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    Altitude and resistance

    Level 4 test question: "At what elevation would the accuracy of ohm readings be effected?"

    Altitude always effects the resistance of air, but I can't find any sources quoting an elevation at which you would need to correct for it on a megger test. Can anyone shed some light?

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    Quote Originally Posted by JoelHolland View Post
    Level 4 test question: "At what elevation would the accuracy of ohm readings be effected?"

    Altitude always effects the resistance of air, but I can't find any sources quoting an elevation at which you would need to correct for it on a megger test. Can anyone shed some light?
    going to do some more digging on this. I did find this article though
    https://nepsi.com/resource/NEPSI%20-...nt%20decreases.

    Meggers specs rate their equipment for 3000m? May not be relevant, that is Alps heights but still thought it was odd they mentioned it. Never noticed it before.

    Rated at 5 kV and 10 kV, the MIT515, MIT525 and MIT1025 are engineered to ensure the highest precision possible. Both 5 kV units offer 5% accuracy up to 1 TΩ, while the 10 kV unit offers 5% accuracy up to 2 TΩ. The 15 kV model, MIT1525, offers a 5% accuracy with a maximum resistance up to 3 TΩ. All units are rated for use at an altitude of 3,000 meters and still maintain CAT IV 600 V safety rating.

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    https://www.ieee-pes.org/presentatio...14P-002640.pdf


    also check IEEE Std 4 section 16

    note from 16.2.3.

    NOTE-The values of exponents m and W have been deduced from experimental values obtained in different conditions.
    These are, however, limited to altitudes between sea level and 2000 m.

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    Quote Originally Posted by JoelHolland View Post
    Level 4 test question: "At what elevation would the accuracy of ohm readings be effected?"

    Altitude always effects the resistance of air, but I can't find any sources quoting an elevation at which you would need to correct for it on a megger test. Can anyone shed some light?
    From Megger:

    The International Electrotechnical Commission (IEC) standard EN61010-1:2001, specifies that the CAT rating must be valid up to an altitude of 2000m. This is because as the altitude increases the density of the air decreases and therefore so does its insulating properties. This is a problem for locations such as open cast mines in chili where altitudes could be around 4000m, which mean the CAT rating of most instruments become invalid. In fact many instruments will breakdown internally under their own test voltage at these altitudes.

    The introduction of the latest range of Megger MIT and S1 series instruments saw the introduction of CAT ratings specified to higher altitudes, exceeding the standard requirement of EN61010-1:2001. The MIT515, MIT525 and MIT1025 can operate and maintain their safety category rating to 3000m and the S1-568, S1-1068 and S1-1568 can maintain their safety category rating to 4000m altitude.

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  9. kyxy13 is offline Junior Member
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    Quote Originally Posted by JoelHolland View Post
    Level 4 test question: "At what elevation would the accuracy of ohm readings be effected?"

    Altitude always effects the resistance of air, but I can't find any sources quoting an elevation at which you would need to correct for it on a megger test. Can anyone shed some light?
    IEEE 48 is the standard for cable termination testing. Table 8 shows the correction factor for elevations greater than 1000m. The table I mentioned can be viewed below.

    Click image for larger version. 

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