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For protection components such as Varistors and Diodes, when an instantaneous fault voltage is applied - how to do these devices protection before the device has time to conduct. For example, a Voltmeter rated to measure up to 1kV, if an instantaneous 5kV voltage is applied at the input terminals, will the overvoltage just smash everything if the input circuit is protecting. If the voltage applied is truly instantaneous, is the circuit designed to have a certain amount of inductance/capacitance to limit the speed the 5kV moves through the circuit?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Not even a direct lightning strike is instantaneous so define what you mean. \$\endgroup\$ – Andy aka May 3 '18 at 11:52
  • \$\begingroup\$ In terms of an arc or spark forming. If there was a spark gap placed on the input and of the circuit and the power jumped. As instantaneous as realistically possible in a real-world - worse case scenario. Then, of course, the engineer would have to have to design to provide enough safety margin for device conduction time. \$\endgroup\$ – user160063 May 3 '18 at 11:57
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If the voltage applied is truly instantaneous

It can't be, so the premise of your question makes no sense and the question can't be answered.

Think about it. A instantaneous step change in voltage would require infinite current to charge up whatever inevitable capacitance there is. Just like there is always some parallel capacitance, there is always some series resistance and inductance. Voltage simply can't change instantaneously. Put another way, the derivative of voltage can't be infinite.

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What Olin and Andy say it's true. However, in terms of a real life situation, since there is a limited dV/dt, or dI/dt, the protection circuit would have to respond as fast as it can, that is, to detect the derivative in enough real-time, and respond in a similar fast manner in order to protect the circuit. How?, where?, for what purpose?, etc, these may be other questions.


If you have values for those components, you can simulate the circuit for yourself and see what happens. If the protection circuit has a TVS, it will likely burn and trip the fuse, as well. If this chain happens before the pulse gets to reach high enough values, then all's well, else, not. Did you have a particular protection in mind? A particular type of pulse? A particular scenario? Your question is very vague, so it can only be answered in a vague, general manner.

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