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I have a small pump rating at 96 Watt, and the input is 12V DC, max current 8A. The product also comes with a 250V 8A glass fuse. Because the pump is 8A and the fuse is also 8A, does that mean the device will draw 8A current flowing through fuse when working on full loading. Will the fuse be always hot and can easily blow when the device is continuously working for a long time?

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    \$\begingroup\$ Hard to say. Ideally, a manufacturer would choose an appropriate fuse for the pump. Can you measure the actual normal current used by the pump in your conditions? Maybe it is quite a bit less than 8A, in which case, maybe there is nothing to worry about. \$\endgroup\$ – mkeith Sep 26 '17 at 3:57
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Like all electronic components, fuse:

  • they must be derated.
  • have their own lifetime (especially as a function of the overstress - transients).

For instance, littlefuse recommends a 25% derating:

  1. NORMAL OPERATING CURRENT: The current rating of a fuse is typically derated 25% for operation at 25ºC to avoid nuisance blowing. For example, a fuse with a current rating of 10A is not usually recommended for operation at more than 7.5A in a 25ºC ambient. For additional details, see RERATING in the previous section and AMBIENT TEMPERATURE below.

Operating temperature is also an important factor. The higher the ambient temperature, the hotter the fuse will be, the sooner it will melt.

Repeated surge also will determine the fuse lifetime, as reported in this article (J. McLinn, "The Simple Fuse", IEEE Reliability Society 2008 Annual Technology Report.).

enter image description here

The higher the current spike, the smaller the number of surges the fuse will withstand.

This is the case of your pump: despite it's rated at 8A continuous load, it will probably have a larger inrush current.

By the way, remember that if the current of your load has large high-frequency components, the equivalent fuse series resistance will be larger, due to skin effect (i.e. it will likely blow earlier at the same RMS component). This might not be your case though. (It's rather a problem if you PWM a load, like an heater).

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A fuse, depending on it's characteristics can conduct a higher currency over shorter times, depending on load. Inductive and capacitive loads have high startup currency spikes for a short period.

And according to ref. [6] in quoted Wikipedia article below, the answer should be yes with a BUT.

Ref.: https://en.wikipedia.org/wiki/Fuse_(electrical)

Rated current IN

A maximum current that the fuse can continuously conduct without interrupting the circuit.[6]

Speed

The speed at which a fuse blows depends on how much current flows through it and the material of which the fuse is made. The operating time is not a fixed interval, but decreases as the current increases. Fuses have different characteristics of operating time compared to current. A standard fuse may require twice its rated current to open in one second, a fast-blow fuse may require twice its rated current to blow in 0.1 seconds, and a slow-blow fuse may require twice its rated current for tens of seconds to blow.

Fuse selection depends on the load's characteristics. Semiconductor devices may use a fast or ultrafast fuse as semiconductor devices heat rapidly when excess current flows. The fastest blowing fuses are designed for the most sensitive electrical equipment, where even a short exposure to an overload current could be very damaging. Normal fast-blow fuses are the most general purpose fuses. The time delay fuse (also known as anti-surge, or slow-blow) are designed to allow a current which is above the rated value of the fuse to flow for a short period of time without the fuse blowing. These types of fuse are used on equipment such as motors, which can draw larger than normal currents for up to several seconds while coming up to speed.

Manufacturers can provide a plot of current vs time, often plotted on logarithmic scales, to characterize the device and to allow comparison with the characteristics of protective devices upstream and downstream of the fuse.

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  • \$\begingroup\$ That is an awfully long quote from wikipedia. I don't think you will find many fuses that blow in 100 ms at 2x rated current. \$\endgroup\$ – mkeith Sep 26 '17 at 5:25
  • \$\begingroup\$ Feel free to edit it to improve it. \$\endgroup\$ – MatsK Sep 26 '17 at 5:46
  • \$\begingroup\$ Some vigilant person will just revert my edit. I have given up on editing wikipedia. \$\endgroup\$ – mkeith Sep 26 '17 at 5:48
  • \$\begingroup\$ I was thinking about editing my answer... \$\endgroup\$ – MatsK Sep 26 '17 at 5:50
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    \$\begingroup\$ This came up recently in electronics.stackexchange.com/questions/316321/… For 210% load, the maximum times are measured in minutes, not milliseconds. ( though the minimum time will be much faster) \$\endgroup\$ – Brian Drummond Sep 26 '17 at 8:47

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