Practical Evaluation of a Power Fuse Interrupting Capacity
The selection of the short-circuit rating of power fuses and low voltage circuit breakers are treated differently than medium and high voltage circuit breakers. According to ANSI/IEEE standards, these protective devices respond to short-circuit instantaneously which means that at the initiation of the fault, fuses or low voltage circuit breakers should not only be able to withstand the magnetic and thermal forces resulting from the magnitude of the short-circuit current but should also be capable to safely interrupt the available short-circuit duty.
A distinction between fused and unfused low voltage circuit breakers with regards to the evaluation of their short-circuit ratings is specified in IEEE C37.13. For fuses and fused low voltage circuit breakers, the evaluation is based on the total asymmetrical RMS first cycle currents whereas for unfused low voltage circuit breakers, the basis of evaluation is the peak first cycle currents.
Furthermore, like symmetrically rated medium and high voltage circuit breakers, fuses and low voltage circuit breakers already has an embedded asymmetry factor based on 20% test power factor (X/R ratio of 4.9) for fuses and fused low voltage circuit breakers and 15% test power factor (X/R ratio of 6.6) for unfused low voltage circuit breakers. The test power factor, however, may be higher than the test power factors specified in IEEE C37.13. For this reason, it is best to consult with the device manufacturer for the technical data specifications.
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In this post, an evaluation of the short-circuit rating of an existing power fuse is presented in a step-by-step procedure based on ANSI/IEEE standards.
Power Fuse Specifications
- Max. Rated Voltage: 15kV
- Rated Frequency: 60Hz
- Rated Continuous Current: 18A
- Rated Short Circuit Interrupting Current: 20kA RMS, Symmetrical
- Test PF: 6.7%
Maximum Available Short Circuit Current
Like low voltage circuit breakers, fuses operate instantaneously on fault thus the 0.5 cycles symmetrical short circuit current is used in calculating the interrupting duty. The fault level information may be requested from the local electrical distribution utility.
I’sym = 12.786kA
with an X/R ratio of
X/R = 9.683
Calculation of Interrupting Duty
The interrupting duty for fuses is calculated by introducing a multiplying factor based on the total asymmetrical RMS first cycle current as shown below.
The calculation of the multiplying factor is very important in order to estimate the maximum interrupting duty of the power fuse. However, since the fault point X/R ratio is lesser than the power fuse test X/R ratio (derived from test PF), calculating the multiplying factor may not be necessary. In this case, the maximum available short circuit current can be used directly to determine the power fuse short circuit interrupting rating.
To illustrate this, let us evaluate the multiplying factor using the fault point X/R ratio and the fuse test PF.
From the fuse technical data specification on the test PF, we can calculate the test X/R ratio.
From this, we can calculate the multiplying factor and ultimately the total asymmetrical RMS first cycle current.
The above calculation confirms that for a fault point X/R ratio that is less than the protective device test X/R ratio, the maximum available short circuit current can be used directly to determine the power fuse short circuit interrupting rating.
In the application example, the maximum available short circuit current of 12.786kA is lesser than the fuse rated short circuit interrupting current 20kA.
References
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