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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!

Parts no longer made and no longer available...what do you do?

A customer of ours is in charge of the electrical distribution of a manufacturing facility and had some switchgear running in his building and his production line. These metal-clad switchgear and circuit breakers have been well maintained and functioned properly but over the years the manufacturer obsoleted one particular product line and stopped supporting its parts as well. He's been able to source parts through the secondary or used equipment market through qualified dealers like National Power Equipment, Inc. but one specific product was nearly impossible to locate. In this case, it was a plastic part the is very fragile and breaks if you even look at it wrong. This customer wanted to find replacements for 60 of them but couldn't seem to find that quantity in the used equipment market. He also felt that it didn't make much sense to replace an inherently inferior part with the same inferior part that is just going to fail in the same way.

The solution...National Power Equipment's Aftermarket Parts Department. In less than 2 weeks, NPE had designed the tooling to make a replacement part using modern materials that are stronger and more resilient than what he had been struggling with for years. A true one-and-done solution.


This part was an electrical connector that brought control power into his Federal Pacific H2 and H3 type circuit breakers. This part was originally manufactured with material that tends to crack and break causing an increased risk of breaker failure, downtime along with the risk of an injury to the operators. Check out this part and other's in NPE's Aftermarket Parts Store. The parts listed on our website all suffer from inherent defects the NPE has worked hard to make stronger, faster, better and they don't cost six million bucks!

Medium Voltage Load Break Switches





Medium voltage load break switches control power to circuits with voltage ratings from 1kV to 52kV. They are used in a variety of applications and are differentiated from non-load break switches because as the name suggests they can safely be opened while power is flowing through the cricuit that they are installed in.

Applications include:

  • Selector Switches - Sometimes called A-B switches which select which power source will be used to energize the primary of a transformer.
  • Power Distribution - A group of switches, can be with or without a main, which direct power to various locations in a factory or other large facility.
  • Primary - For a single transformer powering a secondary switchgear.
  • Isolation - To de-energize equipment not being used or so that the switchgear can safely be serviced.

Typical ratings for the most common switches used on a 2.4kV through 15kV will have a continuous rating of 600 amp but are most often fused well below that.

They can be designed for use both indoor and outdoor and are used to interrupt the flow of power under both normal and emergency situations. They are most often operated manually but some are also equipped with a motor driven mechanism with a shunt trip which allows them to be opened and closed remotely.

The components of a medium voltage load break switch include the switch body and housing, contacts and mechanism and arc extinguishing medium. The arc extinguishing medium can be air, vacuum of SF6 (sulfur hexafluoride) gas. Vacuum and SF6 gas are used when a medium is needed to provide effective current interruption with minimal contact wear. Air is a cost effective option, especially if the switches will see minimum cycling.




As with any mechanical or electromechanical device, regular scheduled maintenance is required. This includes visual inspection, lubrication of mechanism and contact joints, testing of insulators and conductivity testing of the closed switch to ensure minimum internal resistance. Frequency of this maintenance will vary depending on factors including frequency of operation and environmental considerations. Failure to properly maintain these switches can and will result in non-scheduled power outages with damage to surrounding property and can injure or kill personnel in the area.

Many switchgear installations are in service well past the time that the OEM stopped supporting them or items may have long lead times which will extend the time of the unintended outages causing loss of production that run into tens or hundreds of thousands of dollars. This makes your local used equipment dealers like National Power Equipment invaluable. NPE not only has switches and parts in stock and available for quick shipment but also the experience and knowledge to guide you through pitfalls in things like parts interchangeability that wouldn't otherwise be apparent in the field. As always, knowledge and information is critical. Gather not only nameplate information but photos of both the overall switch and details of the parts in question. This will make it much easier for NPE to live up to its motto of "The right part, at the right price, right now!"

Are we in an Economic Recession now?



Many say that we are currently in a recession, others say not yet but one is due to hit soon. Either way we are all dealing with rising prices, longer lead times and shortages in available equipment. At NPE, we are here to help recession-proof your business.

Here's how we can help!

* Used, reconditioned equipment is at least 40%-60% less expensive than buying new.

* Used, reconditioned equipment is always quoted from available inventory so there are no supply chain issues.

* All reconditioned equipment is tested and warranted for up to two years! That's longer the manufacturer warrented it as new!

National Power Equipment specializes in:

Pars for Equipment Maintenance and Repair- We can support you with parts and supplies to keep your breakers, switchgear and medium voltage starters safely maintained and running for years to come. Many of the most problematic parts are available online at Aftermarket Parts Store for Replacement Circuit Breakers or Switchgears ( Many of these parts are available as new and upgraded by NPE (or one of our partners) to eliminate the defect that caused the problem in the first place.

Increase Capacity - As your business grows we can help your switchgear grow with you! From simply supplying a breaker for a future cell or adding and entire unit substation we have the equipment and expertise to help you expand economicly and without down time. Circuit Breakers, Relays, Transformers National Power Equipment (

Equipment Upgrades - Solid State trip upgrades with reputable companies, such as Utility Relay, are one of our many specialties. Do you need to upgrade the ampacity of a line from say 1200 amp to 2000 or 3000 amp? We have the parts and experience to help there, too! Reconditioned Equipment (

Rentals and Exchanges - With an inventory of literally thousands of breakers in every configuration, we can help you avoid longer term outages and stay up and running by purchasing upgraded and warranted breakers and equipment and swapping them out with your existing breakers, then return your original back to us for credit for the core. Quick Ship Inventory for Reconditioned Circuit Breakers & Switchgears (

Technical Support - With decades of hands-on experience, literally 1,000's of breakers on hand for reference and huge online library of technical manuals and renewal parts books. Just give is a call and we will figure it out! Circuit Breakers, Relays, Transformers National Power Equipment (

As Bob Dylan said, "The times they are a-changin'!" but that doesn't mean change has to be bad. With NPE's inventory, staff and decades of experience, we can all navigate these economic changes and grow.

Give us a call 1-800-647-0815 or send us an email and let us know how we can help you.

Remember, at NPE our motto is "The Right Equipment, at the Right Price, Right Now!"

Beating the Supply Chain Crisis with Switchgear and Circuit Breakers

Almost every day we receive an inquiry for switchgear, air and vacuum circuit breakers and related parts because the customer just got notice from the OEM  that their order has been delayed by weeks or even months due to "supply chain issues". These issues have become a fact of life and effect everyone, everyday regardless of industry, right? Wrong!

How NPE Helps Businesses Beat the Switchgear Shortage

When you deal with a company like National Power Equipment (NPE), there are NO supply chain issues! Everything we quote is on the shelf waiting for an order. We generally ship the same day or next day. Custom orders may take a little longer, but still are a fraction of the time versus OEM orders and are a fraction of the cost of new! 

NPE has thousands of used air and vacuum circuit breakers, tons of switchgear and tens of thousands of related parts all in stock. Each order has a money back guarantee and reconditioned equipment has a 1-2 year warranty.

In short, NPE has:

     Equipment is IN STOCK and ready to ship 

     Never had a supply chain issue

     Expert staff that will guide you through finding the proper equipment for your application

     Inventory that is updated daily and searchable online at



So give us a call, send us an email or search or inventory online. Find out why our motto at National Power Equipment, Inc. is "The right equipment, at the right price, right now!"