# Megger test for how long?



## brother (Nov 25, 2008)

Just curious as to how do others do their meggering of motors, wires etc..

If you have a 480v 3phase motor, how long do you megger, 30 seconds? 20 seconds? 1 minute? and at what voltage, 500v or 1000volts? whats your rule of thumb?

We use a 1000 volts on 480 systems. I was taught to use double the voltage of what you are meggering. 240v would gets 500v etc...


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## Zog (Apr 15, 2009)

brother said:


> Just curious as to how do others do their meggering of motors, wires etc..
> 
> If you have a 480v 3phase motor, how long do you megger, 30 seconds? 20 seconds? 1 minute? and at what voltage, 500v or 1000volts? whats your rule of thumb?
> 
> We use a 1000 volts on 480 systems. I was taught to use double the voltage of what you are meggering. 240v would gets 500v etc...


No rules of thumb, there are standards for things like this, ANSI/IEEE/NETA. 

1. Get the correct test voltages and minimum specs for the equipment you are testing. (NETA MTS has them all)
2. Tempature correct your readings, to 20 or 40 C depending on equipment, this makes a huge difference. 
3. Duration also depends on the equipment, for motors it is a 10 minute test and you calculate DAR and PI ratios, whcih are just as important as the resistance value alone. 
4. Trending, the spot reading you get won't tell you much unless it fails, the trending of these values (Temp corrected) over time is where the true value is.


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## jcdpk (Aug 23, 2011)

*megger test*

Megger or insulation testing can be done for 1 min or a 10 min. I've done it this way for years on 480 and 4160 volt motors an transfomers 11,000 volt on up at the Powerplant in which I work at. As a bare minimum megger 1 min which allows the insulation to stablize as the voltage is being applied to it and you can do it at 500 volt or at 1000 volt for a 480 and you can do it at 2500 volt or 5000 volt for a 4160. The 10 min or PI testing is a little more involved and we are looking for more infomation on this test. But here is what I would like to suggest and which will give you a through understanding about insulation measurement testing. Go to:*www.biddlemegger.com*. And select "*A guide to diagnostic insulation testing about 1 KV*," and select also "*A stitch in time*" both on the left side once you get to their website both of these articles are good. And the last one I would like to include is this just type in or copy and paste to website: *"amec understanding insulation resistance testing"*


Dan


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## jcdpk (Aug 23, 2011)

*Megger or Insulation testing*

*Commonly used DC test voltages for AC equipment are:*
*AC equipment rating DC test voltage*
*Up to 100 VAC 100 and 250 VDC*
*440 to 550 VAC 500 and 1000 VDC*
*2400 VAC 1000 to 2500 VDC*
*4160 VAC and above 1000 to 5000 VDC (or higher)*
So, what is “good” insulation? Since we know that insulation has a high resistance to current flow, “good” insulation must be able to provide a high resistance to current flow and be able to maintain that high resistance over a long period of time. In order to evaluate the quality of the insulation certain standard tests have been developed which provide a reliable indicator to determine what comprises “good” insulation.
There are two tests that the production technician can easily perform using a battery powered megger like the one shown in Figure 29. The first test is the short time or spot reading test and the second test is the one minute test.


*The spot reading test*
In the spot reading test you simply connect the “earth” lead of the megger to a good ground and the “line” lead to the conductor and operate the megger for a short time, say for *30 seconds or so*. If the apparatus you are testing has a very small capacitance, such as a short run of cable, the spot reading is all that is necessary. However most equipment (like electric motors and long runs of electrical cable) is capacitive, so the very first spot reading can be only a rough guide as to how good or how bad the insulation is.
Bear in mind that the temperature and humidity will affect the readings and electrical circuits do not have to read infinity (perfect insulation on the megger scale) for the circuit to be serviceable. The *NEMA* standard for *minimal insulation resistance is*: 1 megohm per rated KV plus 1 megohm. What that means is that if you have a 1000 volt circuit, you should have a _minimum_ of 2 megohms to ground for the circuit to be considered safe to energize and operate. If you have a *4000 volt* circuit, you need (*5 megohms**)* and if you have a *460 volt* circuit, you should have *(1-1/2 megohms)*, and so on. *Insulation that is in good* condition will normally have *40 - 50 megohms*, or more, to ground. 


*The one minute reading test*
The other test is the *one minute* reading. This method is fairly independent of the influence of temperature. It is based on the current absorption of good insulation compared to the *current absorption of moist or contaminated insulation*. A characteristic of good insulation is that it will show a *continual increase in resistance* (which means less leakage current is flowing) over a period of time. The initial test current (called the absorption current) is absorbed by the capacitance of the equipment being tested and then after that, any current flowing is the leakage through the insulation. If the insulation contains much moisture or other contaminants, the absorption current is masked by a high leakage current which stays at a fairly constant value, keeping the resistance low.
*The value of the one minute test* is that it can give you a better idea as to the condition of the insulation and alert you to a problem even when the spot reading indicates that everything is OK.
For example, let’s say a spot reading on a *460 volt* induction motor was 10 megohms, which at first glance is well above the minimum requirement. Now lets assume that the one minute test showed the resistance quickly climbing to 10 megohms and then from there, holding *steady* for the rest of the *60 seconds*. This means that there may be *dirt* or *moisture* on the windings. On the other hand, if the *reading gradually increases* between the 30 and 60 second time interval, then you can be reasonably certain that the windings are in good condition.
*The comparison of the 30 second reading to the 60 second reading is called the dielectric absorption ratio (or D.A.R.).* The way the ratio is calculated is to divide the *60* second reading by the *30 *second reading. Using that method, the chart in Figure 30 will give you an idea of how to determine if the insulation is good.

Insulation​ 

condition​ 

60/30 second​ 

ratio​ 

Poor​ 

Less than 1​ 

Questionable​ 

1.0 - 1.25​ 

Good​ 

1.4 – 1.6​ 

Excellent​ 

Above 1.6***​ 

**In some cases, with motors, values approximately 20% higher than shown here indicate a dry, brittle winding which will fail under shock conditions or during starts. For preventive maintenance, the motor winding should be cleaned, treated and dried to restore winding flexibility.*


*Dielectric absorption ratio chart*
There is another megger test called the “*Ten minute test*” which is similar to the one minute test. Because this test requires ten minutes to perform, it is better accomplished by line operated (120 volt) equipment. Essentially the test is performed for ten minutes, the ten minute reading is divided by the one minute reading and the resulting ratio is called the *Polarization Index.* This test is mostly used on *larger equipment* that has *large capacitance* and requires longer time to stabilize the absorption test current. The conclusions drawn from the test are the same as those drawn from the dielectric absorption ratio *but* the actual ratio values of the polarization index are not the same as for the dielectric absorption chart in Figure 30.
Using the some of the symptoms listed in the “Problem” column of the troubleshooting chart, let’s go through the process that should be followed to perform the tests and develop the solution
*Breaker trips free when motor start is attempted.*

If the circuit breaker trips free (or trips during normal motor operation) it should* not* be re-closed and another motor start attempted before testing the *motor* and the *power cables* for an insulation failure. If there is a problem, additional starts will just cause further damage.
The symptom of the circuit breaker tripping is *always *caused by overcurrent; either a phase to phase short circuit or a phase to ground short circuit.


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## Thomasin (Jun 27, 2014)

helpful and learn alot



jcdpk said:


> Megger or insulation testing can be done for 1 min or a 10 min. I've done it this way for years on 480 and 4160 volt motors an transfomers 11,000 volt on up at the Powerplant in which I work at. As a bare minimum megger 1 min which allows the insulation to stablize as the voltage is being applied to it and you can do it at 500 volt or at 1000 volt for a 480 and you can do it at 2500 volt or 5000 volt for a 4160. The 10 min or PI testing is a little more involved and we are looking for more infomation on this test. But here is what I would like to suggest and which will give you a through understanding about insulation measurement testing. Go to:*www.biddlemegger.com*. And select "*A guide to diagnostic insulation testing about 1 KV*," and select also "*A stitch in time*" both on the left side once you get to their website both of these articles are good. And the last one I would like to include is this just type in or copy and paste to website: *"amec understanding insulation resistance testing"*
> 
> 
> Dan


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## sparkyjim (Feb 25, 2011)

Very helpful


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## Montre (Jul 11, 2016)

Bumping this thread back from the dead, but it is bang on relevant to my concerns.

As an apprentice in a motor shop, we have a new piece of equipment to megger with. I don't do them often and am now just learning some of the intricacies.

I'm curious to know how to work in the ratios of the one minute and thirty second tests above, when the megger taps out at >500 MOhms and it reaches that level within 15 seconds.

If both tests are up to 500 MoHm within 15 seconds how does one use hat chart. Or isthe motor deemed fine?


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