ECAD Testing Methodology
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:
- True DC current which is time independent and flows indefinitely.
- 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.
Distributed element data are measured using time domain reflectometry (TDR). Analysis of the TDR data is enhanced by understanding that the distributed elements represent an r-f transmission line that has special properties at special frequencies. This allows circuit components to be separated in time and analyzed individually while measurements are made from a remote location.
By definition, an r-f transmission line is a system of material boundaries forming a continuous path and capable of directing energy along the path. In addition, an r-f line exhibits the following characteristics:
- Delay from input to output.
- Directs AC energy.
- All energy contained in dielectric.
Examples of typical transmission lines include parallel conductors, conductor over a ground plane and concentric coaxial conductors.