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Discussion Starter · #1 ·
Hello experts,

I would like to know if 200 Mega-ohms megger reading between the two cables of 185mm2 or 350MCM size and 100 meters long, is pass or failed with 380VAC supply Voltage and 200 Amps load.

PLs. help me, this is a serious situation.


Thank you in a advance...

-Jairus
 

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Hello experts,

I would like to know if 200 Mega-ohms megger reading between the two cables of 185mm2 or 350MCM size and 100 meters long, is pass or failed with 380VAC supply Voltage and 200 Amps load.

PLs. help me, this is a serious situation.


Thank you in a advance...

-Jairus
has this cable been lying around for a long time? if yes then might be having some moisture. 200megger is a low value. in my view

Sent from my HUAWEI Y210-0100 using Tapatalk 2
 

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Hello experts,

I would like to know if 200 Mega-ohms megger reading between the two cables of 185mm2 or 350MCM size and 100 meters long, is pass or failed with 380VAC supply Voltage and 200 Amps load.

PLs. help me, this is a serious situation.


Thank you in a advance...

-Jairus
How long was the test?
 

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Discussion Starter · #4 ·
Thank you MDShunk.

I did the test for about 3 minutes, I had wait until the reading became stable for at least 1 minute. I am using fluke 1507 insulation tester. my testing insulation voltage was at 1000V.
 

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Discussion Starter · #5 ·
has this cable been lying around for a long time? if yes then might be having some moisture. 200megger is a low value. in my view

Sent from my HUAWEI Y210-0100 using Tapatalk 2
Thank you nickson.

yes, the cable is lying for almost 6 years and my insulation test voltage is at 1000V.
 

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Thank you nickson.

yes, the cable is lying for almost 6 years and my insulation test voltage is at 1000V.
my few years in this field have seen cables staying idle for more than six years and they still give better megger value.I think it depends with the environment of where your cable was.


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Not trying to take over this thread, but can someone explain the whole time part of taking a megger reading? I have a megger that I have used for meggering circuits, motors, etc... Mine has no setting where I can run it for a certain amount of time. Mine basically seems to be connect leads, hit the test button and get your reading. I think I'm missing something...
 

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Not trying to take over this thread, but can someone explain the whole time part of taking a megger reading? I have a megger that I have used for meggering circuits, motors, etc... Mine has no setting where I can run it for a certain amount of time. Mine basically seems to be connect leads, hit the test button and get your reading. I think I'm missing something...
Does it have a time setting for Pi (Polarity Index) and Dar (Dielectric Absorbtion Ratio) tests?;)
 

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...I would like to know if 200 Mega-ohms megger reading between the two cables of 185mm2 or 350MCM size and 100 meters long, is pass or failed....
Especially measured phase-to-phase that's low enough to worry me.

The rule of thumb is 1MΩ/kV+1. So for a 600V system you'd expect a bare minimum of 2MΩ phase-to-ground before catastrophic failure.

That said, for insulation systems produced after 1970 you start at 100MΩ and go up from there. So for newer cabling you're looking at an expected minimum insulation value of 200MΩ phase-to-phase.

And realistically, I often pay perk up on any reading that I get below 1kMΩ because most modern, good insulation systems are well above that, even.

Get phase-to-ground readings on both cables if you can. Try wiping the ends of each cable with alcohol on a clean cloth to see if you're getting tracking over dirty insulation.
...Mine has no setting where I can run it for a certain amount of time. Mine basically seems to be connect leads, hit the test button and get your reading. I think I'm missing something...
PI and DA are just ratios, if your megger doesn't do them you can do the math.

PI = 10 minute / 1 minute
DAR = 1 minute / 30 second
 

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Jairus said:
Hello experts, I would like to know if 200 Mega-ohms megger reading between the two cables of 185mm2 or 350MCM size and 100 meters long, is pass or failed with 380VAC supply Voltage and 200 Amps load. PLs. help me, this is a serious situation. Thank you in a advance... -Jairus
Did the voltage drop on your meter while testing? Or was was it steady and did you test it with1kv or .5?
 

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I doubt it would blow up when energized, but those are not very good readings. What did you have phase to ground?
 

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From my point of view that's absolutely fine. Our legal minimum standard as laid out in our wiring rules, over here is 1 MOHM.
 

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Try wiping the ends of each cable with alcohol on a clean cloth to see if you're getting tracking over dirty insulation.
I had the same reasoning but I think 1000v injected in the cable should be able to pass any corrosion in the cable


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Discussion Starter · #17 ·
I doubt it would blow up when energized, but those are not very good readings. What did you have phase to ground?

What i have here are phase to phase line. will it blow up right away? or it will survive let say for a couple of year.
 

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Not trying to take over this thread, but can someone explain the whole time part of taking a megger reading? I have a megger that I have used for meggering circuits, motors, etc... Mine has no setting where I can run it for a certain amount of time. Mine basically seems to be connect leads, hit the test button and get your reading. I think I'm missing something...
if i remember correctly(and im not sure on that point:laughing:)
it gives time to basically warm up the conductor and insulation
raising the temp in a conductor changes its resistance slightly
but i think its to give time for reading to stabilize
 

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gnuuser said:
if i remember correctly(and im not sure on that point:laughing:) it gives time to basically warm up the conductor and insulation raising the temp in a conductor changes its resistance slightly but i think its to give time for reading to stabilize
Thanks! That definitely makes sense. I guess I'm just wondering how to go about testing for a said amount of time. I think my megger probably does not have that option... Maybe it's just the higher end ones?
 

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When you megger something you are not really measuring resistance, you are measuring current (And calculating resistance) There are 3 currents being measured (Important to understand you are measuring current, no such thing as an ohmeter) this is very important to understand when testing inductive equipment or a long MV cable run like the one being discussed here as it has capacitive properties.

(View attachment before reading on)

When a high DC voltage is first applied the total current (a) consists of three current components;

1. Leakage current
2. Capacitance charging current
3. Absorption current

Conduction/Leakage Current (b) - This current passes through the surface of the insulation. The magnitude of current flow depends on the resistance of the insulation. Surface leakage is usually not a problem because it can be eliminated through external cleaning.

Capacitive Current (c) - The insulating specimen appears, ideally, as a capacitor. As a DC voltage is applied to a capacitor initial charging current flows until the voltage drop across the component equals the source voltage. As the capacitor charges, its charging current decreases to a minimum. This is called its steady-state value.

Dielectric Absorption Current (d) - This current also appears at the initial application of test voltage the same as capacitance current. This current is required to polarize the insulating medium. In other words it is energy absorbed by the insulating system.

To seperate these different qualities of an insulating system you can use the time they take to decay (As you can see on the attachment). Since the currents are decaying the indicated resistance reading on your display will go higher.

When you test, record the values at 30 seconds, 1 minute, and 10 minutes. The 1 mniute reading divided by the 30 second reading is called your Dielectric Absorbtion Ratio (DAR). The 10 minute reading divided by the 1 minute reading is your Polarization Index (PI). The values of these ratios can be used to determine the condition of the entire insulating system and allow you to do a condition assement of your assets.

Another key thing is properly correcting your readings for temature. Most equipment is corrected to 20C, Temp makes a huge difference!!
Attached Images
 
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