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# Polarization Current VS Absorption Current(2)

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 BigJohn1 (Electrical) 5 Mar 12 8:43
 When doing insulation resistance testing, I understand there are two different measurement ratios:  Dielectric Absorption Ratio and Polarization Index.I think I have a good understanding of polarization current being measured during the PI test where the dipoles in the conductor insulation are being re-oriented in the presence of an electric field.However, I can't find any solid explanation of absorption current.  Is the measurement made during a DAR simply a different value of the same current as a PI?If not, what is absorption current?  Anyone care to tackle this?  Thanks.-John
 petronila (Electrical) 5 Mar 12 9:54
 Hello BigJohnA modern instrument(Megger) could calculate both: PI and DAR values, you can read the following manual: Guide to modern insulation testing by Metrel.RegardsCarlos
 electricpete (Electrical) 5 Mar 12 12:59
 IEEE 43 identifies three components of current:1 - Capacitive2 - Resistive (leakage)3 - 'absorption' and 'polarization' currents...both terms have the same meaning.Capacitive decays away quickest, then aborbption current takes long, resistive of course is constant.Dielectric Absorption (D.A.)  = Ration 1 minute reading / 30-sec readingPolarization Index (P.I.) = Ratio 10 minute / 1 minute reading. D.A. and PI both attempt to deduce the same behavior (high resistive current compared to other two components... primarily absorbption.).   The only reason to do D.A. instead of P.I. is because it's easier (particularly with hand-cranked megger where cranking for 10 minutes would be a pain).   However the P.I. is a better test and the only one recognized by IEEE 43  (D.A. has fallen out of favor now that very few people use hand-cranked megger).The values of DA tend to run lower than the values of PI for a given machine/condition,  as suggested by following limits given by Megger "Stich in Time":Questionable:  1 – 1.25 DA,  1-2 PIGood:  1.34 – 1.6 DA,  2 – 4 PIExcellent: > 1.6 DA, > 4 PI  =====================================(2B)+(2B)'  ?
 BigJohn1 (Electrical) 5 Mar 12 13:38
 Thanks for the replies.So both tests are fundamentally measuring the exact same physical principals, the only difference being that obviously a PI would be more comprehensive?Is the DAR called that simply because much of the current increase in that test is also capacitive, so it's not primarily measuring polarization and leakage?  Am I correct in thinking that's the importance of comparing the 30 second and 60 second readings:  You're trying to null out the capacitive charging that occurs in the first 30?-John
 electricpete (Electrical) 5 Mar 12 14:31

#### Quote:

So both tests are fundamentally measuring the exact same physical principals, the only difference being that obviously a PI would be more comprehensive?
Yes, exactly.

#### Quote:

Is the DAR called that simply because much of the current increase in that test is also capacitive, so it's not primarily measuring polarization and leakage?
There is no difference between absorbtion current and polarization current by IEEE43 terminology. Specificaly IEEE Std 43-2000(R2006) Definition 3.1 treats them equivalently:

#### Quote (ieee43):

"3.1 absorption (polarization) current: A current resulting from molecular polarizing and electron drift, which decays with time of voltage application at a decreasing rate from a comparatively high initial value to nearly zero, and depends on the type and condition of the bonding material used in the insulation system."
So personally, I'd be be inclined to think there is not a lot of significance to those particular two choices of names for those particular two tests.

#### Quote:

Am I correct in thinking that's the importance of comparing the 30 second and 60 second readings:  You're trying to null out the capacitive charging that occurs in the first 30?
Let's start with absorption: I'd be inclined to think (from the IEEE43 figures) that the PI has a fairly straightforward meaning:
PI = (Iabs1min + Iresistive) / Iresistive.
Where Iabs1min = 1-minute absorption current.
The capacitive is not included here because it is pretty much gone by 1 minute.
You could also rewrite this as:
(Iabs1min / Iresistive)  = PI -  1 which gives imo a nice simple definition.

The DAR is not as straightforward. For one thing, absorption current is present on both measurements (30 sec and 60 sec) and it is of course not the same for both.  So already we are working with three values (I resistive,  Iabs1min, Iabs30sec where in PI we only had two).  If Icap is not completely gone at 30 seconds, it's even messier.   So there is not as simple an interpretation for DA as for PI.  But I think the values we use as limits tend to be based on empirical-based thumbrules (rather than theory) anyway, so it's not a big problem.

=====================================
(2B)+(2B)'  ?

 electricpete (Electrical) 5 Mar 12 14:39

#### Quote:

But I think the values we use as limits tend to be based on empirical-based thumbrules (rather than theory) anyway, so it's not a big problem.

Perhaps the exception is 1.0 limit.  Below 1.0 limit on either test means current is increasing... insulation getting worse due presumably to breakdown.

=====================================
(2B)+(2B)'  ?

 BigJohn1 (Electrical) 5 Mar 12 16:23

#### Quote:

...There is no difference between absorbtion current and polarization current by IEEE43 terminology. Specificaly IEEE Std 43-2000(R2006) Definition 3.1 treats them equivalently....
That was actually one of the reason I posted this question, because I saw that definition in IEEE 43 and was confused why there would be such drastically different names for what appeared to be the same phenomenon.

I appreciate the replies.  Y'all cleared it up nicely.

-John
 Zogzog (Electrical) 5 Mar 12 16:24
 BigJohn1 (Electrical) 5 Mar 12 20:06
 Hey, I know you!Thanks for the clarification of terms.-John

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