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Routine Field Testing
Routine field testing of the electrical equipment is intended to enable maintenance personal to determine,
without laboratory conditions or complicated equipment, that a particular electrical equipment is able to
perform its basic circuit functions.
The following constitutes a guide to tests that might be performed during routine maintenance.
1.
Insulation Resistance Test
Extreme atmospheres and conditions may reduce the dielectric withstandability of any insulating
material. An instrument commonly known as "megger" is used to perform this test.
The voltage recommended for this test should be at least 50 percent greater than the circuit rating;
however, a minimum of 500 volts is permissible. Tests should be made between phases of
opposite polarity as well as from current carrying parts of the circuit protective device to ground.
Also, a test should be made between the line-and-load terminals with the circuit protective device
in the "OFF" position.
Resistance values below one megaohm are considered unsafe and should be investigated for
possible contamination on the surfaces.
NOTE:
For individual circuit protective device's resistance readings, load and line conductors
should be disconnected. If not disconnected, the test measurements will also include the
characteristics of the attached circuits.
A temperature and humidity reading are recommended and recorded during the testing
period.
Insulation resistivity is markedly effected by temperature and humidity conditions. Based
condition of one (1) megaohm per kV assumes a 20°C wet bulb reading. The following table shall
be used to adjust readings to the 20°C constant.
TABLE OF TEMPERATURE CORRECTION FACTORS
Temp (°C)
Motors
Transformers
Oil filled
Cables
PVC 90
XLPE
0
0.40
0.25
0.22
0.41
5
0.50
0.36
0.37
0.54
10
0.63
0.50
0.58
0.72
15
0.81
0.74
1.00
0.96
20
1.00
1.00
1.53
1.28
25
1.25
1.4
2.48
1.70
30
1.58
1.98
4.03
2.26
35
2.00
2.80
6.53
3.01
40
2.50
3.93
10.70
3.99
45
3.15
5.60
17.10
5.30
50
3.98
7.83
27.85
7.04
55
5.00
11.20
45.00
9.35
60
6.30
15.85
73.00
12.40
2.
Connection Test
Connections to the circuit protective device should be inspected to determine that a proper
electrical joint is present. If overheating in these connections is evident by discoloration or signs
of arcing, the connections should be removed and the connecting surfaces clean before re-
