The Oil Industrial Circuit Breaker

An oil industrial circuit breaker consists of gang-operated, three single pole switches whose contacts are completely immersed in oil. Most units have one grounded tank per phase although some designs may have all three phases housed in a single tank.

The major components of an oil industrial circuit breaker are as follows:

        Two brushing per phase mounted in an oil-filled grounded tank.
        A contact assembly (interrupter) mounted on the bottom terminal of each brushing.
        An insulating operating rod to open and close breaker contacts.
        An insulated guide assembly to keep the operating rod in proper alignment.
        Tank containing a volume of insulating oil to provide quenching.

Some oil industrial circuit breakers also have tank liners, shunt resistors across the interrupters and other auxiliary components.

Oil Industrial circuit breakers and Insulation Tests

The primary objective of the insulation test on the oil industrial circuit breaker is to determine the condition of the oil industrial circuit breaker components (listed above). In specific, the brushings are the most vulnerable components within the system, especially at higher voltages.

Nine overall tests are routinely performed on a three-phase oil industrial circuit breaker (three overall tests per phase). They are further classified by being either open versus closed.

    The open industrial circuit breaker tests measure the insulation of brushing, oil, interrupter, lift rod guide, and tank liner.
    The closed industrial circuit breaker test measure both brushings, oil, both interrupters, lift rod, and tank liner.

Open Vs. Closed Insulation Test and the Oil Industrial Circuit Breaker

The difference between an open industrial circuit breaker and a closed industrial circuit breaker test is that in the closed industrial circuit breaker test a larger dielectric field is established. The watts loss of a closed industrial circuit breaker test is different from the sum of the two open breaker watts loss because during the closed breaker test, the cross-head is energized thereby placing the lift rod in a stronger dielectric field. Also, the average dielectric field within the tank is increased during the closed breaker test.

Therefore, since the difference between a closed oil industrial circuit breaker test is different than the sum of the open oil industrial circuit breaker tests, then any differences must be due to losses in auxiliary insulations which are not stress the same for both conditions. In fact, the amount of such differences can be used to evaluate the condition of the auxiliary insulation, and is referred to as the tank loss index test.

Tank Loss Index and the Oil Industrial Circuit Breaker

The tank loss index test is defined as the difference in watts loss between closed breaker test minus the sum of the two open breaker tests. Finally, the polarity sign of the loss measurement for the tank loss index should be noted in order to indicate the type of problem with the oil industrial circuit breaker.