Metal Case Safety

[Fred]

Hey Everyone

If a transformer is used in a design is it safer to enclose it in a metal
box or a plastic box? Most lab equipment (power supplies, scopes etc) use a
metal case. Wouldn't plastic be safer? Is metal only used for strength?

 

[Vidar Lřkken]

Fred wrote:

Hey Everyone

If a transformer is used in a design is it safer to enclose it in a metal
box or a plastic box? Most lab equipment (power supplies, scopes etc) use a
metal case. Wouldn't plastic be safer? Is metal only used for strength?

Depends. If the metal case is properly grounded, it is at least as safe.
If it is not grounded, plastic is safer.
Because if it is grounded, and a failure happends, current will pass to
gnd, and the ground relay will sense that, turning off power. So, if you
intend to use it in an *dry* envirorment without earthed wall outlets,
pick plastic. Otherwise, I'd stick to metal.
Also reffer to faradays cage...It won't hurt you to touch the metal case
when it is powered, as long as no current is passing trough you, but
rather trough gnd connection which will likely have less resistance than
humans. In norway, the voltage upon touch to an earthed part *shall*
never be above 50V, so then you must adjust ground resistance according
to the maximum current which can pass in a single-phase short (short to
earth). This was slightly offtopic, but hope it explains a bit

 

[Fred]

"Vidar Lřkken" <njus@vidarlo.net> wrote in message
news:tbsod.5223$HA5.626009@juliett.dax.net...

Fred wrote:
Hey Everyone

If a transformer is used in a design is it safer to enclose it in a
metal
box or a plastic box? Most lab equipment (power supplies, scopes etc)
use a
metal case. Wouldn't plastic be safer? Is metal only used for
strength?



Depends. If the metal case is properly grounded, it is at least as safe.
If it is not grounded, plastic is safer.
Because if it is grounded, and a failure happends, current will pass to
gnd, and the ground relay will sense that, turning off power. So, if you
intend to use it in an *dry* envirorment without earthed wall outlets,
pick plastic. Otherwise, I'd stick to metal.
Also reffer to faradays cage...It won't hurt you to touch the metal case
when it is powered, as long as no current is passing trough you, but
rather trough gnd connection which will likely have less resistance than
humans. In norway, the voltage upon touch to an earthed part *shall*
never be above 50V, so then you must adjust ground resistance according
to the maximum current which can pass in a single-phase short (short to
earth). This was slightly offtopic, but hope it explains a bit

That helps, thanks.

The secondary output of the transformer is rectified to produce DC. Should
the ground of the DC be connected to the case, which is also connected to
the ground (earth) in the plug?