250.20 SYSTEM GROUNDING (B)

[K8MHZ]

Absolutely, none of them are "hot" until one of them becomes grounded - whether intentionally grounded or not. It will be safe to touch any conductor of the system, just don't touch more then one conductor at a time. Once one of them is grounded you have a ground reference and you need to be more careful about ensuring you are insulated from ground before it would be safe to touch any other ungrounded conductor.

So by your definition, there are no hot wires in an ungrounded delta?

 

[Smart $]

Absolutely, none of them are "hot" until one of them becomes grounded - whether intentionally grounded or not. It will be safe to touch any conductor of the system, just don't touch more then one conductor at a time. Once one of them is grounded you have a ground reference and you need to be more careful about ensuring you are insulated from ground before it would be safe to touch any other ungrounded conductor.

Can I agree with both of you...

I believe "hot" started as a slang term for energized. Essentially the same as live, as in not dead. For instance a 120V lighting circuit is not "hot" if the light switch is turned off (talking about after the light switch, of course).

Electricians have muddled this slang term to the point of describing certain conductors in an electrical system. But what really matters about the "hot" term is whether one or more conductors has the potential to shock or electrocute you. An example which defies the conductor classified as not "hot" is when the neutral of a standard 120VAC live circuit is opened. It is still a neutral conductor on both sides of the open, but it is "hot" on one side if there is a connected and ON load.

 

[kwired]

So by your definition, there are no hot wires in an ungrounded delta?

Depends on exactly what the definition is, but if it is primarily the ability of that conductor to shock you assuming you are at ground potential, then yes there are no hot wires in an ungrounded delta with no fault conditions present. If you decide to make said system into a corner ground system - you can ground any phase you wish.

In any voltage system it may be customary or desired to ground a particular point, but until there is a ground reference there is no voltage to ground other then capacitively coupled voltages, and those are typically fairly harmless for 600 volt and below systems, usually not enough capacitance to store enough charge to be harmful.

 

[don_resqcapt19]

Depends on exactly what the definition is, but if it is primarily the ability of that conductor to shock you assuming you are at ground potential, then yes there are no hot wires in an ungrounded delta with no fault conditions present. ...

The capacitance from an ungrounded system is often enough to drive a fatal shock. The larger the system the more likely this is. I have worked on ungrounded 480 volt delta systems that had enough capacitance to pull in a solenoid voltage tester when testing from a phase conductor to ground. The voltage tester pulled about 20mA at that voltage ....plenty for a fatal shock.

 

[bobby ocampo]

The capacitance from an ungrounded system is often enough to drive a fatal shock. The larger the system the more likely this is. I have worked on ungrounded 480 volt delta systems that had enough capacitance to pull in a solenoid voltage tester when testing from a phase conductor to ground. The voltage tester pulled about 20mA at that voltage ....plenty for a fatal shock.

What happens if one of the hot conductor touches the metal enclosure that is NOT grounded and bonded, and somebody touches the metal enclosure?

What happens if one of the hot conductor touches the metal enclosure that IS grounded and bonded, and somebody touches the metal enclosure?

Again the assumption is the system is UNGROUNDED or HRG.

 

[GoldDigger]

1. Capacitive current from the ungrounded system (capacitance providing a lower impedance ground reference than the HRG) will source a fatal current that person.
2. The EGC carries enough current that the voltage on the metal is near ground potential and no current (well, not a dangerous amount) flows through the toucher.

 

[bobby ocampo]

1. Capacitive current from the ungrounded system (capacitance providing a lower impedance ground reference than the HRG) will source a fatal current that person.

Fatal current to the person because there is no Equipment grounding conductor?

2. The EGC carries enough current that the voltage on the metal is near ground potential and no current (well, not a dangerous amount) flows through the toucher.

The EGC (equipment grounding conductor) will carry the current so that the voltage on the metal is near ground potential. What does ground potential means? Will the EGC in UNGROUNDED AND HRG save the person touching the metal even without tripping the OCPD?

 

[GoldDigger]

Yes.
Yes.
Can we stop now, please?

 

[bobby ocampo]

Yes.
Yes.
Can we stop now, please?

Does it mean that it is TRUE that the only way to prevent electrocution is for the OCPD to trip and to isolate the system?

https://www.youtube.com/watch?v=3vvvv5QVZoA

 

[GoldDigger]

Does it mean that it is TRUE that the only way to prevent electrocution is for the OCPD to trip and to isolate the system?

https://www.youtube.com/watch?v=3vvvv5QVZoA

No. That is not what you asked before.
An equipotential environment for all exposed metal can (generally will) prevent electric shock whether the OCPD trios or not. The amount of risk that has to mitigated may be lower for an ungrounded or high resistance ground, but the end result will be the same degree of safety.

 

[kwired]

Does it mean that it is TRUE that the only way to prevent electrocution is for the OCPD to trip and to isolate the system?

https://www.youtube.com/watch?v=3vvvv5QVZoA

Electrocution happens when the victim contacts two points of differing voltage - that happens to be high enough and resulting current in victim hits vital enough organs to cause death. Lower currents or currents away from such vital organs may still result in anything from mild discomfort to severe electric shock. Anything you do to lower those possible potential differences helps lower any risks. Bonding all non current carrying components together and insulating or covering all current carrying components is the first big steps at accomplishing this. If all non current carrying components are bonded together with low impedance between them there is not much risk of having high enough voltage potential between two points within reach of a person to be hazardous. Abnormal voltages may be present during a ground fault condition, but generally not enough to matter between objects solidly bonded together.

 

[bobby ocampo]

No. That is not what you asked before.
An equipotential environment for all exposed metal can (generally will) prevent electric shock whether the OCPD trips or not. The amount of risk that has to mitigated may be lower for an ungrounded or high resistance ground, but the end result will be the same degree of safety.

"The only way to prevent electrocution is to turn the supply power off" This is base on a video by Mike Holt in youtube "Electrical Fundamentals Protection against electric shock"

https://www.youtube.com/watch?v=mpgAVE4UwFw

Starts at 15 minute

 

[kwired]

"The only way to prevent electrocution is to turn the supply power off" This is base on a video by Mike Holt in youtube "Electrical Fundamentals Protection against electric shock"

https://www.youtube.com/watch?v=mpgAVE4UwFw

Starts at 15 minute

Opening the circuit to the supply is the best assurance to protect against electric shock, which is why it is important to ensure you have an effective ground fault clearing path so that high enough fault current will flow that the overcurrent device can respond quickly. (I did watch the video you linked to) If conditions don't promote opening of the circuit, it may be possible to minimize risks, but the longer the fault goes on the more likely something may fail and present new hazards.

 

[bobby ocampo]

Electrocution happens when the victim contacts two points of differing voltage - that happens to be high enough and resulting current in victim hits vital enough organs to cause death. Lower currents or currents away from such vital organs may still result in anything from mild discomfort to severe electric shock. Anything you do to lower those possible potential differences helps lower any risks. Bonding all non current carrying components together and insulating or covering all current carrying components is the first big steps at accomplishing this. If all non current carrying components are bonded together with low impedance between them there is not much risk of having high enough voltage potential between two points within reach of a person to be hazardous. Abnormal voltages may be present during a ground fault condition, but generally not enough to matter between objects solidly bonded together.

My concern is the video that says that grounding is only for the purpose of operating the OCPD to prevent electrocution. This is not what IEEE 142 Chapter 2 is saying on the purpose of equipment grounding specially if the system is UNGROUNDED OR HRG.

https://www.youtube.com/watch?v=mpgAVE4UwFw

Starts at the 15 minute mark.

 

[kwired]

My concern is the video that says that grounding is only for the purpose of operating the OCPD to prevent electrocution. This is not what IEEE 142 Chapter 2 is saying on the purpose of equipment grounding specially if the system is UNGROUNDED OR HRG.

https://www.youtube.com/watch?v=mpgAVE4UwFw

Starts at the 15 minute mark.

I saw the video and heard him say it. That is his opinion and not necessarily everyone's opinion. Mine was that a major function of grounding conductors is to facilitate operation of overcurrent devices, but is not necessarily the only function, and watch the context of what you say here grounding and equipment grounding conductors are not always the same thing.

 

[GoldDigger]

What Mike is saying is that for a hard ground fault in a grounded system the wire EGC may only be able to hold the potential of exposed metal to 1/2 the nominal line to ground voltage. This will not be a low enough voltage to prevent injury, so the OCPD trip is a necessary part of safety.
But Mike does not dispute that when the current is limited (to capacitive current in an ungrounded system or to that allowed BT the resistor in HRG systems) the EGC can ensure a safe voltage without any OCPD trip.
He is saying that if there is a high current fault mode available (as with solidly grounded systems or a second fault on ungrounded systems) then the OCPD trip is critical. No more, no less.

 

[mbrooke]

What Mike is saying is that for a hard ground fault in a grounded system the wire EGC may only be able to hold the potential of exposed metal to 1/2 the nominal line to ground voltage. This will not be a low enough voltage to prevent injury, so the OCPD trip is a necessary part of safety.
But Mike does not dispute that when the current is limited (to capacitive current in an ungrounded system or to that allowed BT the resistor in HRG systems) the EGC can ensure a safe voltage without any OCPD trip.
He is saying that if there is a high current fault mode available (as with solidly grounded systems or a second fault on ungrounded systems) then the OCPD trip is critical. No more, no less.

But my understanding is that Engineers have already calculated the impedance and disconnect times in so far that while the fault is occurring the voltage potential will be non lethal?

 

[GoldDigger]

While the fault is occurring the system voltage (actually voltage at breaker) will divide almost equally between VD in the ungrounded conductor and VD in the EGC. Source impedance control and clearing times can do nothing to change that voltage. What they can do is keep the exposure time short enough that even 240V will not be lethal.
This is different in some ways from a bolted ground fault right at the service point, where the relative sizes and lengths of grounded and ungrounded wires may make the voltage divider other than 50%, but the limited exposure time is still what saves lives.
Note also that when the wire EGC is smaller than the ungrounded conductor the voltage will be higher than half the system voltage and when a racewsy EGC is used the voltage may be lower than half.

 

[mbrooke]

While the fault is occurring the system voltage (actually voltage at breaker) will divide almost equally between VD in the ungrounded conductor and VD in the EGC. Source impedance control and clearing times can do nothing to change that voltage. What they can do is keep the exposure time short enough that even 240V will not be lethal.
This is different in some ways from a bolted ground fault right at the service point, where the relative sizes and lengths of grounded and ungrounded wires may make the voltage divider other than 50%, but the limited exposure time is still what saves lives.
Note also that when the wire EGC is smaller than the ungrounded conductor the voltage will be higher than half the system voltage and when a racewsy EGC is used the voltage may be lower than half.

How is this 50% number reached? A larger EGC would defiantly reduce the voltage rise. And, if I bonded the faulting appliance to a mat or UFER below my feet that voltage would go down even more. Even at 277 volts, my understanding is that when the fault does happen in a building that on average the voltage drop across the EGC will cause a voltage around (approximately) 1/3 to show up relative to remote earth.



ID have to crunch the numbers though... but if the impedance, length and available fault current and voltage drop at the service when the fault is happening is known a very good rough number can be deduced.


In The IEC they do talk about earth fault loop impedance and disconnect times. Though its more disconnect time...


I guess I need to think about this more And more explanation.

 

[GoldDigger]

The 50% number was reached by assuming that the hot and EGC wire lengths and sizes were identical and that the only fault return path was to and through the grounded utility neutral. Normally the impedance of the CEE to POCO MGN path will be much higher than that of the neutral wire.
I did bring up the possibility of other voltage ratios later.