What is Polarisation Index?
When testing insulation on electrical equipment, sometimes it is necessary to obtain a polarisation index (PI).
Polarisation index, put simply, is a ratio of the results from the insulation test over 10 minutes to that of 1 minute. It is a more definitive insulation test than a single insulation resistance (IR). It is usually applicable to testing insulation in generators, motors and transformers.
What does it mean?
When a large DC voltage is applied to an insulator, there will be four different currents that will flow. Two of these currents will decay over time, and the other two will be constant throughout the whole test.
In the Polarisation Index test, when the DC Voltage is first applied the insulation is in effect like a dielectric between two capacitor plates. Thus, capacitance will charge up over time and the resulting current will eventually decay to zero as the capacitance reaches maximum charge. This current is omitted from the results as it is merely capacitive, decays to zero well before the first minute, and gives no indication as to the condition of the insulation. This is why the first reading is not taken until after the first minute.
Many impurities (including water molecules) that occur in insulation are polar in nature, especially if the insulation medium is organic. As a result they align themselves to an electric field placed on them. This aligning of the molecules when the DC voltage is applied presents itself in the form of an electric current. This current is expected to decay before ten minutes has expired as all of the molecules have aligned by then. This is why the test is done for ten minutes.
At the end of the ten minutes there are only two currents left flowing in the insulator. A current that flows through the medium, and a current that flows on the outside of the insulator.
Some current will flow through the insulation regardless of what the insulator is made of. However, if the insulation has cracks, or has begun to break down, this current can become larger than desirable. That is, the insulation is no longer at it’s optimum.
The current that flows on the outside of the insulation is due to moisture, dirt, grime, carbon, etc that builds up on the surface and allows for some degree of conductivity.
Here’s where the PI comes in.
From ohm’s law, the total current and supply voltage are used to determine the resistance of the insulation.
After one minute, the capacitive current has decayed away, the polarisation current is still near its highest value, and the internal and external currents are also flowing. By ten minutes, the only currents flowing are internal and external currents.
If the one minute current is high, compared to the ten minute current, then the resistance value at one minute will be low compared to the ten minute value. This is because the majority of initial current is polarisation and the other two currents are comparatively minimal, indicating that the insulation is in good condition.
If the one minute current is NOT high compared to the ten minute current, then the value of resistance taken at 1 minute will be closer to the ten minute resistance value. This indicates that more of the current is due to the internal and external currents, rather than polarisation, and suggests that the insulation is beginning break down.
A polarisation index less than 2 indicates that the insulation is becoming questionable to unreliable. Greater than 4 is excellent. Most companies will have specific information that indicate what test results are acceptable. Be sure to find out, and if not available on a company level, then the information should be sourced through the equipment manufacturer.