10 Critical Factors When Choosing Medium Voltage Circuit Breakers ...

Introduction

Selecting the right medium voltage circuit breaker or load break switch is a critical decision that impacts the safety, reliability, and efficiency of your electrical distribution system. Making the wrong choice can lead to inadequate protection, unnecessary downtime, or even catastrophic failures. Medium voltage equipment typically operates and serves as the backbone of power distribution in commercial buildings, industrial facilities, and utility substations.

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Understanding the differences between a medium voltage circuit breaker and load break switch is essential for making the right selection. While both devices serve important functions in electrical systems, they have distinct purposes and capabilities that make them suitable for different applications. This guide outlines ten critical factors to consider when choosing medium voltage switching and protection equipment, helping you make an informed decision that meets your specific requirements.

1. Understanding the Fundamental Differences

What is a load break switch? It’s a device designed to make or break electric circuits under normal load conditions. Unlike circuit breakers, load break switches have a simpler design focused on manual or motor-operated switching for isolation purposes. They’re cost-effective solutions primarily intended for circuit isolation under normal operating conditions.

In contrast, a medium voltage circuit breaker provides protection against electrical faults and overloads. Equipped with automatic tripping mechanisms and complex protective systems, circuit breakers can detect and interrupt fault currents, preventing damage to equipment and ensuring personnel safety.

When deciding between these options, consider:

• If your primary need is protection against faults: Choose a circuit breaker
• If you only need isolation capabilities under normal conditions: A load break switch may be sufficient
• If both functions are required: You might need both devices in your system

Common Medium Voltage Circuit Breaker Types

The most common medium voltage circuit breaker types include vacuum, air, and SF6 technologies, each with distinct advantages:

• Vacuum circuit breakers: Excellent for frequent operations, environmentally friendly
• Air circuit breakers: Simple maintenance, good visibility of contacts
• SF6 circuit breakers: Superior arc quenching, compact design

2. Voltage and Current Ratings, endurance

Selecting medium voltage switches based on voltage ratings is the foundation of proper equipment selection. The nominal voltage rating must match your system requirements, typically falling within the 1kV to 52kV range for medium voltage applications. Undersized equipment can lead to insulation failure, while oversized equipment wastes resources.

An MV switch must be properly rated for the specific application to ensure safe and reliable operation.

• Continuous current rating: The current the device can carry indefinitely without exceeding temperature limits
• Short-time current rating: The current the device can withstand for a specified duration during fault conditions
• The classification of E, M, C: The endurance of electrical, mechanical and closing time

Guidelines for Medium Voltage Circuit Breaker Sizing

Proper medium voltage circuit breaker sizing depends on system voltage, load current, and fault current levels. When sizing your equipment:

1. Determine the system nominal voltage
2. Calculate the maximum load current
3. Determine the available fault current
4. Select a device with appropriate ratings for all three parameters
5. Include a safety margin for future load growth

Remember that undersized equipment can fail catastrophically, while oversized equipment increases costs unnecessarily.

3. Interrupting Capacity and Short Circuit Ratings

The interrupting capacity of a switching device is critical for system protection. Medium voltage circuit breakers are designed to interrupt fault currents, with ratings typically expressed in kA (kiloamperes). When comparing MV switches, consider both the initial cost and long-term maintenance requirements, as higher interrupting capacities often come with premium prices.

A load interrupter switch is designed specifically to break normal load currents but cannot interrupt fault currents. This fundamental limitation makes them unsuitable for fault protection. If fault protection is required, you must either:

1. Use a circuit breaker instead
2. Combine the load break switch with appropriate fusing
3. Implement a separate protection scheme

The interrupting capacity must exceed the maximum available fault current at the installation point with an appropriate safety margin. Inadequate interrupting capacity can result in catastrophic failure during fault conditions.

4. Arc Extinguishing Technologies

Medium voltage switches come in various designs, each with specific advantages for different applications. The arc extinguishing medium is a key differentiator:

• Vacuum technology: Uses vacuum as the interrupting medium, offering excellent performance with minimal maintenance
• SF6 gas: Provides superior arc quenching in a compact design, but raises environmental concerns
• Air magnetic: Uses air and magnetic fields to extinguish arcs, environmentally friendly but requires more space

Each technology offers different trade-offs between:

• Initial cost
• Maintenance requirements
• Environmental impact
• Space requirements
• Operational life

For environmentally sensitive applications, vacuum technology is increasingly preferred over SF6 due to the latter’s high global warming potential, despite SF6’s excellent technical properties.

5. Operation Types and Mechanisms

The operation mechanism of your MV switch or circuit breaker significantly impacts its application suitability. Options include:

• Manual operation: Requires physical presence to operate, lowest cost
• Motor operation: Allows remote operation, enhances safety
• Solenoid operation: Quick response, suitable for automatic systems
• Spring-charged mechanisms: Stores energy for reliable operation

A medium voltage load break switch typically operates in the 1kV to 52kV range and is rated for specific load currents. When selecting the operation type, consider:

• Frequency of operation
• Need for remote control
• Response time requirements
• Reliability needs
• Maintenance capabilities

For critical applications requiring remote operation or integration with automated systems, motor-operated devices offer significant advantages despite their higher initial cost.

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6. Environmental Considerations

The operating environment significantly impacts equipment selection and longevity. Consider:

• Temperature range: Extreme temperatures can affect insulation and mechanism performance
• Humidity and moisture: Can cause insulation degradation and corrosion
• Altitude: Higher altitudes reduce dielectric strength of air
• Pollution level: Contaminants can affect insulation performance
• Indoor vs. outdoor installation: Requires different protection levels

For outdoor applications, equipment must withstand weather extremes, while indoor installations may face space constraints. Many manufacturers provide detailed medium voltage circuit breaker PDF catalogs with technical specifications that include environmental ratings.

When selecting equipment for harsh environments, consider:

• IP/NEMA ratings for enclosures
• Special coatings for corrosion resistance
• Heaters for condensation prevention
• Special insulation for high humidity areas

7. Main Specifications and Compliance

The Specifications are a critical consideration when selecting a medium voltage disconnect switch. Modern equipment should include:

• The classification of E, M, C: Electrical ,mechanical endurance and closing time
• Visible break points: Allow visual confirmation of isolation
• Mechanical interlocks: Prevent unsafe operations
• Padlocking provisions: Secure equipment during maintenance
• Arc-resistant design: Protects personnel during arc flash events
• Position indicators: Clearly show equipment status

Equipment must comply with relevant standards including:

• ANSI C37.55- for metal-clad assemblies
• IEEE C37.20.2 for metal-clad switchgear
• ANSI/IEEE C37.20.7 for arc-resistant design

These standards ensure equipment meets minimum safety and performance requirements. Non-compliant equipment may create liability issues and safety risks.

8. Installation Requirements

Installation considerations can significantly impact total cost and feasibility. Key factors include:

• Space requirements: Footprint and clearances needed
• Weight and floor loading: Structural support requirements
• Cable entry options: Top or bottom entry flexibility
• Accessibility: Front or rear access for maintenance
• Ventilation needs: Heat dissipation requirements

A load break disconnect switch combines isolation capabilities with the ability to interrupt normal load currents, making it versatile but still requiring proper installation planning.

When comparing different MV switches technologies, consider not just the equipment dimensions but also:

• Required working clearances per electrical codes
• Future expansion needs
• Equipment removal paths
• Ventilation and cooling requirements
• Noise considerations for indoor installations

9. endurance Maintenance Accessibility

Maintenance accessibility directly impacts long-term reliability and operating costs. When selecting equipment, consider:

• Frequency of required maintenance: Varies by technology
• Ease of access to critical components: Affects maintenance time and cost
• Availability of spare parts: Critical for long-term support
• Self-diagnostic capabilities: Reduces troubleshooting time
• Specialized tools or training required: Affects maintenance capabilities

Vacuum technology generally offers the lowest maintenance requirements, while air magnetic designs may require more frequent inspection and maintenance. Review the medium voltage breaker control schematic to ensure compatibility with your existing systems and maintenance capabilities.

Establish a clear maintenance schedule based on:

• Manufacturer recommendations
• Operating environment
• Criticality of the application
• Available maintenance resources
• Historical performance data

10. Cost and Long-Term Value

While initial purchase price is important, the total cost of ownership should guide your decision. Consider:

• Initial equipment cost: Purchase price and delivery
• Installation costs: Labor, materials, and commissioning
• Operational costs: Energy consumption and efficiency
• Maintenance costs: Regular service and parts
• Replacement costs: Expected service life and future replacement
• Reliability costs: Downtime and production losses

When evaluating long-term value, a higher initial investment in quality equipment often results in lower total ownership costs through:

• Reduced maintenance requirements
• Longer service life
• Improved reliability
• Better protection of connected equipment
• Enhanced safety features

Conclusion

Selecting the right medium voltage circuit breaker or load break switch requires careful consideration of multiple factors beyond basic ratings. By thoroughly evaluating the ten critical factors outlined in this guide, you can make an informed decision that balances performance, safety, reliability, and cost.

Remember that the best choice depends on your specific application requirements. A load break switch provides a cost-effective solution for isolation purposes in medium voltage systems, while a circuit breaker offers comprehensive protection against electrical faults. In many cases, a well-designed system will incorporate both types of equipment to provide both isolation and protection functions.

Before making your final selection, consult with qualified electrical engineers familiar with your specific application and system requirements. The right equipment, properly selected and installed, will provide decades of reliable service and protect both your electrical system and personnel.

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Forum,

I'm designing the electrical system for a project in our plant. I plan on feeding a v, hp motor from a breaker inside v switchgear (the sole use of this breaker is for the motor). My question is this: is there anywhere in the code that states I'm required to install a medium-voltage starter between the motor and the breaker? The breaker is of the thermal-magnetic type, so the motor is protected properly. Could I use the breaker as a means of starting/stopping the motor? Please keep in mind that I'm a new engineer out of school, so this is a legit question. I'm not trying to get around using a starter to save money or anything. Your input is appreciated. There's no such animal as a thermal-magnetic medium voltage breaker, so you need to double-check what the protection actually is on this circuit and evaluate whether the settings are suitable for that load.

That said, there's no reason a breaker can't be used in that application, but if this thing cycles frequently, as in every couple of days or more, it's very hard on the breaker and often leads to malfunctions. That's where a contact is valuable in that they can handle a far greater duty cycle than a breaker will.

Just be aware that whatever option you install, you need to have a method of putting a physical air-gap in the circuit because vacuum interrupters are not an effective means of safety isolation. I've.

I've.

Forum,

I'm designing the electrical system for a project in our plant. I plan on feeding a v, hp motor from a breaker inside v switchgear (the sole use of this breaker is for the motor). My question is this: is there anywhere in the code that states I'm required to install a medium-voltage starter between the motor and the breaker? The breaker is of the thermal-magnetic type, so the motor is protected properly. Could I use the breaker as a means of starting/stopping the motor? Please keep in mind that I'm a new engineer out of school, so this is a legit question. I'm not trying to get around using a starter to save money or anything. Your input is appreciated.

How would go about it if this were a 480v system?
Would you feed the motor using a branch aircuit breaker and then control and protect the motor using a combinatin motor starter.
Same with v. You couid feed the branch circuit using a VCB but CV would accomplish it with a fused load interrupter switch which is less expensive. Then control and protect the motor with a MV motor starter such as an ampgard starter which has a vacuum contactor controlled be a solid state programmable relay. The vacuum contactor is capable of being racked out for isolation.
The are are a number of MV starter manufactures but I'm more familiar with the ampgard. What you are talking about is using a MV breaker as a Manual Motor Starter. If you can adjust the thermal trips into the required range for proper motor overload protection, it can be done. It's not a great idea however. Assuming a vacuum circuit breaker, they are not designed, mechanically, to be used like a contactor. But certainly, if you turn the motor on and off only a few times per month or year, not really a problem. I see it done all the time on large seasonal pumps that turn on in the summer and off in the fall.

There is also a phenomenon called "current chop" when using vacuum interrupters. Vacuum contactors, because they are DESIGNED for that purpose, are specially designed to minimize current chop, vacuum circuit breakers are not. So again, if turning the motor on and off frequently with a vacuum breaker, you might experience premature motor insulation failure n

http://www.danaherspecialtyproducts...Clark/Service_and_Support/JC Current Chop.pdf

There's no such animal as a thermal-magnetic medium voltage breaker, so you need to double-check what the protection actually is on this circuit and evaluate whether the settings are suitable for that load.

That said, there's no reason a breaker can't be used in that application, but if this thing cycles frequently, as in every couple of days or more, it's very hard on the breaker and often leads to malfunctions. That's where a contact is valuable in that they can handle a far greater duty cycle than a breaker will.

Just be aware that whatever option you install, you need to have a method of putting a physical air-gap in the circuit because vacuum interrupters are not an effective means of safety isolation.

I disagree. There are medium voltage breakers out there that can supply both thermal (overload) and magnetic (short-circuit) protection. Hence the name thermal-magnetic breaker.

I disagree. There are medium voltage breakers out there that can supply both thermal (overload) and magnetic (short-circuit) protection. Hence the name thermal-magnetic breaker.

Thermal is one type of overload protection, they do not mean the same thing. Magnetic is one type of short circuit protection, again, not the same thing. Thermal magnetic is a term used to describe the methods used in small LV molded case breakers. Larger (modern) LV molded case breakers and modern LV power breakers do not use thermal magnetic trip devices, they have not been used in power breakers for 40 years.

MV breakers do not have ANY internal protective devices, they rely on external inputs to tell them when to trip from relays.

Regarding your OP, you do not HAVE to use a starter but they exist for a good reason, to start motors. Breakers are not designed for that type of duty and while they can be used in that application they will have many failures and failed parts. My company repairs MV breakers as the core of our business and our customers that use breakers as motor starters usually have to send them in every couple years for complete overhaul and repair. (Hmmm, on second thought maybe I should encourage folks to use breakers as motor starters)

Also, as mentioned, using a VCB presents additional problems from chopping and prestrike failures. I do MV work all the time. Most is custom gear. I have used a MV vacuum breaker once as a starter. It works, but...I was using an SEL relay so doubling as a breaker/starter settings was no problem. Still had a fused disconnect upstream, too. Remember that if ONE phase welds shut, you can't detect it easily. The basic issue though is vacuum interrupters for breaker duty are rated like cycles and a crazy high interrupting current rating. The contactor duty interrupters are rated like 30,000+ cycles and a relatively low interrupting limit so typically they are paired with MV fuses and set to trip just as a breaker would.

Also an issue is whether they "latch" or not. Typical breaker service (outside of mining) is to latch even under power loss whereas contactor's do the opposite. The danger here is if you have a fault before the relaying has booted up from power failure and it gets you into using either DC battery chargers or capacitive trip devices, which again may drive a decision to use an under voltage trip. Its not well published but most microprocessor relays have enough "substation duty" capacitors in their switching power supplies to survive low voltage under fault or even total power loss for about 100 ms but its not enough juice to drive a trip coil, thus the CTD. That's the essence of the "controllers" you find in "real" starters. AB's own controller is available as OEM style and works well. Most others (Toshiba, Eaton, and variants) mostly use a small PCB relay board that frankly fails a lot.

So the "breakers as contactor's" worked but production liked to start and stop them A LOT, enough we were approaching the 3K limit on the interrupters but before we got there, lots of springs and PCB relay cards failed prematurely because the manufacturer hadn't done this kind of "accelerated testing" before. In the prior life, cycling was perhaps once or twice a week instead of multiple times per hour. And yes, they were wound rotor motors so starting current was managed. Lesson learned: creative solutions trying to meet crazy unrealistic delivery schedules is flirting with disaster.