# megger readings



## All Current (Jul 31, 2011)

What is a good a megger reading for a 500' run of 350 kcmil cable. We are getting 1.0 m ohms.


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## Zog (Apr 15, 2009)

All Current said:


> What is a good a megger reading for a 500' run of 350 kcmil cable. We are getting 1.0 m ohms.


What is the voltage rating of the cable?
What was your test voltage?
What duration?
What was tempature at time of testing?
Where were your leads connected?

m = milli
M = Meg


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## brian john (Mar 11, 2007)

Zog said:


> m = milli
> M = Meg


In either case not good, especially if it is new cable.

BUT definitely more information is needed.


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## al_smelter (Jan 25, 2011)

Is the cable terminated on either end? To what? Some of our old switchgear in the carbon mill will give this kind of poor reading. We normally live with a lot of that (@< 600 volts).

Need much more information as Zog has alluded.


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## Rick567 (Mar 3, 2011)

Make sure your meter is actually reading megohms and not higher scale.


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## All Current (Jul 31, 2011)

The cable is cleared on both ends, one end is in a man hole safed off with 130c. It is 2kv cable, and it is approximately 15 years old. The overall length of cable is about 2400' this section was seperated out to try to get some good reading. But not working out so well. The entire cable can't be bad. It was definately meg ohms and we were testing at 1000 volts for about 15 seconds. We tested between legs and also each leg to ground. I should mention that it is paralled 350's that are triplex cable's. And every cable reads bad. Nothing got driven through duct bank either.


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## jmsmith (Sep 10, 2011)

All Current said:


> The cable is cleared on both ends, one end is in a man hole safed off with 130c. It is 2kv cable, and it is approximately 15 years old. The overall length of cable is about 2400' this section was seperated out to try to get some good reading. But not working out so well. The entire cable can't be bad. It was definately meg ohms and we were testing at 1000 volts for about 15 seconds. We tested between legs and also each leg to ground. I should mention that it is paralled 350's that are triplex cable's. And every cable reads bad. Nothing got driven through duct bank either.


A couple more questions here.... Is the triplex all one phase? If so, are you megging these as a SINGLE cable (all together), or each conductor separately? If you are treating the three conductors as ONE cable, you would be paralleling resistances. The resistance reading in this case would be less than HALF of what one conductor would be on it's own.


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## brian john (Mar 11, 2007)

What prompted you to megger the cables in sections was there a fault?

Why did they pick 1000 VDC?

I think you may have issues.


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## All Current (Jul 31, 2011)

There are definately issues, the breaker which is rated at 70 amps was tripping after about 2 hours. I was testing each cable individually and were all reading approximately 1 meggohm. I thought it was so strange I tryed a couple different meggers 1 crank style and one newer digital type. The cables were tested as phases then I unspliced the two sets to test them individually. Everything reads about the same. the two sets were spliced down to #2's to fit on breaker.


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## jmsmith (Sep 10, 2011)

brian john said:


> What prompted you to megger the cables in sections was there a fault?
> 
> Why did they pick 1000 VDC?
> 
> I think you may have issues.


That's why I was asking about the arrangement, Brian. After he mentioned triplex, I'm wondering if they are megging the triplex as one phase and not megging EACH conductor individually. Would the triplex act like a standard parallel circuit if all three conductors are measured as a single unit?


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## jefft110 (Jul 7, 2010)

I would think you would want to meg a 500' run for a lot longer than 15sec.


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## RIVETER (Sep 26, 2009)

jefft110 said:


> I would think you would want to meg a 500' run for a lot longer than 15sec.


Why?


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## jefft110 (Jul 7, 2010)

RIVETER said:


> Why?


To the OP, and Riveter... if he's being serious.


http://www.scribd.com/doc/24911026/Megger-Book-The-Complete-Guide-to-Electrical-Insulation-Testing


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## sparky970 (Mar 19, 2008)

All Current said:


> What is a good a megger reading for a 500' run of 350 kcmil cable. We are getting 1.0 m ohms.


Seems a little low.


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## RobRoy (Aug 6, 2009)

I was uploading some readings to my laptop, and I came across this article, whilst multitasking on my iPad2.
I always do my tests for at least 3 minutes....


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## micromind (Aug 11, 2007)

If a 2KV cable is reading 1 meg at 1KV, there are serious issues with it. I'd be concerned with 1 meg at 4KV, let alone 1KV.


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## knowshorts (Jan 9, 2009)

Here is a .pdf version:

http://www.omnicontrols.com/articles/megger-a_stich_in_time.pdf


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## brian john (Mar 11, 2007)

RIVETER said:


> Why?


The cable has to charge up, the readings will change, typically (hopefully) increase.


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## al_smelter (Jan 25, 2011)

If you're telling us that you actually tested six conductors (parallel sets) and every single wire shows the same reading, chances are that the cable(s) is NOT faulty in that stretch. It would be a hell of a phenomenon for every single conductor to show the exact same result.... unless you had a catastrophic failure/fire and the results were a dead short. 

I cannot definitively assess the issue without seeing it, but I would look at my testing methods (not that you are doing anything "wrong") first. Something just doesn't wash with all the same results on every conductor.

But stranger things have happened (my disclaimer)

Before I proclaimed the cables bad with the one test, I'd DC Hi-Pot them as well. I would ramp and hold at least all 2kV for 15 minutes (I would not acceptance test 15 year old cable). If it breaks over, then indeed they were bad to begin with, so nothing lost by snapping them. If not, record the leakage and then put them back in service and look for another issue. Rinse, repeat with other parallel set.

Mark


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## sparky970 (Mar 19, 2008)

Did you check your meter?


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## All Current (Jul 31, 2011)

Tried several different meters. Today we are splicing our cuts. Because we found a burnt up section in one of the manholes. Then we are going to reenergize,probably in the next couple of days . I have a feeling it will hold. Like I have been saying I have a hard time believing every section is bad. I agree the cable is probably going bad but probably has some life. The reading were not all exacly the same across the three sections we cut it in. We could probably energize the first section Coming off breaker today and see if that holds.


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## Zog (Apr 15, 2009)

RIVETER said:


> Why?


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!!


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## Zog (Apr 15, 2009)

al_smelter said:


> Before I proclaimed the cables bad with the one test, I'd DC Hi-Pot them as well. I would ramp and hold at least all 2kV for 15 minutes (I would not acceptance test 15 year old cable). If it breaks over, then indeed they were bad to begin with, so nothing lost by snapping them. If not, record the leakage and then put them back in service and look for another issue. Rinse, repeat with other parallel set.
> 
> Mark


DC hipot is considered a destructive test for service aged MV cables by IEEE, ANSI, NETA, ICEA standards. Not to mention it only reveals gross installation errors, a Tan Delta, VLF, or Partial Discharge test should be done in lieu of a DC hipot test.


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## Zog (Apr 15, 2009)

All Current said:


> We could probably energize the first section Coming off breaker today and see if that holds.


  See if it holds??

I suggest you hire a testing company.


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