Note: This test is to be done only when there is no any power available in the equipment. The equipment must be in shutdown condition. This is because to prevent mixing of AC and DC supply for Equipment and personal safety.
Insulation resistance is one type of Electrical Test which is commonly used for checking healthiness of Insulation of Electrical Equipment. Insulation testers are provided with different voltage rating like 500 V, 1000 V, 2500 V, 5000 V etc. We have to inject the particular voltage to equipment as per equipment's voltage rating. For example the cable is to be used for 430 V AC application than DC injection voltage will be 500 V and if the cable is to be used for 6600 V AC application than DC injection voltage will be 5000 V.
Why we need to check the Insulation Resistance
As the insulation of any electrical equipment is made, ageing process starts in the insulation and this process become faster depending upon the temperature and chemical reactions. So we need to check insulation resistance periodically. If insulation resistance of the electrical equipment becomes weak, there is risk of damage to electrical equipment and person.
We check the Insulation resistance of any electrical equipment by injecting DC supply according to the voltage level of equipment between the windings (or phase connection) and between windings (or phase connections) and earth connection for duration of one minute.
We are injecting only DC supply because we are supposed to find Insulation Resistance (R) and not Impedance (Z). If we inject AC supply then inductance and capacitance of the insulation will also be added with resistance so impedance (Z) will be calculated. Other reason is that if we inject AC supply then the electrical equipment will start to function as it is intended for use. For example we are injecting 5 kV AC supply to motor of 6.6 kV rating than motor will be charged and will start to run.
How we check Insulation Resistance ?
We check the Insulation resistance of any electrical equipment by injecting DC supply according to the voltage level of equipment between the windings (or phase connection) and between windings (or phase connections) and earth connection for duration of one minute.
Why we are injecting only DC supply for checking Insulation Resistance ?
Connection provided for Insulation Resistance Test on Insulation Tester
There are three terminals provided on insulation tester.
1. Line
2. Earth
3. Guard
1)Line: This terminal is the DC voltage injecting terminal.
2)Earth: This terminal is provided to connect it to earth during testing for injection of DC voltage with reference to Earth.
3)Guard: This terminal is used for bypassing the resistance path that we don't want to calculate. For example, we are carrying IR test on bushing. During test there will be two currents flowing, first is the current through the bushing that we want to calculate and the second is the surface leakage current that we don't want to calculate. So to ignore this surface leakage we will connect guard terminal to this point.
Standard value for Insulation Resistance Test
The standard value of insulation resistance test should be (kV + 1 ) M-ohm. We consider 11 kV panel breaker for this example. As per standard its insulation resistance value should be 12 M-ohms. Suppose its insulation resistance is 15 M-ohms than it is acceptable but if it is less than 12 M-ohms than it is not acceptable.
No comments:
Post a Comment