Inappropriate use of control transformer???

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arcsnsparks98

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This past week I was doing some work in an industrial facility where I noticed something I have never came across before. Mounted on the sides of a few of the machines were convenience receptacles. After paying closer attention I discovered they were fed from the 480/120 control transformer inside the control cabinet. I didn't see the exact VA rating but apparently it was enough that it could support a few amps at a receptacle. What concerns me is the fault current path return path in the event of a fault. Control transformers aren't intended to be utilized outside of the control cabinet and associated control wiring of the machine in which they are installed. The 120v secondary is floating in this case because one side of it is not bonded to ground.Am I correct in saying that in the event of a ground fault somewhere on this convenience receptacle circuit, there is not an effective return path to the source to allow for adequate protection? There are primary side fuses for the transformer but without fault current being able to return to it, I don't know that they would do much good. The ground from the receptacle is electrically connected to the ground from the primary so fault current could flow back to the building GES but that may not be sufficient to make it safe. Any thoughts?
 
The secondary of the transformer should be grounded per Art 250.30.
 
arcsnsparks98 said:
This past week I was doing some work in an industrial facility where I noticed something I have never came across before. Mounted on the sides of a few of the machines were convenience receptacles. After paying closer attention I discovered they were fed from the 480/120 control transformer inside the control cabinet. I didn't see the exact VA rating but apparently it was enough that it could support a few amps at a receptacle. What concerns me is the fault current path return path in the event of a fault. Control transformers aren't intended to be utilized outside of the control cabinet and associated control wiring of the machine in which they are installed. The 120v secondary is floating in this case because one side of it is not bonded to ground.Am I correct in saying that in the event of a ground fault somewhere on this convenience receptacle circuit, there is not an effective return path to the source to allow for adequate protection? There are primary side fuses for the transformer but without fault current being able to return to it, I don't know that they would do much good. The ground from the receptacle is electrically connected to the ground from the primary so fault current could flow back to the building GES but that may not be sufficient to make it safe. Any thoughts?
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How and why would a secondary fault cause current to go back to the building GES? It wont, in fact zero current will flow. Grounding electrodes do nothing with fault clearing.
 
arcsnsparks98 said:
This past week I was doing some work in an industrial facility where I noticed something I have never came across before. Mounted on the sides of a few of the machines were convenience receptacles. After paying closer attention I discovered they were fed from the 480/120 control transformer inside the control cabinet. I didn't see the exact VA rating but apparently it was enough that it could support a few amps at a receptacle.
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A very common thing to do.

What concerns me is the fault current path return path in the event of a fault. Control transformers aren't intended to be utilized outside of the control cabinet and associated control wiring of the machine in which they are installed.
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What makes you think that? A convenience receptacle can be used for whatever purpose one wants.

The 120v secondary is floating in this case because one side of it is not bonded to ground.
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It probably should be, note the use of the word "probably".

Am I correct in saying that in the event of a ground fault somewhere on this convenience receptacle circuit, there is not an effective return path to the source to allow for adequate protection?
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Adequate protection from what? If the outlet is a GFCI there is probably adequate protection to prevent electrocution.

There are primary side fuses for the transformer but without fault current being able to return to it, I don't know that they would do much good.
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There should be fuses in the ungrounded conductors on the secondary side as well.

The ground from the receptacle is electrically connected to the ground from the primary so fault current could flow back to the building GES but that may not be sufficient to make it safe. Any thoughts?
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The building GES plays no part in fault clearing in a properly designed system.
 
Woah, lets pull in on the reigns a bit. Bob, a convenience receptacle fed from a lighting and receptacle panel could certainly be used for whatever you want. A control transformer has a specific VA rating sufficient to do the job it is designed to do. They are not sized to turn the control cabinet into a load center. At no point was GFCI protection mentioned because the receptacles are not GFCI protected. And I am well aware of the fact that a grounding electrode is not there to clear a fault. However, when voltage is applied to a grounding electrode, current will flow. I mentioned the GES because, in a worst case scenario, there might just be enough current flowing on it to blow a fuse. Of course that is not ideal or even a correct design.
 
mbrooke said:
How and why would a secondary fault cause current to go back to the building GES? It wont, in fact zero current will flow. Grounding electrodes do nothing with fault clearing.
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While you are partially correct, current will absolutely flow to the GES. Most likely not a sufficient amount to clear a fault but a measurable amount of current will flow.
 
arcsnsparks98 said:
Woah, lets pull in on the reigns a bit. Bob, a convenience receptacle fed from a lighting and receptacle panel could certainly be used for whatever you want. A control transformer has a specific VA rating sufficient to do the job it is designed to do. They are not sized to turn the control cabinet into a load center.
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You did not say it was undersized for whatever was plugged into it. There is no limitation on how large a control transformer can be. It can certainly feed a panelboard if you want to do so and it is sized appropriately.

At no point was GFCI protection mentioned because the receptacles are not GFCI protected.
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If this was a listed panel, UL508a would require the receptacle be a GFCI. Many control panel manufacturers have been making such receptacles GFCI for a long time, listed or not.

And I am well aware of the fact that a grounding electrode is not there to clear a fault. However, when voltage is applied to a grounding electrode, current will flow.
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The is not true at all.Current flows back to the source. If the GE is not part of that path no current will flow. Unless this is an autotransformer there should be isolation between primary and secondary so there should be no path for current to flow in the case you described if the GE was energized via the secondary side of the transformer.

I mentioned the GES because, in a worst case scenario, there might just be enough current flowing on it to blow a fuse. Of course that is not ideal or even a correct design.
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Based on what you said was the case, there likely will be no current flow at all thru the GE under a GF condition.

Look at it this way. You have an ungrounded secondary. If you unintentionally ground one of the legs, it is no different than if you had intentionally grounded it as is the normal case. The electrons have no way to know whether you grounded the wire deliberately or accidentally.
 
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arcsnsparks98 said:
TJust because X2 isnt bonded to the GEC wouldnt stop current from flowing to the GES.
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Actually, yes, it would stop current from flowing to the GES.

The secondary is only magnetically connected to the primary. Without the secondary side being bonded to ground there is not circuit for current to flow on the GES.
 
Lets say I have a pretty standard 480/277v 3phase 4 wire transformer and I dont bond X0 to the GES. I Touch L1 with one finger and I touch the grounding electrode with the other, I am at no risk of electric shock?
 
augie47 said:
The secondary of the transformer should be grounded per Art 250.30.
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I agree with Gus here.

Regardless of it being a control circuit or not as soon as receptacle was added to it, that receptacle has to be supplied from a grounded system.

Once we accept that we can move on to other issues like 'Was it a good idea?' or 'Are we actually allowed to supply a 15 or 20 amp receptacle that can only provide a couple of amps?' etc.
 
arcsnsparks98 said:
Lets say I have a pretty standard 480/277v 3phase 4 wire transformer and I dont bond X0 to the GES.
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OK

I Touch L1 with one finger and I touch the grounding electrode with the other, I am at no risk of electric shock?
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Well you tossed a wrench into the works by using 'L'. :)

GES to H1, your toast

GES to X1, ... You might be toast but the actual current, if any will be minimal and only the result of capacitance coupling. It would depend on the amount of wiring connected to the transformer but the current would be very low. The problem with using a human as the test instrument is that we are really delicate and it does not take much to do us in.


Lets go back to that 480-208Y/120 unbonded transformer.

Kill it, connect X1 to the GES, turn it back on. What will happen?
 
arcsnsparks98 said:
Lets say I have a pretty standard 480/277v 3phase 4 wire transformer and I dont bond X0 to the GES. I Touch L1 with one finger and I touch the grounding electrode with the other, I am at no risk of electric shock?
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Yes, but from capacitive coupling, not because there is an actual physical current path. The amount of current will be based on the physical size of the circuits connected to the transformer. The more wire the more capacitive current. Not enough to do any work, but enough to ruin your day.

With that transformer you could connect any one of the secondary wires to the grounding system and there would not be any current flowing to the grounding system.

In your original example, all that happens when you have a ground fault on something that is connected to the receptacle, is that you just changed the system from an ungrounded system to a grounded system. Now with a second fault there is a current path, and if both sides of the secondary of the CPT are fused, one or both of the fuses should open.
 
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don_resqcapt19 said:
In your original example, all that happens when you have a ground fault on something that is connected to the receptacle, is that you just changed the system from an ungrounded system to a grounded system. Now with a second fault there is a current path, and if both sides of the secondary of the CPT are fused, one or both of the fuses should open.
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Boom. That just made this whole thing make sense. I just gained a new perspective on transformer grounding. :)
 
I was 110% sure that any source of AC energy, no matter how many SDS's down the line, would have potential to a grounding electrode (other than capacitive coupling). It seems this isnt exactly the case. To anyone I may have offended early in this thread by relentlessly insisting there would be current to the GES, my apologies.
 
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