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Old 20th February 2014, 11:33 AM   #1
pointy is offline pointy  United Kingdom
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Exclamation isolation test

Hi I have a power transformer that i wish to test for isolation but I do not have a neon test bulb. Could I put a transformer primary winding in place of the neon lamp and then test the secondary winding with a multimeter, would this be a good isolation test. I would be using a 25w tungsten lamp bulb as a current limiter for mains supply to the DC supply.

Last edited by pointy; 20th February 2014 at 11:45 AM.
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Old 20th February 2014, 11:38 AM   #2
M Gregg is offline M Gregg  United Kingdom
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Originally Posted by pointy View Post
Hi I have a power transformer that i wish to test for isolation but I do not have a neon test bulb. Could I put a transformer primary winding in place of the neon lamp and then test the secondary winding with a multimeter, would this be a good isolation test.
Er No..

The whole point of insulation test is the level of current flow and at what voltage? As soon as you load the one side of insulation you drain the leakage so you get a volt drop. ie no power to drive the indicator.

You should use something like a megger insulation resistance tester.

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Old 20th February 2014, 11:41 AM   #3
M Gregg is offline M Gregg  United Kingdom
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Just for interest,

There is no such thing as an insulator...
Everything has leakage it just depends on voltage level..and at what voltage it will break down.
Ie even the air will conduct if the voltage is high enough (lightning)
So insulation value and voltage<<<insulator >>>not a very good conductor.. (up to a point)

NB there are recommended values under IEE.
http://electrical-engineering-portal...n-resistance-2

The above is just an example...so how would you be able to measure the actual value?


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Old 20th February 2014, 12:24 PM   #4
M Gregg is offline M Gregg  United Kingdom
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Just had a look for the IEE

BS7671 can't find the 17th just the 16th at the moment>>> a bit rushed....

...ah well.

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Old 20th February 2014, 12:32 PM   #5
TheGimp is offline TheGimp  United States
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What are you trying to measure?

The actual resistance between windings, and windings to core? (IR Test per the link M Gregg pointed to)

The breakdown voltage between windings, and from windings to core? (Hi-Pot test)

Or something else?
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Old 20th February 2014, 12:44 PM   #6
pointy is offline pointy  United Kingdom
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Steps to first power on.

Testing the Transformer Part 2.
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Old 20th February 2014, 12:47 PM   #7
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you said isolation
he said insulation

what are you trying to accomplish in the end?

isolation on a transformer IE inter-winding capacitance is tested with a impedance bridge or a capacitance voltage divider over a range of frequencies. using a known value capacitor in series with the unknown inter-winding , the value then can be determined with a little math.

insulation is done with a Hipot tester > most electric motor shops have one
Dielectric Withstanding Voltage Test it takes about 30 seconds to run
Hipot - Wikipedia, the free encyclopedia
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Old 20th February 2014, 12:58 PM   #8
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Thank you M Gregg what I shall do is take it to my local electrical store where I am sure they will have a megger insulation resistance tester.
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Old 20th February 2014, 01:08 PM   #9
infinia is offline infinia  United States
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in the link > he's just doing a simple continuity check with 400Vdc and a neon bulb.
yes u can use a DC ammeter instead of the bulb
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Old 20th February 2014, 01:28 PM   #10
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by pointy View Post
Thank you M Gregg what I shall do is take it to my local electrical store where I am sure they will have a megger insulation resistance tester.

NB never Megger any semiconductors..ie they should not be connected to the Tx under test.

Some tests you can do with a meter just to see.
Test continuity of each winding low resistance.
Test between each winding expect very high resistance. Ie no shorts between HT and heater or any winding to primary.
All windings to the core.

If this is OK then the Megger test.

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