I am working with a high voltage relay (Cynergy3, DBT72410) to discharge a 70 uF capacitor charged to 4 kV using a string of resistors and a relay to ground. I have placed a resistor between one contact (low side) and ground. I am trying to decide if a capacitor connected between the other contact (high side) and ground is a good idea or not.
The thought process is as follows:
With the cap in place, the high side contact sits at whatever the main capacitors voltage is. So when it switches, the voltage difference between the contact and the signal is ~0. This will minimize arcing, but it produces inrush current which would be infinite without the low side resistor (it limits the current to about 8 A).
Without the cap, the high side contact is close to ground. So the voltage between the contact and the switched signal is the full cap voltage (up to 4 kV). This will eliminate the inrush current, but create arcs during the switch over.
Both are not good for the relay, but which is worse?
The power supply is off when the output cap (C2) is charged. The cap I am asking about is C1. During charge C1 is also charged to 4kV. When the relay closes, C1 is at 4kV, so the high side contact is also at 4kV, so no arcing. During relay chatter, the cap is continually recharged to the output cap voltage through the resistor stack (~60uS), as long as the open period of the chatter is greater than 60uS. This will continually keep the upper contact at the output cap voltage.
R1 always limits the current through the relay to no more than 8.5A. The carrying current of the relay is 2A, so inrush is limited to 8.5A for 60uS. Which I think the relay should handle with no problem.