How to De-Energize transformer?

[DDoug]

I don't see LOTO being addressed by the OP....

 

[Courier250]

This thread has been very entertaining. On the educational side, transformers are inductors, not capacitors. They store electricity on the input of power and dissipate that charge when power is disconnected. When not properly grounded, the discharge causes "inductive ringing". Mother earth (properly grounded) sucks ups the voltage spike from the big transformer or 500 hp motor at the factory down the street instead of blowing up the lights at my house when it'sturned off. "Properly grounded" can be an active phase, as the case with corner grounded delta, aka 3P3W. Testing each phase to ground with a VOM (volt-ohm meter) gives a reading of zero or nearly zero when power is disconnected. I mention 3P3W because the capacitors in a rotary phase converter system (when retaining a charge) can give a not-nearly-zero voltage for a period of time. Might not kill you, but doesn't mean it feels good.

 

[SomeoneSomewhere]

Err, no. Grounding is not the fix. Systems with IT earthing exist, where the live conductors are not earth referenced. For an extreme example, consider aircraft.

What can be necessary is snubbing, which is what eats up the inductive ringing.

With inductive loads on AC, they continually store and discharge energy with each half cycle - this is what causes their low power factor. If power is interrupted at a random point in the cycle, the dI/dt is likely *much* steeper than typical in the circuit, leading to a much larger voltage spike.

Your description is more accurate when considering DC systems, where back-emf from disconnected loads can cause damage if not considered during design.

 

[1 Jaw Chuck (Chuck Evans)]

Last question.

What is the correct term I should use to describe removing residual electricity from a device?

Assume we are talking about a VFD or something with capacitors.

Snubbing? Draining?

Chuck
Burbank, CA

 

[Strostkovy]

Last question.

What is the correct term I should use to describe removing residual electricity from a device?

Assume we are talking about a VFD or something with capacitors.

Snubbing? Draining?

Chuck
Burbank, CA

Discharging

 

[awander]

Discharging.

 

[1 Jaw Chuck (Chuck Evans)]

Discharging!

Thank you.

Chuck
Burbank, CA

 

[CarbideBob]

Err, no. Grounding is not the fix. Systems with IT earthing exist, where the live conductors are not earth referenced. For an extreme example, consider aircraft.

Or a space shuttle or even as common as an automobile.
For certain floating ground control systems exist in machine tools.
Ground is not earth.
Bob

 

[JST]

Or a space shuttle or even as common as an automobile.
For certain floating ground control systems exist in machine tools.
Ground is not earth.
Bob

It is with respect to most power systems covered by the NEC.

In case of innards of a device, then it is incorrect to refer to "ground" unless it is referenced to the power ground. In such a case of a "floating" circuit, "circuit common", or similar terms ought to be used.

 

[jim rozen]

...
What is the correct term I should use to describe removing residual electricity from a device?
...

"Impossible." Removing those pesky residual electrons, that is.

Oh, you mean 'removing all stored energy' from something. Because that's the name of the game.

Stored mechanical energy (garage door springs) or stored pneumatic energy (the hose whip on an LN2 hookup), stored electrical enegy, stored thermal energy (don't grab the hot end of a big soldering iron), stored chemical energy (be sure the action is open and there's nothing in the pipe....)

 

[Strostkovy]

It is with respect to most power systems covered by the NEC.

In case of innards of a device, then it is incorrect to refer to "ground" unless it is referenced to the power ground. In such a case of a "floating" circuit, "circuit common", or similar terms ought to be used.

The common terms are "Earth ground" or "Chassis ground". In typical use "ground" just means 0 volts.

Maybe it shouldn't be that way, but it has been for a very long time.

 

[JST]

The common terms are "Earth ground" or "Chassis ground". In typical use "ground" just means 0 volts.

Maybe it shouldn't be that way, but it has been for a very long time.

Correct, it should not.

Very misleading, and can be fatal if taken literally.

For those of us who deal with schematics, and not power wiring diagrams, ground is ground and other things are not.

 

[JST]

Eh, the schematic is wrong or right. If the wire is isolated, and has a ground symbol on the schematic, the schematic is wrong.

 

[Strostkovy]

Eh, the schematic is wrong or right. If the wire is isolated, and has a ground symbol on the schematic, the schematic is wrong.

I don't know how applicable these are still but I see them commonly used. Can't say I've seen any modern profession schematics because they don't share those anymore, but I see lots of documented personal projects, as well as my own.

Then again, resistors are jagged squiggles and logic gates all have distinct shapes and you can't tell me otherwise. Whatever standards body disagrees can f off with their "every component is a box" approach.

 

[Big B]

Wow. Rough crowd.

In my working days we had upwards of 100 journeyman electricians in our plant and one thing that I learned from them was that if there was any doubt you checked it with a meter. A transformer with no power to it isn't a problem but one with wires going to it needs to be checked with a meter. Anything that may have capacitors or batteries must be checked. Just assuming that something is dead is a good way to die.

 

[Strostkovy]

Wow. Rough crowd.

In my working days we had upwards of 100 journeyman electricians in our plant and one thing that I learned from them was that if there was any doubt you checked it with a meter. A transformer with no power to it isn't a problem but one with wires going to it needs to be checked with a meter. Anything that may have capacitors or batteries must be checked. Just assuming that something is dead is a good way to die.

On that note I really wish my meter had like a 6vac output that I could touch the probes to and double check my meter functions. Assuming a zero on my meter means no voltage is equally deadly.

I once worked on an electrical panel that had a main feed, and two separate breakers that were live from the load side from two different services. I shut the whole building down and still had power at the stupid panel.

 

[Strostkovy]

A) Use the Ohms scale or the Diode test scale or the continuity buzzer/horn.

B) WTF are you doing with your experience still wandering about with only ONE meter?

I won't go to the field with fewer that two, usually have a scope plus three on the bench, if not two scopes.

It's just easier than playing one-probe hopalong in a multivariate environment.

Kinda like the randy mouse as took a lady giraffe as a lover.

Between shagging, teasing tit, licking ears, and french kissing, poor bastard ran himself half to death.

I do that, but far prefer to have it to the correct settings and then test it, rather than check it in continuity mode and forget. At my job it's rare I have a complete uninterrupted minute to myself.

I have several meters, but they are more for low voltage stuff. I only have one I trust on higher voltages. Can't say I typically need more than one for running or debugging circuits, and I just about never need my amp clamp for this sort of stuff.

Now if there were an (affordable) meter with the appropriate ratings that only measured AC volts and had both a test voltage source, and noncontact voltage detection, I would buy it immediately.

I just keep it simple and run the appropriate sized wire through the appropriate sized conduit into appropriate sized boxes with appropriate overcurrent protection and disconnects/outlets. If there is a short in a box I look at it and fix it. If there is a short in a run of conduit I rip all of the wire out of it and rerun it. Just not that much in depth debugging needed. The fanciest I get is measuring ground impedance on questionable conduit runs. (I've never seen anything needing repair)

 

[jim rozen]

One brief word of advise: don't use inexpensive, general pupose multi-meters for testing power circuits. Use the correct wigger type testers designed for this.

Bad stuff happens when you stick an old simpson meter, accidententally set to the wrong range, across some main lugs on a panelboard. 'Nuff said.

 

[TDegenhart]

One brief word of advise: don't use inexpensive, general pupose multi-meters for testing power circuits. Use the correct wigger type testers designed for this.

Bad stuff happens when you stick an old simpson meter, accidententally set to the wrong range, across some main lugs on a panelboard. 'Nuff said.

Better yet. If you don't know what you are doing (you many think you do), DO NOT MESS WITH POWER!!! That stuff can kill.

Tom

 

[jim rozen]

As the previous wise man said (paraphasing) 'If you don't know how to work the internet stay OUT of the panelboard.'