Working with Fixed Circuit Breakers

Circuit breakers protect circuits from melting under excess currents. They work by stopping the flow of electrical currents when there is excess current. This, in turn, it prevents fires from starting by making sure the excess current stays confined in the wire.

Since the invention of electricity in the late eighteenth century, electricity has become a basic human necessity. The prevalence of electricity also means an increased need for electrical circuits. This means an increased need for reliable circuit breakers to protect these electrical circuits.

  

Figure 1. Common circuit breakers found in everyday applications

Evolution of the Circuit Breaker

Thomas Edison patented the earliest form of a circuit breaker in 1879. A few decades later, in 1904, Cutler Manufacturing created the first circuit breakers in Philadelphia.

The original circuit breaker utilized oil as the overcurrent trip device, but they tended to combust. As a result, regular maintenance had to be performed to prevent fires. This led to the invention of withdrawable circuit breakers, so they can easily be removed during maintenance.

From Withdrawable to Fixed

There was no incentive to move away from withdrawable electrical circuits until the late 1960s. During this period, working vacuum interrupters became available and vacuum circuit breaker designs came to fruition.

These vacuum interrupters were more expensive than the oil used in traditional circuit breakers. However, they are also permanently sealed and require almost no maintenance. This was the starting point in the development of fixed circuit breakers.

Fixed Breaker Design

Fixed circuit breakers are made up of several parts. Consisting of a bus bar, a change-over switch, a protection relay, a breaker, current transformers and an earthing switch, it does not require the need for a contact like in withdrawable breakers.

Instead, a change-over switch is added to control starts and stops. The entire withdrawable part is also eliminated, making the system more streamlined and compact.

Fixed Breaker Maintenance

Fixed circuit breakers require very little maintenance compared to withdrawable circuit breakers. The first vacuum sealed circuit breakers required the occasional high voltage test to ensure the vacuum is sealed airtight. They can handle many fault interruptions before requiring maintenance, and do not need to be replaced at all.

  

Figure 2. Design for a vacuum-sealed circuit breaker

Maintaining the switching unit and bus bars on a fixed circuit breaker will require grounding or earthing of the entire unit. However, this is a rare occurrence, and it can be accomplished by earthing the bus bars on either side of the unit.

Fixed Breaker Benefits

Due to advancements in vacuum seal technology, modern fixed circuit breakers require no maintenance. This means significant cost savings from regular maintenance. Costs for lifting carriages, shutters and isolating busing required to maintain traditional circuit breakers are non-existent for fixed circuit breakers, giving them a lower life-cycle cost

Fixed circuit breakers are also highly reliable. This is due to its reduced number of components compared to traditional withdrawable circuit breakers. Having fewer parts means a lower probability that one of the components would be faulty. Therefore, malfunctions in the systems will be less common.

Fixed circuit breakers also take up less room. This is also due to its reduced number of components. They don’t require the traditional removable components that allow circuit breakers to be taken out of the circuit.

The environmental impacts associated with fixed circuit breakers are also smaller. This is because having a reduced number of components also means the energy and materials required to manufacture more components in withdrawable circuit breakers can now be eliminated.

Lastly, fixed circuit breakers are easier to install because of their reduced number of components. Accident investigation reports also suggest they are safer. This is because the removable parts in the withdrawable breakers can result in arc flash incidents, which is not an issue in fixed circuit breakers.

Modern Day Improvements

The 1990s trend for fixed circuit breakers has been a vacuum technology with three position disconnection functions that uses the gas SF6 for insulation. Fixed SF6 designs have been in successful operation for more than two decades in secondary distribution networks.

Modern fixed circuit breakers also contain a single moving blade, compared to earlier designs which utilized separate devices to interrupt the circuit.

Figure 3. Modern circuit breaker filled with SF6 gas

Innovations have also been made to rotate the circuit breaker sub-assembly to provide more isolating distance. An interlocked earth switch has also been produced to simplify cable termination during cable tests.