breaker electronic trip unit

Spectra RMS Electronic Trip -Previous Generation Circuit Breakers

Need help migrating from Spectra to Tmax XT?

Spectra breakers are now in the Limited life cycle phase. For circuit breakers, see SACE® Tmax® XT MCCBs . For retrofits of existing equipment, see Tmax XT Retrofit Kits for Spectra Panelboards and Switchboards .

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Other Previous Generation Circuit Breakers

Micrologic 4 Electronic Trip Units

Introduction

The Micrologic 4 electronic trip unit is designed to protect:

o Conductors in commercial and industrial electrical distribution.

o Goods and people in commercial and industrial electrical distribution.

On 4-pole circuit breakers, neutral protection is set on the Micrologic trip unit by using a three-position dial:

o 4P 3D: neutral unprotected

o 4P 3D + N/2: neutral protection at half the value of the phase pickup, 0.5 x Ir (not available on Micrologic trip unit with In ≤ 40 A)

o 4P 4D: neutral fully protected at Ir

The Micrologic 4 electronic trip unit is available in two versions for earth-leakage detection:

o The Trip version trips when earth-leakage is detected.

o The Alarm version measures the earth-leakage current and indicates an earth-leakage fault on the front face with the earth-leakage fault indicator, which changes from gray to yellow.

When the SDx indication contact is present, it signals an earth-leakage fault remotely.

Description

The adjustment dials and indications are on the front face.

The trip unit rating In corresponds to the maximum value of the setting range.

Setting the Long-Time Protection

The long-time protection pickup Ir is set by using two multi-position dials.

o The preset dial allows the pickup to be preset to the value Io (displayed in amperes on the dial).

The maximum preset value (maximum setting on preset dial) equals the trip unit rating value In.

o The adjustment dial can be used to fine-tune the pickup Ir (value displayed in multiples of Io on the dial).

The time delay tr for long-time protection cannot be adjusted.

The following table shows the value of the time delay tr for long-time protection (in seconds) according to the overload current (in multiples of Ir)

The precision range is -20%, +0%.

Setting the Short-Time Protection

The short-time protection pickup Isd is set by using a multi-position dial.

The setting value is expressed in multiples of Ir.

The precision range is +/- 15%.

The time delay tr for short-time protection cannot be adjusted:

o Non-trip time: 20 ms

o Maximum breaking time: 80 ms.

Setting the Instantaneous Protection

The pickup Ii for instantaneous protection cannot be adjusted.

The following table shows the value of the pickup Ii for instantaneous protection (in amperes) according to the trip unit rating In:

The time delay for instantaneous protection cannot be adjusted:

o Non-trip time: 0 ms

o Maximum breaking time: 50 ms.

Setting the Neutral Protection (4P Only)

The neutral selection dial gives a choice of three values for the neutral long-time and short-time protection pickups.

The following table shows the values of the pickup for neutral long-time protection (in multiples of Ir) and neutral short-time protection (in multiples of Isd) according to the dial position:

The time delay for the neutral long-time protection and short-time protection is the same as that for the phases.

Setting the Earth-Leakage Protection

The earth-leakage protection IΔn, type A, is set by using a multi-position dial.

The following table shows the value of the pickup IΔn for earth-leakage protection according to the trip unit rating In:

The OFF setting annuls any earth-leakage protection and the circuit breaker behaves as a standard circuit breaker for cable protection.

Setting the earth-leakage protection to OFF can be used to inhibit earth-leakage protection during periods of setting, commissioning, testing and maintenance.

Setting the Earth-Leakage Protection Time Delay

The time delay of the earth-leakage protection is set by using a multi-position dial.

When IΔn is set to 30 mA, the time delay Δt is always 0 ms regardless of the position of the dial (instantaneous tripping).

When IΔn is set above 30 mA, the time delay Δt can be adjusted to the following values:

o 0 ms

o 60 ms

o 150 ms

o 500 ms

o 1000 ms

Testing the Earth-Leakage Protection

The earth-leakage protection must be tested regularly by using the test button ( T ). Pressing the test button simulates a real leakage current passing through the toroid, and the earth-leakage fault indicator displays the following symbol:

When the earth-leakage protection pickup IΔn is set to the OFF  position, pressing the test button has no effect.

In the case of the Trip version of Micrologic 4, pressing the test button trips the circuit breaker.

In the case of the Alarm version of Micrologic 4, pressing the test button causes the earth-leakage indicator to change to yellow.

If the circuit breaker does not trip, or the earth-leakage indicator does not change to yellow, check that the circuit breaker is energized. If the circuit breaker is energized correctly, and has not tripped or indicated the earth-leakage fault, replace the Micrologic 4 trip unit.

Resetting the Circuit Breaker After an Earth-leakage Fault Trip

Resetting the circuit breaker after an earth-leakage fault trip depends on the version:

o For the Trip version, reset the circuit breaker by moving the handle from Trip  to O (OFF)  position, and then to I (ON) position.

o For the Alarm version, press the test button ( T ) for three seconds.

For Trip and Alarm versions, the earth-leakage fault indicator changes back to gray after the reset.

Examples of Setting the Long-Time Protection

Example 1: Setting the long-time protection pickup Ir to 140 A on a Micrologic 4.2 trip unit rated In 250 A:

Example 2: Setting the long-time protection pickup Ir to 133 A on a Micrologic 4.2 trip unit rated In 250 A:

The actions in steps (2) and (3) on the adjustment dials modify the trip curves as shown:

Example of Setting the Short-Time Protection

Setting the short-time protection pickup Isd to 400 A on a Micrologic 4.2 rated In 250 A on a 133 A feed:

The action in step (2) on the adjustment dial modifies the trip curve as shown:

Example of Setting the Earth-Leakage Protection

Setting the earth-leakage protection pickup IΔn to 1 A with a tripping time delay of 500 ms on a Micrologic 4.2 rated In 250 A:

DOCA0140EN-01

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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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Andrew's Presentation

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Circuit Breaker, PowerPact M, unit mount, electronic trip, 450A, 3 pole, 25 kA, 600 VAC, no lugs

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Electronic Trip Circuit Breaker Basics: Schneider Electric Micrologic Trip Units

What is electronic trip circuit breakers .

An electronic trip circuit breaker is a type of circuit breaker that uses electronic components to control the tripping mechanism instead of traditional thermal-magnetic trip elements. Electronic trip circuit breakers are commonly used in industrial and commercial applications where high reliability and selective coordination are required.

These circuit breakers use a solid-state trip unit to sense and respond to overcurrent conditions. The trip unit monitors the current flowing through the circuit breaker and can be programmed to trip at specific current levels and time delays. This allows for greater precision in protecting against overcurrent conditions and reduces the risk of nuisance trips. Electronic trip circuit breakers can also provide advanced monitoring and diagnostic capabilities. Some models can provide real-time current and voltage measurements, as well as fault event recording and reporting. This information can be used to identify potential issues and optimize system performance.

Related Article:  Circuit Breaker Essentials 

Why Use Electronic Trip Circuit Breakers? 

In most cases, the basic overcurrent protection provided by standard thermal-magnetic circuit breakers will meet the requirements of the electrical system design. In some cases, however, basic overcurrent protection might not be enough. Electronic trip circuit breakers can provide the additional features needed in those cases.

Reasons to use electronic trip circuit breakers includes:

• Enhanced coordination capabilities
• Integral ground-fault detection
• Communication capabilities
• Future growth potential

Enhanced Coordination Capabilities

Schneider electric micrologic electronic trip units.

• Independent adjustments  - allow one dial setting to be changed without affecting the rest of the pickup and delay levels. This allows the designer to better define the tripping characteristics needed on the system. 
• Interchangeable rating plugs-  allow the designer to shift the entire trip characteristic curve (except for ground fault) to improve coordination with other devices. MICROLOGIC rating plugs define the circuit breaker's maximum current rating based on a percentage of the circuit breaker sensor size and can be used on any frame size of circuit breaker within the MICROLOGIC family of circuit breakers. 
• Withstand ratings give the designer a larger window of coordination potential - The withstand rating is the level of rms symmetrical current that a circuit breaker can carry with the contacts in the closed position for a certain period of time. At current levels above the withstand rating (and less than or equal to the interrupting rating), the circuit breaker will trip instantaneously. In other words, the withstand rating is the highest current level at which delay can be introduced to maintain coordination with downstream devices. Withstand ratings are available only on full-function trip systems ordered with the adjustable short-time function.
• Inverse time delay characteristics  - allow for better coordination with fusible switches or thermal-magnetic circuit breakers downstream. Devices that respond to heat generated by current flow (such as fuses and thermal-magnetic circuit breakers) have inverse time tripping characteristics. This means that as current increases, the time that it takes the device to trip will decrease. In order to coordinate better with these types of downstream devices, MICROLOGIC circuit breakers offer inverse time delay characteristics on the long-time, short time and ground-fault functions. 
• Ammeter/trip indicator - displays the level of ground-fault leakage current associated with the circuit. The ground-fault pickup level on the circuit breaker may then be adjusted somewhat higher than the amount of leakage current displayed on the ammeter.

Integral Ground Fault Protection

Communication capabilities.

• History of last trip
• Trip unit pickup and delay levels
• Impending trip conditions
• Operating currents for each phase
• Ground-fault leakage current associated with the circuit
• Ground-fault alarm signal

Standard Function Trip Unit Curves

Long term trip function.

• LONG-TIME PICKUP Switch — switch value (multiplied by the ampere rating) sets the maximum current level which the circuit breaker will carry continuously. If the current exceeds this value for longer than the set delay time, the circuit breaker will trip. 
• LONG-TIME DELAY Switch — sets length of time that the circuit breaker will carry a sustained overload before tripping. Delay bands are labeled in seconds of overcurrent at six times the ampere rating. For maximum coordination, eight delay bands are available.

Short-time Trip Function

• SHORT-TIME PICKUP Switch — switch value (multiplied by the ampere rating) sets the short-circuit current level at which the circuit breaker will trip after the set SHORT-TIME DELAY.
• SHORT-TIME DELAY Switch — sets length of time the circuit breaker will carry a short circuit within the short-time pickup range. Delay bands are labeled in seconds of short-circuit current at 12 times the ampere rating, P. The short-time delay can be set to one of four I^2t ramp operation positions (I^2t IN).

Instantaneous Trip Function

• INSTANTANEOUS PICKUP Switch — switch value (multiplied by the ampere rating) sets the short-circuit current level at which the circuit breaker will trip with no intentional time delay. The instantaneous function will override the short-time function if the INSTANTANEOUS PICKUP is adjusted at the same or lower setting than the SHORT-TIME PICKUP. 
• GROUND-FAULT PICKUP Switch — switch value (multiplied by the sensor size) sets the current level at which the circuit breaker will trip after the set GROUND-FAULT DELAY.
• GROUND-FAULT DELAY Switch — sets the length of time the circuit breaker will carry ground-fault current which exceeds the GROUND-FAULT PICKUP level before tripping. Delay bands are labeled in seconds of ground-fault current at 1 times the sensor size, S. Ground-fault delay can be adjusted to one of four fixed time delay positions (I^2t OUT). 

In Comparison with Thermal Magnetic Circuit Breakers

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• Introduction
• Features and Benefits
• Specifications
• Codes and Standards
• Interrupting Rating
• Application Ratings
• Ampere Rating (Continuous Current Rating)
• Enclosure Sizes
• Temperature
• Extreme Atmospheric Conditions
• Storage Temperature
• MicroLogic Trip System
• Instantaneous OFF Feature
• Motor Circuit Protectors
• Automatic Molded Case Switches
• Manually-Operated Circuit Breakers
• Electrically-Operated Circuit Breakers
• Push-to-Trip Button
• Unit-Mount Circuit Breakers
• I-Line Circuit Breakers
• Drawout Circuit Breakers
• M-Frame, P-Frame and R-Frame Circuit Breaker Catalog Numbers
• NS630b–NS3200 Circuit Breaker Catalog Numbers
• Cradle Catalog Numbers
• UL, NEMA, CSA, and NMX requirements
• IEC Requirements
• PowerPacT and ComPacT Devices with MicroLogic Trip Units
• Communication
• Power Meter Functions
• Display Function
• Instantaneous RMS Measurements
• Maximum/Minimum Ammeter
• Energy Metering
• Demand and Maximum Demand Values
• Power Quality
• MicroLogic A/P/H Trip Units Integrated Power Meter Functions
• Contact Wear
• Circuit Breaker Load Profile
• Management of Installed Devices
• FDM121 Display of MicroLogic Trip Unit Measurements and Alarms
• FDM121 Status Indications and Remote Control
• FDM121 Main Characteristics
• FDM121 Mounting
• FDM121 Connection
• FDM121 Navigation
• Communication Components and FDM121 Connections
• FDM128 Display of MicroLogic Trip Unit Measurements and Trips
• FDM128 Status Indications
• FDM128 Remote Control
• FDM128 Main Characteristics
• FDM128 Mounting
• FDM128 Connection
• FDM128 Navigation
• FDM128 Screens
• FDM128 Communication Components and Connections
• Thermal Imaging
• True RMS Current Sensing
• ET1.0 (M-Frame only)
• ET1.0I (P-Frame and R-Frame only)
• ET1.0M (P-Frame only)
• MicroLogic Electronic Trip Unit Features
• MicroLogic 2.0 and 3.0 Basic Trip Unit Settings
• MicroLogic 5.0 Basic Trip Unit Settings
• MicroLogic 2.0A and 3.0A Trip Unit Settings
• MicroLogic 5.0A and 6.0A Trip Unit Settings
• MicroLogic 6.0A Trip Unit with Ground-Fault Settings
• External Power Supplies for MicroLogic Trip Units
• MicroLogic 5.0P and 6.0P Trip Unit Settings
• MicroLogic 6.0P Trip Unit with Ground-Fault Settings
• MicroLogic 5.0P and 6.0P Trip Unit Settings with Protection Functions
• MicroLogic 5.0P and 6.0P Trip Unit Settings for Current and Power Load-Shedding
• MicroLogic P Trip Units and 24 Vdc Power Supply
• MicroLogic Trip Units Tripping and Alarm Histories
• MicroLogic Trip Unit Metering
• MicroLogic Communication Network
• MicroLogic Event Log
• MicroLogic 5.0H and 6.0H Trip Units with Harmonic Metering
• MicroLogic Trip Unit Functions
• Wiring System ULP
• Smart System Four Functional Levels
• Smart System Modbus Principle
• Smart System Ethernet Principle
• COM Option in PowerPacT and ComPacT Circuit Breakers
• IFE Interface, IFE Interface + Gateway Description
• IFE Interface Mounting
• IFE Interface 24 Vdc Power Supply
• IFE Interface Required Circuit Breaker Communication Modules
• IFM Interface Function
• IFM Interface Characteristics
• IFM Interface Technical Characteristics
• Simplified IFM Interface Installation
• I/O Application Module Description
• I/O (Input/Output) Application Module for Low-Voltage Circuit Breaker Resources
• I/O Application Module Pre-Defined Application
• I/O Application Module User-Defined Applications
• I/O Application Module Mounting
• I/O Module Application Rotary Switch
• I/O Application Module Setting Locking Pad
• I/O Application Module General Characteristics
• Introduction to EcoStruxure Power Commission Software
• Compatible Devices (Configuration and Device Management)
• PowerPacT M-Frame Performance
• PowerPacT M-Frame Catalog Numbers
• PowerPacT M-Frame Interrupting Ratings
• PowerPacT M-Frame Termination Information
• PowerPacT M-Frame Accessories
• PowerPacT M-Frame Control Wiring

PowerPacT P-Frame Performance

Powerpact p-frame catalog numbers, powerpact p-frame continuous current rating, powerpact p-frame interrupting ratings, powerpact p-frame automatic molded case switches, powerpact p-frame motor circuit protectors, powerpact p-frame electrically-operated circuit breakers, powerpact p-frame termination information, accessory control wiring diagrams for manually-operated p-frame circuit breakers, accessory control wiring for manually-operated p-frame circuit breaker, accessory control wiring diagrams for electrically-operated p-frame circuit breakers.

• PowerPacT R-Frame Performance
• PowerPacT R-Frame Catalog Numbers
• PowerPacT R-Frame Interrupting Ratings
• PowerPacT R-Frame Automatic Molded Case Switches
• PowerPacT R-Frame Termination Information
• PowerPacT R-Frame Continuous Current Rating
• Accessory Control Wiring Diagrams for R-Frame Circuit Breakers
• Accessory Wiring for R-Frame Circuit Breaker
• ComPacT NS630b–NS1600 Performance
• ComPacT NS630b–NS1600 Drawout Configuration
• ComPacT NS630b-NS1600 Catalog Numbers
• ComPacT NS630b-NS1600 Interrupting Ratings
• ComPacT NS630b-NS1600 Electrically-Operated Circuit Breakers
• ComPacT NS630b-NS1600 Termination Information
• ComPacT NS630b-NS1600 Accessories
• ComPacT NS630b-NS1600 Control Wiring
• ComPacT NS1600b–NS3200 Performance
• ComPacT NS1600b–NS3200 Termination Information
• ComPacT NS1600b–NS3200 Accessories
• ComPacT NS1600b–NS3200 Control Wiring
• ComPacT NS1600b–NS3200 Catalog Numbers
• M-frame, P-frame, R-frame and NS630b–NS3200 Factory-Installed Accessories
• M-frame, P-frame, R-frame and NS630b–NS3200 Field-Installable Accessories
• M-frame, P-frame, R-frame and NS630b–NS3200 Accessory Availability
• Maximum Wire Length
• Shunt Trip (MX1) and Shunt Close (XF)
• Undervoltage Trip (MN)
• Time-Delay Module for Undervoltage Trip
• Indication Contacts
• Spring-Charging Motors (MCH) for P-Frame Circuit Breakers
• Lead-Free Trip Unit Sealing Kit
• Replacement Covers
• Neutral Current Transformer (CT)
• Ground-Fault Interface Module
• External Sensor for SGR or MDGF Protections
• Sensor Plugs
• Rating Plugs
• Modbus Circuit Breaker Communication Module (BCM-ULP)
• M2C Programmable Contacts
• Zone-Selective Interlocking (ZSI)
• Restraint Interface Module (RIM)
• External Power Supply Module
• External Battery Backup Module
• Hand-Held Test Kit
• Full-Function Test Kit
• Mechanical, Compression, and Distribution Lugs
• I-Line Jaw Configurations
• Power Distribution Connectors
• Control Wire Terminations
• Phase Barriers
• Electric Joint Compound
• Door-Mounted Operating Mechanisms
• Handle Extension
• Rotary Operating Handles
• Replacement Handles
• Door Escutcheons
• PowerPacT M-, P-, and R Frame Locking Accessories
• Sub-Feed Lugs
• P-Frame Circuit Breaker and Cradle Design
• P-Frame Drawout Mechanism
• P-Frame Drawout Connectors
• P-Frame Cradle Position Switches
• Disconnected Position Locking
• Door Interlock
• Cradle Rejection Kits
• Shutter and Shutter Lock
• Door Escutcheon (CDP)
• Transparent Cover for Door Escutcheon (CCP) (P-Frame Only)
• Terminal Layout for Push-In Connector Installation
• Wiring Diagrams for Auxiliary Connections
• Wiring Diagrams for the COM Option
• Dimensions for M-Frame Circuit Breakers
• Dimensions for P-Frame and NS630b–NS1600 Circuit Breakers
• Dimensions for R-Frame and NS1600b–NS3200 Circuit Breakers
• Accessory Dimensional Drawings
• Accessory Wiring
• Section 13—PowerPacT M-, P-, and R-Frame, and ComPacT NS630b–NS3200 Trip Curves

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Section 5—PowerPacT P-Frame Molded Case Circuit Breakers

P-Frame Unit-Mount

P-Frame I-Line

P-Frame Drawout

Providing unparalleled performance and control, the P-frame circuit breakers features the exclusive MicroLogic electronic trip units, which allow for a range of sophisticated applications for metering and monitoring. In addition, trip units can be interchanged in the field to allow for maximum flexibility.

The compact size and small footprint of the P-frame circuit breaker permits high density installations in I-Line panelboards and switchboards. These circuit breakers are available in 100% rated construction for all unit-mount circuit breakers and up to 800 A in I-Line circuit breakers to meet a broad range of commercial and industrial application needs.

Both standard (80%) and 100% rated construction circuit breakers are available in 1200 A with a sensor size range of 250–1200 A.

Interrupting ratings (AIR):

The P-frame circuit breakers with K interrupting rating are recommended for applications having high inrush and/or non-linear loads such as large motors, transformers, motors with soft starts, etc.

ComPacT circuit breaker size allows for small footprint installations using I-Line panelboards and switchboards. Nine inch width provides increased density installation.

Most field-installable accessories are common to all frame sizes for easier stocking and installation

Selection of four interchangeable MicroLogic trip units available, with PoweLogic power metering and monitoring capabilities available in advanced trip units.

Field-installable sensor plugs. See Sensor Plugs .

Compatible with PowerLogic systems and high amperage power circuit breakers.

Built-in Modbus protocol provides an open communications platform and eliminates the need to purchase additional, proprietary network solutions.

Connection options include bus, cable or I-Line for installation flexibility.

Additional options are available for 5-cycle closing, stored energy mechanisms and drawout-mounting.

Catalog Numbers The L interrupting rating at 600 Vac is 25 kA. * for UL/IEC Rated, Unit-Mount Catalog numbers are for circuit breakers with lugs on line and load ends. Consult the Product Selector for catalog numbers for circuit breakers with alternate terminations * , Manually-Operated, Standard-Rated Electronic Trip Circuit Breakers with Basic Electronic Trip and MicroLogic Electronic Trip Unit

Catalog numbers the l interrupting rating at 600 vac is 25 ka. * for ul/iec rated, unit-mount catalog numbers are for circuit breakers with lugs on line and load ends. consult the product selector for catalog numbers for circuit breakers with alternate terminations * , manually-operated, 100%-rated electronic trip circuit breakers with micrologic electronic trip units, catalog numbers the l interrupting rating at 600 vac is 25 ka. * for ul/iec rated, i-line, manually-operated, standard-rated electronic trip circuit breakers with basic electronic trip and micrologic electronic trip units, catalog numbers for ul/iec rated, i-line, manually-operated, 100%-rated electronic trip circuit breakers the l interrupting rating at 600 vac is 25 ka. * with micrologic electronic trip units, p-frame interrupting ratings, p-frame termination options.

All circuit breakers marked as 100% rated can be continuously loaded to 100% of their rating.

Because of additional heat generated when applying circuit breakers at 100% of continuous current rating, the use of specially-designed enclosures and 194°F (90°C) wire is required. The 194°F (90°C) wire must be sized according to the ampacity of the 167°F (75°C) wire column in the NEC. Minimum enclosure size and ventilation specifications are indicated on the circuit breaker, in its instruction bulletin, and in Enclosure Sizes .

Circuit breakers with 100% rating can also be used in applications requiring only 80% continuous loading.

Automatic molded case switches are available in individually-mounted and I-Line constructions from 600–1200 A. Automatic switches are similar in construction to electronic trip circuit breakers except that long-time tripping is not present. The switches open instantaneously at a non-adjustable magnetic trip point calibrated to protect only the molded case switch itself. They must be used in conjunction with a circuit breaker or fuse of equivalent rating.

Motor circuit protectors Catalog numbers for motor circuit protectors are designated by M68, M69, or M70 in positions 11–14 (trip system). See the table below. * are similar in construction to thermal-magnetic circuit breakers, but have only instantaneous trip functions provided by the ET1.0M trip unit. These motor circuit protectors comply with NEC requirements for providing short-circuit protection when installed as part of a listed combination controller having motor overload protection. Interrupting ratings are determined by testing the motor circuit protector in combination with a contactor and overload relay.

Motor circuit protectors are available in PJ and PL individually-mounted and I-Line construction. According to the NEC, the instantaneous trip of the motor circuit protector may be set to a maximum of 8 to 17 times motor Full Load Amps (FLA), but a setting as close as possible to inrush current (without nuisance tripping) results in the best protection. The instantaneous trip pickup level is adjustable within the ranges shown below.

Motor Circuit Protector Trip Range

Select motor circuit protectors as follows:

Select a motor circuit protector with an ampere rating recommended for the hp and voltage involved.

Select an adjustable trip setting of at least 800% but not to exceed 1300% (1700% for high-efficiency motors) of the motor full load amperes (FLA).

The NEC 1300% maximum setting (1700% for high-efficiency motors) may be inadequate for motor circuit protectors to withstand current surges typical of the magnetization current of auto-transformer type reduced voltage starters or open transition wye-delta starters during transfer from “start” to “run,” constant hp multi-speed motors and motors labeled “high efficiency.”

Part-winding motors, per NEC 430-3, should have two motor circuit protectors selected from the above at not more than one-half the allowable trip setting for the horsepower rating. The two circuit protectors should operate simultaneously as a disconnecting means per NEC 430-103.

Electrically-operated P-frame circuit breakers are available in I-Line and unit-mount construction up to 1200 A and are denoted in the catalog number by an “M_” suffix. These come equipped with a two-step stored energy mechanism and come standard with a motor assembly. These are available factory-installed only.

Motor assemblies provide on and off control from remote locations. The assemblies contain a spring-charging motor (MCH), a shunt trip (MX) and a shunt close (XF) and are available in standard or communicating versions. An SDE overcurrent trip switch is also included for trip indication. When remote indication of the circuit breaker status is required, use of a circuit breaker with an OF auxiliary switch for on-off indication. See Indication Contacts for details.

Motors Assembly Voltage Ratings (Vn)

Unit-mount circuit breakers and switches have mechanical lugs standard on both ends. I-Line circuit breakers have lugs standard on the O/OFF end. These lugs accept aluminum or copper wire. Manually-operated P-frame circuit breakers are also available in drawout construction. See Mechanical, Compression, and Distribution Lugs for more lug options.

PowerPacT P-Frame Control Wiring

Control wiring for unit-mount and I-Line construction is connected to terminals located under the circuit breaker accessory cover. Control wiring for drawout construction is connected to terminals located on the cradle.

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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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