Metrel MI300 - Unexpected No Trip Zs Reading

[Thenitwentbang]

I did originally post this in the Tools and Products area - but think that was probably the wrong place.

Recently, I purchased a Metrel MI3000. The initial impression was actually very good - looks robust enough and with very logical controls.

Shortly after getting it and whilst installing a SWA run to an outbuilding through a RCD spur, I had my first opportunity to test the No Trip PEFC facility. The readings were much higher than I expected (about 2 Ohms!!) so much re-checking of connections ensued, followed by another test - readings exactly the same. It's worth pointing out that the automated PSCC readings from the socket in the outbuilding were spot on.

My gut instinct is that the No Trip process is probably giving the unexpectedly high reading, so I took the meter into the house and tested from a socket adjacent to the spur (fed from the same ring). This seemed to confirm suspicion - no trip reading was almost exactly 1.5 Ohms higher than the standard (this socket is not RCD protected, so could perform both tests on it back to back). Reading with the No Trip facility set was 1.94 Ohms (so, that would be outside limits for the 32 A Type B breaker protecting the ring). However, with the No Trip facility switched off, the reading was a much more acceptable (and realistic) 0.43 Ohms.

So, long-wnded question, but can anyone throw some light on this for me? Until I've got it sorted, I'm definitely not going to trust the No Trip readings on this meter.

Cheers.

 

[telectrix]

hope you mean Zs, not pefc when you say ohms. the bad reading could be caused by noise on the circuit. i would deffo be on the phone to metrel, as it's under warranty.

 

[tony mc]

It could be the RCD putting up the reading. What were your dead R1 R2 results for the SWA cct and whats the ZE reading??

 

[Thenitwentbang]

hope you mean Zs, not pefc when you say ohms. the bad reading could be caused by noise on the circuit. i would deffo be on the phone to metrel, as it's under warranty.

DOH! Yes - a slip of my best typing fingers (and inability to proof read!)Thanks for the advice - and correction (embarrassed!)Cheers,Ian.

 

[Thenitwentbang]

It could be the RCD putting up the reading. What were your dead R1 R2 results for the SWA cct and whats the ZE reading??

Hi Tony,I dead tested the SWA at 0.37 Ohms.Vastly different to the auto test reading I'm getting.Cheers,Ian.

 

[Thenitwentbang]

It could be the RCD putting up the reading. What were your dead R1 R2 results for the SWA cct and whats the ZE reading??

Ze for the circuit is 0.18 Ohms.

 

[tony mc]

Sometime when you do the live ZS and a RCD or RCBO is involved on the cct it can (not always) put up your reading and in some cases it can be very high.

 

[telectrix]

RCDcould well be the problem. i tested a job recently where Ze was 0.2. the Zs read 351! retested at CU busbar and still got 349. a lot worse than your discrepancy.

 

[Thenitwentbang]

Thanks very much for the replies Telectrix and Tony MC. I do suspect that the RCD is playing a part in this.

I suppose that RCDs, by their very nature, would introduce resistance.

Thanks again,

Ian.

 

[malcolmsanford]

There was a thread on here a while back about Megger doing the same thing, giving un-realistic readings on the no trip facility, and I believe megger upgraded the MFT free of charge to stop this, but as usual can't find the thread.

As the meter is new I would still contact metrel and tell them what is happening and see if they offer you the same solution.

 

[Octopus]

Were the batteries on the Metrel low? I've noticed that if mine are low "unusual" readings can result, recharge the batteries and readings become "normal"!

 

[Thenitwentbang]

As the meter is new I would still contact metrel and tell them what is happening and see if they offer you the same solution.

Thanks for the advice - I think I will get in touch with them.

 

[Thenitwentbang]

Thanks Murdoch - had not thought of that. The batteries were about half charged, but I will give them a full charge and see if it improves the readings.