Current through body on an ungrounded system

[mull982]

If you touched one of the phase conductors on a 480/277V 3-phase ungrounded system what would be the current traveling through your body assuming we used 500ohms as the standard resistance for a human body. I konw on an ungrounded system there will be no current during a L-G fault so what happens when you come in contact with a conductor on an ungrounded system? Will there still be enough current through the system capacitive coupling to cause sever injury?

Now on a solidly grounded system again using the same 500ohm body resistance if you touced one of the phase conductors would this current through your body be 277V/500ohm = .554A, assuming an infinite bus on the 480V system? This does not seem like much current, but I suppose it is enough to cause severe damage to a human.

Now if someone were to come in contact with one phase of this system at an MCC level would the shock current be the same there as it would be lets say if they came in contact with a phase at a downstream lighting circuit that when through an isolation lighting transforer? Would you still use 277V/500ohms in both cases? Would you assume an infinite bus in both cases since the body resistance would be much greater than system impedance? Or would the current be less from the lighting circuit due to the fact that the upstream impedance would be greater at the lighting circuit as opposed to the MCC?

 

[ericsherman37]

If you touched one of the phase conductors on a 480/277V 3-phase ungrounded system what would be the current traveling through your body assuming we used 500ohms as the standard resistance for a human body. I konw on an ungrounded system there will be no current during a L-G fault so what happens when you come in contact with a conductor on an ungrounded system? Will there still be enough current through the system capacitive coupling to cause sever injury?

I don't know. Seems like 277V probably wouldn't be high enough on an ungrounded system for the capacitive coupling thing to push any significant current through the body. But I don't fully understand that concept to really say anything about it

Now on a solidly grounded system again using the same 500ohm body resistance if you touced one of the phase conductors would this current through your body be 277V/500ohm = .554A, assuming an infinite bus on the 480V system? This does not seem like much current, but I suppose it is enough to cause severe damage to a human.

Over half an amp would light a human up pretty darn good. We're filled with electrolyte-saturated water :grin:

Now if someone were to come in contact with one phase of this system at an MCC level would the shock current be the same there as it would be lets say if they came in contact with a phase at a downstream lighting circuit that when through an isolation lighting transforer? Would you still use 277V/500ohms in both cases? Would you assume an infinite bus in both cases since the body resistance would be much greater than system impedance? Or would the current be less from the lighting circuit due to the fact that the upstream impedance would be greater at the lighting circuit as opposed to the MCC?

I suppose the available fault current at the MCC would be higher than downstream, but even if the fault current going through a 500 ohm human somewhere downstream dropped by a couple tenths of an amp it could still kill you the same.

You figuring out how to electrocute somebody or something?

 

[don_resqcapt19]

If you touched one of the phase conductors on a 480/277V 3-phase ungrounded system what would be the current traveling through your body assuming we used 500ohms as the standard resistance for a human body. I konw on an ungrounded system there will be no current during a L-G fault so what happens when you come in contact with a conductor on an ungrounded system? Will there still be enough current through the system capacitive coupling to cause sever injury?

There will be no continuous current but there will be current flow at the time of the fault. I would expect that the capacitive coupling current can be more than enough to be fatal in some cases. I have seen it pull in my wiggy, a 20 to 30 mA load, a number of times. That is enough current to kill.

Now on a solidly grounded system again using the same 500ohm body resistance if you touced one of the phase conductors would this current through your body be 277V/500ohm = .554A, assuming an infinite bus on the 480V system? This does not seem like much current, but I suppose it is enough to cause severe damage to a human.

The trip point of GFCIs is set at ~5mA to protect people. Your current is 100 times that level.

Now if someone were to come in contact with one phase of this system at an MCC level would the shock current be the same there as it would be lets say if they came in contact with a phase at a downstream lighting circuit that when through an isolation lighting transforer? Would you still use 277V/500ohms in both cases? Would you assume an infinite bus in both cases since the body resistance would be much greater than system impedance? Or would the current be less from the lighting circuit due to the fact that the upstream impedance would be greater at the lighting circuit as opposed to the MCC?

It is the voltage at the point of contact that drives the current through the person. If the voltage is the same the current is the same. The point at which you contact the circuit does not change this. There is no need to look at the impedance of the system for this. It doesn't matter.

 

[rcwilson]

I know from personal experience that you can get hurt bad on a 480V ungrounded system. The system cpacitance is enough to knock you around. I was lucky with only bruises and sore muscles. (An old journeyman lineman used us first-time summer student engineers as voltage testers to "teach" us.)

It only takes a few milliamps through your heart to kill you.

As far as calculating possible fault current, the system is not "ungrounded", it is just not intentionally grounded. You don't know if another phase is already grounded. Use 480V as the driving voltage.

 

[jumper]

I know from personal experience that you can get hurt bad on a 480V ungrounded system. The system cpacitance is enough to knock you around. I was lucky with only bruises and sore muscles. (An old journeyman lineman used us first-time summer student engineers as voltage testers to "teach" us.)

Please tell me you are kidding, this gives me the willies(creeps) bad.

 

[nollij]

I was trying to come up with an equation recently that correlated the resistance to ground from a phase of an ungrounded system and the current flowing through that resistance.

After a few hours of floundering around, I devised that I was either approaching it incorrectly or that a numerical iterative method would be needed.

However, it is still possible to think in the extremes. Were an ungrounded system to have a short to ground on one phase (mutual reference for the three phases), the charging current flowing through that connection would be equal to the vector summation of the capacitive coupled current through the other two phases.

The question I have is: I see 3*Ico referenced for charging current during a ground fault (for HRG or ungrounded systems). However, I cannot determine more than sqrt(3)*Ico charging current during a HRG ground fault. (Ico is the magnitude of the charging current of one phase of the system during normal operation.) Where is this 3*Ico coming from?

 

[rcwilson]

That was over 41 years ago. That cantankerous lineman/relay tech/ electrician was a couple months from retiring. We were working on 1920 vintage voltage regulators with ungrounded 440V delta control power. The regulator's meters and controls were mounted on a marble slab with the power and control wiring connected to studs that extended through holes in the marble. After the regulator was supposedly cleared, he handed me a wrench and pointed at the connections to break open. I took the hit from my right palm to the upper arm which was touching a support. Nothing through the heart. Fortunately there was no tissue damage just pain and bruises from slamming into the railing behind me. I don't recall any near-miss investigation. He did retire that summer.

I learned a lot. (Maybe there was brain damage-which would explain why I post so much.)

 

[jumper]

That was over 41 years ago. That cantankerous lineman/relay tech/ electrician was a couple months from retiring. We were working on 1920 vintage voltage regulators with ungrounded 440V delta control power. The regulator's meters and controls were mounted on a marble slab with the power and control wiring connected to studs that extended through holes in the marble. After the regulator was supposedly cleared, he handed me a wrench and pointed at the connections to break open. I took the hit from my right palm to the upper arm which was touching a support. Nothing through the heart. Fortunately there was no tissue damage just pain and bruises from slamming into the railing behind me. I don't recall any near-miss investigation. He did retire that summer.

I learned a lot. (Maybe there was brain damage-which would explain why I post so much.)

Forum rules prevent from me saying what I want to in response to this guy, so I will say this:

!@#$%^&*&^%$#@!*&^^%$$#!!!!

 

[LarryFine]

Maybe there was brain damage-which would explain why I post so much.

My I borrow that excuse? :grin:

 

[netaguy]

Maybe this will help or is a good first hand example.

We just performed a system capacitive charging test to determine the correct neutral grounding resistor for a HRG installation we are installing.

We used a 500 ohm variable resistor typical to what is shown in the attachment and got test results as follows:

500 ohms - 224 mA
400 ohms - 235 mA
300 ohms - 242 mA
200 ohms - 252 mA
100 ohms - 292 mA
0 ohms - 550 mA

So the answer is yes, it could kill or cause severe injury.

 

[mull982]

There will be no continuous current but there will be current flow at the time of the fault. I would expect that the capacitive coupling current can be more than enough to be fatal in some cases. I have seen it pull in my wiggy, a 20 to 30 mA load, a number of times. That is enough current to kill..

O.k. so it looks like on an ungrounded system the current through the capacitance coupling is still enough to kill. The current runs through the body and returns on the capacitance coupling of the other two phases.

Did you simply connect your wiggy between one of the phases and ground?

The trip point of GFCIs is set at ~5mA to protect people. Your current is 100 times that level..

Is 500ohms a typical value that is used for the human body for such rough calculations? I thought I have seen this number used somewhere before?

So using this resistnace value the maximum current that will flow through the body at 277V will be about .554A? I have heard others state that 100's of amps would flow through body which I never chose to believe.

It is the voltage at the point of contact that drives the current through the person. If the voltage is the same the current is the same. The point at which you contact the circuit does not change this. There is no need to look at the impedance of the system for this. It doesn't matter.

Does the impedance of the system then only come into play for a bolted fault condition? For such faults with a fault resistance there is no need to consider upstream impedance? Therefore shock would be that same at 480V MCC bus as it would at a 480V lighting circuit?

 

[don_resqcapt19]

O.k. so it looks like on an ungrounded system the current through the capacitance coupling is still enough to kill. The current runs through the body and returns on the capacitance coupling of the other two phases.

Did you simply connect your wiggy between one of the phases and ground?

Yes the wiggy was between an ungrounded conductor and non-current carrying metal parts.

Is 500ohms a typical value that is used for the human body for such rough calculations? I thought I have seen this number used somewhere before?

So using this resistnace value the maximum current that will flow through the body at 277V will be about .554A? I have heard others state that 100's of amps would flow through body which I never chose to believe.

I am not sure what resistance is normally used for these types of calculations. The current will be limited by the impedance of the path and the available voltage. I think it would be almost impossible to get 100s of amps to flow through a person.

Does the impedance of the system then only come into play for a bolted fault condition? For such faults with a fault resistance there is no need to consider upstream impedance? Therefore shock would be that same at 480V MCC bus as it would at a 480V lighting circuit?

Yes the impedance only comes into play when there is enough current flowing that it limits the amount of current that can flow. With a person as the load in the circuit you only look at the voltage across the person and the impedance of the path through the person. The impedance of the system itself does not enter into this.

 

[iwire]

I think it would be almost impossible to get 100s of amps to flow through a person.

I think we would have fire long before that. EEK

 

[ELA]

I think it would be almost impossible to get 100s of amps to flow through a person.

Isn't that about what it took to wake up Herr Frankenstein ? If I remember correctly he only smoked a little bit.

 

[gadfly56]

Body Resistance

Body Resistance

A quick cruise through Wikipedia gives a wide range of resistances for the human body. Hand-to-hand with dry skin can be as high as 100K ohms, wet skin as low as 1K ohms. Dielectric breakdown at high (~450) volts can drive resistance as low as 500 ohms.

 

[mull982]

I would think then that with such little amount of current flowing through the body (less than 1/2A) this would not be enough to trip a breaker on a 480V system. The only way a breaker would trip is if it had ground fault protection settings, and typically on a 480V system these are such greater then 1/2A so most likely no OCPD would clear the circuit?

 

[rcwilson]

Resistance of the Body

Resistance of the Body

IEEE 80, "IEEE Guide for Safety in AC Substation Grounding" lists the internal resistance of the body as 300 ohms with skin resistance ranging from 500 to 5,000 ohms or more. Most of our resistance is in the skin, once that is punctured, the resistance drops.

The same reference quotes Dalziel's work where he passed current through student "volunteers" whose hands and feet were wet with saltwater. He calculated body resistances of 2330 ohms for hand-to-hand and 1130 ohms hand-to-foot.

The subjects experienced "no let go" at currents of 6-18 mA. At 60 mA ventricular fibrillation occurred. These current values vary person to person and are affected by their size and the duration of shock.

Using 50 mA as lethal threshold, 277V/.05 A = 5,540 ohms. Your body + contact resistance has to be 5 kohms or greater to survive at 277V.

Some other safety standards use 1500 ohms as the body resistance when investigating shock hazards on ungrounded systems, like the hot working zone of DC bus in chlorine and aluminum potlines. (See NEC Art 668). Connect a 1.5kohm resistor across the voltmeter and measure for shock hazards. If you read over a 100V there is a hazard.

 

[gadfly56]

Some other safety standards use 1500 ohms as the body resistance when investigating shock hazards on ungrounded systems, like the hot working zone of DC bus in chlorine and aluminum potlines. (See NEC Art 668). Connect a 1.5kohm resistor across the voltmeter and measure for shock hazards. If you read over a 100V there is a hazard.

Using V=I*R, it looks like the threshold would be more like 75V.

 

[zog]

Using V=I*R, it looks like the threshold would be more like 75V.

Actually it is 50V, based on a 500 Ohm body resistance and a 100mA threthhold of fibrilation, which is almost always fatal.

That is why you see 50V referenced all over OSHA standards.

 

[rcwilson]

Actually it is 50V, based on a 500 Ohm body resistance and a 100mA threthhold of fibrilation, which is almost always fatal.

That is why you see 50V referenced all over OSHA standards.

Correct, Zog.

50 V is a better threshold.

I was typing from memory on what we did in aluminum plants 20 years ago.