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Circuit Breaker Large Over Current Time Delays

December 29th, 2010

In MIDWEST’s training classes for qualified personnel, there is a segment where we explain the long time delay range within which a Square D 1000 amp circuit breaker should trip due to an overload. This information is received with anything from amazement to skepticism to outright disbelief, even though we show the Square D circuit breaker characteristic trip curve.  The overload time delay information is not restricted to Square D circuit breakers. It’s the same with Cutler Hammer, GE General Electric, Siemens, ITE, Westinghouse, Merlin Gerin, or Federal Pacific circuit breakers.

 

In our training example we use an old Square D 1000 amp MA type circuit breaker.  If we tested this circuit breaker at 3000 amps, that’s 300%, the minimum to maximum trip range is about 45 seconds to 340 seconds. It might trip in 45 seconds or it might not trip for 340 seconds.  This is an old thermo-magnetic circuit breaker, which typically works by heating a bi-metal in the over current trip device. Many newer breakers use electronic over current devices which have more repeatable overload time delay test results.

 

The illusion is that these Square D, Cutler Hammer, Westinghouse circuit breakers are designed to directly protect people. They are not. The breakers protect the equipment connected to them and they protect the electrical system. They are designed for the characteristics of the equipment connected, such that connected equipment will not be damaged by an overload or fault. This is a basic limited explanation. So, when you think of molded case circuit breakers, power circuit breakers or air circuit breakers, it’s important to know these breakers don’t just trip right at the breaker trip device rating.       

  1. Nathan
    February 11th, 2012 at 07:09 | #1

    Looking at table for GE values for Overcurrent Trip Test at 300 percent brker rated current. GE has columm Maximum trip with values, but the defindition reads “These values are based on Heat test on conductors in conduit”. Not understanding Meaning? If anyone can explain would help. Thanks.

  2. Nathan
    February 11th, 2012 at 07:18 | #2

    GE breaker question non-adjustable

    Reproduction of table 5-4 from NEMA publication AB4-1996

    Table and explaination below it seems to say two diffent things. Table shows trip range +25%–High side, -25%–Low side.
    The explaination below that saids +40%–High side, -30%–Low side.

    Can someone explain the reason that theres a differance?

  3. Nathan
    February 11th, 2012 at 07:20 | #3

    Nathan :Looking at table for GE values for Overcurrent Trip Test at 300 percent brker rated current. GE has columm Maximum trip with values, but the defindition reads “These values are based on Heat test on conductors in conduit”. Not understanding Meaning? If anyone can explain would help. Thanks.

    Nathan :GE breaker question non-adjustable
    Reproduction of table 5-4 from NEMA publication AB4-1996
    Table and explaination below it seems to say two diffent things. Table shows trip range +25%–High side, -25%–Low side.The explaination below that saids +40%–High side, -30%–Low side.
    Can someone explain the reason that theres a differance?

  4. MIDWEST
    February 13th, 2012 at 15:00 | #4

    The manufacturers have a very specific setup for the “overcurrent trip tests” on their protective circuit breakers. These tests are performed in a controlled laboratory environment. The manufacturers take into account heat that may be transferred from connected equipment, ie cables, to the circuit breaker under test. If the circuit breaker overcurrent device is thermomagnetic, this heat will have some impact on the trip time. Service and maintenance companies do not duplicate exactly the mfg’s lab setup. It is not practical in the maintenance world. In addition, the difference does not result in a dramatic difference in test results, relative to the real world operation of the equipment. The key point is to recognize the large range of overcurrent trip times possible for circuit breakers. A particular breaker may ‘cook’ for a long time before it trips on an overload.

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