Miniature Circuit Breakers

Miniature Circuit Breakers

As previously stated, a miniature circuit breaker is a device that switches and/or protects the lowest common distributed voltage in an electrical system. It is designed to protect conductors and insulation from damage due to Overload (or Overcurrent) and Short Circuit.
For most people, the easiest way to visualize this application picture is to think in terms of a loadcenter in a home (Figure 3). The circuit breakers inside the loadcenter are miniature circuit breakers.
Think about the electrical utility and where the electricity is generated. The residential loadcenter is certainly at the end of the distribution system. It is here that the voltages are the lowest of the distributed voltages in the electric utility's system.

End of the Line Distribution System (Loadcenter)

Miniature circuit breakers are not just for residential applications only. They are used in residential, commercial and industrial applications.
In an industrial or commercial application, miniature circuit breakers can be found in loadcenters, lighting Panelboards and individual mountings.
Applications
Miniature circuit breakers fall into two categories. These are:
• Residential-Residential miniature breakers are only of the Plug-In type. These are designed for residential loadcenters, commercial units, and light industrial applications. They typically range from 10 to 125 amps, with an interrupting rating of 10 or 22 KAIC.
• Industrial-These breakers are designed for three types of mounting applications: plug-in, Bolt-On, and Cable-In/Cable-Out. (We will look at mounting methods shortly.)
Industrial miniature breakers are designed to protect small branch circuits in commercial or industrial electrical distribution systems. They are applied in loadcenters, lighting panelboards or individual mounting applications. They typically range from 6 to 125 amps, with an interrupting ratings as high as 65 KAIC.
Some potential customers are original equipment manufacturers (OEMs) involved in industrial control panels and electrical machinery, such as machine tool equipment, material handling and packaging systems. In addition, look for involvement with printing machines, food-processing systems, uninterruptable power supplies (UPS) and HVAC (heating, ventilation and air conditioning).
Pictured here is a typical residential loadcenter. Each miniature circuit breaker in the loadcenter protects a branch circuit. Two branch circuits are shown, each providing power to common residential loads.



Each miniature breaker is rated to handle a specific load. For example, a circuit breaker protecting a branch used with kitchen appliances has a higher rating than a circuit breaker protecting a branch with an overhead lighting fixture on it.

Introduction

We will discuss two types of products in this module. These are the Miniature Circuit Breaker and the Supplementary Protector.
We group these products together because they perform the same function. They both switch and protect the lowest common distribution voltage in an electrical system.
Some other similarities include:
• Both have molded case enclosures
• Both are used in low voltage (under 600 volts) systems
• Both devices are small: Typically 1" wide
The big difference between the two is that the supplementary protector is not UL 489 (Underwriters Laboratory) approved. For this reason, it cannot be used as a Branch Circuit Overcurrent Protective Device, or in the place of the branch circuit protector. A miniature circuit protector protects the whole branch circuit, but the protector is only used to protect a particular device.
Figure shows the difference between using just a miniature circuit breaker (shown on the left) and using a supplementary protector.
Figure Branch Circuit Overcurrent Protection (on left) Vs.
Supplementary Protector (on right)

Therefore, this module will concentrate on miniature circuit breakers. Supplementary protectors will be discussed only briefly

Welcome to Module 9

Welcome to Module 9, which is about miniature circuit breakers and supplementary protectors.

Typical Miniature Molded Case Circuit Breakers


Like the other modules in this series, this one presents small, manageable sections of new material followed by a series of questions about that material. Study the material carefully, and then answer the questions without referring back to what you've just read.
You are the best judge of how well you grasp the material. Review the material as often as you think necessary. The most important thing is establishing a solid foundation to build on as you move from topic to topic and module to module.

Mounting and Enclosures

Generally, molded case circuit breakers can be mounted in any position. Mounting them up, down, horizontal or vertical does not affect the tripping or interrupting characteristics of the breaker. However, mounting them in a vertical position with the ON position as anything other than up, is in violation of National Electric Code.
In some cases because of the physical arrangement of a panelboard or switchboard, it is necessary to reverse feed a circuit breaker. The circuit breaker must be tested and listed accordingly for this type of application. Only breakers that have fixed trips can be used, and they usually have sealed covers. They often do not have "Line" and "Load" marked on the cover, so the power source can be connected to either the line or the load terminations.
In addition to being mounted in motor control centers, switchboards and panelboards, molded case circuit breakers are mounted individually in separate enclosures. The National Electric Code and local electric codes determine the proper selection of an enclosure type for a particular application. The National Electrical Manufacturers Association (NEMA) and the International Electro-technical Commission (IEC) have set standards for the protection of devices in various environmental situations. Enclosure types are rated to withstand water, dust, oil, and other environmental conditions. NEMA assigns Type classifications to enclosures. When an enclosure is rated a particular type, it means it is made of the specified materials and has passed specific tests. IEC also has tests and standards that enclosures must conform to. They assign an IP classification.

Mounting Circuit Breakers

The most common types of enclosures are:
NEMA Type 1 (Conforms to IP40) - These enclosures are designed for indoor applications. They are suitable for installations where unusual conditions do not exist, but where a measure of protection from accidental contact is required. They are commonly used in commercial buildings and apartment buildings. They are often referred to as general purpose enclosures.
NEMA Type 3R (Conforms to IP52) - These enclosures are designed for outdoor use where falling rain, sleet or external ice might form. They have a gasket on the cover to keep out water. Some versions have a top hinged front cover which must be opened to gain access to the circuit breaker handle. Other versions have an external side operated handle mechanism. These enclosures are often referred to as raintight enclosures.
NEMA Type 4 (Conforms to IP65) - These enclosures are designed for use either indoors or outdoors. They provide protection from splashing water, wind blown dust or rain. They even protect the circuit breaker from hose directed water. They are well suited for application in dairies, breweries, paper mills, food processing plants and other process industries. These enclosures are often referred to as watertight enclosures.
NEMA Type 4X (Conforms to IP65) - These are much the same as the Type 4 except that they are made of gasketed, stainless steel. In some designs, they are made of a nonmetallic material. They provide better resistance to corrosion than the Type 4. Industries that have a high amount of corrosive liquids, require a high measure of hose-down cleaning, or are in a salt-water environment use these enclosures. They are often referred to as corrosion-proof enclosures.
NEMA Type 12 (Conforms to IP62) - These enclosures are designed for indoor use in dirty and dusty applications. They are constructed of sheet metal and provide protection from dripping liquids (non-corrosive), falling dirt and dust. A special NEMA 12K version provides Knockouts for conduit. These enclosures are often referred to as dust-tight enclosures.
There are other ratings of enclosure types, but these are the most commonly used.

The selection of molded case circuit breakers is generally determined in two ways:
• For protection of non-motor circuits.
• For protection of motor circuits.
We will discuss protection of non-motor circuits first.
Protecting Non-Motor Circuits
Non-motor circuit applications usually center around cable protection and require molded case circuit breakers with both overload and short circuit capabilities. They should be able to distinguish between harmless and destructive conditions, and function appropriately for its application. It is very important that the MCCB selected be adequately rated and equipped for all the electrical and physical conditions that are likely to exist when the system is energized.
The standard selection factors for molded case circuit breakers include:
• Voltage Rating
• Frequency
• Continuous Current Rating
• Interrupting Rating
• Number of Poles
• Fixed or Interchangeable Trip Unit
• Trip Unit Functions
• Accessories
Protecting Motor Circuits
The selection of an HMCP is based on the full load current of the motor it is to protect. Data shown in the National Electric Code (tables 430-148 and 430-150) list the full load currents of induction motors running at speeds normal for belted motors and with normal torque characteristics. However, actual motor nameplate ratings should be used for selecting the motor running overload protection.
Other considerations in the selection of HMCPs include:
• The ambient temperature outside the enclosure should not exceed 40°C (104°F).
• Infrequent starting, stopping and reversing of the motor.
• Motor accelerating time of 10 seconds or less.
• Locked rotor rating is a maximum of six times the motor FLA rating.

Accessories and Modifications

Handle Operating Devices
Handle operating devices provide indirect electrical or manual operation of the circuit breaker handle.Electrical operators provide complete remote control of a molded case circuit breaker by means of a pushbutton or similar pilot device. When energized from a remote location, the operator mechanism moves the circuit breaker handle to either the ON or OFF position. Shunt trips and undervoltage release mechanisms can only be used to trip the breaker. Electrical operators deliver a positive switching action. In case of a power failure, means are provided for manual operation. They come in a variety of designs and, depending on the circuit breaker type, are mounted in different ways.

The newer designs are front mounted on the breaker cover and fit within the trim line of the circuit breaker. For smaller frame breakers, a solenoid is used. On larger frame sizes, a motor is used to provide the increased operational force required to move the breaker handle.

Front Mounted Electrical Operator (Solenoid Type)

Some designs are side mounted and use an extended arm to move the circuit breaker handle. These older motor driven operators are not usually suitable for generator synchronizing because of the time it takes for operation.

 Side Mounted Electrical Operator (Motor Type)


Contemporary electrical operator designs, whether solenoid or motor driven, are capable of performing a closing operation in five cycles or less. This makes them very suitable for generator synchronizing applications.

Handle mechanisms allow for the manual operation of the circuit breaker toggle handle. These mechanical devices allow personnel to open and close the breaker without opening the enclosure in which the breaker is mounted. They come in a variety of designs for different applications and enclosure types.

The flex shaft type handle mechanism is an extra heavy-duty mechanism designed for mounting in flange-type enclosures. An operating handle, flexible shaft and mechanism are required for standard application.

Flex Shaft Handle Mechanism

Handle mechanisms are often designed to indicate the status of the circuit breaker because the breaker itself is often not visible when the handle operates. Some designs allow for protection against tampering, or to tag and lockout during maintenance. The unit shown here will accept up to three padlock shackles.

 Safety Handle Mechanism



Rotary handle mechanisms mechanically transfer the rotating operation to the in-line toggle operation of the circuit breaker handle. Depending on their design, they may allow mounting an auxiliary switch on the handle for undervoltage release. Some devices are available with red handles and yellow background labels to meet local codes.

Rotary Handle Mechanisms


Lock and Interlock Devices
Lock and interlock devices are used to deter undesired circuit breaker operation and establish interlocked control systems. They do not interfere with the trip-free operation of the molded case circuit breaker.

The nonlockable handle block secures the breaker handle in either the ON or OFF position to prevent accidental operation.

 Nonlockable Handle Block

The padlockable hasp mounts on the circuit breaker cover and will allow multiple padlocks to secure the handle in either the ON or OFF position.

Padlockable Hasp



The cylinder lock internally blocks the breaker trip bar in the tripped position. This prevents the breaker from being switched ON. However, use of a cylinder lock may reduce the interrupting rating of the breaker.

 Cylinder Lock

A key interlock is used to externally lock a circuit breaker handle in the OFF position. An extended deadbolt blocks movement of the breaker handle. The key is only removable in the locked position.
 
Key Interlock


To prevent adjacent breakers from being switched to the ON position, a sliding bar interlock can be used. When the sliding bar handle is moved from one side to the other, a bar extends to alternately block the breaker handle.
 Sliding Bar Interlock

Along the same lines, the walking beam interlock provides mechanical interlocking between breakers of the same pole configuration. It mounts on a bracket behind and between the breakers.

Walking Beam Interlock


Miscellaneous Devices
Certain molded case circuit breakers can have their interrupting rating increased to 200,000A symmetrical at up to 600V. The device used for this is called a current limiting attachment. It is bolted to the load end of the circuit breaker so that it will interrupt normal fault current up to 50 or more times the breaker's continuous rating. The limiter will only trip when a very high fault is encountered. As the limiter trips, the breaker is also tripped magnetically, preventing a single-phasing condition.
A spring loaded indicator is on each pole of the limiter to identify the faulted phase. When a current limiter trips, it is an indication of a serious circuit problem that must be corrected before restoring service. To ensure the proper limiter is used with a compatible breaker, they are always of a noninterchangeable design.

 Current Limiting Attachment


Another device designed to protect personnel and equipment is the low level earth leakage (ground fault) protector. This device usually trips the circuit breaker through a shunt trip or undervoltage release mechanism when it detects a low level ground fault.

 Earth Leakage Protection

Accessories and Modifications

When a comparison is made between a fusible switch and a molded case circuit breaker, it is easy to see the application flexibility MCCBs provide. This is even more apparent when you look at the array of accessories and modifications available. We are going to review in general what is available with a brief explanation of their purpose and applications. We will not cover specific accessories for specific lines of breakers in this module.

We are going to divide our review into the following categories:
• Operational Devices
• Termination Devices
• Handle Operating Devices
• Lock and Interlock Devices
• Miscellaneous Devices

Operational Devices
A shunt trip provides remote controlled tripping of a molded case circuit breaker. A solenoid coil is remotely energized using a pilot device, such as a pushbutton. That moves the plunger to activate the trip bar of the breaker. At the same time, a cutoff switch operates which disconnects power to the solenoid so the coil doesn't burn out. Often, pigtail leads are supplied for connecting the shunt trip to either an AC or DC control power source

In the workplace
One application for a shunt trip is for use on a welding machine. Normal thermal overload protection is not practical because of the high, frequent cycling of the machine. Often, the machine is equipped with a Thermistor to provide overload protection. The Normally Open (NO) contacts of the thermistor close when it reaches a preset temperature.

 Shunt Trip Application Example

The thermistor is connected to a shunt trip on a magnetic only circuit breaker. The breaker provides short circuit protection, the thermistor provides the overload protection.
An undervoltage release mechanism trips the breaker whenever the voltage falls below a predetermined level. These undervoltage release mechanisms (UVRs) come in two different styles:
The handle reset UVR (standard on current breakers) consists of a continuous rated solenoid with a plunger and tripping lever. The UVR mechanism is reset by a tripping lever when normal voltage has been restored, and the circuit breaker handle is moved to the reset OFF position. With no voltage applied to the UVR, the circuit breaker contacts will not touch when a closing operation is attempted.

Handle Reset UVR

An automatic reset UVR (standard replacement type breakers) has a tripping lever extension for resetting during the tripping action cycle. It works like the manual reset UVR except there is no plunger to be reset. When the breaker trips, it resets the UVR mechanism.
It is important to point out, that undervoltage release mechanisms are not designed to be used as circuit interlocks.

Due to long lengths of cables from power supplies in underground mining, low voltage conditions are common. To protect personnel and equipment, the circuit breaker trips. It cannot be energized until the power has been restored to at least 85% of the coil rating on the undervoltage release mechanism.

Providing circuit breaker main contact status, an auxiliary switch is mounted in the breaker. In this diagram, the contacts are shown as "A" and "B". An "A" contact is open when the breaker is open or tripped. A "B" contact is closed when the breaker is open or tripped. The contacts are rated 120V for pilot duty.

 Auxiliary Switch Contacts

If you wanted to give a visual indication that a circuit is energized, you could mount an indicating light on the panel. Using an auxiliary switch with an "A" contact would allow the light to be illuminated whenever the breaker is closed. When the breaker trips, the light goes off, letting you know the breaker has tripped or been opened.
Auxiliary switches can be used for circuit interlocking purposes. The NEC requires that motor control circuits be disconnected from all sources of supply when the disconnect means is in the open position. For starters with common control, one circuit breaker would disconnect the voltage for both the power circuit and the control circuit. If the control circuit has a separate power supply, the circuit breaker would not disconnect that supply source. You could use separate disconnect means, or simply use an auxiliary switch in the breaker. When the breaker is open, the auxiliary opens the control circuit, disconnecting it from its supply source.

 Auxiliary Contacts Disconnect Separate Control Power Source


Alarm switches differ from auxiliary switches in that they function only when the breaker trips automatically. The normally open contact of the switch closes when the breaker trips due to a short circuit, an overload condition, or when operated by a shunt trip. An indicating light, buzzer or bell can be placed in the circuit to provide indication that the circuit has tripped. When the breaker is reset, the alarm switch is reset. A manual opening of the circuit breaker does not affect the alarm switch contact.

Termination Devices
Line and load terminals provide the means for connecting the circuit breaker to the power source and the load. They are sized for various continuous current capabilities and wire types. Depending on the breaker, it may be supplied factory equipped with both line and load lugs, load terminals only, or line and load terminals for field installation.
The terminal body is usually made of aluminum to accommodate aluminum or copper wire or cable. Terminals with copper or steel bodies are also available. Terminals are available that support single or multiple conductors.

 Typical Termination for Use with 3/0-250 MCM Conductors


A keeper nut slides onto the line or load conductor and acts as a threaded adapter to accept a ring terminal or other bolt-on connector.

 Typical Keeper Nut


A plug nut is used where screw connected ring terminals are preferred to connect cables to the breaker conductors. They are press-fit into the opening in the breaker terminal conductor.

 Plug Nut with Ring Terminal


Endcap kits are used to connect bus bar or similar electrical connections.

 Endcap Kit


Rear connected studs are available in different sizes to accommodate specific fixed mounted breaker applications and ratings. The breakers are front removable from switchboards and other equipment by unscrewing the nut that holds the stud to the breaker. Studs must be assembled in accordance with UL required clearances.

 Rear Connected Studs


As with the rear connected studs, plug-in adapters simplify installation and front removal of the breaker from applications such as switchboards. Tulip-type connectors and threaded studs or flat bus bars are built into the molded support block and connect to the main bus.

 Plug-in Adapters


To connect circuit breaker terminals to the panelboard bus, panelboard connecting straps can be used. Because depth and bus spacing vary depending on type of panel and manufacturer, the panelboard builder should be consulted to determine the correct strap.

 Panelboard Connecting Straps

So far, these termination devices have been a means for connection of the circuit breaker to the power source and load. The next few items offer terminal isolation and protection.
In motor control center applications, because of confined spaces, line side conductors are often custom fitted. Terminal end covers fit together with the circuit breaker case to form terminal compartments. This allows isolation of the discharged ionizing gasses that form during the tripping operation.

 Terminal End Covers

To provide protection against accidental contact with live line side terminations, terminal shields fasten over the front terminal access openings.

 Terminal Shield


Interphase barriers are high dielectric insulating plates that fit between the terminals to provide additional electrical clearance between circuit breaker poles.

 Interphase Barriers


Terminal covers provide the required electrical clearance between circuit breaker poles when extended terminals are used.

 Terminal Covers

Motor Circuit Protectors

Because special considerations need to be taken when using circuit breakers with motors, we will dedicate this section on their particular characteristics and applications.

Most faults on a motor circuit are caused by a breakdown of the insulation within the motor windings. The initial fault current is usually low when compared to the overall system capacity. However, because it causes an Arcing condition, it could cascade and short out more and more of the motor windings. If the fault is allowed to continue, serious motor and starter damages occur, increasing repair costs. Although fusible switches and thermal magnetic breakers can provide motor branch circuit protection, the level of protection is not as effective against this type of fault.
For this reason, the motor circuit protector was developed. A motor circuit protector (MCP) operates on a magnetic only principle. It has a specially designed current sensing coil in each of its three poles to provide sensitive low level protection. It can clear a fault faster than a fusible device. It does not, however, provide overload protection for the motor. As a result, a contactor with an overload relay or motor starter must be used in conjunction with the motor circuit protector. (See Module 19 for information on contactors, overload protection and starters.)

Fuse vs. Circuit Breaker
This chart shows the typical fault Clearing Time (Fuse) of a dual element fuse on Low Level Faults. It required from 12 to 84 cycles to clear the fault. This test was based on a 3 hp motor with an FLA of approximately 4.2A. The short circuit was 150A, or 35 times the full load current. The fusible device had a 5A dual element fuse, sized at 120% the FLA. The NEC allows fuses to be sized up to 175% of motor full load current.

Fault Clearing Time of Dual Element Fuse

When an MCP rated at 7A was used, it was able to clear the fault in less than one cycle. It was set to trip at 51A, or approximately 12 times the full load current. The MCP also did not create a single-phasing condition, whereas the dual element fuse did. The fuse cleared the fault in phase B in only 12 cycles, but did not clear it in phases A and C until 22 cycles had elapsed.

Fault Clearing Time of MCP

In a single-phasing condition, the current to a motor in the remaining phases increases significantly above normal. This can lead to severe equipment damage. Because the motor circuit protector cleared the fault in all three phases in less than one cycle, the motor failure did not result in starting an electrical fire.

Components
The components of motor circuit protectors are very similar to molded case circuit breakers. They are:
• Molded Case
• Operating Mechanism
• Arc Extinguishers
• Contacts
• Trip Mechanism
• External Adjusting Mechanism
 Motor Circuit Protector Components


How It Operates
Motor circuit protectors disconnect the motor load from an electrical supply under three conditions. They are:

• When the handle is switched to OFF.
• When an automatic trip operation occurs.
• When a manual trip is initiated with a push-to-trip button.

As with the molded case circuit breakers, the operating mechanism is a spring-loaded toggle that provides quick-make, quick-break and trip-free operation.
Its design provides an increased air gap between the stationary and moveable contacts when in the tripped position. This air gap results in greater arc extinguishing during contact opening and provides higher interrupt ratings.

The magnetic trip unit operates when a fault current exceeds the magnetic pickup setting. It consists of an electromagnetic coil and plunger assembly. Certain HMCPs also have a transient inrush trip suppression device. This allows the startup of energy efficient motors without nuisance tripping the sensitive short circuit protection of the current sensing coil.
A tuned spring introduces a time delay of approximately 8 ms into the trip sequence under normal conditions. It allows the HMCP to ignore the initial high inrush current during the first half-cycle of start-up. A true fault current would supply a magnetic force to override the spring action and provide instantaneous tripping of the device.

Transient Inrush Suppressor

Larger HMCPs (600A) and above may use electronic trip units to provide an adjustable three-phase instantaneous trip setting. At start-up, an electronic time delay acts as the inrush trip suppressor. However, any current in excess of the predetermined setting level, such as with a short circuit, would override the time delay and trip the HCMP.
A trip setting adjustment allows for precise motor protection. Press in on a cam and turn the arrow until it is aligned with the required trip setting shown on the nameplate.
However, in keeping with NEC requirements, they cannot be set at more than 1300% of the motor full load current rating.

 Trip Setting Adjustment


Applications
Motor circuit protectors can be used in combination starter units within a motor control center. They allow for protection against both low and high level fault currents without requiring current limiters. They can also be applied in standalone combination starters.
When properly sized, they can provide short circuit protection for resistance welding devices. The normal high welding currents can flow, but the HMCP trips instantaneously if a short circuit develops.

HMCPs can be used in panelboards. You can have both distribution branch circuit protection and protection of the motor circuits within the same enclosure.