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Glossary of Current Fuse

  • Fuse-link
A Fuse-link in which the Fuse-element is totally enclosed, so that during operation within its rating it can not produce any harmful external effects, e.g. due to development of an arc, the release of gas or the ejection of flame or metallic particles.
  • Fuse-element
A part of the Fuse-link designed to melt when the fuse operates.
  • Ambient Temperature
Refers to the temperature of the air immediately surrounding the fuse. The current carrying capacity tests of fuses are performed at 25 and will be affected by changes in ambient temperature. The higher the ambient temperature, the hotter the fuse will operate, and the shorter its life. The chart below shows typical ambient temperature effects on current carrying capacity of SetFuse products.

  • Fuse Characteristics
Also known as Blowing Characteristics, this characteristic of a Fuse-link design refers to how rapidly it responds to various current overloads. Fuse Characteristics can be classified into three general categories: Very fast-acting, fast-acting, or time-delay (or time-lag, Slo-Blo) fuse.
  • Current Rating(In)
The nominal amperage value of the fuse, also known as Amperage Rating. It is established by the manufacturer as a value of current which the fuse can carry, based on a controlled set of test conditions.
  • Voltage Rating(V)
The voltage rating, as marked on a fuse, indicates that the fuse can be relied upon to safely interrupt its rated short circuit current in a circuit where the voltage is equal to, or less than, its rated voltage.
  • Interrupting Rating(A)
Also known as Breaking Capacity, or Short Circuit Rating, this is the maximum approved current which the fuse can safely break at Rated Voltage.
  • Cold Resistance(mΩ)
Refers to the resistance obtained using a measuring current of no more than 10% of the fuse’s nominal rated current.
  • Hot Resistance(mΩ)
Hot Resistance is the resistance calculated from the stabilized voltage drop across the fuse, with current equal to the nominal rate current flowing through it.
  • Voltage Drop(mV)
Voltage Drop is the reduction in voltage while applying the rated voltage to the Fuse-link. The voltage drop across the Fuse-links at their rated current shall not exceed the maximum values given on the relevant standard sheet. Individual values shall not deviate from the mean value determined for the model under test during type tests by more than 15%.
  • Time-Current Characteristics
Also known as Time-Current Curves.
  1. For A.C.: A curve giving, under stated conditions of operation, the value of time expressed as virtual time as a function of the prospective symmetrical current, expressed as the R.M.S. value.
  2. For D.C.: A curve giving, under stated conditions of operation, the value of time expressed as actual time as a function of the D.C. prospective current.
Time-Current Characteristics usually stated for a Fuse-link relate to the Pre-arcing Time and Operating time. The following Figure offers an example.

  • Pre-arcing Time
Also Known as Melting Time, the interval of time between the beginning of a current large enough to cause a break in the Fuse-element and the instant when an arc is initiated.
  • Arcing Time
The interval of time between the instant of the initiation of the arc and the instant of final arc extinction.
  • Operating Time
Or Total clearing time, refers to the sum of the pre-arcing time and the arcing time.
  • Surge and Pulse Current Characteristics
Transient surge or pulse currents are used to describe wave shapes that result from any startup, inrush, surge, or transient currents in a circuit. It is therefore important to size the fuse properly to allow these pulses to pass without nuisance openings or degradation of the fuse element. The fuse must then open within the limits specified by UL and CSA if the overload condition continues. The ability to resist surges is a function of the fuse design and/or classification relative to the surge pulse, duration frequency etc.
Pulse currents can produce thermal energy that may not be large enough to open the fuse but could possible cause element fatigue and decrease the life of the fuse. To properly size a fuse and determine its surge withstand capability, the circuit’s pulse energy should be determined and compared to the time current curve and I²t rating of the fuse. The fuse’s melting I²t value must be greater than or equal to the pulse I²t multiplied by a pulse factor.
The peak current and decay time define the pulse current characteristic or waveform. Pulses can generate different waveform shapes, which determines the formula used to calculate the pulse energy or I²t. Refer to Table below to select the appropriate waveform and its corresponding pulse I²t calculation.