Specifications

PIP ELTFT01
Field Inspection and Testing of March 2001
New Electrical Equipment
Process Industry Practices Page 6 of 23
Transformer (V) Liquid (Oil) Dry
0 - 600 100 megohms 500 megohms
601 - 5000 1000 megohms 5000 megohms
5001 - 15,000 5000 megohms 25,000 megohms
5.1.3 Switchgear buses, circuit breakers, motor control centers, and panelboards:
250 volt and below 25 megohms
600-volt equipment 100 megohms
2400-volt equipment 500 megohms
Equipment rated above
2400 volts
2000 megohms
5.1.4 For 600-volt-rated cables, insulation resistance shall not be less than
50 megohms.
5.1.5 For instrument and control conductors rated 300 volts and below, the
insulation resistance shall not be less than 1 megohm.
5.1.6 For cables rated above 600 volts, minimum insulation resistance per
1000 feet of cable shall not be less than that calculated with the formula
below:
R = k log10 D/d, where:
R = insulation resistance in megohms/1000 ft
k = insulation constant
D = diameter over insulation
d = conductor diameter
The calculated values of insulation resistance shall be rounded to the next
lower 100-megohm value. For armored cables, the calculated insulation
resistance shall be reduced to 80 percent and then rounded off to the nearest
100-megohm value. For lengths other than 1000 feet, the insulation
resistance is inversely proportional to the cable length.
Insulation constant (k) values for different insulations are as follows:
Insulation Type k
Ethylene propylene rubber (EPR) 20,000
Cross-linked polyethylene (XLP) 20,000
Butyl rubber 10,000
Thermoplastic 2000
5.2 The values from transformer turns ratio (TTR) tests shall not deviate more than
1/2 percent from either the adjacent coils or the calculated ratio given in NETA
ATS-1999, paragraph 7.2.2.3.6.