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DAR and PI insulation test voltages are selected on the central
rotary switch by simply aligning the switch opposite to the
required insulation test voltage. Press and hold TEST to start a
DAR/PI test.
DAR is defined as the ratio of insulation resistance at 1 minute
divided by insulation resistance at 30 seconds, although a 1
minute, 15 second DAR is also popular:
DAR = IR
60s
/ IR
30s
Insulation Condition
DAR result
Poor
< 1
Acceptable
1 – 1.4
Excellent
1.4 – 1.6
IEEE standard 43-2000, Recommended Practice for Testing
Insulation Resistance for Rotating Machines, defines PI as the
ratio of insulation resistance at 10 minutes divided by insulation
resistance at 1 minute:
PI = IR
10min
/ IR
1min
If IR
1min
> 5000 M
Ω
the PI may or
may
not be an indication of
insulation condition and is therefore not recommended by IEEE
std. 43.
Insulation Condition
PI result
Poor
< 1
Questionable
1 - 2
Acceptable
2 - 4
Good
> 4
PI results > 1.5 are regarded as acceptable by IEC60085:-
01:1984 for thermal class rating A, and PI results > 2.0 for
thermal class ratings B, F and H.
Dielectric Discharge Test
The Dielectric Discharge
(DD) or
reabsorption
current
test operates during
the discharge of the
dielectric under test.
Originally developed by
EDF, France’s power utility
company, it is a diagnostic
insulation test that allows
ageing, deterioration, and voids in the insulation to be assessed. The
result is dependent on the discharge characteristic which tests the
internal condition of the insulation and is largely independent of
surface contamination.
The insulator must be charged until the only remaining component of
current is leakage current. On discharge the capacitive component
of the discharge current decays from a high value with a relatively
short time constant of a few seconds. The released reabsorption
current decays from a lower value with but has relatively long time
constant of up to several minutes.
The DD timer (t1) defaults to 30 minutes of charge, which is
generally sufficient time for full absorption to take place in an
insulation material. The default test voltage is set to 500 V so the
primary rotary switch must be set at 500 V. The discharge timer (t2)
defaults to 1 minute. Timer settings t1 and t2 are adjustable.
DD should be selected on the test mode rotary switch and settings
on the central rotary switch.
The ‘DD’ test requires the instrument to measure the discharge
current 1 minute after the removal of the test voltage. On completion
of the test, the instrument uses the current, the test voltage and
calculated capacitance to produce a figure of merit indicating the
quality of the insulation.
