Chat with AIExam 18: Short-Circuit Calculations and Coordination of Overcurrent Protective Devices
What is the intent of NEC 110.9 and 110.10?
(30) 
VerifiedThe student must understand the intent of NEC 110.9 and 110.10.That is, ensure that the fuses and/or circuit breakers selected for an installation are capable of interrupting the current at the rated voltage under any condition (overload, short circuit, or ground fault) with complete safety to personnel and without damage to the panel, load center, switch, or electrical equipment in which the protective devices are installed.
An overloaded condition resulting from a miscalculation of load currents will cause a fuse to open or a circuit breaker to trip in a normal manner.However, a miscalculation, a guess, or ignorance of the magnitude of the available short-circuit currents may result in the installation of breakers or fuses having inadequate interrupting ratings.Such a situation can occur even though the load currents in the circuit are checked carefully.Breakers or fuses having inadequate interrupting ratings need only be subjected to a short circuit to cause them to explode, resulting in injury to personnel and serious damage to the electrical equipment.The interrupting rating of an overcurrent device is its maximum rating and must not be exceeded.
The amount of short-circuit current from a motor is equal approximately to the starting ( ) current for each motor.
(33) VerifiedB
A situation known as occurs when a fault on a branch circuit opens not only the branch-circuit overcurrent device but also the feeder overcurrent device.
(32) VerifiedA
To meet the requirements of NEC 110.9 and 110.10, it is absolutely necessary to determine the available fault currents at various points on the electrical system.If a study is not done, the selection of overcurrent devices may be in error, resulting in a hazard to life and property.
(40) The actual impedance value on a UL-listed 25-kilovolt-ampere or larger transformer can vary plus or minus percent from the transformer's marked impedance.
(37) What is the importance of selective coordination? What guidelines cover it?
(39) The NEC includes a requirement that the short-circuit current that is available at the service be field marked at or on the service equipment.
(38) To obtain the approximate short-circuit current values when two hot conductors of a 3-phase system are shorted together, use percent of the 3-phase current value.
(37) What is the approximate line-to-ground arcing fault value on a 480/277-volt system where the line-line-line fault current has been calculated to be 30,000 amperes?
(43) The amount of available short-circuit current decreases as the from the transformer increases.
(39) The transformer multiplier M2 is calculated by the formula .
(41) If noncurrent-limiting overcurrent devices (standard molded-case circuit breakers) are to be installed in the main switchboard, breakers having adequate must be installed in the panelboards.
(40) The value of the square root of three ( ) is used frequently in 3-phase calculations.
(51) A simple method of determining the available short-circuit currents (also referred to as fault current) at various distances from a given location is the method.
(35) power systems are power systems that are generally installed in vital infrastructure facilities that, if destroyed or incapacitated, would disrupt national security, the economy, public health or safety, and where enhanced electrical infrastructure for continuity of operation has been deemed necessary by governmental authority.
(34) 
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