing and talking to Megger.

 

[spark 68]

Personally, I never use the 2-wire no-trip setting now, as without the reference neutral the reading is not accurate at all. Megger have told me that with an RCD in circuit, an inaccurate reading isn't a problem as we have a few hundred ohms to play with.

The above comments are purely my opinion after my own experience, testing and talking to Megger.

This is pretty poor, I would have expected better from a premium brand such as Megger!, I have older Megger Equipment (in two instruments) the LRCD 220 and MIT320, and there are mistakes in the user manuals and in the updated online ones too!
Although the instruments themselves are OK, but again the (3-wire) no trip readings always give slightly higher readings than the high current readings, I now just record on the sheets the calculated Zs where possible, Ze+(R1+R2), and still carry out the no-trip reading just to double check.

 

[Guitarist]

My elecsa assessor advised calculating Zs by Ze +(R1+R2) on installation work, and measuring it with the MFT when testing and inspecting. High readings can then warrant further investigation. Seems like sound advice to me.
Spark 68, I was also very disappointed, but understood the limitations of the MFT once I'd had it explained. What I find almost unforgivable is that there was nothing in any documentation about this, and neither did tech support at megger help me on the phone. It was my 3rd e-mail which clarified it all. What a waste of my time and energy!
Actually, there is a line somewhere about 2-wire no-trip resting not always being accurate...

 

[pencilpusher]

Well, an update. I had my 1720 calibrated this afternoon. All very accurate except the low current setting no trip facility. With three wire it was about 0.15 ohms too low and with a two lead test it was up to 0.3 ohms too low!

I have spoken to Megger and they will look at it. I have firmware version 1.06 so its rather behind the new firmware now out. I will report back once they have looked at it and my calibrator is going to re check it for me. That will be interesting. Megger state that "0.2 or 0.3 isn't bad"

Guitarist - This backs up exactly what you said above.

 

[Guitarist]

Thanks for the update pp. Like everyone else, I just want a meter that I KNOW is accurate. I am at peace with mine now that I know its limitations.

 

[pencilpusher]

Yep, me too. For now my cali cert means I have to add 0.15 ohms to any three wire low current test I do. At least I know where I stand.

PP