Testing of Switchgear - Part 1 (Breaker Insulation resistance test)

What is Switchgear ?

Switchgear is one type of electrical equipment (called Panel) which receives power supply from upstream like generators, greater capacity switchgear, transformers etc and provides supply to down stream like lower capacity switchgear, transformers, motors etc. It consists of breakers, switches, MCBs, control circuits, heaters, relays, closing and tripping circuits etc.

What are the tests we need to do for testing Switchgear on site ?

Following are the tests required for testing switchgear on site.

1) Breaker Insulation resistance test

Suppose we conducting insulation resistance test on 6.6 kV switchgear breaker, so the DC voltage injection will be 5 kV. If the switchgear is of low voltage rating than DC voltage injection using insulation tester will be low. Breaker will have three poles for R,Y and B phase. 

Why we are injecting only DC supply for checking Insulation Resistance ?

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.

So when the breaker is closed, conduct the test for 1 minute at 

R phase to Earth

Y phase to Earth
B phase to Earth

R phase to Y phase

Y phase to B phase

B phase to R phase 

When the breaker is open, the pole will have two part. One is moving contact and other is fix contact. 

Conduct the test between moving and fix contact for 1 minute at

R phase to R' phase

Y phase to Y' phase

B phase to B' phase

Thus we can ensure that there is no any other path for current to flow in the breaker and for checking the insulation of breaker pole circuit.

Here the value we will get in ohms. As per standard, the value should be more than or equal to (kV + 1) M-ohms. Where kV is the voltage rating of the switchgear. 

If you do not get the values within the range, check the power circuit for any material in between the circuit and clean the current carrying part through electrical contact cleaner spray. The current carrying part should be without any moisture to get the value of insulation resistance test.

For further test details i will post "Testing of Switchgear - Part 2" within few days.