###### Low and medium voltage electrical components of the electrical parameters, many are similar, such as short-time withstand current Icw, are required to switchgear in the case of short-circuit faults, due to the circuit breaker tripping time required or faults caused by the absence of tripping, or sectional protection needs to be switched to withstand a certain amount of time of the short-circuit current, and therefore there is a parameter requirement of the Icw.

Icw includes two parts, one is to withstand the rated short-circuit current rms (rms), such as medium voltage short-time withstand current 31.5, 40, 50kA, etc., low-voltage short-time withstand current 50, 63, 80, 100, 150kA, etc., and the other is to be able to withstand the time, the low-voltage general requirements of the 1 second, the medium-voltage requirements of the 3 / 4 seconds.

This is based on the protection with the need to develop, low-voltage at the end of the line, near the short-circuit source, the current current, the protection logic requires as soon as possible to cut off the power supply, the time is short, the current is large. The medium voltage is far away from the fault, in front of the protection circuit breaker, so the withstand time to be long, with the rapid decay of the short-circuit current, the medium-voltage section required to withstand the short-circuit current is also small, for some special applications such as offshore oil platforms, generators and loads are very close to the cable length is short, so the short-circuit current is large, the requirement of short disconnection time, in order to protect the safety of the equipment.

Short time withstand is more on the conductor thermal effect of the consideration, such as the conductor cross-sectional area, the contact size of the contact pressure, etc., for the cross-sectional area can be calculated in accordance with the GB/T3906 appendix minimum cross-sectional area, for the contact, can be calculated in accordance with the company’s resistance, which in turn leads to a short-circuit current generated by the voltage drop, such as the voltage drop of > 0.41V, then the contact part of the contact may melt welding, that is, can not withstand the short time withstand current.

What is contained in the short-circuit current? The first is the frequency component, this part is the main force of the short-circuit current, if the power grid to think of an infinite, the final short-circuit current of the frequency component will ultimately remain constant; the second part is the harmonic component, asymmetric component, this part of the short-circuit current in the frequency is not the part of the 50Hz, due to this part of the ratio is very small, generally do not go to study it.

What is the DC component? We know that the short circuit occurs instantly, the short-circuit current will reach the peak value, and then slowly decay to a constant value. Why is the beginning of a very large back very small it, the most important reason is due to the industrial frequency short-circuit current superimposed DC component, simply put is the industrial frequency sinusoidal wave by the DC component to the “elevated”. Then where the DC component, it is very simple, along the peaks and valleys of each do an envelope, and then look for the middle of the two that is the DC component, you can see that the DC component is in constant attenuation. DC component attenuation is complete, the short-circuit current will be restored to the steady state value, that is, we usually call the “cyclic component”, then the part before the cyclic component, we usually call the “non-cyclic component”. The first half-wave of the short-circuit current has a maximum value, the peak value of the short-circuit current, Ipk, which is often used to check the dynamic stability of electrical equipment.

Therefore, the short-time withstand current corresponds to the peak withstand, that is, need to withstand the short-circuit caused by the maximum value of the current value of the side of the bias, for the low-voltage system, the peak withstand current is 2.2 times the short-time withstand current, that is, the short-time withstand current of 100kA RMS value, the corresponding peak withstand current is 220kA, and for the medium-voltage system, the peak current and the frequency related to the 50Hz is 2.5 times, 60Hz is 2.6 times, that is, for the 50Hz is 2.6 times, that is, for the 50Hz is 2.5 times, 60Hz is 2.6 times. 2.6 times, i.e. for 50kA short-time withstand current RMS value, the peak withstand current in 50Hz application environment is 125kA, while the application in foreign 60Hz countries needs to reach 130kA, which is a test for the mechanical endurance of switchgear. For special applications such as generator outlets, the peak withstand current requirement is 2.74 times, i.e. 50 kA requires a peak withstand current of 137 kA, due to the huge peak short-circuit current caused by a short-circuit in the power supply measurement.

Peak withstand that is, dynamic stability, the mechanical stress caused by electric force, that is, the support of the copper row, contacts, etc. by the deformation caused by electric force does not collapse.