14
TABLE C.1 — NORMAL INSULATION RESISTANCE VALUES BETWEEN ALL LEGS AND
GROUND
Insulation resistance does not vary with rating. Motors of all HP, voltage and phase rating have the same insulation
resistance ranges.
METER READING
CONDITION OF MOTORS
R x 100K
R x 10K
AND LEADS
OHMS MEGOHMS
or
or
R x 100,000
R x 10,000
Scale Scale
BENCH TESTS
• A new motor (without drop cable).
20,000,000+
20+
200+
2000+ or 2K+
• A used motor which can be reinstalled
10,000,000+
10+
100+
1000+ or 1K+
in the well.
• Cable splice after immersion for one
2,000,000+
2+
20+
200+
hour in water.
WELL TESTS Ohm readings are for drop
cable plus motor.
• A new motor or used motor in good
2,000,000+
2+
20+
200+
condition.
• A motor in reasonably good condition.
500,000–
0.5–2.0
5–20
50–200
2,000,000
• A motor which may have been damaged
20,000–
0.02–0.5
0.2–5
2–50
by lightning or with damaged leads.
500,000
Do not pull the pump for this reason.
• A motor which definitely has been
10,000–
0.01–0.02
0.1–0.2
1–2
damaged or with damaged cable. The
20,000
pump should be pulled and repairs
made to the cable or the motor replaced.
The motor will not fail for this reason
alone, but will probably not operate for
long.
• A motor which has failed or with
Less than
0–0.01
0–0.1
0–1
completely destroyed cable insulation.
10,000
The pump must be pulled and the
cable repaired or the motor replaced.
+ Indicates that the reading should be the value shown or greater. Higher readings indicate better insulation.
Resistance measured between the power cable leads after
splicing the power cable to the motor leads will indicate
the resistance of the power cable plus the motor wind-
ings. The motor winding resistance is obtained by the
formula below. The calculated value should be within the
limits specified in the motor manual.
Motor Winding = Reading taken – Cable Resistance
Resistance
at Power Cable from Table 2.
A higher winding resistance than shown in the motor
manual indicates a possible burned (open) winding, an
open cable, a loose connection or the wrong motor (dif-
ferent HP or voltage than readings being referenced).
A considerably lower winding resistance than shown in
the motor manual indicates a possible shorted (burned
together) winding or the wrong motor.
Unequal resistance between the windings on a three
phase motor indicates a burned winding or a faulty con-
nection.
