Let-through current

From a manufacturer s data sheet a 125 A MCCB has a let-through current Ip of 25 kA gat for a prospective fault current C of 100 kA m. ... 

The actual value of let-through current for a given fuse will depend upon the nature and magnitude of the prospective fault current e.g. asynunetrical or synunetrical. This is because a greater current has to be reached in the symmetrical case than in the asynunetrical case to create the same amount of melting energy. This is due to the shape of the current waveform in the first cycle, which can be seen in Figure 8.1. 

The maximum value of the let-through current is called the peak let-through current Ip. ... 

The importance of the peak let-through current is in relation to the thermal and mechanical stresses that occur in the downstream equipment e.g. contactors, cables. 

When cables are to be sized for a particular project with regard to their short-circuit performance it is necessary to consider the let-through current of the protective device in the circuit e.g., fuse, circuit breaker. It is also necessary to determine whether the consnmer has fixed equipment such as a motor, or temporary equipment such as a portable tool plugged into a socket, because this establishes the minimum time dnration. This aspect is described in more detail when the earth-loop impedance is being considered, see sub-section 9.4.3.6. 

It is therefore common practice to use 5 seconds in (9.7) as the disconnection time for motor cables. This choice also corresponds with standardised data given by the manufacturers of fuses and moulded case circuit breakers for their let-through current as calculated from their graphical data. 

This extremely high inrush current at a frequency of almost 3-7 kHz will release an enormous amount of let-through energy during contact making,... 

When the bridge is balanced, let the current through P be and through R be Since no current flows through the galvanometer, the current through Q and S must also be equal to ij and 2 respectively. Moreover, the potentials at B and D are equal... 

When contactors are backed up by fuses it is possible to reduce the fault levels considerations. The current versus let-through-time characteristics of the fuses need to be studied if the above fault levels are to be reduced. 

When high voltage motors are being considered, it is usually found that the minimum conductor size of the cable is determined by the let-through fault withstand capability rather than the full-load or starting current. Cable manufacturers provide graphical data for fault withstand capabilities of their cables, which are based on practical tests. These aspects are also associated with the protection system used for the motor, e.g. a contactor-fuse combination, a circuit breaker, the protective relay characteristics (thermal, inverse time with or without instantaneous or earth fault elements). 

Let us trace the flow of electric current through the cell. 

Influence on Electrolyte Conductivity In porous separators the ionic current passes through the liquid electrolyte present in the separator pores. Therefore, the electrolyte s resistance in the pores has to be calculated for known values of porosity of the separator and of conductivity, o, of the free liquid electrolyte. Such a calculation is highly complex in the general case. Consider the very simple model where a separator of thickness d has cylindrical pores of radius r which are parallel and completely electrolyte-filled (Fig. 18.2). Let / be the pore length and N the number of pores (all calculations refer to the unit surface area of the separator). The ratio p = Ud (where P = cos a > 1) characterizes the tilt of the pores and is called the tortuosity factor of the pores. The total pore volume is given by NnrH, the porosity by... 

It is well known that the flow of either the positive or the negative ions, or of both, is responsible for the electric conduction process through the body of the solution of an electrolyte. Let the fraction of the current carried by each ion in a given electrolyte be considered. For this purpose, the transference numbers mentioned in the previous section are introduced according to the definitions... 

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