How much clearance do I need between floating ground and earth referenced ground?

Question

I have two circuits where one side is earth reference and another floating. The floating circuit is isolated by a transformer and talk to the earth reference circuit via opto-coupler.

During final phase of development, i noticed my floating ground shield got too close of the chassis ground with only less than 0.1mm of gap. I immediately tagged it as issue and as usual people will ask me how much gap I needed. (I don't really have an answer, I am only a mfg engineer). I go through some safety standard but I still don't have an clear answer.

Here is my question:

1. Do I need to have clearance between two grounds to avoid hazard or arc?

2. Since both points are concealed within a metal cover which are grounded, do I need to follow the clearance requirement as required by the safety standard?

3. if that is not required, then how much of gap do I need to prevent a arc over? Should I follow the safety standard clearance guideline or should i follow the 30kV/cm rules?

Answer 1

Do I need to have clearance between two grounds to avoid hazard or arc?

Yes. Is suspect this is functional isolation required for the operation of the device, but not to protect users from shock.

Since both points are concealed within a metal cover which are grounded, do I need to follow the clearance requirement as required by the safety standard?

You need to comply to the relevant standard. They will have requirements with a test scenario for the isolation barrier.

Eg: 50178:1997 (A.9.4.6.5)
- insulation resistance: 100 MΩ;
- dielectric strength: a.c. 1 kV or d.c. 1,5 kV (60 s duration);
- pulse withstand: 1,2/50 ms, 2 kV (peak value)

Even if the standards are not a requirement, use them as guideline.

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If it is safety isolation then all of the above is invalid, the standards are mandatory and significant distances and strict requirements on parts apply. And have your device tested by a certified auditor.