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#191062 - 12/16/09 10:03 PM Using your Megohmeter
njelectricmaster Offline
Member

Registered: 04/29/01
Posts: 110
Loc: Lead, South Dakota
Hello from Washington State!!!!!

I know it has been a while since I have made a post, and I am sorry for being gone so long. I have moved from South Dakota to Washington and taken a position as the Electrical and Fixed maintenance forman at a gold mine.

Even after such a long time I am turning to the wealth of knowledge I have always found here.

I am looking for training material for my employees on the proper way to use the Megger. I am specifically looking for the proper way to use the measurements. I know there are a lot of different tests that can be performed with the meter, such as testing insulation, motor windings, and transformer cores.
What I want is a way for them to use all the different numbers to determine whether or not a winding is good or bad, and a way for me to track all of the information they record.

Any Megger Gurus out there?
Thanks in advance,
Jon

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#191068 - 12/17/09 01:21 AM Re: Using your Megohmeter [Re: njelectricmaster]
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Gidday Jon,
Welcome back, mate. wave

An interesting question you ask, it is also one that can't be answered with a "broad-brush" answer.
Insulation resistance testing was taught to me at night school.
There are a few "fish-hooks" involved in meggering gear as well.
AVO Megger came out with a really good interactive CD ROM, about 6 years ago, that explained the in's and out's of proper use of the Megger, if you Google that, it may point you in the right direction

In a general sort of a way, insulation testing with equipment like motors and transformers, is really only ever done between any of the line terminals on the equipment and (Ground/Earth/The Metallic body of the equipment).

Transformers are however slightly different, in that you test between the core of the transformer and the winding ends.
You also test between the primary and secondary sides of the transformer to make sure there is no "blow-through".

At the end of the day, the readings you get with each test are meaningless, if you don't know what they should be in the first place.
Only experience can really tell you how high or low a reading should be.
Take for instance an electrical cable, it is say 100' long, it will have an insulation resistance of say 300 Megohms, if you then double it's length to 200', the insulation resistance will halve to 150 Megohms.
This is because of the capacitance between the cores in the cable.

{Please note that these figures are not what you would get from an actual cable, they are only an example}

One other thing about Insulation Resistance testing is that it is defined as:

A test that will stress the insulation of wiring and components, to test thier electrical integrity.
This test is required to be done, using a DC voltage of twice the working voltage of the circuit/equipment.

{Foot-note:I'm going to move this thread to the Electrical Theory and Applications Forum, I'd like to see what Scott35 has to say about this subject}
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#191090 - 12/17/09 06:44 PM Re: Using your Megohmeter [Re: Trumpy]
aussie240 Online   content
Member

Registered: 07/06/05
Posts: 221
Loc: Blue Mountains, NSW, Australia
Originally Posted By: Trumpy
This is because of the capacitance between the cores in the cable.

Capacitance between cable cores will only have an effect if the test was done with AC, in which case the leakage test would be meaningless. Not only that, the higher the frequency used for the test, the greater the "leakage" would appear to be, when in fact the actual resistance is considerably higher.
Insulation testing is done using DC to avoid this error.
It is important to differentiate between capacitive reactance (which exists only on AC) and resistance which shows on both AC and DC.
For example, a 100m roll of three core flex has about 6nF between any two of the conductors. At 50 cycles, a capacitance of 6nF has a reactance of about 530K ohms. One would assume poor insulation if this was the DC resistance.
The modern Megger tester is essentially a high voltage DC inverter with a calibrated high output resistance. The output is applied to the device under test and the resulting resistance reading is determined by measuring the voltage across the D.U.T. in the same way a digital multimeter works on its resistance ranges. The high output resistance is required also to protect the D.U.T when stress testing. Basically, you can put very high voltages into things without damaging them provided the current is kept very low.

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#193744 - 04/17/10 03:19 AM Re: Using your Megohmeter [Re: aussie240]
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Originally Posted By: aussie240

Capacitance between cable cores will only have an effect if the test was done with AC, in which case the leakage test would be meaningless.

Exactly where, did I say that a megger test was done with AC?
You know (or should do) that testing long lengths of cable with a megger can cause some serious voltage build-up in the cable under test, this is caused by the cable storing the current from the megger during the test, not unlike a capacitor.
Has no-one else ever seen this at all?
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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