megger vs hi-pot

[augie47]

Over the years I have used a "megger" on numerous ocassions and I have heard the term "hi-pot" testing, however, I do not understand the difference between the two (tests & equipment). can someone explain, please ?

 

[bob]

Hi-Pot is a generic term for High Potential Test. Megger is a brand which has become a generic term for a similar testing device for lower voltage equipment. In different applications, "high potential" has vastly different meanings. In integrated electronics, high potential may be 150 volts, whereas in the power industry, high potential is in the 15000 volts range and above.

Both hi-pots and meggers can be used for testing insulation resistance. However, testing only resistance does not give a complete picture of the health of the insulation. Meggers usually apply 600 to 2000 v for 1 minute or less, for a "snapshot" of the insulation at existing ambient conditions. Motor, generator and transformer windings, as well as medium and high voltage power cables. When testing Hi-Voltage cable, the high potential is gradually applied over a period of about 15 minutes minutes, with readings taken at the 1 minute intervals. The reading are the cable leakage current in micro-amps. The maximum applied test voltage is 300 volts per mil of insulation. A 15 kv cable with 175 mils would be tested at about 50000 volts.
As the voltage is applied, the tester notes the leakage current at the end of
each minute interval. As long as the leakage current remains constant or
decreases, the cable is considered satisfactory.

Hi-pots are also useful for locating damage in faulted underground cables, a process called "thumping". The cable is disconnected at both ends, and the hi-pot is connected between the conductor and ground. A closed spark gap is attached in parallel with the hi-pot. As the spark gap is widened, the voltage output of the hi-pot will rise further each time before the spark to ground collapses the voltage. At some point, the spark gap becomes a higher impedance path to ground than the cable fault. The sparks no longer jump the gap. The sparking now occurs at the location of the fault. The cable arcs create a fairly loud sound and the location can usually be found readily with a ground microphone and headset.

 

[augie47]

Thank you !
BTW, One of the "best" answers I have ever received on the Forum. Impressive.

 

[electricalperson]

Hi-Pot is a generic term for High Potential Test. Megger is a brand which has become a generic term for a similar testing device for lower voltage equipment. In different applications, "high potential" has vastly different meanings. In integrated electronics, high potential may be 150 volts, whereas in the power industry, high potential is in the 15000 volts range and above.

Both hi-pots and meggers can be used for testing insulation resistance. However, testing only resistance does not give a complete picture of the health of the insulation. Meggers are usually apply 600 to 2000 v for 1 minute or less, for a "snapshot" of the insulation at existing ambient conditions. Motor, generator and transformer windings, as well as medium and high voltage power cables. When testing Hi-Voltage cable, the high potential is gradually applied over a period of about 15 minutes minutes, with readings taken at the 1 minute intervals. The reading are the cable leakage current in micro-amps. The maximum applied test voltage is 300 volts per mil of insulation. A 15 kv cable with 175 mils would be tested at about 50000 volts.
As the voltage is applied, the tester notes the leakage current at the end of
each minute interval. As long as the leakage current remains constant or
decreases, the cable is considered satisfactory.

Hi-pots are also useful for locating damage in faulted underground cables, a process called "thumping". The cable is disconnected at both ends, and the hi-pot is connected between the conductor and ground. A closed spark gap is attached in parallel with the hi-pot. As the spark gap is widened, the voltage output of the hi-pot will rise further each time before the spark to ground collapses the voltage. At some point, the spark gap becomes a higher impedance path to ground than the cable fault. The sparks no longer jump the gap. The sparking now occurs at the location of the fault. The cable arcs create a fairly loud sound and the location can usually be found readily with a ground microphone and headset.

your a genius. also hi-pot could be declared as a destructive test since sometimes if its used wrong it can destroy good insulation, when a megger is generally a nondestructive test. whenever we pull in a primary cable for the power company on private property we have to do a 60kv hi pot test and submit the results to the power company. the load break elbow can fail at that test voltage if it was installed wrong or contaiminated.
better to fail during a hipot test then when its energized

 

[wptski]

Some insulation testers like the Fluke 1507 have the 1 minute DAR and 10 minutes PI test functions built in which can be perform on any voltage setting up to 1000V.

 

[electricalperson]

Some insulation testers like the Fluke 1507 have the 1 minute DAR and 10 minutes PI test functions built in which can be perform on any voltage setting up to 1000V.

thats a nice tester. goes up to 10Giga ohms also. come with a nice test lead with a button so you can test without letting go of the leads if you choose too. cheap price also compared to other insulation testers. only downside is that it doesnt come with a case

 

[Special K]

Very impressive answer Bob

 

[wptski]

thats a nice tester. goes up to 10Giga ohms also. come with a nice test lead with a button so you can test without letting go of the leads if you choose too. cheap price also compared to other insulation testers. only downside is that it doesnt come with a case

It shows that it takes a C101 hard case but the 1587 combo meter comes with a hard case. I wonder if it's the C101 with the foam cut outs?

 

[steved]

One area where hipot testing is required is for factory-built housing such as motorhomes. Article 551.60 of the 2008 NEC requires a 1-second long 1080VAC or 1530VDC dielectric withstand test for RV's. There are similar requirements for park trailers, manufactured homes, and mobile homes.

Here are some hipot tester manufacturer's sites:

http://cirris.com/testing/guidelines/hipot_testing.html
http://www.hipot.com/faqs/index.shtml
http://www.rodl.com/rodl/faqs.html
http://www.compwest.com/library.html

 

[danickstr]

as mentioned elsewhere, high resistance in motors does not necessarily mean high dielectric strength. As the temp of the motor under load increases, the resistance of the wire will decrease quite a bit.

 

[zog]

"When testing Hi-Voltage cable, the high potential is gradually applied over a period of about 15 minutes minutes, with readings taken at the 1 minute intervals. The reading are the cable leakage current in micro-amps. The maximum applied test voltage is 300 volts per mil of insulation. A 15 kv cable with 175 mils would be tested at about 50000 volts."

DC hipot is now considered a destructive test on service aged MV cables, IEEE and ICEA standards both agree on this issue, and even back when DC hipotting of cables was accepted in the industry it was a go-no-go test that is proven to not find even gross installation defects in new cable.

Also, a hipot cannot be used to thump a cable, you need a lot more current than a hipot can put out to do any location at all.

 

[bob]

Also, a hipot cannot be used to thump a cable, you need a lot more current than a hipot can put out to do any location at all.

The Hi-Pot test set can not provide the required current to "thump". A large
capacitor is used along with the test set to provide the necessary current.
The Hi-Pot set charges the capacitor to a high voltage and when the cable fault arcs, the capacitor discharges causing the "thump" sound.

 

[zog]

Yep, you need the accessory

 

[lexkyphil]

That was a fine response on the hi pot question -

There was a post by danstrikr that was about motor insulation. It stated that as the temperature in the motor under load rises, the resistance of the wire decreases. I belive it would increase with temperature, not decrease. Was this one of those clinton mis-speaks?

Phil in Kentucky

 

[zog]

That was a fine response on the hi pot question -

There was a post by danstrikr that was about motor insulation. It stated that as the temperature in the motor under load rises, the resistance of the wire decreases. I belive it would increase with temperature, not decrease. Was this one of those clinton mis-speaks?

Phil in Kentucky

You aree correct, however, what I think danstrikr meant to say was the resistance of the INSULATION decreases as tempature increases. Which is true, thats why a IR (or megger) test is worthless unless it is tempature corrected.