Q:
Fuse
I²t
Ratings, Melting Point and Breaking Ratings
The purpose of this rating is to ensure that the heat generated on the fuse during a surge does not have enough time to thermally conduct from the fuse to the external circuitry. Once the measurements of current (I) and time (t) are determined, the melting point
I²t
can be simply calculated
. When the melting stage is complete, an arc occurs immediately before the fuse element "breaks".
The higher the melting point (I²t
)
, the longer the fuse will operate. When considering a fuse's ability to withstand a transient overcurrent event without unexpected opening or "nuisance tripping," it is important to understand the nature of the transient event and the energy it contains. Depending on the expected characteristics of the transient event, each transient pulse will have an associated energy or I²t
value
. In order for a fuse to withstand this transient energy repeatedly without opening, its melting point I²t
must
be higher than the
I²t
value
of the transient pulse
.
Electrical pulses can affect fuse life by creating thermal cycling and possible mechanical fatigue. Therefore, it is important to understand a fuse's pulse cycle withstand capability, which is defined as the number of pulses for a given
value of
I²t
that the fuse can withstand without opening, assuming sufficient cooling time between pulses.
This is different from a fuse's breaking capacity or breaking rating, where these are the maximum amount of current a fuse can handle at a specific voltage before causing physical damage to the fuse body, exploding, or creating an excessive arc upon opening. If you've ever seen a glass fuse break, it means the energy has exceeded the fuse's ability to open. Sometimes worse, if the breaking capacity is too low, the fuse can shatter or explode. This can cause danger to people and the surrounding environment, especially if flammable gases are present.
As with most electronic components, there are derating factors such as the temperature of the application. The current carrying performance of the fuse is tested at 25˚C and will be affected by ambient temperature changes. At higher ambient temperatures, the fuse will respond more quickly to a given overcurrent condition. Conversely, at lower ambient temperatures, the fuse responds more slowly to a given overcurrent condition. Additionally, the temperature of the fuse increases as the current carried by the fuse approaches or exceeds the fuse's rating. Different fuses will have different rating curves depending on construction. Some fuses have minimal changes in fuse ratings over a wide temperature range, while other fuses vary significantly within a given temperature range.
For more technical information on
fuses
, see the following:
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