Lumped measurements include DC Resistance. AC Resistance, Impedance,
Capacitance, Inductance, Quality Factor, Dissipation Factor, Insulation
Resistance and Polarization Ratio.
Lumped measurements treat the circuit as a "Black Box" combination of
resistors (R), inductors (L) and capacitors (C). If the black box is composed of
linear elements (R, L, C), the contents of the black box can be determined by
analyzing the steady state, transient and frequency responses to the respective
forcing functions. Lumped data measurements are obtained using the DMM Card
(Multimeter), impedance card and the megohmmeter.
Insulation Resistance & Polarization Ratio
The most typical measurement of insulation quality is
Insulation Resistance (IR). IR is an indication of the overall quality of a
cable's insulation. A conductor insulated with an ideal dielectric will not lose
any current through the insulating material when a voltage is applied to the
conductor. Since the ideal insulating material has not yet been discovered, some
amount of leakage current will flow through the insulation. Resistance to
leakage current flow is defined as the ratio of test voltage (V) to the leakage
current (I). This simple ratio can be affected by conditions such as temperature
or ionizing radiation. The ratio is also time and frequency dependent.
Current flow through the dielectric or insulating material has two components:
1. True DC current which is time independent and flows indefinitely.
2. Polarization or absorption current which decreases inversely as a negative
exponent of the time.
True DC current is the transport of charge from an electrode
through the insulator to the other electrode. Polarization or absorption current
does not involve charge transfer, but results from the displacement of charge in
the dielectric.
In the first few nanoseconds the displacement of electronic and intramolecular
charged atoms takes place. This realignment of charged atoms in a dielectric is
responsible for very-high-frequency permittivity observed in some dielectric
materials. On a more human time scale, on the order of seconds or minutes, the
polarization current that is seen to flow is due to to the rotation of dipolar
molecules and groups of molecules that are relatively free to move. This
component of polarization current is most commonly observed in DC measurements
and is a good indicator of insulation quality.
Measuring current flow through a dielectric at different time intervals allows
for the calculation of a temperature independent ratio called the Polarization
Index (PI). IEEE Standard 62-1978 defines Polarization Index (PI)
as the ratio of the insulation resistance value measured at ten minutes to the
insulation resistance value measured at one minute. Other indicators of
polarization are used such as Polarization Ratio (PR) which is the ratio
of the insulation resistance at 3 minutes to the insulation resistance at 15
seconds, and the Dielectric Absorption Ratio (DAR) which is the ratio of
the insulation resistance at 60 seconds to the insulation resistance at 30
seconds.
Low ratios (less than 1.0) associated with a low resistance sometimes indicates
parallel paths through the insulating material. The most usual cause of this
sort of degradation is an absorbed water film on the surface of the dielectric
which allows for increased ionic concentrations and greater mobility of the
dielectric's atoms.
The ECAD System 2005 calculates the PI, PR or DAR and reports the final
insulation resistance measurement. Regardless of the ratio selected by the user,
the ECAD will measure and store 12 data points over the time period of the IR
test. For calculation of DAR and PR, measurements are taken at 5 and 15 second
intervals respectively. If the PI is selected, measurements are taken at 15, 30,
and 60 seconds then every 60 seconds for the duration of the test. The ECAD System 2005 calculates the ratio, stores the value and displays the number
immediately.
