IR testing not settling on a measurement

[Sbc]

Morning to all the good people out there.

When insulation resistance testing using Megger MFT 1710, sometimes the value never get to >999, it will keep rolling over random numbers and never settle which i do for about 15-20 seconds and give up. I may expect to see this on older cable but i've been getting this on new cable, 1.5mm & 2.5mm.

I got the megger with the 7 week C&G course i did, so im now starting out and learning in the real world

Could anyone tell me if this is normal?

Should it settle on a measurement?

How long should I do the IR test for?

What measurement should i enter on the cert?


Thanks in advance to all who reply

 

[Silly Sausage]

This is worth reading...

View attachment Insulation Testing Fluke.pdf

If your reading is just oscillating around a certain value, record the lowest.
If it's continually falling, you need to carry on the test until the reading stabalises.

 

[telectrix]

if you are holding the leads or breathing on the connections, it can affect the reading. bloody stupid having meters read up to hundreds of Megs. IMO.

 

[Octopus]

Are you testing a circuit which has recently been plastered over? Moisture will affect the readings

 

[Knobhead]

if you are holding the leads or breathing on the connections, it can affect the reading. bloody stupid having meters read up to hundreds of Megs. IMO.

My old works meter would read 999 Gigs. Bad weather would play havock with it if you didn't set the leads out correctly.

I've corrected this now a couple have had a go at me.

 

[Silly Sausage]

My old works meter would read 100 Gigs. Bad weather would play havock with it if you didn't set the leads out correctly.

Hahahahaa....and there was me going to say my IR tester goes to 2G!

 

[sparks1973]

there may be a resistor thats gone in this tester as well......get it on a checkbox to verefy....

 

[Brightspark2]

My old works meter would read 100 Gigs. Bad weather would play havock with it if you didn't set the leads out correctly.

Well mine used to read infinity....



*Insert tongue in cheek smilie here*

Sent from my Xperia S using next doors WIFI

 

[Amp David]

Leave the test on for minute.

Had it before where when doing a global test the reading is up and down and never stabilising. Keeping the test on for 60 secs sorted it and it stabilized as it mostly will imo

 

[Deleted member 26818]

This instability is usually due to damp.
As you hold the test buton, current is heating up the conductors causing the damp to evaporate.
If cable ends are left exposed for any amount of time, damp will travel up the cables from the exposed ends.
It does appear that the newer low smoke cables are more prone to this.

 

[Sbc]

Thanks all, good stuff, gonna try some of those recommendations

 

[Silly Sausage]

If cable ends are left exposed for any amount of time, damp will travel up the cables from the exposed ends.

A good reason for taping up the ends of cables.

 

[Engineer54]

How long should I do the IR test for?

You have been given the various reasons why you are experiencing fluctuating IR readings. So i will just give you what the recommended time durations should be.

When conducting final IR tests after an installation has been completed or for other official recording, the time duration should be a minimum of one minute, during which time the meters reading should be stable. For general checking, 10 to 20 sec's should be sufficient to see if there is a problem present or not!!

As for ranges on IR test meters, it is standard practice to use a 1Tg ohm or more, meter on sub-station IR testing, at anything between the test equipments 1KV to 10kV voltage ranges...