TL inverters IR testing

sambotc · Feb 20, 2012

Sorry to raise this question again. What are you guys getting on your DC IR test's on new installs? I've been doing some DC IR testing on Fronius 3.6 IGTL inverters and they are coming back as 50-100M ohms which is fine as far as DTI guide says, but not what I was expecting ie >999M ohms type AC results? I am presuming this is due to lack of galvanic isolation in the TL but wanted to clarify? None of the arrays are bonded and all to date are TNCS

Cheers in advance

SolarCity · Feb 20, 2012

Don't want to go over old ground too much but you should be bonding those arrays if you're using a TL inverter.

What exactly are you testing? The IR tests should be done with the strings disconnected from the inverter - perhaps this is why you're getting those readings?

sambotc · Feb 20, 2012

Yeah I know the arrays should be bonded, but this crowd don't do it! I am carrying out test's to collect lost data as far as I can tell (just doing what I am told to earn a living due to recent redundancy) large well known company making up the rules to suit I guess!

As for inverter, your right I am being a dunce the inverter is out of the equation as the DC isolator is off when I test sorry,i'm testing CPC at AC isolator by inverter to DC - & + (2 separate tests noting lowest reading on paperwork)

For some reason the DC IR tests just aren't making sense to me which is why I asked the question. If it was an isolated result I would be concerned, but it's every job where the results are lower than I expect so their must be a reason, I just can't figure out what!

Cheers for the reply

sambotc · Feb 20, 2012

Just to add, using a metrel mi3200 MFT if its relevent, calibrated and all OK on AC tests so don't think thats the issue

sambotc · Feb 21, 2012

Any ideas or do you all think i'm bonkers?

Are you all getting infinite readings when doing IR on DC ?

SolarCity · Feb 21, 2012

I'm not sure why you test to your CPC as your PV installation uses Class II equipment anyway. You should test the strings to the metal work of your array (or at least that's what I do)

And I'm not sure why you're getting those readings either.

mmccx · Feb 21, 2012

are all the modules connected to the strings?

yellowvanman · Feb 21, 2012

SolarCity said:
I'm not sure why you test to your CPC as your PV installation uses Class II equipment anyway. You should test the strings to the metal work of your array (or at least that's what I do)

And I'm not sure why you're getting those readings either.

Page 53 of DTI Guide

sambotc · Feb 21, 2012

Taken from DTI guide

NOTE REGARDING TEST METHOD
Two test methods are possible:
a) Test between Array Negative and Earth followed by a test between Array Positive and Earth
b) Test between Earth and short-circuited Array Positive & Negative
Where the structure/frame is bonded to earth, the earth connection may be to any suitable
earth connection or to the array frame (where the array frame is utilised, ensure a good
contact and that there is continuity over the whole metallic frame).
For systems where the array frame is not bonded to earth (eg where there is a class II
installation) a commissioning engineer may choose to do two tests: i) between Array cables
and Earth and an additional test ii) between Array cables and Frame.
For Arrays that have no accessible conductive parts (eg PV roof tiles) the test should be between
Array cables and Building Earth

Still none the wiser as to why i'm getting a reading on my meter on more than one occasion? I've got a new install to do tomorrow so I will see if its the same reading on a job I have wired, I was carrying out IR DC testing wrong when I was installing before Dec 12th so can't comment.