Hipot versus Isulation Resistance Test 4

[akrus]

I have been reading a lot about the distinctions between Dielectric Withstand (Hipot) and Insulation Resistance testing. There is a lot of conflicting information out there about the merits of each, especially if you read a response from a manufacturer of particular test equipment.

I understand that generally hipot is a pass/fail test to determine whether the equipment can withstand the test voltage and IR is a quantitative test to measure the resistance at the test voltage. I can see that often hipot is run at a higher AC voltage and IR is run at a relatively lower DC voltage.

Now for my question...

In my application, we are often requested to run both of these tests at the production level. The requirement is to run hipot at 500 VAC and IR at 500 VDC. The capacitance in the unit under test is minimal because there are no capacitive components and the wire length is short. I am struggling to understand how there is any difference between these tests. In both cases, ~500V is applied (I know AC is RMS) and the test equipment is measuring the leakage current. How would any defect (creepage, clearance, damaged insulation, etc.) be detected in one and not the other? My thought is that IR testing covers everything unless the hipot is run at a much higher test voltage. Am I missing something here? Any discussion is welcome. Thanks!

 

[btrueblood]

I agree with you that Hipot is for a peak withstand voltage (spikes) and verifies breakdown (arc) voltage above a level that might cause insulation degradation over time. IR pretty much covers everything else. But, am not the expert and hope those chime in here.

edit: I learned something new - you can do Hipot testing with either AC or DC voltages.

 

[IRstuff]

They measure different things; consider measuring the "resistance" of a capacitor using AC and DC. In the former, you get AC conductance, which infers the dielectric quality of the insulation, while DC conductance infer direct leakage of the insulation

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[analogkid2digitalman]

See pages 10-11 and 16 for HP and IR, slso some good safety/operator guidelines in there.

It's been a few decades since I performed such tests. We used Slaughter units (now SCI?), I found it amusing that their tagline was 'Slaughter, The Pro Testers'
Requirement of 500VAC for HP seems rather low.
Usually IR is performed prior to HP.

https://20996632.fs1.hubspotusercon...papers/Guide-To-Electrical-Safety-Testing.pdf

-AK2DM



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[akrus]

Thanks to all for your input.

@AK2DM
Your post is a good example of what I'm talking about with conflicting information. I agree that usually IR is performed before hipot, and could easily find some sources saying so, but the pdf you linked says to do it afterwards. I also agree that 500VAC is low for a traditional hipot. Essentially that is my question: if the hipot test voltage is the same as IR, then isn't doing both tests just a performative box-checking exercise?

@IRstuff
I agree with the AC/DC comment generally, but this application is resistive loads only and short (<3') wire runs. Is there going to be a practical difference here? My question is not about generic testing, but rather if there is anything to be gained by running both tests in this situation.

 

[IRstuff]

if there is anything to be gained by running both tests in this situation.

It's not out of the realm of possibility; the circuits involved might be sensitive to capacitive loads, or the customer might simply have a "template" for testing that they don't have the expertise or inclination to modify, and they seemingly don't care about spending the extra money.

Certainly, you could ask them if they want to save some bucks by not doing the AC test; that's the sort of thing that could seal a long-term relationship


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[che12345]

Dear Mr. akrus (Electrical)(OP)11 Jan 23 16:14
#204A87]"#1. ...I have been reading a lot about the distinctions between Dielectric Withstand (Hipot) and Insulation Resistance testing. There is a lot of conflicting information out there about the merits of each......"
There is NO conflicting information between (Hipot) and ((IR) test. Both are required. They serve different purposes and reveal different information.

" #2. .... we are often requested to run both of these tests at the production level. The requirement is to run hipot at 500 VAC and IR at 500 VDC......"
Please tell us a little more on: 1. What is the [operation and rated insulation voltage] AC or DC of the product. 2. Which standard IEC, ANSI, UL ,CCC.....etc or military std the product is to be tested/complied with? If available, which standard publication No. (e.g. IEC 61XYZ).
Note: With these additional information, we may be able to offer our opinion.

"#3. ....My thought is that IR testing covers everything unless the hipot is run at a much higher test voltage. Am I missing something here?"
Hipot test voltage (AC/DC) is usually higher than the IR test voltage. But, you state your case with Hipot 500 VAC and IR at 500 VDC need further study. See above 2.
Che Kuan Yau (Singapore)

 

[BrianE22]

They serve different purposes and reveal different information.

But what are those different purposes and information? What fault does one reveal that the other does not reveal?

 

[akrus]

There is NO conflicting information between (Hipot) and ((IR) test. Both are required. They serve different purposes and reveal different information.

I agree with this in a general sense and I'm not trying to argue otherwise. The conflicting information is things like which test to run first, whether a particular test is for qualification or production, etc.

Please tell us a little more on: 1. What is the [operation and rated insulation voltage] AC or DC of the product. 2. Which standard IEC, ANSI, UL ,CCC.....etc or military std the product is to be tested/complied with? If available, which standard publication No. (e.g. IEC 61XYZ).

The application is typically low voltage DC and tested per MIL-STD-202G Methods 301 & 302.

Hipot test voltage (AC/DC) is usually higher than the IR test voltage. But, you state your case with Hipot 500 VAC and IR at 500 VDC need further study. See above 2.

I agree hipot is usually higher than IR and this is exactly my question. The applied voltage levels are a settled matter and not up for debate. So given that information, is there a practical distinction between the tests?

In other words, people will insist DWV & IR are different, but then ask why do you test DWV so low? And my question is that given the constraint of the test voltages, what is the benefit of running both tests in a production acceptance environment? Isn't the reason you are asking about the voltage level because the hipot is not distinct unless you raise it?

To be clear, I don't mean to be argumentative. I only want to have a good discussion about this. I'm happy to have my mind changed if someone can tell me about a fault which might only be detected on one test. The closest thing in my opinion is the AC/DC discussion, but even there it seems academic for this application at the given voltage levels.

 

[Gr8blu]

Insulation resistance is performed per IEEE 43. The most current version that I know of is 2013. This test is performed at a voltage that is NO MORE THAN the operating voltage of the winding being tested. Typically, it is at a somewhat lower voltage (test at 500 Vdc for windings rated up to 1000 V, for example). The purpose of the test is to determine if the insulation is suitable for testing at higher voltages (for instance, dielectric testing such as high potential or surge). It indirectly measures the moisture content of the insulation, as well as the presence of tracking paths to ground along the exterior surface of the insulating material.

A surge test (performed per IEEE 522) is used to verify the quality of the turn-to-turn insulation within a winding. It is a series of higher-than-operating-voltage pulses often applied at something above line frequency. The purpose here is to stress the insulation inside the coil, between individual turns.

A dielectric test (high potential or hipot) is performed per IEEE 433 and can use either AC or DC. It is done at elevated voltage levels relative to the operating voltage of the winding. The purpose of this testing is to discover voids or other impurities within the insulation itself which may, under certain circumstances, cause the conductor to go to ground. As a basic rule, new windings are tested at an AC value that is twice the rated voltage (in volts) plus 1000, AFTER installation and vacuum pressure impregnation. This means something rated for 500 V (AC or DC) would receive a 2000 Vac hipot for 1 minute. For a DC test, the AC value is multiplied by 1.7. Thus, the same winding undergoing a DC hipot would be tested at 3400 Vdc.

Hipot testing at different in-process points are at higher voltages than the "final" test. Usually testing an installed (but not yet impregnated) winding would occur at the 2E+1000 level plus an additional ten percent. Testing sections of winding as it is installed would be another 10% higher, and testing individual coils as they are manufactured would be another 10%. The idea is to catch a problem before too much work gets put into it: and more handling means more likelihood of damage to the coil. Hipot testing after the machine has been in service is at 65-75 percent of the "final" factory value, depending on whether you look at IEEE or IEC standards.

The three types of test (insulation resistance, surge, and hipot) should be performed IN THAT SPECIFIC ORDER. Think of pushing on a balloon with the palm of your hand: that's an insulation resistance test. Then poke the balloon quickly and repeatedly with your finger: that's a surge test. Then poke the balloon once with a very sharp metal pin: that's the hipot.

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[onatirec]

I'm only familiar with these tests in regards to ac motors.. The primary difference (other than the magnitude) is the electric stress distribution across the groundwall between ac and dc tests. The ac hipot yields an electric stress distribution more similar to that found during normal operation; this makes it superior to the dc hipot - especially with modern insulation systems.

The consensus of dc hipot breakdown voltage being 1.7x higher than ac was made in the 1950's, based on older insulation systems. Ratio of dc to ac breakdown voltage was found to be on average 4.3x, with epoxy-mica insulation (Gupta et al - Use of AC and DC hipot tests to assess stator winding insulation). 1.7x ratio was maintained in IEEE literature, but there is reason to suspect it being larger. See also Stone - Recent important changes in IEEE motor and generator winding insulation diagnostic testing standards.

The minimum recommended IR test voltage I've seen is 500V, so it's not necessarily less than the operating voltage of the winding. I have not heard of increasing hipot voltage by 10% at each subsequent stage of assembly. My understanding is that it's a stressful test and "repeated application of the high-potential test voltage is not recommended" (IEEE Std 112); we test at ~1.2x(2xV + 1000VAC) for a very short duration.

 

[che12345]

Dear akrus (Electrical)
@ Mr. BrianE22 (Specifier/Regulator)
1. You mentioned : The application is typically low voltage DC and tested per MIL-STD-202G Methods 301 & 302[/color]. Information: Superseded by MLT-STD-202H . method 301 and 302,
2. See method 301, Purpose 1.1 for dielectric test. See method 302, Purposes 1.1 for IR test.
3. See method 302 Purpose which states ".....insulation resistance measurement shall not be considered the equivalent of dielectric withstand voltage test. A clean, dry insulation may have a high resistance, and yet processes a mechanical fault that would cause failure in the dielectric withstand test. Conversely, a dirty .....".
3. In ....202H, (a) Method 301 does NOT specify the dielectric AC/DC test voltage i.e. dependent on rated insulation rating. Refer to product standard.
(b) Method 302 , the insulation test voltage range 100, 500, 1000Vdc. i.e. any DUT rated < 100V, shall be tested at 100Vdc not any lower, and DUT rated < 1kV can be tested at 1kVdc not required to be higher.
4. From various "white paper" the IR is to be carried out before Hipot. Do NOT hipot if IR is < 1 Mohm !
Che Kuan Yau (Singapore)