To Ground Or Not To Ground, That Is The Question...

[vanessa]

A while ago I picked up some vintage test equipment from a garage sale. Recently I found use for some of it in my DIY tinkering.

Two items I'm concerned about both are older pieces, before they started to put that third earth ground plugs on outlets (USA). One piece is an isolation transformer (excellent for personal safety when working on high voltage items like tube pedals and amps), the other a variac (excellent for circuit testing and circuit safety for the same).

Q: I've seen isolation transformers without an earth ground before and I'm wondering... On the one's that do have them is the earth ground internally lifted, connected straight through, or ground to the chassis (bell/frame)? It would seem like you would not want to have an earth ground on them (safety, ground loops)?

Q: With the variac, if I wanted to add an earth ground, would I tie the earth ground to the chassis for both the line input and the output? Or would I take it from the line and directly connect it to the output bypassing the chassis?

Any help would be greatly appreciated as I don't want to die... 

[John Lyons]

I think that the Ground would/should be a chassis ground and passed on to the outlet for the Variac.
Same for the isolation xformer.
Just in case a winding shorts to the case...
Neither use ground to do their magic. That's why they didn't need the earth/chassis ground in the good old days...a safety thing...

John

[vanessa]

Thanks John,

I agree with you and I too was thinking that the variac should have both input and output earth grounds to chassis, but I was wondering if earth ground skips the chassis and goes directly to the output jack what does it really matter?

Also I'm confused about the isolation transformer. If you are trying to isolate the pedal (or amp) you are working on from the house power source why would you want to complete a path to ground (earth)?

[jrem]

The point of the transformer is that the output legs and supply and ground are isolated . . .      on the primary side, neutral and ground are close to the same potential.  The ground is there in case of a fault, the potential has some place to go.  Add a variac or isolation transformer (a variac IS an isolation transformer by default), and the secondaries are 'isolated' from the primaries, so neither secondary lead can go back through the primaries.

Come to think of it, I need one of these.

[R.G.]

Sorry, I was tied up with some layout work earlier.

1. The point of having a third-wire ground is that you cannot rely on the cold (neutral) side of the AC line to shunt hot wire currents to ground. Neutral is tied to earth ground back at the power pole, and possibly at your breaker box, but all of the hot-side(line) current returns through it, so it is not really at 0V with respect to earth. The third wire does not (normally) carry return current so it is at ground potential. Therefore the third wire does not need to be and is not fused or protected by breakers, and it will ALWAYS be available to shunt fault currents to ground, just in case the cold/neutral side is open.

2. The point of isolation transformers is to completely break the connection between the AC power line and earth ground. With one side of the AC power line kind of grounded (cold/neutral side) then the hot/line side is always hot to other things - like the ground you're standing on, cold water pipes, concrete floors (which are good conductors for electrocution purposes). If you happen to have an open cold/neutral wire the equipment looks dead - no lights come on, nothing moves, etc - but if you poke around in it and touch the hot/line conductor and also happen to touch one of the sneaky connections to ground, you probably die. Notice that if you happen to have on shoes, you can touch neutral repeatedly until you happen to rest your other arm on a grounded metal desk or cabinet, then touch the same wires you touched a minute ago.

Isolation transformers prevent this by making NEITHER side of the AC line connected to earth ground, and you can touch either side while holding a cold water pipe and not be in danger at all. This is why they are so good. They prevent inadvertent snake-in-the-grass electrocutions. Notice that if you happen to get across the isolated side of the isolation transformer, it will happily kill you dead too, isolated or not. This is another good reason for the one-hand rule. But it's better than the whole world being the second contact for electrocution.

Isolation transformers are also good for situations where you simply must make measurements on the AC power line itself, or simply must make equipment that has the internal power ground connected to the cold/neutral side of the AC power line. In the first case, you can float your equipment on the isolation transformer and connect your ground probes to the AC line without melting the ground wire down in to copper droplets. I have actually seen that happen back when I was designing switching power supplies. (It killed the $10K oscilloscope, too!) In the second case, lots of cheapo consumer stuff from the 50's and early 60's was designed with the internal ground connected to the cold side of the AC line. They ran the speaker through an output transformer that then became all that was between you and electrocution. Had headphone outputs that way too. This kind of stuff was called "hot chassis" stuff, for obvious reasons. Zillions of TVs were made this way to save the price of a power transformer. Isolation transformers are the only safe way to repair this stuff.

The useful form of an isolation transformer is to have the third wire safety ground coming into the metal cabinet around the isolator, and then to carry that to the isolated output plug, but to offer the option to switch-disconnect the output third wire when needed, and to make that switch be hard to get wrong and kill yourself with. Sometimes you need it both ways. There are now on the market isolated centertapped transformers that produce isolated 120Vac with a centertap so you can have both sides of your AC line moving around ground, instead of one near ground and one swinging +/-170V peak. I've heard that this is good for low noise measurements, but I've never done anything that sensitive.

3. The point of a variac is to VARY the AC line. Variac is a brand name. They are variable autotransformers and in general are not isolated from the AC power line at all. The Cold/neutral goes to the cold side of the transformer and to the cold/neutral side of the output jack. The hot side goes to the hot side of the transformer and the wiper goes from the transformer to the hot side of the output jack. Safety ground should be carried from input supply wire to the output jack and never disconnected. Since the thing is not isolated, the output of the variac is always hot to the rest of the planet, just like the AC power line itself, and needs the same cautions. Never, ever disconnect safety ground from the output of a variac, thereby removing half a century of making electrical power safer.

[vanessa]

Thank you R.G.!

2. The point of isolation transformers is to completely break the connection between the AC power line and earth ground.

So based off what you are saying if you have an isolation transformer unit (say 120V 1:1) that has a three prong input and output the earth grounds are lifted on both the primary and secondary?

3. Safety ground should be carried from input supply wire to the output jack and never disconnected. Since the thing is not isolated, the output of the variac is always hot to the rest of the planet, just like the AC power line itself, and needs the same cautions. Never, ever disconnect safety ground from the output of a variac, thereby removing half a century of making electrical power safer.

What would be the pro's & con's of both safety (earth) grounds tied to the chassis?

[PerroGrande]

One quick related note --

While I like to have an isolation transformer when working on anything that is mains-connected, it is imperative to have one when repairing or building tube equipment.  The current love for "old" gear has brought some hot-chassis stuff back into the picture.  I've recently encountered two amps (from the 1950's) that operate in exactly this manner.  Fortunately, they were in the hands of someone who knew what the heck they were doing.  However, if someone inexperienced were to get a hold of those things and start wrenching, the results could be, well, unpleasant.

An autotransformer is also a must-have when working on Tube gear.  If you've got an amp that wants to blow fuses, you can slowly bring up the voltage while watching the current and avoid damage to the other components.

[R.G.]

So based off what you are saying if you have an isolation transformer unit (say 120V 1:1) that has a three prong input and output the earth grounds are lifted on both the primary and secondary?

I would be very suspicious - so suspicious that I would pull the thing open and find out if it's possible. You can definitely check for solid connections by using your DMM to measure which wires are connected to the safety ground prong, including most especially any metal you can touch on the outside case of the transformer. If safety ground doesn't connect to anything, I would not use it. It's probably OK if it connects only to exterior touchable metal parts and NOT to either of the output wires.

What would be the pro's & con's of both safety (earth) grounds tied to the chassis?

Pro:
- safer because third wire ground is carried through to the equipment being tested

Con:
- you probably need some way to disconnect it from the secondary only for some cases where you want the stuff it powers to be truly floating.

[R.G.]

While I like to have an isolation transformer when working on anything that is mains-connected, it is imperative to have one when repairing or building tube equipment.  The current love for "old" gear has brought some hot-chassis stuff back into the picture.  I've recently encountered two amps (from the 1950's) that operate in exactly this manner.  Fortunately, they were in the hands of someone who knew what the heck they were doing.  However, if someone inexperienced were to get a hold of those things and start wrenching, the results could be, well, unpleasant.

An autotransformer is also a must-have when working on Tube gear.  If you've got an amp that wants to blow fuses, you can slowly bring up the voltage while watching the current and avoid damage to the other components.

You can say both of those again, but in bold-face, underlined italic fonts.

The other thing anyone serious about fixing amps really needs is a light-bulb limiter. It's the only way to fix some things that want to blow fuses, because it's dynamic. That was the only way I could find some soft-failing filter caps one time. They leaked, and heated up as they leaked then blew fuses. I could not balance them on the variac long enough to find which ones. The bulb limiter followed them and I could use my meter.

[vanessa]

So based off what you are saying if you have an isolation transformer unit (say 120V 1:1) that has a three prong input and output the earth grounds are lifted on both the primary and secondary?

I would be very suspicious - so suspicious that I would pull the thing open and find out if it's possible. You can definitely check for solid connections by using your DMM to measure which wires are connected to the safety ground prong, including most especially any metal you can touch on the outside case of the transformer. If safety ground doesn't connect to anything, I would not use it. It's probably OK if it connects only to exterior touchable metal parts and NOT to either of the output wires.

So on my older two prong version I should replace the input and output jacks with modern three prong (earth ground) jacks and solder the earth grounds to the transformer's end bells?

What would be the pro's & con's of both safety (earth) grounds tied to the chassis?

Pro:
- safer because third wire ground is carried through to the equipment being tested

Con:
- you probably need some way to disconnect it from the secondary only for some cases where you want the stuff it powers to be truly floating.

I agree that both to chassis would be safer and my reasons would be should there be a failure with the transformer, switches or in this case the volt meter you have that as a backup.

Where would you want to use something that's floating and if so would I want to instead have a ground lift period?

[RedHouse]

I think that the Ground would/should be a chassis ground and passed on to the outlet for the Variac.
Same for the isolation xformer. Just in case a winding shorts to the case...

Absoulutely, correct.

You can go to the NEC website and see diagrams in case you're not sure vanessa.

Funny thing, a lot of musicians don't have a clue about the case-ground system, and often lift a ground to stop hum problems in multiple equipment scenario's (PA systems etc)

The Rane website and the Jensen website also have some real good white papers on this too, they make some excellent additional reading along with RG's description above. I cant think of how many time this sort of thing comes up on the DIY amplifier message boards, it's not small thing to ignore the proper grounding method of power equipment (even low power)

[brett]

Funny thing, a lot of musicians don't have a clue about the case-ground system, and often lift a ground to stop hum problems

Here in Oz, we have two systems that are combining to make that less common:
1. extra voltage (240 V 50Hz), and
2. survival of the less stupid.