Tuesday, January 27, 2009

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

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