knights of 3

Hi all,
One of my guys was working on a control ciruit for a fan bank. The control circuit in powered by a 480-120 V control xfmr. His voltages from x1-x2 were 123 Vac. Across all energized relays: 123 Vac. But when he took voltages to ground he got around 40 Vac from the X1 bus to ground and 80 Vac from the X2 bus to ground.
I asked him if the transfromer's X2 terminal was properly bonded to ground. He said he could not definetly find where the control circuit was grounded.
I told him there is a reference to ground somewhere in the circuit. My guess is that the XFMR is not grounded and he is reading a voltage divider from the high resistance (megaohms?) leakage to ground form each leg of the circuit.
Unfortunately we cannot tag out this circuit to do furthur testing so this all the info I have right now.
Can anyone give their thoughts and the theory behind these strange voltages? Remember I THINK the ground is not connected to X2, but I don't know if it is for sure, I haven't seen it myself.
thanks in advance for your theories.
Danny

eddy current

Usually X2 is grounded right away. Trace out the wires leaving X1 and X2 and see where they go. If X2 is not grounded you should turn off the circuit and ground it.

John

What eddy current said. Plus I see a safety issue with the control circut not being bonded.

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knights of 3

We all kind of agree the reason is becuase X2 in probably not grounded.

HOWEVER, I am looking for the ENGINEERING THEORY or formulas of why I am getting 80 v to grd from one leg and 40 V to grd from the other leg.
My question is WHY doe these voltages occur? And what does the term "floating ground" mean? I have heard it before but don't quite know the true definition.

ALSO, this circuit cannot be tagged out right now due to its importance. so I can't turn it off and I can't take megger readings to ground.





thanks Danny

InPhase277

The circuit cannot be grounded, otherwise you would read 0 V to ground from one side, and 120 V to ground on the other. The reason why you read the voltages you do is due to capacitive coupling. One leg is more strongly coupled, probably due to its location or proximity to other wires. Try to power a small load from the transformer to ground, like a wiggy. I bet nothing happens. I also bet that the circuit could be ground while it is energized, but this is probably a delicate operation.

Mike_586

I know what it means and why its happening, I just can't put it into words properly.

But the short version is that the 'floating ground' is giving the circuit a reference to ground somewhere in the middle of the circuit and X1/X2 are giving different values because their potential to ground is different relative to the point where the circuit is actually grounded at. Were X2 grounded properly, this 'floating ground' would be a short to ground. Can't have more than one reference point to ground on a circuit.

IMM_Doctor

RE: Floating Ground

You do NOT have a floating ground, you do not have a GROUND. An AC inductive transformer has a primary winding and a secondary winding, they are electrically isolated form each other. The primary is fed with 480v (ground referenced system) and the secondary energy is trasferred via non-conductive magnetic flux. The secondary winding has no grond reference. There is a VOLTAGE POTENTIAL and watts available at the two secondary conductors, but if no intentional bond is made to ground, then no real voltage measurment is valid.

Now, if you test or measure the voltage on one of the current carrying control circuit conductors (in reference to GROUND) with a high impedence digital multimeter, you will see various voltages. I dislike the terms "floating" and "phantom", because you are really measuring a voltage pontential. Insulated conductors in any equipment are typically are in close proximity of metalic enclosures. This is true of both sides of the control circuit. AC can pass though a capacitor. A few feet of copper insulated wire in a raceway or an enclosure is a small capacitor, and will pass very small amounts of AC current. This current is enough to register on a hign impedance DMM.

If there are NO real problems (ground faults within the circutry fed by isolated control voltage) then....

....the root answer to your question "Why do these voltages occur?" is,
CAPACITANCE.

nick

Well ill give it a try - A [ floating neutral ] is a high or low resistive voltage drop the path thur a load or other connection point thur load or source winding to opposite polarity of source not a solid connection just a found connection a radom connection .
We see the lowest level or point of resistance to opposite side or opposite potential of polarity from that source returning point meaning degree of wave positive to negative of ac [ ~ ] sine wave cycle thur zero crossing any resistance or load thur which that point can be found is at what ever voltage drop . E= IXR like a voltage divider circuit any path any way to get back to source .

Meaning its kinda that resistance of contact that determines that voltage at that point it can be any voltage it can change in voltage due to resistance or ambent temp freq and at higher freq capacitance may be added in circuit thur loads or path thur which current flows or finds a path back to source. Well its the best i can give ya hope it makes sense ? look up see IEEE STD 141 CHAPTER 7 lots of fun reading . ITS CALLED FLOATING VOLTAGE take care

knights of 3

I wanted to test the theory of capacitive coupling, so I had a control XFMR laying around and wired up on the bench. I plugged in 120 from the wall outlet and set it up to put out 60 on the secondary winding. I did NOT ground the secondary output.

I took a fluke DMM and went from X1-X2 got 65 Vac.
Went X1 to ground got 29 Vac
Went X2 to ground got 30 Vac

I then took a wiggins tester and got nothing to ground on either leg.

Then I bonded X2 to ground and got:
Went X1 to ground got 65 Vac
Went X2 to ground got 0 Vac

I am very happy with the results and this proves the theory to me

Attached Images

120-60.bmp (58.4 KB, 20 views)

zgozvrm

... other than line-to-line, which is of course 120VAC. Line-to-ground measurements are invalid though.

If you don't want (or need a ground), this is perfectly okay to do. Not grounding one of the legs doesn't make the circuit necessarily unsafe, and it is done all the time (within NEC guidelines). For instance, look at the plug-in transformer that runs your typical sprinkler timer; it puts out 24VAC ungrounded. So does your typical doorbell circuit.

RIVETER

The reason you ground the service of a residence is to LIMIT voltage to ground to 150volts. Otherwise it will work normally if left ungrounded but will be pervious to outside influences such a transient over voltages. PeterD, what do you think?

zgozvrm

If you've ever seen an older house (i.e. pre-1970's?), you'll know that the wiring was usually ungrounded (hence, the 2-prong-only receptacles). Grounding the system allows us to have a safe path for current to return to the distribution panel in the event of a short circuit. In other words, the ground is not necessary, it's there for safety reasons only.

Just as a seat belt is not necessary in a car. The current laws state that you must ground residential circuits, and that you must not only have, but actually wear a seat belt.

wwilson174

Even though the wiring in the older houses consisted of two conductors (hot and neutral) the neutral conductor was always grounded. the third ( ground) wire was added to provide a bonding method for the appliances and raceway systems.

macmikeman

I don't know what Peter D is going to say, but I certainly have seen enough residences where the voltage during system overvoltages exceeded 150 volts to ground while the surge was happening. KABLOOYEY would be a word to describe the aftermath's. Those places were all grounded. Myth busted.....

Many transformers are purposely left floating in respect to ground, like in hospitals. Without problems.

macmikeman

Many short circuits occur with no current flow on the equipment ground conductors or frames of electrical equipment at all. It does nothing if line to neutral short circuits occur.

Bonding equipment will do what you describe in the event of a line to frame fault.

The practice of connecting a wire into some dirt on a dwelling exterior doesn't do much about any of the above unless some real high voltage gets imposed onto the dwelling system wiring by accident, and wants to find its way back to Niagra Falls....

brian john

very nice explanation

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brian john

OV= Over Voltage

Transients OV's to ground often exceed 150 VAC grounding, the systems will NOT limit OV. Grounding the system can minimize over voltages due to utility primary to secondary faults.

If a grounding electrode was that important the NEC would have a rule 25 ohms or 5 ohms or 1 ohms or less and you have to add electrodes till this level is achieved. NOT 25 ohms or less add a electrode and walk away no matter what the ohm reading.

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brian john

And way too many electricians, engineers and inspectors think the Earth is the answer when in fact all metallic components of the structure are what matter.

Even in an ungrounded system bonding is important, WAY IMPORTANT.

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micromind

Here's what can (and did) happen to an ungrounded system.

Sever years ago, I did a control house for a power plant. The utility voltage was 120KV, and there was a transformer from 120KV to 13.8KV.There was a 15KV metal-clad switchgear inside the control house, with power-operated circuit breakers. There was an overhead line from this plant to another plant about 12 miles away. One of the breakers fed this line.

The 13.8KV side of the transformer was delta, and the generators (13.8KV) were wye. Each generator had a grounding transformer with resistor connected from the center of the wye to the grounding system. The grounding system consists of a 4/0 CU ground grid around the perimeter of the plant, and connected to anything metal.

After the overhead line was connected to the load side of the breaker, the cut-outs on the other end were left open. Before the breaker was closed in, I suggested to the group of engineers who were present, something like "This line is nothing more than a big capacitor, there is no grounding transformer in the system, and no load on the other end."

"We have studied the line, one of the generators is running, its ground transformer is enough. Close the breaker."

I gave it one more shot, "What if the line is energized with no generators running, or what if the generators all trip out while the line is energized?" "We have studied the line, it's OK" was the response.

My thoughts were; the overhead line is 12 miles long, the humidity around here is usually in the single digits, there's no load on the other end, and it has the possibility of being ungrounded. But, as we all know, engineers are always right, especially when there is a group of them. More especially if the counter-argument is being made by just a dumb electrician.

I closed the breaker. The generator breaker tripped out instantly. Bam-bam; just like that. I thought.....this can't possibly be good! A second or two later, it sounded like a bunch of arc welders in the back of the main breaker cubicle. This is where the wire from the transformer comes in. 5-750 CU 15KV. A LOT of energy. I'll never forget the dumbfounded look on the engineers faces as I exited the building, at a VERY rapid pace.

I tripped out the 120KV breaker at the POCO meterhouse, and after the smoke cleared, we found that the surge capacitors had burned up. No huge explosion, just a lot of smoke.

My suggestions of a grounding transformer, and a trip switch for the 120KV breaker at the control house were taken a bit more seriously after this.

Capacitive coupling of an ungrounded system can result in VERY high voltages to grounded parts, including the earth itself. But more importantly, to grounded metal parts. I realize that 12 miles of overhead line is a bit different than a 120 volt control circuit, but the idea is the same.

Rob

P.S. The POCO meterhouse is a good 100 yards away, I bet I outran any NFL player!!