As for grounding, it shouldn't be an issue here, but consider a wall wart adaptor. This normally doesn't have a ground wire as it is in a plastic box, so the secondary is happily floating.
Would this be a safety concern in any circumstance?
You mean the secondary, right?You don't want to force a neutral. Leave the primary side floating if there is not a 3rd ground wire.
1- is straightforward,1. Unpowered insure there are megaohms between the primary and secondary circuit grounds.
2. With the circuit powered check for <100uA AC leakage between the secondary circuit ground and a real earth ground.
You mean the secondary, right?
---------- Post added at 05:23 ---------- Previous post was at 05:20 ----------
1- is straightforward,
As for 2, I am not sure what you mean.... leakage through what? an ammeter between secondary mass and earth?
thanks!
I read the original post wrong. The choice of a secondary neutral or ground has no bearing on safety. You do what you like there.
Yes, connect an AC ammeter between any point on the secondary circuit and earth. This is the fault path that a person, or cat, would see if they touched some part of the low voltage side of the circuit and earth. If you get more than a few hundred microamps you'll start to feel the juice if get between ground and the secondary side.
Gotcha. All that matters is isolation btw primary and secondary, right?
What was worrying me is that there could be a potential difference between the secondary circuit and ground. But that would be a static potential independent of the circuit being the secondary of a transformer, right?
A DC ammeter wouldn't do the trick, would it?
---- few more questions:
- does it make sense to have one fuse for each secondary terminal? or is one enough? What difference does it make if I put the secondary fuse before or after the rectifier?
- if there was ever the risk that a primary wire could touch the secondary then I'd have to ground the secondary, wouldn't I?
- how do you properly ground the secondary? connect the DC gnd to primary earth? (there's no such a thing a mass wire, it's called ground, right? Damn Italian)
thanks again
Since most equipment (at least here in the USA) is not grounded there are very strict safety requirements (UL / CSA / CE / etc) that do a very good job of preventing primary faults to user controls or metal parts of the chassis.
If you are going to fuse the secondary you only need one (assuming you are not using a center tapped arrangement) and I would definitely put it before the rectifiers. They are a likely failure point and could short the transformer output.
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