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Thread: Inverse VS Instantaneous Circuit Breakers

  1. #1
    Join Date
    Jan 2004
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    6

    Inverse VS Instantaneous Circuit Breakers

    My question deals with Inverse Time Circuit Breakers(CBs) and Instantaneous CBs. According to an email that Mike Holt sent recently, I gathered that Inverse Time CBs are found in a typical CB panel? Also, where are Instantaneous CBs used? How does one distinguish between the two when you are trying to determine which is inverse and which is instantaneous? Any information you could give me on this would be appreciated.

    Dan

  2. #2
    Join Date
    Feb 2003
    Location
    N.C.
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    Re: Inverse VS Instantaneous Circuit Breakers

    Go here, then type "Inverse Time" in the search window, then after that, back out and type "Instantaneous" in the search window for some FAQ's.

    Roger
    Moderator

  3. #3
    Join Date
    Feb 2003
    Location
    Wisconsin
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    3,245

    Re: Inverse VS Instantaneous Circuit Breakers

    This may be a bit long and review be eagle eyes but I trust that it will give you a good idea of the differences of these devices and how they are applies.
    Inverse time relates to a thermal characteristic that a breaker has. These breaker have commonly used a bimetallic element that bends to hit a trip release when it is heated. With a low overcurrent thy heating is low the element bends slowly. The more overcurrent the more heating the faster it bends. Thus I2t or inverse time. These breakers also have and instantaneous or magnetic element which response to high magnitudes of current such as one would have from a short circuit condition. On small residential breakers this magnetic pickup ay be calibrated at around 6-7x the rating of the breaker and others to 10x. Commercial and industrial (thermal magnetic)TM breakers are calibrated commonly at 10x +-20%. All of these breakers are listed UL489 as stand alone devices.
    The instantaneous (magnetic only)breakers have no thermal protection and will not protect form an overcurrent, not even for themselves. As such if these beakers experience an overcurrent they can be severely damaged. They only respond to instantaneous currents such as short circuits. The most common use of these breakers is in combination with motor starters. A motor starter is made of a contactor and overload relay. The contactor is completely stupid and provides no protection for the motor, the motor circuit nor itself. The overload relay provides motor overload protection as well as cable overload protection as long as the cable is size properly.
    Remember that the "motor starter" still does not have short circuit protection and, as such are commonly rated with a withstand of 5ka.
    Add the instantaneous mag only breaker provides the short circuit capability. When added to the motor starter it is now called a combination starter.
    Instantaneous mag only breakers are component listed "reverse UR" breakers and ARE NOT STAND ALONE DEVICES. These breakers are commonly known as "motor circuit protectors (MCP). When tested in combination with a motor starter in a listed enclosure the assembly now can get an interrupting rating. It should be noted that this interrupting label is affixed to the enclosure.
    These breakers also have and adjustable magnetic pickup to coordinate with the motor "inrush" to provide as close of short circuit protection as possible but yet not nuisance trip when the motor is energized. The maximum setting of this device is referred to in NEC art 430-52.
    It should also be noted that the magnetic elements in the TM breaker and the mag only breaker are often constructed differently. Smaller mag only breakers commonly have a coil of wire that forms a solenoid. This coil will be smoked and the breaker destroyed if it is subjected to overcurrent. This often occurs when an unqualified person tries to "test" this device. Testing current CAN NOT BE RAMPED to determine the trip point as you would a TM breaker.

  4. #4
    Join Date
    Jan 2004
    Posts
    6

    Re: Inverse VS Instantaneous Circuit Breakers

    Thanks to Roger and Templdl for the good information in regards to my question!

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