On Mar 3, 9:49*pm, Michael Black wrote:
On Sun, 4 Mar 2013, Jim Mueller wrote:
On Sat, 02 Mar 2013 20:00:49 -0800, Frank wrote:

Clamshell-looking enclosure on small AC power transformers. *Magnetic
shielding to prevent interference with nearby wires?

Magnetic fields are hard to shield. *In a power transformer, it would be
more effective to use more iron in the core and more turns in the
windings to reduce the flux density. *While a steel shell probably
provides a limited amount of shielding, it's more likely there to provide
mechanical protection. *At the same time, it gives a better appearance.

Maybe protect the windings, too.

Definitely, nobody specifies between just a frame transformer and a metal
cased one. *I'd honestly never given it any thought. *Bigger transformers
often seem to be more likely to be cased in metal, so structural support
may apply. *I don'thave it handy, but that one Hammond catalog I got
decades ago I think offered the same voltage/amps in open frame and metal
enclosed versions, which would add more to "looks" than anything else.

I have heard of the leads breaking off open frame transofrmers, right
where they come off the winding, which isn't a good thing. *Any metal
cased transformer, I don't recall that happening, which may mean there is
some sort of terminal block inside, rather than just bringing out the
wire. *Certainly I've seen open frame transformers where the wire from the
windings are brought to terminal points, which helps protect the winding
wire. *it also exposes the line voltage going into that transformer, which
in some cases can't be a good thing.

* *Michael

Or if it's installed in a perforated enclosure. Paper and wire
varnish
don't protect as well against sharp objects.