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.

What is CT and why we need small value for measurement and protection in electrical circuits ?

What is Current Transformer  ?

Current transformer is one type of instrument transformer which is useful for measuring AC current in any electrical equipment. Current transformer primary is connected in series with the circuit whose current is to be measured and it's secondary is connected to relay,  ammeter,  watt-meter,  etc. This current transformer measures current in primary by producing proportional current in secondary circuit.  All current transformer have particular ratio like 200/1, 400/5 etc upon it's requirement. So for ratio of 200/1, this current transformer will produce 1 ampere in secondary circuit if there is 200 ampere in primary circuit. 

Why we need small scale value for measuring the current in electrical circuits ?

Current transformer is converting high value of current to low value of current and we use low value of current for measuring and protection purpose. We need low value of current because If we will use large value current for measurement and protection, then the instruments like ammeter, wattmeter and relay rating will have to be larger to bear the high current and the  costing of this devices will also be high. Large currents are also not easy to handle without circuit breakers.

Insulation Resistance Test in Electrical Equipment

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.

What is Insulation Resistance (I.R.) Test ?

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.

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 ?

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.

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.