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drewcrew6 December 6th, 2002 02:12 PM

grounding power sources
 
Since we did 24vdc on this lets continue. What about a control panel with a transformer supplying 120vac (actaully any ac voltage) should 1 terminal be grounded?

I believe that you should. But have seen a decent amount of panels built with this spec'd to not be grounded. I have never been able to come up with a reason for this other than if a wire gets grounded for some reason (wear or cut into). Then the piece of equipment will continue to run with blowing a fuse or breaker. That would benefiet a continuos operation that has considerable down time due to control power loss.

But what if one of those wires come in contact with say a 480 vac line then the potetial of the 120vac is now up at possibly 480+120. Depending on the phasing that could be well over 600 volts the limit of most control wiring.

Sounds like a saftey risk .

Not to mention that the person servicing this knows its 120vac. And works with a little less care than if it was a 480vac potential which we all do but shouldn't.

Any comments gladly accepted



Drewcrew6

awalinski December 6th, 2002 02:48 PM

In our application few weeks ago, none of the 120 VAC relays or valves were working, even though they were energized by the program and we were passing power to PLC output terminals.
Everything went fine when we grounded the neutral at the transformer.
Obviously there wasn't a required potential difference before grounding.

drewcrew6 December 6th, 2002 03:02 PM

awalinski : Sounds like you're now using the ground for the return current path. That's not a good thing.

You said that everthing worked after you grounded the neutral at the transformer. And that there wasn't a potential difference before grounding.

That potential difference was at the transformer but not at the relays and you just completed the loop by using the ground to carry the return current. Putting the ground on the neutral won't make the transformer to just start working it COULD just complete the ability to do its job.

Sounds like you need to look at the complete path of the neutral I think you will find a problem there.

Drewcrew6

okiebob December 6th, 2002 03:43 PM

oh boy here goes
 
Your 120VAC should go to its last device that needs it and return to to source NEUTRAL. That said this is NOT a ground yet may be at ground potential. This is just a return current path. The ground itself is there to CATCH and DUMP a faulty (shorted) circuit SAFELY to SAFE potential. With this being the case the chance of a LIFE threatening fault is reduced. If the neutral goes to ground prior to its completion of the circuit the process should STOP (no power) and the GROUND installed at that enclosure should safely dump. This is NOT ideal for equipment and or processes (smoke gets out of the box) but it works well for us soft two leggers. Another cool thing is that a little ground goes a LONG way. Throw major volts at it and it is still a GROUND. Blondie did a song about what is fixin' to happen. The name is something like RIP ME/HER TO SHREDS. Remember the order Big bricks / Little Bricks

Eric Nelson December 6th, 2002 03:48 PM

120vAC Control Circuit Transformer...
 
I always ground one side of a control transformer (X2 out of habit). I look at it as a new power source, so by grounding X2, you create a "neutral" conductor. Although, there's no requirement that it MUST be grounded...

From NFPA 79...

9 Control circuits

9.3 Grounding of control circuits

Grounded or ungrounded control circuits shall be permitted
as provided in 19.7. Ground faults on any control circuit shall
not cause unintentional starting or dangerous movements, or
prevent stopping of the machine.

19 Grounded circuits and equipment grounding

19.7 Control circuits

Control circuits shall be permitted to be grounded or un-
grounded. Where grounding is provided, that side of the
circuit common to the coils shall be grounded at the control
transformer if alternating current or at the power suppy termi-
nal if direct current.

Exception No. 1: Exposed control circuits as permitted by 9.2.1, Ex-
ception No. 2, shall be grounded.

Exception No. 2: Overload relay contacts shall be permitted to be
connected between the coil and the grounded conductor where the con-
ductors between such contacts and coils of magnetic devices do not ex-
tend beyond the control enclosure.


9.2 Control circuit voltages

9.2.1 Alternating-current (ac) control voltage shall be 120 volts
or less, single phase. Where the supply voltage is greater than
120 volts, the control voltage shall be provided from a trans-
former with an isolated secondary winding.

Exception No. 1: Other voltages shall be provided, where necessary,
for the operation of electronic, precision, static, or similar devices used
in the control circuit.

Exception No. 2: Exposed, grounded control circuits shall be permit-
ted where supplied by a transformer having a primary rating of not
more than 120 volts, a secondary rating of not more than 25 volts,
and a capacity of not more than 50 volt-amperes

Exception No. 3: Any electromechanical magnetic device having an
inrush current exceeding 20 amperes at 120 volts shall be permitted to
be energized at line voltage through contactor or relay contacts. The
contactor or relay contacts shall break both sides of the line voltage cir-
cuit to the magnatic device. The relay coil shall be connected to the con-
trol circuit.


I guess that answers that... :D

beerchug

-Eric

tomneth December 6th, 2002 07:08 PM

And this is where a question comes up as to grounding/shielding. I recently installed some 120 VAC transformers to run some heaters. Our plant requires us to ground the neutral at the transformer. We were also told to run a ground wire from the casing to the grounding bar inside the cabinet. This grounding bar also has some shield wires attached from analog input signals. Is this a good practice or would I be better off with a seperate 'shield' bar for the shield wires even though it would be attached to the grounding bar?

Eric Nelson December 6th, 2002 07:16 PM

Quote:

Originally posted by tomneth
...This grounding bar also has some shield wires attached from analog input signals. Is this a good practice or would I be better off with a seperate 'shield' bar for the shield wires even though it would be attached to the grounding bar?...
It shouldn't make a difference, but one advantage of a seperate bar for the shields might be that you'll have the ability to isolate the shields (or experiment with grounding them at a different locations) in the future if problems arise...

-Eric

rsdoran December 6th, 2002 07:59 PM

Quote:

I always ground one side of a control transformer (X2 out of habit). I look at it as a new power source, so by grounding X2, you create a "neutral" conductor. Although, there's no requirement that it MUST be grounded...
So if you need 220/240 single phase and use a transformer to step down from 480 you GROUND one side of the transformer?

BTW doesnt using X2 imply the of use 120v single phase control transformers? Control voltage isnt always 120volt...is it?

Eric Nelson December 6th, 2002 08:08 PM

Quote:

Originally posted by rsdoran
So if you need 220/240 single phase and use a transformer to step down from 480 you GROUND one side of the transformer?
Absolutely not!... Notice the subject line of my post...
--- 120vAC Control Circuit Transformer... ---

Quote:

BTW doesnt using X2 imply the of use 120v single phase control transformers?
Yup! http://www.plctalk.net/qanda/images/icons/icon14.gif

Quote:

Control voltage isnt always 120volt...is it?
Nope! http://www.plctalk.net/qanda/images/icons/icon13.gif

beerchug

-Eric

godfrey December 6th, 2002 08:19 PM

X2 simply implies the secondary not the voltage. There are many different flavors.

Eric Nelson December 6th, 2002 09:08 PM

Good point Godfrey... 95% of my transformer applications are 120v secondaries, so I forgot that the "X"s are simply secondary lead/terminal markings.

Ron, see THIS PAGE from Sola

beerchug

-Eric

rsdoran December 6th, 2002 09:31 PM

thanks but thats one area that I dont need assistance with. I deal with transformers of all FLAVORS...that was part of my point with the X2 comment. I will state though that X2 is common to 120vac transformers in alot of cases but not all.

I asked that question for one reason though, my lead man and I were discussing this subject this afternoon. He had a situation where when setup up some machines that were 240v single phase the person in charge demanded that one side of the xformer be grounded and that some of that old system is still around. He didnt like it. What this does as most know is when using a meter is make one side HOT compared to ground, the same as a 120vac xformer does when one leg is grounded. Ever checked a xformer not grounded or doesnt have a center tap that is grounded. When you read to ground you may not get a reading at all or half voltage reading.

Good topic for my new website

rogerhollingsworth December 11th, 2002 02:06 AM

Grounding
 
Awalinski

Drewcrew6 is correct, that is not a good situation, follow up before someone gets hurt.

I believe in separation and isolation, I do not agree with grounding one side of a control transformer and the situation with Awalinski is case and point.
I like to layout the circuitry visually and physically separated with separate grounds for AC and DC. As I once read, compare your control system to your body, if you put bad food in you are going to get sick. The same goes with your control circuit.

When designing your control system you should know what your power source is, starting at the high voltage substation and follow that grounding/power scheme through out.

(going back in time)
Tying the Negative leg of the DC power supply to ground is not a good idea, it compromises the isolation between the AC and regulated DC.
All DC power supplies offer a plus (+) and minus (-) output connection that is isolated from the AC input.
The AC ground should in no way be connected to either the positive or negative connection of the 24VDC power supply.
Harmonic current flow in the ground wire is a good reason to keep the AC input separate and isolated from the DC output.

Noise does not necessarily need to be directed to ground, merely returned to its source.

This is strictly my opinion which is subject to change without notice.
(only small stones please) :D

drewcrew6 December 11th, 2002 03:31 AM

rogerhollingsworth states
Quote:

Tying the Negative leg of the DC power supply to ground is not a good idea, it compromises the isolation between the AC and regulated DC.
What isolation do you mean by that? I'm not sure if I'm missing something. The regulation is between the ouput points and shouldn't be affected by a ground potential at one.

It makes sense to me that you have a possible safety situation when things aren't grounded .

As I had stated I always like to see almost any power source (controls are a must!!) "grounded" at one point (preferably the comm on dc and must be done at the source termination point!!)to keep things at a certain reference. Thermocouple cards don't seem to like non grounded supply voltages.There are some exceptions :any thing higher than 120v for specific reasons like heaters


Drewcrew6

rogerhollingsworth December 11th, 2002 06:13 AM

Ground
 
The Isolation I was speaking of was the transformer isolation between the AC input and the (induced) rectified output.

The term GROUND is a loosely used term that has strayed from its true meaning. Here I have been guilty of stating Ground, when I should have stated AC return leg. With the AC return leg of the control transformer connected to ground. Now you have AC return, AC ground, DC Neg. at the same potential. Unless I am wrong I think that is the "common ground" approach, That is a widely used and generally accepted method. I'm not condemning it I just prefer the "separate ground" approach.

Any harmonic distortion concerns here? utoh


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