IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Insulation test leads
Topic Summary:
Created On: 15 April 2014 07:07 PM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 15 April 2014 07:07 PM
User is offline View Users Profile Print this message



Fox123

Posts: 9
Joined: 13 April 2014

I may be asking the obvious but how do I specify connector cable for a good quality resistance meter (Megger MIT481) without paying the Megger premium. I have tried several lower cost connectors marked as 1000V Cat III, but when the new leads are paired alongside each other the open circuit reading drops from 200G? to only 30-60G?, presumably due leakage through the insulation. What must I be asking for to get reasonable quality cable from third party suppliers ? Thanks Peter.
 15 April 2014 07:12 PM
User is offline View Users Profile Print this message



aligarjon

Posts: 2896
Joined: 09 September 2005

have you tried the kewtec ones. i have had some for a couple of years and am very happy with them good crocs as well.

Gary

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 16 April 2014 06:00 AM
User is offline View Users Profile Print this message



Fox123

Posts: 9
Joined: 13 April 2014

Thanks Gary but I believe the Kewtech instruments are rated up to 2 G/ohm. What I am wondering is how to specify a 1kV cable that will test up to 200 G/ohms without loss.
 16 April 2014 08:38 AM
User is offline View Users Profile Print this message



aligarjon

Posts: 2896
Joined: 09 September 2005

oh sorry. that one is beyond me.

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 16 April 2014 09:56 PM
User is offline View Users Profile Print this message



paulskyrme

Posts: 833
Joined: 12 February 2003

Presumably then you are testing outside BS7671?
The commercial leads will be made to the instrument standard, which is oriented around BS7671/ EN60364 I would expect.
 17 April 2014 08:58 AM
User is offline View Users Profile Print this message



dlane

Posts: 690
Joined: 28 September 2007

The problem when trying to read very high insulation resistance readings is that your leakage currents are very low and the instruments tend to become a little unstable when testing out in the field.

At higher test voltages what you rest the leads on, your air gaps between connections, cleanliness and the ambient conditions, especially humidity, are all going to affect the readings.

So I would always go for the highest quality leads, look after them and keep them clean.

It also depends what type of insulation tests you are doing and what you are trying to prove. 1kV isn't a particularily high test voltage and to achieve even up to 30GOhms for the installation in 1 minute has proved the installation is in an accpeptable condition.

If you are carrying out PI tests and are achieving those values after 10 minutes it is still a good installation. Achieving an IR of greater than 5 GOhms after 1 minute can give a dubious PI test anyway and although we continue the test and record it we would note in the test protocol that the PI value may not be truly representative. We may also find that the reading would become erratic throughout the whole 10 minutes and we don't get a good charging curve result.

Kind regards

Donald Lane
 18 April 2014 06:52 AM
User is offline View Users Profile Print this message



Fox123

Posts: 9
Joined: 13 April 2014

This is all extremely helpful. I should have explained that I am not testing electrical installations. I am a water engineer and I am using resistance readings on a Megger MIT481 to measure different levels of moisture in wood. This is more precise and reliable than conventional moisture meters which cannot be calibrated for purposes of legal evidence. I have a test rig which has two pins 1cm apart and inserted 1cm into the wood samples. Resistivity readings on the Megger range between 4 M/ohms in air saturated wood and go up to 200 G/ohms in samples of room dry wood (around 7% moisture). I calibrate the full resistivity curve using a range of wood samples with known moisture contents (i.e. kiln dried & weighed).

It is the electrical connections that are stumping me. I need to purchase suitable cables for the test rig which can be paired together (i.e.. run alongside each other) and yet still register up to the full 200 G/Ohm capability of the Megger. The original leads that came with the Megger will do that but they are too short. I have tried three other types of cable, all specified as 1000V Cat III, but the open circuit resistance readings drop to around 30-60G/ohms when the cables are held alongside each other.

The question is how do I specify to purchase flexible cable that is 4mm diameter but has good enough insulation around it to read up to 200G/ohm ?
 18 April 2014 04:17 PM
User is offline View Users Profile Print this message



dlane

Posts: 690
Joined: 28 September 2007

Hi

There are 2 solutions that I can think of. The first is to order cable with a higher voltage rating than 1kV, go up to 2.5 or 5.0kV. DigiKey will sell tet cable on drums but it is likely to be expensive. Spark Plug lead cable can also be purchased on drums and you could make an adapter to go to the 4mm test plug or solder a 4mm connector on and insulate with heat shrink.

The second solution I can think of is to put sleeving over the existing cables. Heat shrink sleeving and electrical sleeving is readily available and will increase the insulation properties of the test leads. You can get heatshrink with an adhesive inside that will glue the sleeving to the test lead. You will be able to check the specification of the sleeving when you buy it, but it is usually rated from 1kV upwards.

Kind regards

Donald Lane
 18 April 2014 04:43 PM
User is offline View Users Profile Print this message



Fox123

Posts: 9
Joined: 13 April 2014

Thanks Donald, I hadn't thought of heat shrink sleeving. I'll certainly try that. I must say that coming into this for the first time I am a little surprised that there isn't some form of an insulation specification as well as the voltage, stamped onto the outside of cables.
 19 April 2014 02:52 PM
User is offline View Users Profile Print this message



Fox123

Posts: 9
Joined: 13 April 2014

Donald, a short trip to Maplins & two metres of 5mm shrink wrap on each lead has done the trick. Everything is back up to 200 G/ohms again. It does beg the question though, why there isn't a specification for the level of electrical insulation around a cable ? The 4mm cables that came with the Megger clearly have better insulated sheathing than other brands so that is good to be aware of. I see that the 10 T/ohm insulation testers do have thicker cable, but there must still be a specification of some sort.

Thanks for the tip, Peter
 22 April 2014 11:30 AM
User is offline View Users Profile Print this message



Fox123

Posts: 9
Joined: 13 April 2014

I think thicker cable may be the only answer. After 20-30 tests at 1000V the additional insulation provided by the shrink-wrap also broke down and started to leak current. I have now spoken with Megger and they say that all of their high voltage test leads are silicone sheathed and specified to EN 61010-031. Silicone apparently gives much better resistance than PVC/PU/PE. Rgds Peter
 22 April 2014 04:12 PM
User is offline View Users Profile Print this message


Avatar for potential.
potential

Posts: 1285
Joined: 01 February 2007

You've discovered that PVC isn't a good insulator by demanding open circuit results @ 1000v of >200Gohm.
Rubber insulation is electrically superior to PVC for any given thickness.

I'm puzzled how you expect to get consistent results with your technique of measuring different levels of moisture in wood using its resistance when wood is not a homogeneous material?
 22 April 2014 05:41 PM
User is offline View Users Profile Print this message



Fox123

Posts: 9
Joined: 13 April 2014

Hi, I thought that too but it turned out much more consistent than expected. The following download link shows the resistance curve for Scotts pine.

https://dl.dropboxusercontent.com/u/76319093/Electrical%20resistance%20and%20wood%20moisture.pdf

You can see that at the driest end of the scale the Megger starts to struggle and levels off at 200 Gig/ohms A more powerful instrument would no doubt go further. Above 7% moisture the response is better right down to fibre saturation point around 28% moisture around 3-4 Meg/ohms. Provided the wood blocks are left to acclimatise for long enough at differing humidity levels the curve is pretty consistent and repeatable. For a denser wood such as oak the resistance curve follows the same pattern but lies just below.
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.