Step-down transformer grounding. How to do?

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Ilya

Well-known member
Joined
Feb 25, 2005
Messages
751
Location
Moscow
We're going to install the new console which was originally intended to work from 120 V AC. So we need to order a step-down transformer to be able to power the desk from 220V. The installation guide says the circuitry is wired for "isolated grounding". In this case should the transformer have some kind of special design? Or do we just connect the ground wire to the chassis?
And BTW, should we fuse the trafo or is the desks power supply fuse enough for that?
Any ideas?
 
[quote author="Ilya"]The installation guide says the circuitry is wired for "isolated grounding". [/quote]

It may just mean that it works best with a proper separate earth, as opposed to what you find in some countries where earth is tied to neutral. In this case, a transformer would be a solution to the problem and will provide balanced mains, assuming the secondary is not tied to neutral on one side.

See here:

http://www.dself.dsl.pipex.com/ampins/groundloops/grndloop.htm#bal
 
Thanks, rodabod.
Although I need specific info regarding if I need to connect the ground wire to chassis where the trafo is housed and where should it be done. Or do I need to order a center-tapped thing and make a balanced supply?..
 
Hey! It shouldn't be verry complicated, I just want to be sure and not to screw something.
Anybody, advise please...
 
I'm not an expert on power stuff but I think I'd wire things as follows:
* connect the neutral sides of primary and secondary winding together (will get you something similar to an autoformer)
* connect chassis to ground
* fuse secondary windings (not absolutely necessary but it won't hurt)

Samuel
 
Samuel!
When you say neutral you mean the transformer with 2 wires or with center tap?
 
Ilya, I have a broadcast engineers guide to mains electricity (including balanced mains which is pretty common). I don't have a scanner though - it would need to be a photo.

Let me know if you are interested.

PS. Did you buy a boxed step-down transformer unit, or did you just buy a transformer with no casing?


Roddy
 
Rod, the guide would be very usefull to me, especially now. If you can take some photos of it that would be great!
I'm ordering a custom step-down trafo, not boxed, and I'm planning to put it inside a rack chassis, which I'll buy separately.
 
Ilya, the links below are of a document describing the use of balanced mains for broadcast which may be handy for you. You could wire the transformer as unbalanced with one side tied to 0V, but I don't really see the point...

Anyway, the most important thing is safety. Test the system before using it.

I probably have more documentation on mains wiring - I'll have a look.

Please don't share these with anyone else.

page 1

page 2

page 3

page 4

page 5

page 6
 
All this is about balanced mains which is not the case in my situation. If I order the balanced trafo for the desk then I need to order another one for everything else in the studio which is near to impossible.
So the question remains. How should I pass the ground to the desk, just with a pice of wire from our in-house ground? And should I ground the trafo core?

P.S. It may be a rather simple question, but unfortunately where I live nobody seems to be competent enough (at least I can't find anybody) to give me appropriate directions.
 
Hi Ilya,

I just realised that the floating mains would not be so effective since it would be connected to "normal" equipment too....

Can you tell us anything about mains electricity in Russia? Do you have a dedicated earth and neutral, or are they just tied together?

It should work just to tie earth to earth, live and neutral to the primary, and then tie one of the secondary terminals to neutral to form a return path (the remaining terminal would be live).
 
Well, earthing in Russia is kinda tricky subject. It wasn't long ago when we had mains with only 2 wires: live and neutral. At least in consumer environment. Now 3-contact outlets are pretty common although most of them have neutral and earth tied together somewhere.
In our studio we have a dedicated earth which (I believe) was installed by certified electricians. At least when the other guys checked it they said it was fine.
So, regarding the trafo, did I get you right that I need to connect neutral wires of the primary and secondary windings? Wouldn't it be common to an autoformer though? I thought it would be better to isolate mains hot and neutral completely...
 

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