Can anyone explain how to interpret a megger test result on a cable?
SITUATION: Chiller upgrade, power cables (480V, 3 phase)were scraped on sharp edge of bushing during pull. Contractor says only the pulling jacket was effected, not the insulation. Megger test was performed and all resistance tests "were in excess of 100 Megohms". What exactly does this mean? Does this mean, definitively, that the insulation has not been damaged?
thank you in advance for your response.

If the megger indicated greater than 100M ohms your insulation is OK.

In a nutshell if a megger reads open like above, the test passed. If you get a reading the test failed.

A megger is simular to a ohm meter on a DVM, but it uses a AC voltage at 50 volts or higher. This gives it the ability to punch through pin holes or arc tracts. So what you are looking for is any type of a reading.

The old hand crank ones are great for driving worms out of the ground.

A megger tests with high-voltage DC.

If you tried measuring with AC you'd read the overall impedance, which includes the reactance formed by the capacitance in the cable.

FYI, it was about 1965 it was dropped, but the NEC used to reference "1 meg-ohm for 1 minute" as a passing score on insulation resistance. The many different manufacturers as well as styles of insulation, varying conditions, etc, made them drop the quote, but it is still a very good guideline. On several occasions manufacturers were out of business, and it was the one I've referenced.

For 480V-AC circuits, usually 1kV-DC and sometimes 500V-DC is used as a test voltage—versus around 3 volts for the typical digital multimeter set for a resistance function. "Megg-o-Testers" are available roughly in the 100V to 5kV-DC range.

You can visualize a "Megg-o-Tester" insulation-resistance test set as simply an ohmmeter with the equivalent of a high-voltage battery.

Producers of these instruments are AEMC, Megger, [formerly Biddle/AVO] Fluke and others. { Er, see www.aemc.com .. www.megger.com .. www.fluke.com }




[This message has been edited by Bjarney (edited 04-04-2003).]

Don't let PETA find out!

The good megger reading doesn't necessarily mean that the insulation has not been damaged. It just means that under the current conditions that there are no phase to phase or phase to ground shorts. The insulation could be cut through to the copper and still get this reading if there are no conductive substances (wire pulling lube, water, ect.) in the area of the damage, and as long as the bare copper is not touching the conduit.
Don

seeks,
Please be aware that an Insulation- Resistance test, only tests between the cores(and their insulation) of the cable.
It does not include the outer sheating, unless the cable has been severely damaged,
in which case, you would get a dead short.

ThinkGood,
When I first started my time as a Line Mechanic, I was sent with a guy to disconnect some old Lines, they were across a river but were still live, long story short, when the line hit the water, all the fish came to the surface!,lying down, stunned!.
Trout for tea, that night!.

For what it's worth, in England we test 240V 1-ph and 240/415V 3-ph systems with 500V DC, and 1 MEG is the lowest acceptable reading for insulation, either between conductors or from any conductor to ground.

If I obtained a reading that low on a new system, however, I'd be concerned that something had been damaged during installation.

Don you said:


.. and I got to ask, then why megger at all? And why trust our Tools, if we can not rely on these tools,meters, meggers , clamp-ons, then troubleshooting becomes impossible.
I admit using a VOM or digital meter to find a ground fault is not a good idea but I have always had good results with a megger in such situations.
And you are right, up to a point, because if there is a fault in the wireing in conduit say some 50 to 75 feet from the controller and the controller is determined to be ok the problem is located somewhere between the motor and the controller, aside from the obvious damaged conduit, or a smoking motor, you must rely on these tools.

Just my two cents..

WOC

WOC,
Take the leads of your Megger and hold them a very small distance apart. Push the button and take the reading. It will read infinity. Air is a good insulator and 1000 volts won't jump very far. If there is bare copper in the raceway, there are no conductive contaminants, and the copper is separated from other copper and the conductive raceway, the Megger test will show that the cable is good. The Megger is giving you a resistance test between the two points where you connected the test leads. If there is an air gap, then the test will read good. If I was the owner and had any real concerns about the integrity of the insulation, I would ask that the conduit be filled with water and the test repeated.
I'm not saying that the Megger test is no good, just that you have to understand exactly what it does and how it works. This is much different from when troubleshooting a circuit that has a fault. In this case, we know for sure that there is a fault and the Megger will be very helpful in locating this fault. With a new cable we can only prove that there is not a fault, but we can't prove that the cable insulation has not been damaged.
Don

I would like to thank everyone for their input. The owner paid for a new cable, and that is what he deserves. On the other hand, the electrical contractor shouldn't have to replace the cable if it's not required. We are trying to determine whether or not the insulation has been damaged. This has proven to be quite complicated in that, while everyone agrees that the test number we got shows the cable satisfactory for use, is the cable "as-good-as-new?". Have the insulation properties gone from 100% to 90%? Whatwould the megger readings be then? My research has shown that the megger test is effected by several things - temperature, length and type of cable, even the method of testing, which varies by voltage and time of voltage application - 30 seconds, 60 seconds, etc. (BTW - this test was done with a Biddle No. 210900 digital megger tester. This instrument charges a DC capacitor, which sends a 500 volt charge through the cable.
DBC1 - I've heard that bass like their worms cooked! regular or extra-crispy?

I don't agree with that statement. It only shows that there were no faults at the time of the testing. It in no way shows that the cable is satisfactory for use. There could easily be exposed conductor in the raceway, but just not touching anything that is conductive. This could change if the conduit were to fill with water, or when the conductors move around as the result of temperature expansion and contraction. Yes, I know that the use of a megger is a commonly required acceptance test, but is does not prove that the insulation has not been damaged. It just proves the existence or nonexistence of a fault at the time of testing. The assumption is that if the cable is damaged, there will be a fault and that the megger will detect it. It will detect the fault if it exists, but the installation process can damage the insulation without producing an immediate fault. In some cases this insulation damage will never result in a fault, in others it will.
Don

resq - you're right. I should have said that everyone agrees that the test indicates no faults.
Is filling the conduit with water the only way to definitively test this cable? Or is there another test we could perform?
And the big question - how the heck do you fill the conduit with water?

I should add that this is an aboveground, RGS conduit, with cables installed, going from a 480/277 volt panel to a combination starter.

As Wolcot said the megger is testing between ground and Line for basic insulation. If the outer jacket was destroyed this will not make a difference. Basic insulation is provided on the conductor independent of the outer jacket. You are proven that there is insulation between ground and line and nothing else. The outer jacket is meant to provide reinforced insulation to the person per applicable safety standards.

ccaserta,
Even if the insulation itself is missing and there is bare copper, the megger will still show it to be ok, unless the bare copper is in contact with some other conductive object.

seeks,
I don't know what else you could do to prove that the insulation has not been damaged.

Don

Are there any gasses that would lessen the resistance of the ambient air, even briefly?

If said gas were to be pumped into the conduit while the megger test is being performed, then when the "Conductive" gas passes the point of damage, the resistance should drop momentariliy...

No?

</stupid idea>

acetylene?