The Theory of Insulation Testing
September 4, 2020
Insulation resistance (IR) testing is commonly used to monitor the insulation integrity of a given electrical system. Such testing can determine whether the insulation is beginning to fail and can indicate any impending problems before the system fails. Perhaps surprisingly, there are a variety of different tests that can be performed to measure the IR, some of which are outlined in this blog post.
IR testing conditions are generally dictated by the system under test and any time constraints the user may have. IR testing is based on a fairly simple concept – apply a voltage between the cable conductor and earth, measure the leakage current and subsequently calculate the IR. The IR is calculated based on one of the most fundamental equations in electronics, Ohm’s Law. Depending on the parameters chosen and the system under test, much more detail can be gathered about the state of insulation.
IR Testing: How and What is Measured
In routine testing direct current (DC) voltage is normally used, as with a megohmmeter. However continuous monitoring with an IR tester, such as an insulation monitoring device (IMD), usually utilises some form of alternating current (AC) waveform. This is because DC measurements are susceptible to interference from noise or stray currents. The testing discussed herein predominantly concerns DC voltage testing.
When a voltage E (Volts, V) is applied between a metal conductor and earth, current flows through the cable insulation. This is known as leakage current. However, the current measured by an IR tester is the total current which includes capacitive and absorption currents. Capacitive current is associated with the charging of a capacitor. A capacitor consists of two conductive mediums separated by a dielectric material (an insulator), such as an insulated copper cable submerged in seawater. When a DC voltage is applied between the two conductors separated by an insulator, a build up of equal and opposite charge at the facing conductor surfaces occurs. The dielectric on the other hand develops an electric field with which molecular dipoles in the dielectric align. It is this movement of chargecarriers to/from the surface of the conductors which produces capacitive leakage current. Absorption currents stem from the movement of dipoles within the insulator due to the presence of an electric field.
So, how do capacitive and absorptive currents affect an IR test? At the start of a DC voltage IR test, capacitive, absorption and leakage currents are present. Capacitive current dominates at the start and is typically much larger than leakage and absorption currents. With time, capacitance and absorption currents will dissipate, dictated by the ability of a material to store charge (which insulators do more effectively). Capacitive current (associated with the conductors) dissipates quickly compared to absorption current (associated with the insulator). The reduction in current therefore causes a continual increase in IR during an IR test. Most IR testing methods, however, take these effects into account.
IR Testing: Methods
IR testing serves as a useful troubleshooting tool to monitor and respond to known problems. IR testing can indicate whether an insulation fault is developing, and whether a system might need maintenance or, in some cases, replacing. It should be noted that repeated IR testing on a single system should be performed under the same test conditions and test equipment if possible.
The simplest form of IR test is a constant voltage test performed for a specified period and recording the IR at a set time. Choosing an ideal DC voltage and timescale depends on the system at hand, such as the cable length and withstand voltage, and time constraints. The IR test time can be 60 seconds long, known as a ‘Spot Test’, or longer. A one-minute minimum is advised intending to avoid effects from capacitive current, where the IR increases rapidly in the first instance. Indeed, absorption currents will also be present. Spot testing therefore only gives a rough idea of the insulation integrity. Increasing the test time, however, can improve accuracy, known as a ‘time-resistance’ test.
Dielectric Absorption Ratio (DAR) and Polarisation Index (PI) Testing
Further to observing IR trends, quantitative measures can be obtained to determine the possible condition of the insulation. The dielectric absorption ratio (DAR) is one of them, which describes the ratio of two time-resistance values. A DAR < 1 indicates that the IR at a larger timescale is smaller than that at a shorter timescale. This means that absorption current is masked by leakage current, in turn indicating poor insulation. The higher the DAR the better the insulation integrity due to a high absorption current. Similar to Spot Tests, DAR values offer a rough indication of insulation integrity. Some would argue that retrieving a DAR at 10 mins:1 ‑min would be more accurate, also known as the polarisation index (PI).
Information gathered from DAR and PI tests depends on the system tested. A PI value between 1-2 could be ‘satisfactory’ for short sections of house wiring, but ‘questionable’ for long offshore cables. All the same, these are useful measures which help to determine whether to intervene or investigate problems further. In addition, more than one test type should be performed to ensure accuracy and consistent outcomes.
Another useful IR testing method is ‘Ramp Testing’ where the voltage is continually increased at a constant rate to a specified voltage. For example, a sweep rate of 100 V minute−1 up to 500 V. It is advised that spot and time-resistance testing is performed prior to ramp testing and the results of which should be taken into account when deciding on ramp testing conditions. The response of insulation to a ramp test provides detail on the condition of the system and can allow the user to detect small defects in the insulation. For this test, the leakage current is plotted as a function of voltage. Certain voltage-current trends can indicate ingress and localised faults.
A smooth almost linear increase in current with voltage is expected for ‘good’ insulation condition. The increase comes from capacitive and absorption current which do not dissipate due to the continually changing voltage. When the behaviour deviates from this ‘ideal’ behaviour, this warns that the test is tending towards insulation breakdown. If a large spike in current is observed, this could indicate water/moisture ingress. If small ‘blips’ in current are observed, this could indicate local weaknesses in the insulation.
Overall, it is evident that these IR testing methods can provide the user information not only on the insulation integrity of the given system, but also the type of faults (if any) that are present.
Read our full Technical Paper ‘The Theory of Insulating Testing’ written by Dr A. R. Langley here: https://www.viperinnovations.com/wp-content/uploads/THE-THEORY-OF-INSULATION-TESTING-Technical-paper.pdf
Read our Failure Investigation ‘Root-Cause Analysis of Failed Submerged Electrical Cables’ written by Dr A. R. Langley here: https://www.viperinnovations.com/wp-content/uploads/FAILURE-INVESTIGATION-Technical-Paper.pdf
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