Newbie help with Zs test result please

[NJG28]

Hi

New to trade and 1[SUP]st[/SUP] post – any help/advice appreciated

Testing DIY installation approx. 10 years old. Final circuit for sockets – a radial (not ring) wired in 2.5mm T&E protected by 20A mcb to BSEN 60898. Not logically wired, several JBs in loft and a number of spurs so not easy to identify “end” of circuit as cable runs all over place in loft backwards & forwards.

I make max Zs from OSG table B6 to be 1.85 ohms.

Using Megger 1720 only a few months old. Testing Zs with the Megger plug adaptor
Zs results confusing me so have tested at every socket in final circuit.
1) Zs varies in each double socket depending on if use LHS or RHS of double socket e.g. LHS 1.98 (fail) RHS 1.63 (pass). Is this to be expected and which reading do I take as proving 0.4 sec disconnection time?

2) Results vary in same room, some sockets pass & some fail. Should I be taking highest reading of complete circuit (several rooms) to show disconnection compliance?

3) Have located several JBs in loft & tightened the screw terminals. Retested every socket. All except 1 have increased Zs from original readings (the 1 decreased). I would have expected Zs to be lower with tighter connections. Can anyone explain why this could be happening?

Basically confused now as to what is going on and why. Any help/advice/pointers really appreciated
Thanks
Nige

 

[Octopus]

With double gang sockets which give different readings, I'd recommend you push the plug adapter in and out a number of times to see if the reading improves.

Failing that take the socket off and take the reading from the cables - you may have worn sockets involved.

 

[telectrix]

if you are using no trip test, then results can vary. if they're borderline on a 20AMCB, you could consider changing it for a 16A.

 

[spark 68]

You could also try the Ze + (R1+R2) method, by measuring the R1+R2 and adding to your Ze, this is an acceptable method.

As Tel said I have had some odd readings on the no trip settings too.

 

[Deleted member 9648]

As long as there's no RCD you should use the high current setting on your megger...the no trip test is very random,10 tests on the same socket will give 10 different readings...sometimes wildly different....how does your Ze fit in with the readings you are getting?

 

[NJG28]

Computer blew up so late reply/thanks: Sockets cheap (not mine) decent reading on replaced socket - thanks you were spot on.

 

[NJG28]

Computer blew up so late reply/thanks: Readings OK on sockets I replaced or some otheres after several attempt to get best positive connection. Cheers

 

[NJG28]

Computer blew up so late reply/thanks: All good with calculation method - thanks. Sockets quite cheap by look/feel - same as other issues in house - poor previous DIYer. Cheers again

 

[pencilpusher]

Megger have been having problems with their 1700 series no trip tests. In fact the advice is to take "several" ZS tests and take an average. If you need to use the no trip test then try this method. I personally would take 5 readings and average them out.

 

[Guitarist]

I have the same issues on my 1730. Basically, the plug adaptor can add quite a bit of resistance to the reading. Drop the socket and carefully test directly at the terminals. Always make sure you carry out a 3-wire test when using no-trip or the reading will be way out.
Also, the RCD/MCB may be adding resistance to your Zs so if you do as already suggested and carry out a Ze + (R1+R2), I have no doubt you will get a lower reading.
Yes, the highest reading on the circuit is the one you record.

 

[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