HT Cable and HVAC Ducts

below or attached to the underside?

It's in a cable tray underneath the HVAC duct, and it doesnt touch, but the clearance is shown very minimum, around 100 mm.

It depends on the application and local codes.

Ask the Consultant to give a sketch.

The consultant has already given a sketch, we are bit skeptical on that , hence the doubt!

Could you post here?.

Suppose the skin of the HVAC duct is non-insulated and below dew point with the HT tray below it. Do you really want conductive water dripping on that? Do you want the tray supporting the HT to corrode? Are you serious? This is a complete no brainer.

I think you have loosely used the descriptor "IS".

The question posed has nothing to do with IS as understood in Electrical Theory.

IS has to do with apparatus capable of generating sufficient energy under fault conditions to ignite a surrounding flammable/ explosive atmospere.

Perhaps there is another meaning for IS (other than Intrinsic Safety) that I am too dim to latch onto , on this otherwise fine day ?

Maybe meant short hand for "International Standard". Usually referred to as ISO?

Aaah..there you go... I stand corrected, thank you!

The years are starting to haul me in.

IEEE Std 525-1992, IEEE Guide for the Design and Installation of Cable Systems in Substations.

ch.12.3.2 Tray system design

1. In general, vertical spacing for cable trays should be 12 in (30.5 cm), measured from the bottom of the upper tray to the top of the lower tray. A minimum clearance of 9 in (22.9 cm) should be maintained between the top of a tray and beams, piping, etc., to facilitate installation of cables in the tray.
2. Cables installed in stacked cable trays should be arranged by descending voltage levels, with the higher voltage at the top.

That means-if "etc." =ventilation duct also-it is permissible to install a cable tray for medium voltage cables below the duct if the clearance of 9 inches is provided.

when buried in earth does the same rule apply?.

For underground installation: in NESC Rule 353 and 354 minimum radial separation required it is 300 mm [12 inches].However, adequate vertical separation shall be maintained to permit access to and maintenance of either facility without damage to the other cables.

Thank you Sir!