I am reading through a HIPOT / Dielectric Strength requirement that states in no so many words: "withstand damage when 1KVac is applied between all mutually insulated circuits"

I search every where and it must be my English but, I am not sure I understand what is meant by mutually insulated circuits.

Let's say I have a connector with the following pins going outside an enclosed box:

  1. 120Vac Phase A
  2. 120Vac Phase B
  3. Chassis Ground
  4. Sensor 1 (0-5Vdc)
  5. Sensor 2 (0-5Vdc)
  6. 24Vdc
  7. 24Vdc_Return
  8. 208Vac "Line" Load
  9. 208Vac "Return" Load

What would be considered mutually insulated circuits?

Since I have surge protection on sensor lines a 1KVac will trip those and fail the test. If I remove the protection a 1KVac it might kill the circuit, and perhaps that is OK as long as it doesn't fail short?

  • \$\begingroup\$ You need to read the standard that applies to your device. This will tell what has to be connected and what can be disconnected during the test. \$\endgroup\$
    – Spoon
    Apr 4, 2017 at 19:49
  • \$\begingroup\$ Certainly have: snebulos.mit.edu/projects/reference/MIL-STD/MIL-STD-202G.pdf pg 154, section 3.3: 3.3 Points of application. The test voltage shall be applied between mutually insulated portions of the specimen or between insulated portions and ground as specified. The method of connection of the test voltage to the specimen should be specified only when it is a significant factor. \$\endgroup\$
    – DIODEX
    Apr 4, 2017 at 20:16

1 Answer 1


So the idea is (depending on the spec) that two portions of the circuit or cable should be isolated. Take for example an AC power supply that is rated for a 3kV isolation, if the DC side and AC side are isolated (with an isolation transformer) then you could safely connect a 3kV source on one part of the supply and current should not flow to the other (unless there was a dielectric breakdown at the transformer). This test would also ensure that the creepage and clearance on the PCB is doing it's job. The current is measured on other conductors to see how much bleeding from one insulated portion of the circuit to the next. Another thing that is usually tested on most products is leakage current from the Line or Neutral to ground. A product could shock someone if the ground wasn't connected.

Some manufacturers\specs require cables to be tested so the insulation strength can be tested, so you would test the cable connected to a hi-pot tester and test the parts of the circuit that are 'mutually insulated' or the sections of the cable that touch. This would depend on the structure of the cable and connector.

I can't say by your diagram which circuits are mutually insulated and I don't know if you need a cable test or a power supply test, by the standards you are required to test to. Work with the regulatory entity that your product needs to pass to find out what kind of testing needs to occur.

  • \$\begingroup\$ I think I see what you are saying. Based on my diagram, the input voltage 208V line-line powers a transformer to generate 5V which is used to drive sensor logic. The 24Vdc is power to drive some outputs and shares ground with the 5V logic. SO what you are saying is to apply hipot between the line and a sensor for example? \$\endgroup\$
    – DIODEX
    Apr 5, 2017 at 2:26
  • \$\begingroup\$ I've only done cable testing and hi pot testing on power supplies. I think there would be other situations that you could run into where you would have to do hi pot testing on a PCB with isolation. You wouldn't want to do hi pot testing on signals with a direct pathway, bad things would happen, like your board going up in smoke. That being said, your circuits would have to be insulated for them to be tested, so to say anything further I'd have to see the whole design. \$\endgroup\$
    – Voltage Spike
    Apr 5, 2017 at 3:35
  • \$\begingroup\$ What you should do Is carefully short all lines in each group. I.e 120A and 120B together, then 24V, 24V return, sensor1 and 2 together and so on, grouping all lines which are supposed to have a "limited" voltage across. Then you connected all groups together but one at your choice and check their isolation. Now repeat changing the lone group among all the possibilities \$\endgroup\$
    – carloc
    May 29, 2019 at 19:34

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