trip unit meaning

Molded Case Circuit Breaker Trip Units, Types and Applications

A fundamental element of all low voltage circuit breakers is the trip unit or ‘brain’ of the circuit breaker. Several different trip unit technologies are available, but which one is the best choice for my application? This presentation, given by Andrew Legro P.E. at ABB, provides an overview of each trip unit technology from thermal magnetic to multi-function digital. Cost versus function will be reviewed and as well as a walk-through of some example applications.  

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By definition a circuit breaker is an electrical safety device, a switch that automatically interrupts the current of an overloaded electric circuit, ground faults, or short circuits. Circuit breakers "trip", shut off, current flow after protective relays detect a fault. Unlike fuses that were used previously, circuit breakers are not usually damaged so they can be reset as opposed to being replaced. Circuit breakers are used in residential and in industrial applications.

There are 5 basic components used in every circuit breaker:

• Tripping or Protective Mechanism – Also known as the trip unit, this triggers the operating mechanism once an electrical fault happens.
• Operating Mechanism – Opens or shuts the breaker to fulfill its protective role.
• Molded Frame – Outer protective and supportive case of most breakers. It shelters the other component of the breaker providing insulation.
• Arc Chutes – Located near the contacts, chutes prevent damage and mostly heat from intervening with the circuit breaker’s functionality and move apart when a fault occurs.
• Contacts – There are three types of contacts: arcing, auxiliary or main contacts that are used to ensure optimal airflow inside the breaker.

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•  Operating Mechanism – Opens or shuts the breaker to fulfill its protective role.
•  Molded Frame – Outer protective and supportive case of most breakers. It shelters the other component of the breaker providing insulation.
•  Arc Chutes – Located near the contacts, chutes prevent damage and mostly heat from intervening with the circuit breaker’s functionality and move apart when a fault occurs.
•  Contacts – There are three types of contacts: arcing, auxiliary or main contacts that are used to ensure optimal airflow inside the breaker.

4 categories of circuit breakers:

Molded case circuit breakers (mccb).

15-1200A Tmax XT Tested UL 489 and meets NEMA AB Standards

Residential to Industrial

single, two pole or three pole

Lighted panelboards

Insulated Case Circuit Breakers

Powerbreak II Tested UL 489 and meet NEMA AB Standards

Frame Sizes 800-4000A frames - mains and large

Electrically operated breaker

Commercial to Light Industrial - office buildings, schools, shopping malls

Somewhat maintainable but contacts not replaceable

Rugged design

Air power circuit breakers

Heavy industrial application - can involve switchgear

Optimal reliability - Hospitals, Data Centers

Serviceable in the field

Best choice for heavy switching (Contacts replaceable)

30 Cycle Withstand

Frame sizes 250-6000A built on ANSI rated

True switchgear breakers

Used in industrial environments where maintenance staff and customer needs high reliability

All electronic eKip Touch and eKip - Bluetooth standard

3 and 4 pole versions

Miniature Circuit Breakers (MCB)

Used for low-energy requirements, like home wiring, offices, or small electronic circuits.

MCBs are equipped with two tripping mechanisms: the delayed thermal tripping mechanism for overload protection and the magnetic tripping mechanism for short circuit protection.

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Evolution of the molded case circuit breaker trip units and their value to customers.

Even though an 1879 patent filed by Thomas Edison provided a glimpse of the definition of what would become circuit breakers, fuses (use once and throw away) were the standard for the first 30-40 years in power distribution systems.1 In 1924, German inventor Hugo Stotz created and patented what was marketed as a re-settable fuse (Figure 1). It was a direct retrofit into common fuse panels of the day. The Stotz fuse incorporated a thermal element to detect and open contacts to clear overloaded or shorted circuits.2 This was a forerunner of the thermal-magnetic breaker (Figure 2) widely used in today’s power distribution systems.

How have circuit breakers evolved since the Stotz? More importantly, how can you take advantage of new circuit breaker technology to deliver to your clients a better tailored and user-friendly project? This brief article will focus specifically on the evolution of the breaker trip unit and the value this evolution provides to customers.

Circuit Breaker and Trip Unit

In order to understand what a trip unit is, let’s revisit the definition of a circuit breaker. A circuit breaker is a mechanical switching device designed to automatically detect and eliminate short circuits and overload current. A trip unit, specifically, is the “brain” of the circuit breaker as its function is to measure physical parameters such as electrical current and decide when to “trip” or rapidly open the mechanical contacts of the circuit breaker. At the bare minimum, a trip unit needs to offer overload and short circuit protection. In regard to the topic of evolution, the trip unit can be as simple as a bi-metallic strip, or now, as advanced as a computer. This evolution has opened the door to so much more than overload and short circuit protection – it’s opened a whole new world of protection, measurement, and control.

Let’s take a look at the evolution of the circuit breaker trip unit in four stages, starting with the basic thermal magnetic circuit trip unit, which is still the most widely used trip mechanism today.

Thermal Magnetic Circuit Trip Unit

The basic thermal magnetic circuit trip unit still provides a cost-effective solution for basic circuit protection and remains in widespread use. With the growth of critical electrical loads, the need for accurate and coordinated circuit protection has become much more important. However, the lower accuracy sensitivity offered by a thermal magnetic breaker cannot fully address this increasing demand. These shortcomings are amplified when you need breakers to trip in a coordinated fashion where only the problematic circuit is taken out of service. This is called selectivity and was a primary driver in the evolution from the thermal magnetic trip unit to the electronic trip unit which can provide a much higher degree of accuracy in sensing and responding to trip events.

Figure 3 exemplifies the typical response of a thermal magnetic breaker in the form of a time current curve (TCC). The X axis represents current and Y axis represents time, in seconds. The grid is logarithmic on both the X and Y axis. The breaker has two elements – ‘L’ or long time for the thermal, and ‘I’ for the magnetic. Note the width of the long-time element indicates a substantial lack of accuracy. Also note that the breaker’s response is significantly affected by temperature. There are two long time curve sections shown. The blue section is the ‘cold’ response and the orange section is the ‘hot’ response. The lack of accuracy makes coordination between thermal magnetic breakers difficult.

First Generation Electronic Trip Units

As noted earlier, this lack of accuracy, along with the growing need for coordinated circuit protection, drove the development of the electronic trip unit. First generation electronic trip units (Figure 4) were simple analog circuits comprised of resistors, capacitors, inductors, and transistors, however, they offered increased accuracy over their thermal magnetic cousins. Electronic trip breakers could be reasonably coordinated and be used to build a selectively coordinated distribution system.

Over the years, electronic trip units underwent incremental improvements including:

• Limited Adjustability – Provided ability to make basic adjustments to instantaneous and overload response to improve selectivity
• True RMS Sensing – Improved accuracy, bringing measurement much closer to the thermal response (not just looking at peak) of the current
• Thermal Memory – Ensured (even lacking the inherent “heater” present in original thermal magnetic breakers) that trip data could be retained and remembered for reporting
• Overall Improvement in Equipment Protection – Due to these enhancements which allowed more selectivity and eliminated the nuisance of premature trips which can damage the equipment

Modern Microprocessor Trip Units

As these electronic trip units continued to evolve, manufacturers used more and more sophisticated and integrated circuits which slowly evolved trip units into the modern microprocessor trip unit. The microprocessor trip unit provides even more improved protection accuracy and adjustability (ability to coordinate breakers closer together thus allowing additional breakers to operate in series IE levels of protection). Electronic trip unit breakers are commonly referred to as ‘LSI’ or ‘LSIG’ where ‘L’ is the long-time trip (60-600 sec), ‘S’ is the short time trip (0.1 to 60 sec), and ‘I’ is the instantaneous trip. ‘G’ is the optional ground fault trip. The ‘L’ and ‘S’ functions replace the thermal element in the thermal magnetic circuit breaker and the ‘I’ replaces the magnetic element. Figures 5 and 6 show the difference in response and adjustability between thermal magnetic and LSIG circuit breakers.

Figure 7 shows the time current curve of a typical breaker with a microprocessor LSI trip unit. Note the increased accuracy and adjustability in comparison with the thermal magnetic breaker.

This improved accuracy and adjustability of LSI breakers allowed for more advanced coordination of increasing layers of panels/circuit breakers in series.

Application Example

A building with a 2000A main switchboard and multiple power panels scattered throughout. Thermal magnetic breakers may allow up to three levels of coordination – switchboard main to switchboard feeder to power panel branch. Suppose the power panels were then feeding lighting panels. The lighting panel branch circuits can not be coordinated as it is the fourth level of coordination. If LSI breakers were used, the same system could be coordinated through the lighting panel and possibly with an additional panel in between (5 levels).

These evolving microprocessor trip units also provide much improved coordination with different types of protective devices such as motor starters, fuses, and relays, as well as the key ZSI (Zone Selective Interlocking) ability which allows planned overlap to gain maximum protection.

One big weakness that had yet to evolve was the advancement of sensors. So, while these electronic microprocessor trip units along with the right add-on equipment could provide early versions of metering from the circuit itself, the data was very inaccurate.

Today’s Advanced Next Generation Microprocessor Trip Units

Finally, we come to today’s advanced microprocessor trip units which are still microprocessor based, but because of the continued miniaturization in electronics to provide additional power, memory, and storage, and with a big change in sensor technology, these new breakers are a quantum leap ahead of their predecessors.

With the evolution of breaker trip units starting with basic overcurrent protection, you now have advanced capabilities that offer a host of additional protective functions nearly equal to functions offered by medium/high voltage multi-function relays. A few key features to look for include monitoring capabilities such as voltage, power quality, and even temperature of external sensors connected to the breaker.

Much of the new functionality is made possible by the replacement of the lower accuracy non-linear iron core current transformers with highly accurate linear current sensors. These sensors are based on the Rogowski coil concept. With traditional iron core current transformers, there is a tradeoff between measurement range and accuracy. Circuit breakers require sensing a large range of currents and accuracy is not as important. Today’s demands for metering require a smaller sensing range and much greater accuracy. The Rogowski coil sensor can cover a wide range of currents and has a very linear response. It is the perfect sensor for both protection and metering.

As mentioned, the real leap in value is moving so many functions “on board” the breaker trip unit that, in the past, could only be delivered by purchasing, integrating, and programming separate devices. A few examples (many more to explore) include:

• Built in programmable logic – Moves functions formerly available only through the addition of one or more PLCs, such as automatic source transfer, load shedding, load control, and generator control
• Communications – Standard network connections, additional communications technology such as IEC6185/GOOSE to high-speed breaker to breaker communications and coordination, including serving as a bridge between LV/MV applications.
• Metering – Ability to delivery revenue-class metering (typically 1% accuracy), harmonic measurement and reporting, and power quality monitoring
• Commissioning – Allows direct access to trip units via HMI panel (one panel for multiple breakers), or USB device (just copy over the settings), or even Bluetooth connection (outside the arc-flash zone)

The Evolution Will Certainly Continue

We’ve touched on the evolution of the circuit breaker trip unit across a century. Generally, three key technical advancements have opened up the possibilities of today’s advanced circuit protection with a molded case breaker – increased processor power (intelligence) due to advances in circuit board/component miniaturization, increased sensor accuracy as advances allowed for the application of the Rogowski coil for linear measurement, and the continued improvements to high-speed communications both in the processor capabilities and communication protocols. Overall, these three things combine to deliver the key cornerstone values required in smarter, safer, and more reliable power – accuracy plus the ability to make decisions and execute responses in milliseconds.

These capabilities will continue to evolve, and you and your customers will continue to benefit from the advancement of cheaper and more available raw computing power and communications over time.

• Friedel, R., & Israel, P. (1987). Edison’s electric light biography of an invention. New Brunswick, NJ, NJ: Rutgers Univ. Pr.
• Riemensperger, S. (2014, October 31). Miniature Breakers Stop Overloads, Short Circuits. Retrieved July 22, 2020, from com/conversations
• Electrical installation handbook – Protection, control and electrical devices (Sixth ed., ABB Technical Guide). (2010). Bergamo Italy: ABB SACE.
• Figure 3 – 3. (n.d.). In A Working Manual on Molded Case Circuit Breakers (Third ed.). Beaver, PA: Westinghouse Electric Corporation

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Characteristics of Micrologic Electronic Trip Units

Introduction

Micrologic electronic trip units provide the following functions:

o Protection of the electrical distribution or specific applications

o Measurement of instantaneous values and measurement of average values (demand) for electrical quantities

o Kilowatt hour metering

o Operational assistance such as peak demand, customized alarms, and operation counters

o Communication

Identification

Identify the trip unit installed on the circuit breaker by using the four characters on the front face:

Micrologic Trip Unit Families

The range of Micrologic trip units is made up of several families:

o Micrologic 1, 2, and 4 without display screen

o Micrologic 5, 6, and 7 with display screen.

On Micrologic 1, 2, and 4 trip units, the protection functions are set using adjustment dials on the front face of the trip unit:

On Micrologic 5, 6, and 7 trip units, the protection functions are set:

o By using the adjustment dials

o By additional settings on the keypad. Setting values are displayed on the screen

o Through EcoStruxure Power Commission software.

For more information about the Micrologic 5, 6, and 7 trip units, refer to DOCA0141EN , Compact NSX Micrologic 5/6/7 Electronic Trip Units - User Guide .

In Rating of Micrologic Trip Units

The In rating (in amps) of a Micrologic trip unit corresponds to the maximum value of the long-time protection (Ir) setting range for the trip unit. The setting range is indicated on the label on the front face of the trip unit (this label is visible on the front face of the Compact NSX circuit breaker after the trip unit has been fitted).

Example: Micrologic 5.2 A 250 trip unit:

o Setting range: 100-250 A

o In rating = 250 A

Distribution Trip Unit

The following figure and table define the protection functions for distribution-type Micrologic trip units.

Thermal Memory

The thermal memory is used to simulate temperature build-up and cooling in conductors caused by current variations, according to a time constant. In the event of an overload, the trip units with a thermal memory memorize the build-up temperature caused by the current. Memorizing the build-up temperature leads to a reduction in the trip time.

All Micrologic trip units incorporate a thermal memory as standard:

o For Micrologic 2 and 4 trip units, the time constant is 15 minutes.

o For Micrologic 5, 6, and 7 trip units, the time constant is 20 minutes.

Motor Trip Units

The following figure and table define the protection functions for Micrologic type M trip units.

Motor Trip Unit: Additional Protection

Micrologic type M trip units (in particular Micrologic 6 E-M) also incorporate additional protection for the motor application. For more information, refer to DOCA0141EN , Compact NSX Micrologic 5/6/7 Electronic Trip Units - User Guide .

Indication LEDs

Indication LEDs on the front of the trip unit indicate its operational state.

The LEDs and their meaning depend on the type of Micrologic trip unit.

The indication LEDs function for circuit breaker load currents:

o Above 15 A on a Micrologic trip unit rated 40 A

o Above 30 A on Micrologic trip units rated > 40 A

The limit value is indicated on the front panel, above the Ready LED of the Micrologic trip unit.

NOTE: For the Micrologic 4 and 7 trip units, the protection functions are supplied by a second power supply, in addition to the current transformer supply. The Ready LED blinks irrespective of the load, indicating that the standard protection functions are operational.

To activate the Ready LED when the load current is below the limit value, you can:

o Install a 24 Vdc external power supply module which allows the trip unit to be monitored continuously, even when the circuit breaker is open. For more information, refer to Compact NSX & NSXm Catalogue .

o Or, during maintenance visits, connect the pocket battery to monitor the trip unit.

NOTE: If the pre-alarm and alarm LEDs keep lighting up, perform load shedding to avoid tripping due to a circuit breaker overload.

Micrologic trip units come with a test port specifically for testing trip unit operation .

This port is designed for:

o Connecting the pocket battery for local Micrologic testing

o Connecting the USB maintenance interface for testing, setting the Micrologic trip unit, or for installation diagnostics

Interchangeability of Micrologic Trip Units

Onsite replacement of trip units is simple:

o No connections to make

o No special tools (for example, calibrated torque wrench)

o Compatibility of trip units provided by mechanical cap

o Torque limited screw provides correct torque

The simplicity of the replacement process means that it is easy to make the necessary adjustments as operation and maintenance processes evolve.

NOTE: The screw head is accessible when the trip unit is installed, so the trip unit can still be removed.

NOTE: On the Compact NSX with R, HB1 and HB2 breaking performances the trip units are not interchangeable.

Sealing the Protection

Seal the transparent cover on Micrologic trip units to prevent modification of the protection.

On Micrologic 5, 6, and 7 trip units, it is possible to use the keypad, with the cover sealed, to read the protection settings and measurements.

DOCA0140EN-01

© 2020 Schneider Electric. All rights reserved.

• Hazard Categories and Special Symbols
• Please Note
• Introduction
• Communications
• Power and Control Settings
• MicroLogic 5.0P Trip Unit
• MicroLogic 6.0P Trip Unit
• Long-Time Protection
• Short-Time Protection
• Instantaneous Protection
• Ground-Fault Protection for Equipment
• Control Power
• External Power Supply
• MicroLogic Setup
• Neutral Protection
• Minimum (Under) and Maximum (Over) Demand Current and Voltage Protection
• Current or Voltage Unbalance Protection
• Reverse Power Protection (rPmax)
• Minimum (Under) and Maximum (Over) Frequency Protection
• Load Shedding
• Phase Rotation Protection
• M2C and M6C Programmable Contact Kits
• Zone-selective Interlocking
• Trip Unit Testing
• Operation Counter
• Overload Indicator Light
• Trip Indicator Lights
• Test/Reset Button
• Graphic Display Screen
• Contact Wear Indicator
• Graphic Display Navigation Buttons
• History Logs
• M2C/M6C Programmable Contacts
• Metering Setup
• Communication Setup
• Amperage Protection
• Voltage Protection
• Other Protection
• Current Load Shedding
• Power Load Shedding
• MicroLogic Trip Unit Setup
• Communication Module Setup
• Switch Settings Adjustment
• Zone-Selective Interlocking (ZSI)
• Trip Unit Operation Verification
• Trip Unit Resetting
• Equipment Ground-Fault Trip Functions Testing
• Trip Unit Status Check
• Current Levels
• Voltage Levels
• Power Levels
• Energy Levels
• Trip History
• Alarm History
• Contact Wear
• Required Tools
• Record Switch Settings
• Circuit Breaker Disconnection
• Circuit Breaker Accessory Cover Removal
• Rating Plug Removal
• Trip Unit Removal
• Battery Installation
• Trip Unit Installation
• Circuit Breaker Accessory Cover Replacement
• Secondary Injection Testing
• Primary Injection Testing
• Check Accessory Operation
• Trip Unit Setup
• Circuit Breaker Reconnection
• Remove Rating Plug
• Install New Rating Plug
• Accessory Cover Removal
• Withstand Module Shifting
• Battery Replacement
• Withstand Module Replacement
• Accessory Cover Replacement
• Metering Menu Flowchart
• Maintenance Menu Flowchart
• Protection Menu Flowchart
• Default Settings
• Metering Range and Accuracy
• Remotely Readable Values
• List of Registers
• Minimum Voltage Protection
• Voltage Unbalance Protection
• Loss of Multiple Phases
• Appendix E—Trip Unit Settings

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