Insulation Testing: Going Beyond a Single Resistance Measurement
Insulation is fundamental to the safe and reliable operation of electrical equipment. Whether in motors, generators, transformers or cables, the integrity of insulation helps protect equipment from failure while also reducing the potential danger to those working with electrical systems.
However, insulation does not remain unchanged throughout the life of equipment. Thermal stress, electrical stress, contamination and moisture can all affect its performance over time. For engineers involved in installation, servicing and maintenance, therefore, establishing the condition and integrity of insulation is an important part of electrical testing.
While a conventional insulation resistance measurement provides a useful indication of insulation condition at a particular point in time, there are situations where a single measurement does not provide the complete picture. By observing how insulation resistance changes over a defined period, engineers can obtain additional diagnostic information. Two established methods for doing this are the Polarisation Index (PI) and Dielectric Absorption Ratio (DAR).
Understanding insulation testing
At its simplest, insulation testing measures the condition of a conductor’s insulation and the internal insulation of electrical equipment. The objective is to establish whether the insulation is performing as expected and to identify indications of deterioration.
The choice of test equipment and test method depends on the application. Equipment may need to be tested during installation, routine servicing, or longer-term maintenance, and the requirements can vary considerably between applications.
Important considerations include the range of equipment to be tested, the voltage requirements for the test and whether the testing programme involves straightforward one-off measurements or longer-term monitoring. The environment in which testing will take place is also relevant, as are the safety rating of the instrument, the skill level of the operator and the conditions in which the tester will be used.
Where the objective is to monitor insulation degradation or obtain a more detailed indication of insulation condition, diagnostic measurements such as PI and DAR can provide information beyond a single insulation resistance value.
Polarisation Index: measuring change over time
The Polarisation Index is a diagnostic measurement used mainly to assess the insulation condition of high-voltage equipment such as motors, generators and transformers.
PI is calculated from the ratio of the insulation resistance measured at 10 minutes to the insulation resistance measured at 1 minute after applying a DC voltage test.
The behaviour of the insulation during the test provides important information. When a DC voltage is applied, the current through the insulation initially includes capacitive current due to the insulation, absorption current due to polarisation, and leakage current.
In a healthy insulation system, current decreases over time, meaning that resistance increases. This produces a high PI. If the insulation is wet, dirty or deteriorated, however, the resistance does not rise as much after the first minute, resulting in a lower PI.
PI can consequently help identify moisture ingress, surface contamination and insulation aging. It is the preferred measurement for large rotating machines and transformers.
Dielectric Absorption Ratio: a faster diagnostic approach
The Dielectric Absorption Ratio provides a similar diagnostic approach to PI, but over a shorter measurement period. DAR is the ratio of insulation resistance (IR) measured at 60 seconds to the IR measured at 30 seconds after applying a DC test voltage.
The shorter test period makes DAR particularly useful when time does not allow for the full 10 minute PI test. It is more commonly applied to smaller equipment or in field service applications, where a rapid indication of insulation condition can be valuable.
The distinction between the two methods is therefore primarily one of measurement period and application. DAR provides a fast screening method, while PI provides a more thorough evaluation of long-term dielectric performance.
Selecting an insulation tester
The requirements placed on an insulation tester extend beyond its ability to measure resistance. For engineers working across installation, servicing and maintenance applications, a versatile instrument can reduce the need to carry multiple test instruments while providing access to the diagnostic functions required for more detailed insulation assessment.
The Martindale IN2203 Advanced Insulation Tester is designed for both installation and service applications. The handheld instrument combines a wide range of test functions in a single compact unit, including a fully featured insulation tester.
The IN2203 provides five selectable test voltages, from 50V up to 1000V, and can measure up to 20G Ohms. It also incorporates a Lo-ohms earth-bond continuity (200 mA) test.
For applications where the performance of insulation over time needs to be assessed, the IN2203 includes dedicated measurement capability for both PI and DAR. Automatic calculation of these indices allows engineers to obtain diagnostic information without having to calculate the ratios manually.
Designed for installation and maintenance environments
Practical considerations can be just as important as measurement capability when selecting test equipment. Insulation testing may be carried out in installation environments, service locations or potentially demanding maintenance conditions.
The IN2203 features a compact design and rugged outer casing for use in harsh environments. In poorly lit applications, its extra-large LC display with background lighting provides a high level of visibility.
The instrument also incorporates additional multimeter-type measurement functions as standard. These include AC and DC voltage measurements up to 1000V, standard resistance measurements up to 600K Ohms, and continuity testing with an audible buzzer.
Safety is another important consideration when carrying out electrical testing. The IN2203 has a safety rating of EN 61010 CAT IV / CAT III 1000V. A live circuit detection check also inhibits testing on live circuits, providing an additional safety feature during insulation testing.
Conclusion
Insulation testing remains an essential part of electrical installation, servicing and maintenance. Establishing the integrity of insulation can help identify conditions associated with moisture, contamination and aging before they develop into more serious problems.
With insulation resistance testing, automated PI and DAR calculations, five selectable test voltages from 50V up to 1000V, measurement capability up to 20G Ohms and a safety rating of EN 61010 CAT IV 600V / CAT III 1000V, the Martindale IN2203 Advanced Insulation Tester brings a broad range of testing capabilities together in a compact instrument for installation, servicing and maintenance applications
