A1-12 Fuses

• What is a fuse?

  overcurrent device designed to open the circuit should the current exceed the rating of the fuse. The fuse will melt open first and protect the circuit from damage
• Does a fuse have a high or low resistance?

  High (higher than the conductors it protects)
• The time it takes to melt and open the fuse is _____ proportional to the magnitude of the current

  inversely
• A fuse will open almost instantly on a ______ current, but more slowly on a ______ current.

  heavy fault, overload
• A fuse rated for 600V will be ____ than fuses rated for 250V

  longer
• What is a renewable link fuse?

  a replaceable fuse (unlike a one-time fuse)
• What is a disadvantage of a fuse?

  Has to be removed and replaced every time it opened
• What is a circuit breaker?

  an electromagnetic device that will open the electric circuit when amperage is exceeded without injury to the device itself and that can be used over and over
• what are plug fuses?

  screw-in fuses (also known as Edison-base fuses)
• circuit breakers may be ___ pole, ___ pole or ___ pole.

  single, double, three
• Are circuit breakers interchangeable amongst different manufactuers?

  No
• What are two types of trip elements in circuit breakers?

  -thermal trip element

  -magnetic trip element
• What is meant by trip-free with reference to a circuit breaker?

  It cannot be held in the closed position (it cannot be prevented from tripping by holding the handle ON)
• A circuit breaker is a ___ free device

  trip free. Someone can't just hold it on to stop it from tripping. They must fix the overcurrent or overload
• Does a fuse instantly open when the current exceeds its rated value?

  No, the amount of time it takes to break open the fuse is inversely proportional to the magnitude of current. A fuse will open instantly on a heavy fault current but not as fast on an overload current (too many devices causing too much current)
• What is an overcurrent / heavy fault current?

  A short circuit is created. It trips a lot faster than a over load current
• What is an overload current?

  Too much load on the circuit causes it to trip (too many devices), but it trips more slowly than an overcurrent
• What are 2 types of cartridge fuses?

  ferrule-contact and knife-blade contact
• What are the different types of fuses?

  Cartridge fuses, plug fuses, and renewable link fuses
• Circuit breakers can be operated by ____ or ____ action

  magnetic, thermal
• What are trip elements?

  Cause the circuit breaker to open when there is prolonged overload or excessive current magnitudes
• Which trip element saves you during overload?

  The thermal trip element (metals heat together, bend, and cause breaker to trip)
• What trip element saves you during overcurrent?

  the magnetic trip element
• What is an advantage and disadvantage of a renewable link fuse?

  +: cost saving, can be used repeatedly

  -: you can install a larger link, compromising the integrity of the installation (dangerous)
• What is the most common type of circuit breaker used today?

  moulded case CB