Properly mounting foot mount transformers

[queennikki1972]

I know, more dumb questions, Im happy I found all of you.

On my first little power supply i had used such a small transformer I mounted it directly on a raised circuit board with nylon stand offs, so it was in no way touching the enclosure. However, my question is, when mounting larger (Foot Mount) type transformers, can the foot be mounted directly to the bottom of the metal enclosure or does it needed to be suspended above the metal enclosure to prevent any contact?

[ataradov]

It depends on the transformer design. Typically large transformers like that have a plastic spool on which the wire is wound. That spool provides plenty of isolation.

But if it is a cheaper version, it might have just a thin insulating wire between the core and the winding. In that case I would err on the safe side and not expose the transformer to the user in any way.

[queennikki1972]

Here is the types I mean. Seems to have plastic in the center

[ataradov]

Just mount to the chassis, don't worry about it.

The core is not electrically connected and it does not care if it is connected to the earth (as hopefully your case is).

The only concern is if the insulation fails, but in this case insulation is provided by that while plastic, and that's enough plastic.

[T3sl4co1l]

Indeed, the core should be grounded in case of failure of that plastic layer: that way fault current flows to GND, away from possible touch.

Tim

[Brumby]

If mounting to a metal panel, then just bolt it straight on.

But if you're going to mount one in a plastic case, then I would suggest mounting it to a metal plate that has a larger footprint and mount that plate to the plastic.  The reasoning is that if something happens and the transformer overheats, then by having it on a larger plate, the plate will slow the passage of heat to the plastic which is holding it in place.  An overheating transformer directly mounted to a plastic case could melt it's way to freedom ... and chaos.

[TERRA Operative]

I bolt them straight down to the chassis then run the earthing wire from one of the mounting bolts to the earth lug on the power connector.

That way I know it it well earthed right where the mains voltage is and I don't have to drill another hole, and I save a bolt and nut too.