Creepage and air clearance under vacuum conditions

[Roland chung]

Hi all,

I am wondering how to take into account the creepage and clearance under vacuum conditions (high voltage generator of X-ray, in my case). I can not find an answer from IEC 60664.

I will appreciate if you can provide some guidance to me.

Thanks,
Roland

 

[Peter Selvey]

Is it for functional or safety distances?

Normally the parts inside the vacuum tube would be functional, so you can base the distance on widely available research on breakdown in a vacuum (e.g. one paper I found gave a figure of ~45kV/mm, wikipedia gives 20-40kV/mm depending on shape/material, both of which far exceed the value for air which is around 3kV/mm).

Since there is no dust, pollution etc in vacuum, the distance would purely a function of clearance, not creepage.

Although some parts of the tube might be earthed, the circuit should be such that under failure conditions, the currents will only circulate inside the equipment, and cannot get outside.

 

[Roland chung]

Thank you.

Regarding the separation of high voltage, I find the answer from IEC 60601-2-44 CT scanners. No distances required but Hi-Pot test.

NOTE For CT SCANNERS installed with a fixed and PERMANENTLY INSTALLED PROTECTIVE EARTH CONDUCTOR it is assumed that there is no unacceptable RISK with regard to the reliability of the PROTECTIVE EARTH CONNECTION. The components with high reference voltages are: parts of X-RAY GENERATOR, X-RAY TUBE ASSEMBLY and in some cases parts of the digital acquisition system therefore, a reliable PROTECTIVE EARTH CONNECTION and sufficient dielectric strength of insulation are considered as equivalent to two MOPs. See also 201.8.8.3.

This also answers my another question, that is permanently installed protective earth alone can not be considered to provide 2 MOPs although the standard does not permit to interrupt such protective earth for SFC.