quiz: could I get shocked if.....

[thinfool]

utility distribution systems.
For a delta, why would it be grounded?

In theory it wouldn't. In real life, virtually all utility distribution networks are 'Y' (grounded systems) in the 12-13kv range. This is how they get 7200 +/- volts for the primary of a typical residential xfmr. The utility neutral is grounded at every xfmr location. So if you have a large residential area with say...30 xfmrs, the common neutral/ground will connect to the earth in 30 locations. In some areas there are upgrades happening to 24-25kv. I suspect that these are also 'Y' systems.

You could have a dedicated feed from a substation to an industrial site. Some utilities provide a 'y' on the primary of a padmount, some provide a delta. Either way, the neutral/ground cable provides solid earthing for the system.

I cannot speak for transmission systems.

For a wye, is the neutral usually grounded (again talking about utility distribution not lines/transformers serving a customer).

Yes, see first answer.

Say it was an ungrounded system.

"It was an ungrounded system"

what kind of voltage would be required for me to get a shock due to stray capacitance? I have a neon sign transformer that puts out 7500 volts and we like to play with that sometimes - just touching one lead bites a little..

Licking your finger will enhance the experience, or you could try using a voltmeter (way safer).

Also I am trying to convince someone that electricity seeks the source, NOT the ground.

In real life, electricity will always have a relationship with ground. Whether you can have a direct or capacitave experience with it depends on several factors.

not planning a stunt or demonstration

Great! There are enough bad stories in the news these days.

 

[winnie]

Actually, 'hundreds of microfarads' (uf) would be more accurate. A Farad is one heck of a capacitance, and is capable of storing quite a large amount of energy, approaching the storage capacity of a battery.

No, I really meant hundreds of farads.

http://www.maxwell.com/ultracapacitors/products/large-cell/bcap3000.asp

Though I guess I should have said kilofarad

The energy density of these devices is perhaps 1/30 of that of good lithium ion cells, but the power density is tremendous.

-Jon

 

[tallgirl]

No, I really meant hundreds of farads.

http://www.maxwell.com/ultracapacitors/products/large-cell/bcap3000.asp

Though I guess I should have said kilofarad

The energy density of these devices is perhaps 1/30 of that of good lithium ion cells, but the power density is tremendous.

There was a time back in the early 1980's, I think it was, when some vendor (Fairchild, I believe) sold devices between 1 and 10F for use as short term backup power. My initial reaction was the same as Larry's -- surely they meant mF and not F, but nope, they were greater than 1 Farad capacitors that could be mounted on a PCB.

I don't recall why they went out of fashion, but I suspect the use of lithium batteries on PCBs for BIOS NVRAM backup obsoleted them.

 

[steve66]

I'm still guessing that a shock caused by touching one phase on a ungrounded shipboard system is caused by a ground fault monitoring system. If you have an ungrounded system, you are going to want a warning when one phase does become grounded. That warning is probably going to take the form of alternately connecting the phases to ground through a high resistance, and seeing if any current flows. If enough current flows, you get a ground fault indicator light. And if you are standing on a metal deck, and you touch one phase wire, the current will probably flow through your body, to the ships hull, and back through this high resistance to the source.

When we talk about currents flowing through a parasitic capacitance, don't forget about Kirchoffs current law. Current can't flow from a system to some ungrounded object without flowing back to the system again. And to get back to the system, it has to go through another parasitic capacitor.

Like Rattus said, the capacitance forms a wye. But the center point of the wye is unconnected, so the current has to flow through two of the capacitors.

 

[charlie b]

I was out of touch this weekend, and didn?t have a chance to pipe in on this one until now.

I'll bet Charlie B. has some insight . . . .

You are too kind. I?ll offer what I can.

I?ll start by saying Winnie?s contribution (post #12) was a good summary. But I would not apply the word ?parasitic? to this situation. Any two things that are conductive and that are not in contact with each other will create a capacitor. There is capacitance between the watch on my wrist and the paper clip on your desk. Not a large one, to be sure, but it is what it is.

To Winnie?s description I will add that when you first turn on the circuit, and when electrons start to build up on one face of the capacitor, what do you suppose it looks like to the electrons on the opposite face? It looks like a large build up of negative charge. Recalling that ?like charges repel,? you could imagine that the electrons on the opposite face will feel a force that tends to push them away from their position. As they move away from the capacitor?s plate, where do they go? Down the wire, and through the rest of the circuit. That is why, from the perspective of a person looking at the circuit from the outside (i.e., can?t see what is happening inside the capacitor), it just looks like current is flowing into one wire of the capacitor and out the other wire.

If the system isn't grounded, there should be very little capacitive coupling through the ships frame. Any coupling through the ship would have to be from one phase of the power system, to the ships frame, and back to another phase of the power system.

But the center point of the wye is unconnected, so the current has to flow through two of the capacitors.

Not true. Only one capacitor need be involved, and the shock can be fatal. Imagine holding (carefully, wearing rubber gloves) a resistor in one hand and a charged capacitor in the other hand. Bring them together and touch their wires to each other, so that they become in parallel. That is a completed circuit, and the capacitor will ?discharge? through the resistor.

Now I repeat that there is capacitance between any two metal items on the planet. There is capacitance between the wires (running from one part of a ship to another), and the metal structure of the ship. It can be a large capacitance, because of the size of the hull, the proximity of the wires to the hull, and the length of the wires. If you touch a wire while standing on the hull, you become the resistor I mention in the paragraph above. The completed circuit consists of your body (the resistor) and the capacitance between the wire (to which your hand is connected) and the hull (to which your feet are connected).

Here is another theory. Maybe the shock is from some ground fault monitoring circuits.

Not necessary. That grounding circuit is tested by pushing a button once an hour. You would be shocked if you touched a live wire, even if nobody is pushing that button at the time.

 

[rattus]

Not so Steve:

Not so Steve:

I'm still guessing that a shock caused by touching one phase on a ungrounded shipboard system is caused by a ground fault monitoring system. If you have an ungrounded system, you are going to want a warning when one phase does become grounded. That warning is probably going to take the form of alternately connecting the phases to ground through a high resistance, and seeing if any current flows. If enough current flows, you get a ground fault indicator light. And if you are standing on a metal deck, and you touch one phase wire, the current will probably flow through your body, to the ships hull, and back through this high resistance to the source.

When we talk about currents flowing through a parasitic capacitance, don't forget about Kirchoffs current law. Current can't flow from a system to some ungrounded object without flowing back to the system again. And to get back to the system, it has to go through another parasitic capacitor.

Like Rattus said, the capacitance forms a wye. But the center point of the wye is unconnected, so the current has to flow through two of the capacitors.

Steve, the wye is grounded! We are talking of stray capacitance to ground. There is also capacitance between the lines as well, but that has no effect in this problem.

"Ground" in this case may be a metal deck, concrete, earth, or whatever.

 

[steve66]

Well, Its starting to feel kind of lonely on this side of the debate, so maybe I am missing something. But I am not ready to concede yet. Maybe I'm not thinking of the same situation everyone else is. I was thinking of Carl's post:

On Navy ships the 480 is ungrounded. I have it on very good authority that touching one of the phases will deliver a very nasty shock.

carl

So we have a 480V ungrounded system. We also have a metal ship. I assume that touching one phase, means the person is wearing insulated shoes (or were you all thinking of someone standing barefoot)?

So if you want to include the ship in the circuit, the current has two capacitors to go through:

1. The capacitor formed between the person's feet and the ship
2. And the capacitor between the ship and the other parts of the 480V wiring.

If you ignore the ship, the current has to go through a capactior formed between the person themselves, and all the other parts of the 480V wiring. system. But neither of these two items have large plated (like the ship) so I don't think you could get much of a shock through this capacitor.

I still don't see how one could recieve a shock that only includes #1 above. And I don't think the combinations of the two capacitors above would be any more than the capacitor formed directly between the person and the wiring.

Steve

 

[steve66]

And I don't think the combinations of the two capacitors above would be any more than the capacitor formed directly between the person and the wiring.

Steve

Looks like the very last sentance I posted above is wrong. I think the current does have to make it through both capacitors (#1 and #2 in my last post), but because of the large amount of wiring on a ship, that makes a better capacitor than the capacitance between the person and the wiring by itself.

http://av.rds.yahoo.com/_ylt=A9ibyK...mbia.edu/Members/dale/how-to/shipboardpwr.pdf

Steve

 

[charlie b]

Steve, for the purposes of this discussion we should consider the person to be barefoot, or wearing shoes that provide very little insulation. Current is presumed to pass from foot to floor with very little resistance. You are right to say that there is capacitance between foot and floor, and that that value of capacitance will not permit a significant current to flow. But the current flow through the shoes does not have to be very high, since as little as 0.1 amps can be fatal.

For the purposes of modeling, let?s use that resistor and that capacitor I mentioned earlier. Connect one wire of the resistor to one wire of the capacitor, and hold the other two wires close to each other. Be ready to touch the other two to each other, when the word is given to conduct the test. That is the same situation as a person (the resistor) standing on the deck of the ship. We will add the resistance of the shoes to the resistance of the body, for the sake of the experimental model, so that the connection between foot and deck has no resistance (just like they were two wires that have been connected together). If the person touches a live wire, the capacitor (i.e., between wire and hull) and the resistor (i.e., the person) are placed in parallel.

In this scenario, current will not pass from the energized wire, through the body, into the hull, then through the hull to the point at which the WYE of a transformer is bonded to the hull. That is because the WYE is not bonded to the hull. Actually, the 480 V shipboard distribution is a DELTA, and no portion of that system is connected to the hull.

But current will pass from the wire, through the body, to the hull, and will continue to do so until the capacitor ?discharges.? However, since this is an AC system, the capacitor will continue to charge and recharge on every half cycle. That will not bode well for the person touching the wire.

 

[winnie]

When we talk about currents flowing through a parasitic capacitance, don't forget about Kirchoffs current law. Current can't flow from a system to some ungrounded object without flowing back to the system again. And to get back to the system, it has to go through another parasitic capacitor.

Like Rattus said, the capacitance forms a wye. But the center point of the wye is unconnected, so the current has to flow through two of the capacitors.

I think that the current flow though the ground detectors is an important point, one that makes an ungrounded system into an 'unintentional' resistance grounded system. In the case of intentionally resistance grounded systems, the grounding resistance is designed to pass several amps, more than enough to kill.

However I think that you underestimate the parasitic capacitance issue. Presumably the metal deck has capacitive coupling to the wiring system. You you touch the metal deck and a wire in the ungrounded system, the circuit consists of the wire, you, the metal deck, and then a _single_ capacitor to either of the other phases is enough to complete a circuit. So you have two capacitors, but they are 'parallel' to the other two phases.

-Jon

 

[rattus]

Too Hard:

Too Hard:

You guys are making this too hard!

Consider a delta service with no resistive connection to ground anywhere.

Stray capacitance exists between each line and ground.

This capacitance appears as a reactive load in a wye configuration to ground.

Current flows through these caps, therefore voltage appears across these caps.

Anyone foolish enough to place his bod across one of these caps conducts current through his foolish bod. In other words, he gets knocked on his can!

 

[LarryFine]

I think the current does have to make it through both capacitors . . . , but because of the large amount of wiring on a ship, that makes a better capacitor than the capacitance between the person and the wiring by itself.

If by "better capacitor", you mean a larger value and thus a lower impedance, then keep in mind that the voltage will be greater across the higher impedance (your body) when two impedances are placed in series.

 

[johnny watt]

I have used a zero to 150VAC isolated variable power supply to work on troubleshooting electronic equipment.

It has saved me from getting a shock on many occasions.

No doubt, if there were enough capacitive coupling to ground then I would have still gotten shocked.