Mains AC protection

[shrubbery]

I am building a rack device which will include power supply but I am a bit confused about how grounds connect.  This is what I understand/plan to do so please correct me if there's anything wrong with it (or if you have any other suggestions about safety):

- AC L (through fuse) and N wires into transformer which provides dual rail on the other side (not sure If I'll use 78xx/79xx or LM317/LM337 yet)
- AC PE connected to chassis ground point
- on the DC side I have all signal grounds connected to DC ground (between +/- V)
- I also plan to use toggle switch to cut AC power

Now this is a bit unclear to me:

- if I don't connect DC ground to chassis ground point I think that's called "floating ground" and still safe
- if I connect DC ground to chassis ground point it's not floating

I wanted to connect DC side to ground point just to be safe but I'm not 100% sure I can mix AC and DC side.  I can easily provide toggle if both are safe but I'm not sure what either does because I have another rack with such switch and there's no difference...

[GibsonGM]

Thanks for asking this, shrub.  Now, if you're a bit confused about this, that's good, because it means you should read up on how this all works! 
There's a time you jump over to the mains part of things, but when one does, it's best to be VERY VERY sure about what's going on BEFORE you make any moves.     And it's great that you are.

All I can suggest to you is that you get on some websites and understand how mains wiring works, what that ground wire is for and how it plays in with devices that are attached to the mains.    Look at schematics that are doing just about what YOU want to do, see how they set it up.  Sometimes this is called "bonding" or "earth bonding", and it's literally a lifeline.

That ground wire is there so that if a hot wire touches your metal chassis, it will instantly cause current to flow to ground rather than make the chassis hot (which may kill you). That hot chassis could make a guitar hot, as well.  The almost infinite current that would flow in this case trips the breaker/blows a fuse.    So, without going too far into your project, may I suggest that you NOT build anything floating, but have a very nice, solid, reliable and assured connection to earth ground from guitar back thru whatever you build?   There are ways to make a floating system safe, of course, but unless you're "up on that", I'd stay with the old fashioned tried and true continuous ground.

Get a lot of eyes on whatever you set up, as a schematic, ok?  Draw something up when you get close to done with design.

[R.G.]

AC power line safety is a huge topic. Experts in safety certification never say "this is safe", they say "I can't prove that's NOT safe".

I am not an expert. But I've been chewed on by experts.  

In my dim understanding,  this sounds OK as far as I know:

- AC L (through fuse) and N wires into transformer which provides dual rail on the other side (not sure If I'll use 78xx/79xx or LM317/LM337 yet)
- AC PE connected to chassis ground point
- on the DC side I have all signal grounds connected to DC ground (between +/- V)
- I also plan to use toggle switch to cut AC power

Except that the AC power line safety ground (PE, I think) should be attached to the chassis ground point with toothed washer to bite into the chassis metal, and the earth line should be on a ring terminal, not wrapped wire, for the same reason, to bite into the metal. There shouldn't be any other connections to this point.

A toggle switch that cuts both sides of the AC power line is good, in case you're plugged into an outlet that some electrician may have wired backwards.

- if I don't connect DC ground to chassis ground point I think that's called "floating ground" and still safe
- if I connect DC ground to chassis ground point it's not floating

Floating ground is not safe (I've been chewed on about this one). A safety expert would never go for anything that didn't tie them together solidly. However, some people have used two pairs of back to back diodes in series paralleled with a 10R resistor for a partial float. That won't pass safety certification either, but I've seen it done. I've also seen fully floating, which is a definite mine field; what happens if something you plug into this with a signal cable has a ground shield and the other equipment is defective? This ground becomes "hot", and a hazard. If the ground is not floating, you get horrible hum in this case, but live.  

There is a lot of finesse involved in grounding multiple units. As Gibson simul posted, at least your life is at stake in doing this right. Others may be too.

[shrubbery]

Except that the AC power line safety ground (PE, I think) should be attached to the chassis ground point with toothed washer to bite into the chassis metal, and the earth line should be on a ring terminal, not wrapped wire, for the same reason, to bite into the metal. There shouldn't be any other connections to this point.

That's what I'll do, except I don't have such washer so I just wanted to scrape off the paint and test this connection for continuity.

As for the floating ground, I never planned to not connect PE to chassis.  That is definitely not allowed and I don't understand how commercially sold devices would be able to provide ground lift feature without double insulation if this was it.

This thing about not connecting anything else to ground point was my main concern.  Now I even realized that metallic jacks and other panel mounted components will likely connect DC ground via inside of the holes anyway.  I didn't want to rely on this though so I wanted explicit connection, that was also part of my question.

Now I'm pretty sure this is safe but I'm utterly confused. Why can I connect my DC ground to PE at all?  PE is earth, my DC ground is actualy at +V since it's dual rail.  It just sounds like short circuit to me...

Actually now that I'm thinking about it, it might have something to do with galvanic separation provided by transformer.  There is just no potential between these points, am I right?

[R.G.]

Actually now that I'm thinking about it, it might have something to do with galvanic separation provided by transformer.  There is just no potential between these points, am I right?

Good thinking! The transformer provides a complete insulator between primary and secondary.  So a transformer secondary can have any ONE wire connected to any other potential  - as long as that doesn't exceed the breakdown voltage limits of its insulation, anyway.

That's what I'll do, except I don't have such washer so I just wanted to scrape off the paint and test this connection for continuity.

That's probably OK for the near term, but the use of a star/tooth washer isn't optional. Over time, oxidation corrodes the flat contact area between the ring terminal and the chassis. The idea of star washers is that there is so much pressure on the tips of the tooth that they bite into the metal, and make many areas where the metal has flowed together so well that it excludes oxygen from the air, so it is a durable connection for a very, very long time, just like you want a safety contact to be. It's called a "gas tight high pressure contact".

As for the floating ground, I never planned to not connect PE to chassis.  That is definitely not allowed and I don't understand how commercially sold devices would be able to provide ground lift feature without double insulation if this was it.

Again, good thinking.

This thing about not connecting anything else to ground point was my main concern.  Now I even realized that metallic jacks and other panel mounted components will likely connect DC ground via inside of the holes anyway.  I didn't want to rely on this though so I wanted explicit connection, that was also part of my question.

Safety standards at one time allowed some structural connection, like the bolt holding the power transformer to the chassis, to carry the ground connection. It was found that such connections can come loose and oxidize because of the weight of the other connections; further, the more ring terminals and stuff stacked on the safety ground connection, the more likely it is to come loose. So they just said "only one".

The connection of secondary ground to the chassis is another matter entirely. It needs to be connected to chassis for RF shielding and hum purposes. Just not at the safety ground point. The input jacks connection is one common one.

As one other bit of subtlety, in a certification test, they will measure the slack in the wires from the AC power cord, and ensure that if the power cord was pulled out of the chassis, the ground wire would be the last one to break.

I've picked up a lot of tidbits, but there is even more for me to learn.

[shrubbery]

That's probably OK for the near term, but the use of a star/tooth washer isn't optional. Over time, oxidation corrodes the flat contact area between the ring terminal and the chassis. The idea of star washers is that there is so much pressure on the tips of the tooth that they bite into the metal, and make many areas where the metal has flowed together so well that it excludes oxygen from the air, so it is a durable connection for a very, very long time, just like you want a safety contact to be. It's called a "gas tight high pressure contact".

Thanks for pointing this out.

The connection of secondary ground to the chassis is another matter entirely. It needs to be connected to chassis for RF shielding and hum purposes. Just not at the safety ground point. The input jacks connection is one common one.

Ok then, I think I won't be drilling another hole for this.  I'm getting continuity even on the looser holes so it should be fine.

[davent]

For bringing AC into a chassis i like IEC sockets with an integrated fuse socket. Cord will detach from socket cleanly should it need to and power cord must be removed from socket if the fuse needs to be replaced. They cost relatively little.

(Random photo from the web.)

[shrubbery]

I have exactly one like this but most people should get whatever chassis can hold and add fuse holder if needed, I wouldn't be able to cut rectangular opening.  It's best to get chassis pre-fabricated for this socket.  There are also sockets with added AC filter if you have enough place for it inside or with added button but those require different opening.

[Seljer]

It's not that hard to cut a hole for the IEC sockets. Drill out the majority of it and finish it up with some small files, they quickly eat away at sheet metal and even the thicker cast aluminium enclosures.

[greaser_au]

...and even when using an IEC socket, I'd suggest making sure the wiring is arranged such that the protective earth is the last to come adrift.  I once did a country town service call where the mounting ears had come off the IEC socket (with an integrated filter), the wiring was all that was stopping uninsulated mains terminals from becoming exposed.

david

[davent]

...and even when using an IEC socket, I'd suggest making sure the wiring is arranged such that the protective earth is the last to come adrift.  I once did a country town service call where the mounting ears had come off the IEC socket (with an integrated filter), the wiring was all that was stopping uninsulated mains terminals from becoming exposed.

david

That must have been some pretty severe abuse, wonder how a snap-in socket would have fared?

And squared holes, a few strategically drilled holes then a Dremel with a fiber reinforced cutoff wheel (plus your face shield) will get you a roughly sized hole in a hurry then easily refined with a small file.
dave