Overcurrent Protection

Basic Electricity

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  • Question 1

    What hazards might be posed by a circuit with excessive current going through its conductors (excessive as defined by the conductors’ ampacity)? In other words, what would be bad about a wire carrying too much current?

    Also determine which type of component fault, an open or a short, would most likely be the cause of excessive current in a circuit.

    Reveal answer
  • Question 2

    Explain the construction and purpose of an electrical fuse.

    Reveal answer
  • Question 3

    What is the difference between a fuse and a circuit breaker?

    Reveal answer
  • Question 4

    Show the proper placement of the fuse in this circuit, where an electric motor will be powered by utility (120 volt AC) power:

    Reveal answer
  • Question 5

    Are fuses and circuit breakers located at a power distribution panel rated to protect the wiring from overcurrent, or to protect the load devices from overcurrent? Explain your answer.

    Reveal answer
  • Question 6

    In an effort to obtain greater overcurrent ratings than a single fuse can provide, an engineer decides to wire two 100 amp fuses in parallel, for a combined rating of 200 amps:

    However, after a few years of operation, the system begins blowing fuses even when the ammeter registers less than 200 amps of load current. Upon investigation, it is found that one of the fuse holders had developed corrosion on a terminal lug where one of the wire connects:

    Explain how a small accumulation of corrosion led to this condition of fuses blowing when there was no overcurrent condition (load current less than 200 amps), and also why connecting fuses in parallel like this is generally not a good idea.

    Reveal answer
  • Question 7

    Two 150-amp circuit breakers are connected in parallel to obtain a total ampacity of 300 amperes for an electric motor service. The system works just fine for several years, but then both breakers begin to spuriously trip:

    An electrician measures motor current using a clamp-on ammeter, and discovers the motor’s current is no more than 228 amperes at full mechanical load. Describe what might possibly be wrong that is causing both circuit breakers to trip.

    Reveal answer
  • Question 8

    A large industrial electric motor is supplied power through a pair of fuses:

    One day the motor suddenly stops running, even though the switch is still in the “on” position. An electrician is summoned to troubleshoot the failed motor, and this person decides to perform some voltage measurements to determine whether or not one of the fuses has “blown” open before doing anything else. The measurements taken by the electrician are as such (with the switch in the “on” position):

    • Between A and ground = 120 volts AC
    • Between B and ground = 120 volts AC
    • Between C and ground = 120 volts AC
    • Between D and ground = 120 volts AC

    Based on these measurements, the electrician decides that both fuses are still in good condition, and that the problem lies elsewhere in the circuit. Do you agree with this assessment? Why or why not?

    Reveal answer
  • Question 9

    Explain the meaning of this fuse curve:

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  • Question 10

    Magnetic circuit breakers trip by the action of an electromagnet coil, through which all the load current passes. When the attractive force of the magnetic field is strong enough, a mechanism triggers to snap the breaker contacts open, thus interrupting the circuit current.

    What would the time-current curve for such a circuit breaker look like?

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  • Question 11

    Shown here is the time-current curve for a dual-element fuse. Thermal-magnetic circuit breakers exhibit similar time-current curves:

    Based on this curve, what do you think the purpose of a “dual-element” fuse or “thermal-magnetic” circuit breaker is? Why would this style of overcurrent protection device be chosen over a “normal” fuse or circuit breaker?

    Reveal answer
  • Question 12

    In addition to possessing a primary current rating, fuses and circuit breakers also possess an interruption current rating, usually far in excess of their primary ratings. For example, a typical 15 amp circuit breaker for 120 volt residential use may have an interruption rating of 10,000 amps (10 kA)! Under what conditions could such a circuit ever bear so much current, and why is this rating different than the breaker’s primary current rating of 15 amps?

    Reveal answer
  • Question 13

    Find one or two real fuses and bring them with you to class for discussion. Identify as much information as you can about your fuses prior to discussion:

    • Current rating
    • Voltage rating
    • Interruption rating
    • Fuse curve (opening characteristics: fast-acting, slow-blow, etc.)
    • Status of fuse (blown or not)
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