# Voltage peaks after opening of a contactor in an inductive circuit

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Iván Torres Játiva on 26 Dec 2023
Hello everybody,
I have been analysing a system where the power supply is a 1000 V battery. This battery has a contactor that controls the discharge circuit. This circuit connects to an inverter, which in turn connects to a motor. The cable between the battery and the inverter is of considerable length, and simulating the opening of the contactor (necessary in an emergency situation) produces very high voltage peaks, in the order of tens of kV for a very short period of time. My question is, is this possible or is my simulation wrong? What can cause a voltage spike of this magnitude during such a short period of time?
I add the circuit diagram, with the parasitic inductance of the cable and the parasitic capacitances of the cable:
With this inductance, the peak voltage that should be produced (VL = L * dI/dt), taking into account that the contactor used (according to tests carried out) takes 50 ns to cut off the current completely and the rated current is 500 A, is 17.2 kV. Is this possible?
Furthermore, in the simulations performed, the voltage peaks are even higher.
Best regards and thanks,

David Goodmanson on 27 Dec 2023
Edited: David Goodmanson on 28 Dec 2023
Hello Ivan,
I'm not sure how you obtain 17.5kV from 500A and 50ns, and 50ns seems really fast if the contactor is mechanical, but regardless, voltages in the kV range can occur quite easily by opening a switch in series with an inductor. Usually what happens is that you get air breakdown (~3kV/mm) and arcing across the slightly open switch contacts , which limits the voltage considerably but shortens the operating life of the contacts. A capacitor in parallel with the switch is ofter used to eliminate or reduce the arcing.
Iván Torres Játiva on 28 Dec 2023
Hi David,
The formula I use to calculate the voltage peak analytically is dV = L * dI/dt. My main question is whether such high voltage peaks during such a small period of time are really possible and if they are a big problem for the system, or on the contrary as the time interval is very small it is not a problem.
As a solution to the problem a capacitor could be used, as you have indicated. I had also thought of an anti-parallel diode or a dV/dt filter.
Best regards and thanks for your help

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