What will the voltage be?
- Thread starter Ponchik
- Start date
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Measured with a high impedance meter, probably about the same as if grounded. With a low impedance meter, hard to say.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
If there is no defined reference, than the voltage is undefined. Because of capacitive coupling the measured voltage will be affected by the meter itself. I would expect to see something in the range of 277V to 960V on a normal 480Y/277V system.
Are these phantom voltage or measurable voltage?
In this case, since the earth does not have a path back to the source, then a person should not get shocked.
Phantom voltage is a real voltage, but it is capacitive in nature, so the amount of energy that can be transferred across that capacitor is PROBABLY small, but not zero. I wouldn't make it a habit of touching live parts just because there is no direct conductive path between me and ground.
I agree that I should not make it a habit of getting between live parts and the ground given in my example. However, if I happen to touch, will one get shocked??
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
“Voltage to Ground” for ungrounded circuits is defined in Article 100.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Absolutely, with a high-impedance meter that will not make the voltage collapse.
That depends on many variables, but I wouldn't bet yours or anyone else's life on it.
How high is the voltage, how long is the transmission, how well are you earthed, etc?
Lots of variables come into play to determine if you can get shocked through capacitive coupling. I have done a demonstration where I hold the non-contact voltage tester in one hand and touch a live 120 volt wire with the other hand, while standing on a pallet. I don't feel anything but the tester beeps. I did it with 277 volts and could feel a tingle when I touched it. I suspect that if the humidity were higher I would feel it more, but I won't chance it anymore over 120.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Your body becomes the bonding jumper, not totally different than a high impedance ground. Don't be doing this when another fault occurs.
ptonsparky
Tom
- Occupation
- EC - retired
I don't see it in the '20. Is it new?
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
In the tiny (70 kVA 480/277V) impedance grounded system that I worked with, the capacitive current to ground was above 1A when the VFDs were operating.
My guess is that the same system ungrounded would deliver quite the jolt if you came between hot and ground.
My guess is that the same system ungrounded would deliver quite the jolt if you came between hot and ground.
- Location
- Illinois
- Occupation
- retired electrician
I have been able to pull in a wiggy measuring from a conductor of an ungrounded system to earth a few times....more than enough current for a serious shock.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Looks like you missed it. From the 2020 NEC:
"Voltage to Ground. For grounded circuits, the voltage between the given conductor and that point or conductor of the circuit that is grounded; for ungrounded circuits, the greatest voltage between the given conductor and any other conductor of the circuit. (CMP-1)"
Of course, the second clause on ungrounded circuits is a defined value for regulatory (NEC) purposes, and not an expression of what you'll actually get if you measure voltage to ground.
Cheers, Wayne
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
The definition has been around since I’ve been in the business. It’s primary use is for clearances
ptonsparky
Tom
- Occupation
- EC - retired
I didn't see it in art 100. I'll look again after my nap.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Anyone ever put an Amprobe on the GEC for a pole transformer?
I didn't do it at the pole, but I just did it on the GEC at this office I am at. With the main breaker off, there is 2.7 amps... Meaning that the current is actually flowing INTO the service neutral from the earth. I disconnected the GEC and measured the voltage between it and the neutral as 0.15 V. So the entire circuit must be 0.05 ohms.
ptonsparky
Tom
- Occupation
- EC - retired
Yup, look in Part I, not II
The reason for my, as to why we ground the electrical system. If I understand it correctly, If the system is not grounded the voltage could be any value, even though capacitive, there is a chance of getting shocked. Now, if the system is grounded the voltage value is stable at 120V and for sure one will get shocked and possibly killed.
Then why do we ground the system? what value does a grounded system offer vs a non grounded system.
Then why do we ground the system? what value does a grounded system offer vs a non grounded system.