You have no items in your shopping cart.
Available 24/7 (800) 647-0815


Understanding Air and Vacuum Circuit Breaker Mechanisms

The operating mechanism of a vacuum or air circuit breaker is a critical component and requires not only care and maintenance but a proper understanding of how it functions. For this article, we will only be concerned with the operating mechanisms of circuit breakers in the low voltage class (600 vac or less) and medium voltage class  (15,000 vac or less) although many of the details apply to others as well.


Breakers with manually operated mechanisms are confined to low voltage breakers. They are operated by hand using a handle mounted to the front of the breaker. This handle operates and moves all three phase worth of contacts at the same time. While being simple to maintain, they are the least efficient of all the mechanisms and are considered to be responsible for premature contact wear because of the amount of electric arc they give off when opening. Good examples of this would be Westinghouse's DB, ITE's K series (KA, KB, KC) and General Electric's AE, AL and AK-1 series product lines. 


The addition of springs to the mechanism add speed and a consistently smooth closing and opening of the breakers. This will reduce the amount of arcing and burning of the contacts, decreasing the downtime needed for scheduled maintenance and parts replacement. Some designs will use a racking mechanism to charge (compress) the springs and hold that charge until the operator releases the springs and closes the breaker. Another design uses a racking mechanism which releases at the end of the cycle. Some designs like, Westinghouse's DB series offers this "spring assist" option as an add-on, others like ITE's K-Line and General Electric's AKD-5 and later breakers use it as an integral part of the design.


Electrically operated breakers include both low voltage and medium voltage breakers. While it is optional on low voltage breakers, it is standard on medium voltage breakers (2,300 vac to 15,000 vac). There are three types of mechanisms - solenoid, stored energy and hydraulic.

Solenoid operated breakers use a large coil which pulls a slug through the center and operates the mechanism  Once the mechanism is closed, a provision is included to disengage the power and hold it in place mechanically. The actual closing operation is designated as "X" in diagrams and instructions and the function to disengage the power is designated as "Y". The "X" function always includes a closing solenoid, the "Y" function can use a relay or simple mechanical action to turn the power off to the "X" function. Examples of the mechanical "Y" are most obvious on Westinghouse DB style breakers which have a tab attached to the bottom of the slug on the close solenoid. When the slug raises to close the breaker the tab engages the "X" relay and trips it out which shuts the power down to the entire close circuit. Examples of medium voltage solenoid operated breakers include ITE HV series, Westinghouse DH and General Electric Magneblast's with MS Style (MS-9, MS-13 etc.) mechanisms. While these mechanisms became very popular through the 1960's, they required a power supply that could deliver a strong source of power which could require up to 100 amps of service just to close a single breaker. This required the use of large banks of batteries to supply the power and as a result all solenoid breakers are rated for DC, most ofter 125 vdc but 48 vdc are also seen in the field.

Stored energy breakers, often designated as "SE" on nameplates, use a motor circuit to charge large coil springs. Once charged and latched, a small solenoid or "latch release" can be engaged and then release the stored energy in the springs to quickly close the breaker. Examples of low voltage breakers using this system would be the GE AK and AKR's, Westinghouse/Square D/Eaton DS series and ITE/BBC/ABB K-Line and LK series. Medium voltage stored energy breakers include ITE/BBC/ABB HK series, GE Magneblast breakers with ML-11 through ML-13 mechanisms and then later Westinghouse DHP breakers. The use of a motor to charge the springs greatly reduces the need for large heavy sources of DC for control power. Many stored energy style mechanisms draw as little as 7 amps and allow for both AC or DC circuits to power the breaker. If AC power is used from the in-house service, a separate DC source is usually used so the breaker can be electrically tripped or shut off if the house power goes out. This can also be done with a capacitor trip device which stores power until it is needed to open the breaker. It should be noted that all medium voltage vacuum circuit breakers use this type of mechanism.

Hydraulic stored energy mechanisms are far and few between, thankfully. Due to chronic issues with the hydraulic leaks, etc. they never were popular. The prime example of a hydraulic mechanism is the one on the FPE DST medium voltage breaker which uses an electric motor to drive a hydraulic pump.

The good news is that you don't need to know or understand all of these details to get the right breaker for your system. The team at NPE will be happy to walk you through the process and get you "The Right Breaker, at the Right Price, Right Now". So give us a call, drop us an email or go to our NPE on the Go wizard. The wizard was designed to walk you through all of the steps you will need to correctly specify your next order and then send the information to our sales team. We look forward to working with you!