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Methods Of Mounting LV Circuit Breakers

Methods Of Mounting LV Circuit Breakers

Three general groups

Methods used to mount circuit breakers can be divided into three general groups, those being:

A review of these mounting methods follows.

Fixed Mounted Circuit Breakers

A circuit breaker that is bolted in its enclosure and wired to the load frame, we can call a fixed mounted circuitbreaker. These units are typically rated 600 volts or less and are front mountable. Power is provided to thebreaker typically by wires or sectional type bus bars.

Power feeding the circuit breaker must be turned off in order to physically remove the fixed mounted breaker.

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Removable Mounted Circuit Breakers (Disconnectable units)

A removable circuit breaker (Disconnectable unit) has two parts, a base, which is bolted to and wired to the frame,and the actual breaker, which has insulated parts that electrically mate with the base. This means of mounting

Siemens fixed air circuit breaker

Removable disconnectable circuit breaker (photo credit: electrical-installation.org)

allows the unit to be replaced with out re-wiring the unit on the line side of the breaker.

This type of mounting is typically used forbreakers rated 600 volts or less.

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Drawout (Withdrawable) MountedCircuit Breakers

Withdrawable Circuit Breakers (on photo: ABB’s Emax Low voltage air circuit-breaker)

A drawout circuit breaker also has two parts, the base, which is bolted and wired to the frame and the actualbreaker, which slides into and electrically mates with the base. This allows the unit to be replaced without having toturn off the power feeding the breaker.

The load must be turned off in order to test, remove or replace the unit .

As a safety feature these units are interlocked to automatically turn the power off just before removal of the breakerbegins. By design, only the circuit breaker’s load must be turned off to remove the breaker. This method ofmounting allows for a single breaker to be disconnected from the power supply.

That is to say that it does not require that all of the power be disconnected from all of the breakers installed inthe larger enclosure such as a motor control center.

There are various designs used to facilitate the “racking-in”(installation) and “racking out”(withdrawal) of thedrawout type circuit breaker. Commonly some form of jacking screw is used to initially move and thus electricallydisengage the breaker, then a traveling trolley type of hoist (somewhat like a small boat winch) supports thebreaker during removal and re-installation.

A transient supporting device is necessary as these sizes of breakers are too heavy and too bulky to besafely moved into and out of position by one person.

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Reference: Electrical Circuit Breakers – L.W. BRITTIAN Mechanical & Electrical Instructor