Megger operation / circuit paramaters

Status
Not open for further replies.
P

Pitt123

Senior Member
I started to ask some questions in a previous post regarding megger operation but I figured I'd start a new thread to ask more specific questions.

Specifically I wanted to understand exactly how a megger functioned and how to reperestent the different parts of a megger with circuit equivelents.

So looking at my 1000V AMEC 1040 megger I notice that for 1000V DC it has a nominal output current of 1mA and a measurement range of up to 2000Mohms.

From the previous post it was stated that the current output in a megger is current limited and in my case it appears that its limited to 1mA. What is it that limits this output current to 1mA at 1000V? Output impedance?

I believe a Megger works by outputting this 1mA output current and measuring the voltage drop between leads in order to calculate a resistance? So does the unit always output 1mA regardless of what the measured circuit resistance is? What if measured circuit is a dead short?

It was also mentioned in previous post that voltage on measured circuit may be less than 1000V due to voltage drop across megger output. So when measuring at circuit does the primary point of the circuit actually see 1000V or is it less depending on output impedance and current?

I have seen some cases where when measuring a circuit the voltage will not increase, on an adjustable megger and will stay at a miniumum voltage. What is happening here? Is there not enough voltage in these cases to push the current?

I also assume that DC is used for this instrument to avoid issues with capacitance coupling in circuits.
 
Pitt123 said:
I started to ask some questions in a previous post regarding megger operation but I figured I'd start a new thread to ask more specific questions.

Specifically I wanted to understand exactly how a megger functioned and how to reperestent the different parts of a megger with circuit equivelents.
Click to expand...

You have a DC source powering a series circuit with the load and a calibrated resistor, the voltage drop is measured across the resistor. Knowing the source voltage, voltage drop, and current the meter does the math for you

As seen my attached sketch your ?Ohmmeter? is actually measuring current. The meter applies a test voltage and measures the current flow through the circuit and uses Ohms law to calculate resistance which is displayed on the meter in Ohms. If the applied voltage is not high enough, the amount of current passed through the resistance is below the sensitivity range of the meter.

Ohm meters are connected to a component which is removed from the circuit, Ro is an adjustable resistor whose purpose is to zero the ohm meter and correct for battery aging. It is also a current-limiting resistor which includes the meter resistance Rm. Zeroing the ohm meter is accomplished by shorting the ohm meter terminals and adjusting Ro to give full-scale deflection; this is done automatically by modern digital ohmmeters.

When the unknown resistance Rx is connected across the ohm meter terminals, the current is measured by calculating the total series resistance. Using the following equation:

I= V/ (RO+RX)

A digital ohmmeter will automatically do this calculation for you while an analog meter reading 0 ohms will cause full meter deflection. The meter face says ohms, but it is actually indicating amps, therefore full deflection (All the way to the right) will indicate 0 ohms while a reading of near 0 amps will cause little reflection of the meter and indicate near infinite resistance. (There is also no such thing as infinite resistance but we will save that argument for future discussion)

For digital displays, if the applied voltage of the circuit under test is not high enough to cause enough current flow through the circuit for the meter to sense, the display will read ?O.L?. Contrary to popular belief, this does not mean overload, it means ?Outside Limits?, in other words, the measured current flow is below the sensitivity capabilities of the meter.

The only real difference with a Megohmeter is the test voltage and ratings of the resitor, the same concepts apply as I described above for digital and analog Megohmeters.

Pitt123 said:
So looking at my 1000V AMEC 1040 megger I notice that for 1000V DC it has a nominal output current of 1mA and a measurement range of up to 2000Mohms.

From the previous post it was stated that the current output in a megger is current limited and in my case it appears that its limited to 1mA. What is it that limits this output current to 1mA at 1000V? Output impedance?
Click to expand...
1Ma is the max output at that voltage, based on the power supply, which also determines the max reading you can read. In your case yours will have current too low to measure (Accurately) when you have >2G of resistance. Your megger displaying >2G is the same as your ohmeter displaying "O.L."

Pitt123 said:
I believe a Megger works by outputting this 1mA output current and measuring the voltage drop between leads in order to calculate a resistance? So does the unit always output 1mA regardless of what the measured circuit resistance is? What if measured circuit is a dead short?
Click to expand...

I think I answered the 1st part of this above. The current will vary depending on the resistance of the circuit, current is what the meter uses as the variable to calclate resistance.

Pitt123 said:
I have seen some cases where when measuring a circuit the voltage will not increase, on an adjustable megger and will stay at a miniumum voltage. What is happening here? Is there not enough voltage in these cases to push the current?
Click to expand...


There are 3 currents being measured, leakage current, capacitive charging current, and absorbtion current (See 2nd attachment). Leakage current should stay the same over time, the other 2 will decay with the decay rate depending on the type of and condition of the equipment you are testing. As the current decays the meter output voltage tends to creep up at times before correctiing itself. The rate of these decays is calulated to determine the Polarization Index and Dielectric Absorbtion Ratio of the insulation. The PI and DAR are key indicators of the overall condition of the insulation of inductive equipment such as transformers and rotating equipment.

Hope this helps, I tried to keep it basic so all you EE's don't need to nitpick, we can start another thread if you want to get all EE on me :)
 
zog said:
1Ma is the max output at that voltage, based on the power supply, which also determines the max reading you can read. In your case yours will have current too low to measure (Accurately) when you have >2G of resistance. Your megger displaying >2G is the same as your ohmeter displaying "O.L."
Click to expand...
Since 2G is the max for the AEMC 1040, anything over that shows "OL" just like a ohmeter.

A max. of 1ma at 1000V is the same spec for the AEMC 1050/1060 which at 1000V goes up to 4T. Are you saying that 1ma is inadequate? As I recall, don't you own either a 1050 or 1060 and have often recomended the AEMC brand?
 
wptski said:
Since 2G is the max for the AEMC 1040, anything over that shows "OL" just like a ohmeter.

A max. of 1ma at 1000V is the same spec for the AEMC 1050/1060 which at 1000V goes up to 4T. Are you saying that 1ma is inadequate? As I recall, don't you own either a 1050 or 1060 and have often recomended the AEMC brand?
Click to expand...

My AEMC never says O.L., just >4T, which is rare. The 1060 has a larger power supply which is why it has a 1mA output with a 4T load compared to 2G.

And yes, AEMC is my Megohmeter of choice. But I also use a Megger for big jobs with hundreds of tests needed so I can export the data right to my test reports in Power Db.
 
zog said:
My AEMC never says O.L., just >4T, which is rare. The 1060 has a larger power supply which is why it has a 1mA output with a 4T load compared to 2G.

And yes, AEMC is my Megohmeter of choice. But I also use a Megger for big jobs with hundreds of tests needed so I can export the data right to my test reports in Power Db.
Click to expand...
The 1040 does show "OL" but the 1050/1060's screen will blink or flash which is a "out of range" error or equal to "OL".
 
With my AMEC 1040 giving an output of 1000V DC and a max output current of 1mA, can it then be assumed that the output impedance of this meter is 1Megohm. I figure this by looking what happens if we are measuing a circuit with zero resistance (not possible but just going to use for this example. The because all of the resistance for the total series circuit (meter + measured circuit) would be just that of the meter, and since it is a max of 1mA then 1000V / 1ma gives us 1Megohm. Is this correct?

Also I guess the voltage seen across the measured circuit would be a function of what the circuits impedance was. If the circuit impedance was the same as that of the meter (lets say 1meg) then half of the 1000V output would drop across the meter, and the other half would drop across the measured circuit. So in this case the measured circuit would only have aprox 500V DC across it?

I have seen times where analog meggers (15kV) with a needle dont let you increase the voltage when measuring a circuit. You will dial up the voltage, but but the voltage needle will not increase, it will stay at a low value and not increase. What is happening in this case. Is the megger limiting its voltage ouput for some reason as a result of the circuit condition being measured?
 
Status
Not open for further replies.
Top