Mains product design for the USA

[DenzilPenberthy]

Hi All,

 I'm building a piece of equipment which essentially consists of a 19" rack case with a 24v PSU in it and four amplifiers. When I make kit like this for use here in the UK I'd use an IEC inlet with a single pole switch and a fuse both in the live conductor. This piece of equipment is going to end up being sent out to some folks in the states at some point.

When you design things like this in the states, is the polarity of your supply guaranteed like it is here or do I need a double pole switch and double pole fusing?

Cheers.

[ConKbot]

Is it going to be 120V only or 120/208/240?  If the latter, you definitely need double pole switching/fusing as both connections will be line.  The usual NEMA outlets are polarized, however they werent always, so there can be older outlets in older facilities that are reversed.

[Richard Crowley]

A properly-wired, grounded NEMA mains outlet will have the neutral node nominally at ground potential for a common 120V circuit. However a 240V circuit is practically guaranteed to have both conductors 120V above ground.

However we we under the impression that in the rest of Europe, the "polarity" is undefined so, BOTH nodes must be assumed to be "hot".

[Len]

When you design things like this in the states, is the polarity of your supply guaranteed like it is here or do I need a double pole switch and double pole fusing?

Yes, the polarity is guaranteed whether you use a 3-prong plug or a polarized 2-prong plug. These plugs won't fit into ancient unpolarized outlets, but such outlets are hard to find nowadays.

Of course, the guarantee is only as good as the person who wired the outlet, but that's true anywhere.

[DenzilPenberthy]

Ace, so I'll stick with a single pole switch and fuse then with an IEC inlet. This will be single phase 120v.

Re polarity in Europe: I believe you're correct in that polarity is not ensured in continental Europe but here in the UK our wiring regulations do ensure it and sockets have a (usually single pole) switch in the live conductor and the plug has a single pole fuse in the live conductor.

[TimFox]

When fitting a normal "IEC" line filter on a rear panel, I was surprised that the label did not distinguish the "line" and "neutral" connections to the load side.  To be safe, I connected the power cord to a good outlet and verified which pin was close to chassis ground and which was hot before continuing.