To be realistic the inductance and the resistance of the string is taken into account as follows.
• Inductance. All connecting wires used in the test set-up set inductances, which slow down rise time of current available for the secondary arcing. It is important that they are representative of the flight configuration.
• Resistances effects. The insertion of a serial resistance, during laboratory tests, in the loop of the secondary arcing allows sustaining the arc. It avoids under damping in the oscillating circuit and consequently avoids stopping the secondary arcing. (Its increase R with respect to L). However, to obtain a representative simulation of the flight configuration, it is important that this resistance is carefully chosen in order to have realistic behaviour.
In the real case, there are two serial resistances; the arc resistance (value is typically some Ohms) and the string serial resistance whose value is lower. In fact, without any more information we consider that it is better not to add any resistance and let the arc develop its own resistance. When the string dumping conditions are well known, they can be simulated by inserting a pertinent serial resistance.
The following lines describe the effect of total resistance R on the sustaining conditions. Z is impedance, L is inductance and C is capacitance.
Over damping condition exists if
If there is an added resistance to the discharging circuit, it also affects the operating point on the discharge curve.
In conclusion, it is important to let the arc disconnect if it is realistic.
Figure E-8: Discharging circuit oscillations
Figure E-9: Effect of an added resistance in the discharging circuit (SAS + resistance)