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jr
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Join Date: Apr 2011
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Allowable Resistance of Motor Insulation and Cable Insulation

06/23/2011 2:59 AM

What is the allowable resistance of motor windings insulation and also Cable/Conductor insulation in Megger Test?
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Anonymous Poster #1
#1

Re: allowable resistance of motor insulation and cable insulation

06/23/2011 3:10 AM

As Carl Sagan would put it, billions and billions of ohms.
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TonyS
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#2

Re: allowable resistance of motor insulation and cable insulation

06/23/2011 3:18 AM

A very old rule of thumb I was given as an apprentice was that the leakage current should not exceed 1/10000th of the full load current. So just use Ohms law for the calculation.

This was based on the M&Q Act 1956, I was less than a year old!

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Gibson81
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#3

Re: allowable resistance of motor insulation and cable insulation

06/23/2011 4:19 AM

IEEE 2000-43 states 100 Mohm anything less on high voltage motors is a concern. We also use a rule of thumb for motors that is pre 1970 and our cables that for every 1kV (on Nameplate) = 1 Mohm + 1 so say its a 6.6 kV motor the minimum Mohm reading one should get is then 6.6 Mohm + 1 = 7.6Mohm

Also use your PI values to look at insulation integrity.
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Salunkhem
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#4

Re: Allowable Resistance of Motor Insulation and Cable Insulation

06/27/2011 9:49 AM

As per IEEE minimum is kV + 1 Mohm as informed by our members, but, if you go for inspection then insist for value of 2MOhm per kV. This is industrial practice. If equipment insulation is in good condition this practice will not let you fail, instead you will be very sure that eqpt insulation is in good condition.

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electricalexpert65
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#5

Re: Allowable Resistance of Motor Insulation and Cable Insulation

06/27/2011 11:09 AM

As per IE Rule 48, the minimum insulation resistance for low voltage installations is 1 Mega Ohm when tested with a 500V megger and for high voltage installations it is 5 Mega Ohms when tested with a 2500V megger. However, in case of motors, I would rather be more interested in checking the PI (Poarisation Index) value rather than plain insulation resistance.
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Salunkhem
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#6
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Re: Allowable Resistance of Motor Insulation and Cable Insulation

06/27/2011 2:41 PM

Just a small correction, as per IS (i Will tell exact number tomorrow), LV installations is checked with500V as you said and HV with 5000V not with 2500V. I agree about PI bur, megger is common routine test for all equipments.

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electricalexpert65
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#7
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Re: Allowable Resistance of Motor Insulation and Cable Insulation

06/27/2011 8:39 PM

I reproduce below the IE Rule 48:

48 Precautions against leakage before connection-

(1) The supplier shall not connect with his works the installation or apparatus on the premises of any applicant for supply unless he is reasonably satisfied that the connection will not at the time of making the connection cause a leakage from that installation or apparatus of a magnitude detrimental to safety. Compliance with this rule shall be checked by measuring the insulation resistance as provided below:

(i) All the electrical equipment shall have the 'IR' values as stipulated in the relevant Indian Standards.

(ii) At a pressure of 500V DC applied between each live conductor and earth for a period of one minute, the insulation resistance of low-voltage and medium voltage equipments shall be at least 1 MEGA-OHM or as specified in the relevant Indian Standard.

(iii) At a pressure of 2.5kV DC applied between each live conductor and earth for a period of one minute, the insulation resistance of high-voltage equipments shall be at least 5 MEGA-OHM or as specified in the relevant Indian Standard.

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Salunkhem
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#8
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Re: Allowable Resistance of Motor Insulation and Cable Insulation

06/27/2011 11:05 PM

IS 7816-2002-Guide for testing insulation resistance of rotating machines Assistance for this standard has been derived from IEEE Code No. 43 'Recommended guide for testing insulation resistance of rotating machinery'

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msamad
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#9

Re: Allowable Resistance of Motor Insulation and Cable Insulation

07/14/2011 9:34 PM

See document may also be helpful: http://www.megger.com/common/documents/Above1kV_UG_en_V04.pdf

- MS

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