Semiconductor Fuses: Terms and Explanations

Posted on 18 April 2012

 

Below is a guide to terms commonly used to characterize semiconductor fuses.

Rated voltage VN

Maximum permissible voltage with maximum 10% permissible short-time increase. If the rated voltage is an AC voltage, this will be specified as an effective value. This applies to a sinusoidal voltage of 50 - 60 Hz. For non-sinusoidal voltage, neither the effective nor the crest value of the operating voltage may exceed the rated voltage or their sqrt(2) value.

Rated current IN

Maximum permissible operating current with which the fuse mount may be continually subject to without changes that may be detrimental to its function.

Rated breaking capacity IPM

Maximum prospective current that a fuse can turn off under certain conditions (e.g. recovery voltage).

Let-through current (of a fuse) I(LT)

This refers to the maximum instantaneous current reached during fuse operation, if this turn-off prevents the prospective current from reaching its maximum value (Figure 1). Note that this value for current has nothing to do with the forward current of a rectifier diode or thyristor.

Prospective current IP

Current which occurs if one imagines the fuse being replaced by an impedance-free conducting connection (dotted line in Figure 1).

Prospective short-circuit current IPS

Effective prospective current under short-circuit conditions directly behind the fuse. This value might be needed to determine the precise arcing i²t value.

Pre-arc time tpa

This refers to the time range between the onset of a current that is large enough to melt the fusible element and the emergence of the arc (Figure 1).

Arcing time ta

This refers to the time lapse between the generation of the arc and its (final) extinction.

Operating time top

This is the sum of the pre-arcing time and the arcing time.

Current characteristic

Figure 1. Current Characteristic for short-circuit "turn-off" by a semiconductor fuse with current clamping (let-through current smaller than the peak prospective current); Pre-arc time + arcing time = operating time

 

Time/current characteristic

Curve of the virtual pre-arc time or operating time as a function of the effective prospective current under certain conditions. This normally applies to 20°C initial temperature.

Pre-arc i²t value (i²t)pa

Integral of the square of the prospective current over pre-arc time:

(i^2t)_{pa} = \int_{t_0}^{t_{pa}}I_P^2dt

This depends on the initial temperature and the prospective short-circuit current. It is not time-dependent for pre-arc times of less than 10 ms.

Extinction i²t value (i²t) a

Integral of the square of the prospective current over arcing time:

(i^2t)_a = \int_{t_{pa}}^{t_a}I_P^2dt

This depends on the recovery voltage, the prospective current, and the power factor.

Operating i²t value (i²t)op

This is the sum of the pre-arc i²t and the arcing i²t values.

Virtual time t ,tva, tvop

Resulting time if an i²t value is divided by the square of the prospective current, e.g.:

t_{va} = \frac{(i^2t)_a}{I_P^2}

Virtual values for pre-arc time, arcing time (extinction time) and operating time are obtained in accordance with the various i²t values.

Switching voltage VaM

Switching voltage refers to peak voltage which occurs at the terminals of the fuse during fuse operation. This depends on the operating voltage and the power factor of the electric circuit. The quicker a fuse "clears", the higher the switching voltage.

Recovery voltage VWRMS

Voltage that occurs at the terminals of a fuse once the current has cut off.

Current limiting

Short-circuit current cut-off using a fuse, where the let-through current is smaller than the peak prospective short-circuit current. Whether current limiting takes place depends not only on the properties of the fuse but also on the amount of prospective short-circuit current IPS , as well as on the operating frequency. For small IPS, the pre-arc time is longer than a quarter of an oscillation; likewise for operating frequencies greater than around 100 Hz. In this case, current limitation cannot happen.

 

For more information, please read the following articles:

Introduction to Semiconductor Fuses

Semiconductor Fuses: General

Parallel and Series Connection of Fuses

Dimensioning Semiconductor Fuses

 

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