Megging:

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Greg Goodell

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sacramento ca
We just had a electrical contractor megger out 4 conduit runs with 3 ea 750 mcm alum. cables per. =1600 amp. They told us everything was fine with there finding & that we would have the results in writin in a few days.
Within 48 hours 1 of the 750's blew a hole in the gutter it has been running thru for for many years. [10+ years]
There finding were each conductor [12ea] had a megaohms reading of 1000 to 1500 or more.
Can anyone tell me how this could happen?
 
I do not have a certain answer for you. But do keep in mind that a megger test will not tell you everything about the condition of the insulation system. It gives you the resistance between the wire being tested and the surrounding area. But if the insulation has a weak spot, and if that weak spot is not in the vicinity of another wire nor in the vicinity of the metal conduit, the megger will not tell you about that problem. A megger will also not tell you if there is any defect or corrosion taking place within the conductor itself. Nor can it predict an upcoming short circuit.
 
Over the years, megger testing when done right, has proved a real value to identify insulation problems.
http://www.anixter.com/AXECOM/AXEDocLib.nsf/(UnID)/874BBCCFEDFCB5D480256F2400744785/$file/F-1_16.pdf
 
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Greg Goodell said:
We just had a electrical contractor megger out 4 conduit runs with 3 ea 750 mcm alum. cables per. =1600 amp. They told us everything was fine with there finding & that we would have the results in writin in a few days.
Within 48 hours 1 of the 750's blew a hole in the gutter it has been running thru for for many years. [10+ years]
There finding were each conductor [12ea] had a megaohms reading of 1000 to 1500 or more.
Can anyone tell me how this could happen?
Click to expand...

Greg
What was the reason for the megger test? Were there any problems that triggered this test.

Marc
Meggering in regards to AFCI issues...how did you come up with the figure of 20 megs?
When meggering branch circuits after a fire or possibly a flood, what number of megs do you reference as a cut off for conductors? Do you reference different values for cloth covered conductors as opposed to THW-THHN?
 
Pierre C Belarge said:
Marc
Meggering in regards to AFCI issues...how did you come up with the figure of 20 megs?
When meggering branch circuits after a fire or possibly a flood, what number of megs do you reference as a cut off for conductors? Do you reference different values for cloth covered conductors as opposed to THW-THHN?
Click to expand...
I've noticed that when I meg regular building wire, it's mostly all very high readings. The suspect stuff has historicly been less than 20, which stands out like a sore thumb when compared to all the other historical readings of good wire and cable. That's why I choose 20. I know The Stitch In Time book talks about 1 meg, but holy smokes... that's darned low. 1 meg on a dry day is darned near a hard short on a damp day.
 
Several things could happen:

1. Were the conductors shifted after testing ie. what type of loads? High inrush currents upon re-energizing can cause conductors to shift, a open/bad spot can megger acceptable if in open air, but when it shifts to a grounded surface....

2. As noted improper testing, I have folllowed several techs that did not test properly.


3. With all types of testing there are limitations and possible issues of missing something. BUT in the long run when performed preoperly you will ALWAYS be further ahead if you test prior to engergizing..
 
brian john said:
Several things could happen:

1. Were the conductors shifted after testing ie. what type of loads?
Click to expand...
Oh yeah. I've heard conductors slapping the insides of conduits on high-inrush loads already. Pretty scary. It's one of those, "that can't be right" sort of moments.
 
brian john said:
Several things could happen:

1. Were the conductors shifted after testing ie. what type of loads? High inrush currents upon re-energizing can cause conductors to shift, a open/bad spot can megger acceptable if in open air, but when it shifts to a grounded surface....

2. As noted improper testing, I have folllowed several techs that did not test properly.


3. With all types of testing there are limitations and possible issues of missing something. BUT in the long run when performed preoperly you will ALWAYS be further ahead if you test prior to engergizing..
Click to expand...


What would you consider improper testing methods?
 
#1 We were having voltage problems, 1 leg was 20 to 25 volts low. Come to find out our power company had 3 bad capacitors. They replaced & all is well.
#2 The cables were removed off the breaker, which is directly under the gutters 90 degrees turn where the cable shorted.
 
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