DIELECTRIC VS. MEGGAR TESTING

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lob

Member
Hi, I have to perform testing on some wiring that has been covered before inspected. I've always heard it referred to as meggar testing until this time I was told to perform a dielectric test. so, 1) whats the difference?, 2) can both be done with a meggar tester?, 3) what readings am I looking to find as "acceptable" & "not acceptable"? thanks for any info.
 

MD84

Senior Member
Location
Stow, Ohio, USA
Megger is short trade slang for megohmmeter. It is also a brand name. Megohmmeters are just ohmmeters designed to read very high resistance. Like millions of ohms.

The test most commonly performed as referred to as megging would be an insulation resistance test. In this test you are verifying the dielectric integrity of the insulation. It is all the same.

Your results will vary depending on the system you are testing. The test voltage and duration of test are also considerations that depend on what you are testing. I would recommend contracting this work to someone that performs this type of work regularly.
 

Fulthrotl

~Autocorrect is My Worst Enema.~
Temperature has a huge effect as well and all readings need to be temperature corrected to have any real meaning.

i'd never be one to argue meggers with ya, but i'm guessing the
guy just needs to megger some branch circuits that have been covered,
so i'm guessing romex, etc. and wonders what a good reading
would be.

i'd think if you meggered at the working voltage, assuming 120 volts,
and got a 10 megohm reading, you'd be in pretty good shape. you aren't
going to smoke anything with the megger, and you'll know the insulation
isn't damaged.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Hi, I have to perform testing on some wiring that has been covered before inspected. I've always heard it referred to as meggar testing until this time I was told to perform a dielectric test. so, 1) whats the difference?, 2) can both be done with a meggar tester?, 3) what readings am I looking to find as "acceptable" & "not acceptable"? thanks for any info.

what the other guys said.

there are forms you can find on the Internet to document what you did.

most cases of common conductors under normal circumstances if a conductor has an issue it will be pretty obvious.
 

topgone

Senior Member
Hi, I have to perform testing on some wiring that has been covered before inspected. I've always heard it referred to as meggar testing until this time I was told to perform a dielectric test. so, 1) whats the difference?, 2) can both be done with a meggar tester?, 3) what readings am I looking to find as "acceptable" & "not acceptable"? thanks for any info.

Please take note of these:
  1. To do megger is to measure the insulation resistance of your wires, that is the resistance in between your conductor to the ground.
  2. When you do a dielectric withstand testing, you want to measure the highest voltage that your wire insulations can handle before it starts to leak considerable current to the ground.
A megger will use a fixed voltage across the insulation (250V for 120/240 systems, 1000V for 480/600 systems, etc) and indicate the insulation resistance value (megohms) while a dielectric test uses a voltage that you will ramp up until the built-in trip setting (usually 1 mA) stops the test! Crystal!
 

MD84

Senior Member
Location
Stow, Ohio, USA
A megger will use a fixed voltage across the insulation (250V for 120/240 systems, 1000V for 480/600 systems, etc) and indicate the insulation resistance value (megohms) while a dielectric test uses a voltage that you will ramp up until the built-in trip setting (usually 1 mA) stops the test! Crystal!

Thank you for that info. I did not realize there was a difference between an insulation resistance test and a dielectric withstand test. I have always ramped up to a set voltage and held for a set duration while recording leakage current. I do not recall using a test set that had a built in trip setting as low as 1 mA.
 

Smart $

Esteemed Member
Location
Ohio
A megger will use a fixed voltage across the insulation (250V for 120/240 systems, 1000V for 480/600 systems, etc) and indicate the insulation resistance value (megohms) while a dielectric test uses a voltage that you will ramp up until the built-in trip setting (usually 1 mA) stops the test! Crystal!
Thank you for that info. I did not realize there was a difference between an insulation resistance test and a dielectric withstand test. I have always ramped up to a set voltage and held for a set duration while recording leakage current. I do not recall using a test set that had a built in trip setting as low as 1 mA.
Be aware there are two different dielectric tests, and that description sounds more like the destructive dielectric breakdown test.
wikipedia said:
Dielectric Breakdown Test. The test voltage is increased until the dielectric fails, or breaks down, allowing too much current to flow. The dielectric is often destroyed by this test so this test is used on a random sample basis. This test allows designers to estimate the breakdown voltage of a product's design and to see where the breakdown occurred.

Dielectric Withstand Test. A standard test voltage is applied (below the established Breakdown Voltage) and the resulting leakage current is monitored. The leakage current must be below a preset limit or the test is considered to have failed. This test is non-destructive providing that it does not fail and is usually required by safety agencies to be performed as a 100% production line test on all products before they leave the factory.

It also adds that...
Insulation Resistance Test. This test is used to provide a quantifiable resistance value for all of a product's insulation. The test voltage is applied in the same fashion as a standard hipot test, but is specified to be Direct Current (DC). The voltage and measured current value are used to calculate the resistance of the insulation.

https://en.wikipedia.org/wiki/Hipot
 

Smart $

Esteemed Member
Location
Ohio
smart$,
Do you have a list of the types of wires that gets damaged with 1mA of leakage current?
Don't need a list. If you are trying to say a destructive test isn't really destructive, you are not going to make any headway challenging me.

First of all, whether that 1mA is a result of destructive testing depends on the voltage applied at that point. Second, 1mA of leakage current may well be an near infinite amount (%) more than the 0mA the cable may have had when you started. And if that 1mA of leakage current was a result of testing, you have now opened the door to advanced, premature degradation.
 

kwired

Electron manager
Location
NE Nebraska
Don't need a list. If you are trying to say a destructive test isn't really destructive, you are not going to make any headway challenging me.

First of all, whether that 1mA is a result of destructive testing depends on the voltage applied at that point. Second, 1mA of leakage current may well be an near infinite amount (%) more than the 0mA the cable may have had when you started. And if that 1mA of leakage current was a result of testing, you have now opened the door to advanced, premature degradation.
Like poking a pinhole in a membrane.
 

lob

Member
i'd never be one to argue meggers with ya, but i'm guessing the
guy just needs to megger some branch circuits that have been covered,
so i'm guessing romex, etc. and wonders what a good reading
would be.

i'd think if you meggered at the working voltage, assuming 120 volts,
and got a 10 megohm reading, you'd be in pretty good shape. you aren't
going to smoke anything with the megger, and you'll know the insulation
isn't damaged.


Thanks Fulthrotl, you are correct in what I'm going to perform the test on.
 

policarpio0523

New member
Location
Philippines
Meggar Testing

Meggar Testing

In line with this topic,
May I ask how do you conduct meggar test if its for Control Panels?

Set up is that we have MCC/PLC Panel.
2 section of our panel comprises of
Motor circuit breakers, Contactors and Motors.

on the third section we have also automation part which is Omron PLC, Terminal blocks and relays.

How do we conduct insulation resistance test?


Thanks in advance sirs.
 

kwired

Electron manager
Location
NE Nebraska
In line with this topic,
May I ask how do you conduct meggar test if its for Control Panels?

Set up is that we have MCC/PLC Panel.
2 section of our panel comprises of
Motor circuit breakers, Contactors and Motors.

on the third section we have also automation part which is Omron PLC, Terminal blocks and relays.

How do we conduct insulation resistance test?


Thanks in advance sirs.

You will be turning off power and checking individual segments of the system at a time, you will also be taking any sensitive components that can't take the test voltage out of the circuit before testing circuits that contain such items.
 

big john

Senior Member
Location
Portland, ME
What somebody calls it is not really relevant, what voltage you are expected to apply is what decides what type of test it is.

Generally a megger or insulation resistance test will be close to or less than the nameplate voltage of the equipment. This is considered a non destructive test.

Whereas a hipot or dielectric withstand test will often apply a voltage several times greater than the nameplate voltage of the equipment. In service-aged equipment this is considered a destructive test and should only be performed with the understanding that it can cause an incipient insulation failure to progress catastrophically. This would take your equipment out of service.

It's best to talk to the people specifying the test to try and pin down exactly what they expect, but my guess is that it's a simple megger to try and demonstrate there is not gross insulation damage.
 

Tony S

Senior Member
Temperature has a huge effect as well and all readings need to be temperature corrected to have any real meaning.

On one of our weekly 2.5kV two minute insulation tests the report form asked for the following,
Final MΩ reading
Temperature
Humidity
General weather conditions
There was an early cutoff “if 1TΩ is achieved, record the time” there’s no point stressing the machine more than necessary.

One I’d plotted the results for about six months I could more or less predict the results by looking out of the office window. It also showed when something abnormal was happening within the machine and required remedial action.
 
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