Equipment bonding and earthing

[electrofelon]

along the same lines as a recent post that I missed, but thought I should let that die.

Q1. consider an ungrounded system that does not have non current carrying metallic parts earthed. Capacitance can/will develop between the metallic parts/raceways and earth and in extreme cases could be a shock and fire hazard. Is this correct?

Q2. consider an ungrounded system that does not have non current carrying metallic parts earthed. Imbalances in current in conductors due to things such as improper grouping or multiple SAME Phase faults can/will lead to voltages on metallic parts/raceways due to electromagnetic induction and in extreme cases could be a shock and fire hazard. Is this correct?

Q3. Other than providing a short circuit for a second and different phase fault, and the operation of ground detectors, does equipment bonding and earthing accomplish anything else not mentioned? What about lightning? Please give a specific hypothetical "step by step" example.

Thanks!

 

[Smart $]

... Please give a specific hypothetical "step by step" example.

1) Without moving your left foot, put your right foot in front of it.
2) Without moving your right foot, put your left foot in front of it.
3) Repeat steps 1 and 2.

 

[augie47]

I have no input on lightning, but would assume the other scenarios are factual. In any event Art 250 Part V requires bonding in both grounded and ungrounded systems.

 

[kwired]

Answers in the quote:

along the same lines as a recent post that I missed, but thought I should let that die.

Q1. consider an ungrounded system that does not have non current carrying metallic parts earthed. Capacitance can/will develop between the metallic parts/raceways and earth and in extreme cases could be a shock and fire hazard. Is this correct?True

Q2. consider an ungrounded system that does not have non current carrying metallic parts earthed. Imbalances in current in conductors due to things such as improper grouping or multiple SAME Phase faults can/will lead to voltages on metallic parts/raceways due to electromagnetic induction and in extreme cases could be a shock and fire hazard. Is this correct? Those induced currents from improper grouping of conductors happen in grounded systems as well. Earthing and bonding just gives more possible paths for those currents to take in either case.

Q3. Other than providing a short circuit for a second and different phase fault, and the operation of ground detectors, does equipment bonding and earthing accomplish anything else not mentioned? What about lightning? Please give a specific hypothetical "step by step" example. It equalizes touch potential between non current carrying objects - whether the system is grounded or not.

Thanks!

 

[electrofelon]

Ok K',

Hold on you're not getting off that easy - just throwing out some buzzwords and going home for the weekend

Ok touch potential. I think capacitance and inductance are CAUSES of touch potential. Are there others? I am used to thinking utility voltages and the "Steep" voltage gradient from the high resistance of dirt when talking step and touch potential, but of course it can happen on low voltage; just less likely/less severe.

I thought about during a fault event, but I dont see how bonding would help (in terms of touch potential between 2 parts DURING the fault event). Consider a RMC raceway during a fault that has/is an effective ground fault path. Say its a grounded 480/277 system. Call it raceway "A". Consider another Properly grounded/bonded RMC raceway with no fault and call it "B". Say I am touching raceway "A" where the voltage to ground because of the fault event is 100V, and I am touching "B" with the other hand. So thats 100V I get bit with. If there was a relatively thick bonding jumper just randomly connecting A and B right near where I was touching, the potential difference is much less - whatever ohms law says based on the circumstances. So (bear with me while I work this out as I type...) obviously there is a limit to how many bonding jumpers we can put between everything, but I guess that bonding jumper in the example represents what we are trying to do with pools? Is this correct and logical?

 

[electrofelon]

...So just summed up, bonding and earthing doesn't increase safety* from shock DURING a fault, other than obviously the bonding that is helping the fault clear?

(*exception for elaborate equipotential grids used in special situations)

 

[Dennis Alwon]

...So just summed up, bonding and earthing doesn't increase safety* from shock DURING a fault, other than obviously the bonding that is helping the fault clear?

(*exception for elaborate equipotential grids used in special situations)

Well doesn't clearing a fault increase safety?

 

[kwired]

Ok K',

Hold on you're not getting off that easy - just throwing out some buzzwords and going home for the weekend

Ok touch potential. I think capacitance and inductance are CAUSES of touch potential. Are there others? I am used to thinking utility voltages and the "Steep" voltage gradient from the high resistance of dirt when talking step and touch potential, but of course it can happen on low voltage; just less likely/less severe.

I thought about during a fault event, but I dont see how bonding would help (in terms of touch potential between 2 parts DURING the fault event). Consider a RMC raceway during a fault that has/is an effective ground fault path. Say its a grounded 480/277 system. Call it raceway "A". Consider another Properly grounded/bonded RMC raceway with no fault and call it "B". Say I am touching raceway "A" where the voltage to ground because of the fault event is 100V, and I am touching "B" with the other hand. So thats 100V I get bit with. If there was a relatively thick bonding jumper just randomly connecting A and B right near where I was touching, the potential difference is much less - whatever ohms law says based on the circumstances. So (bear with me while I work this out as I type...) obviously there is a limit to how many bonding jumpers we can put between everything, but I guess that bonding jumper in the example represents what we are trying to do with pools? Is this correct and logical?

Bonding and earthing are two different things. Bonding will lower impedance between objects that are bonded together and if the connection between them is low resistance it brings them to nearly the same potential. Earthing is mostly for bleeding off lightning transients and to give a system a reference to ground, but is not always easy to obtain a low impedance electrode - you will see voltage differences between earth and other objects sometimes because of this. That is part of why we are supposed to bond the grounded system conductor at the service equipment or first disconnecting means and then keep current carrying grounded conductors separated from equipment grounding conductors beyond that point, because voltage drop on the current carrying grounded conductor could raise voltage to ground on frames of equipment if we did not separate them, then you could have touch potential issues all around you in some instances. The fact that utilities bond the current carrying neutral (both on primary and secondary distribution) at nearly every structure does sometimes contribute to this problem and is part of why we need an elaborate equipotential bonding grid at places like swimming pools.

 

[electrofelon]

Well doesn't clearing a fault increase safety?

Certainly it does. Reread that statement again, I just want to discuss and make sure I understand the OTHER THINGS that bonding and earthing does, OTHER THAN the fault clearing process. These would be equipotential issues, during normal operation, and potentially DURING a fault event, although as I questioned last post I dont see how bonding can lower shock risk DURING a fault, unless the bond is very close to the touch point. Am I correct on that?

Bonding and earthing are two different things.

Sure but in terms of touch potential and human shock which require very little current, the earth can be considered a conductor like everything else, so we just want the earth AND all the metal stuff "bonded" together. But this is for low current events like capacitance, of course once we have significant current faulting through the earth, we would have to be just about standing on the electrode due to the high voltage drop of the dirt. Do you agree with this statement?

That is part of why we are supposed to bond the grounded system conductor at the service equipment or first disconnecting means and then keep current carrying grounded conductors separated from equipment grounding conductors beyond that point, because voltage drop on the current carrying grounded conductor could raise voltage to ground on frames of equipment if we did not separate them, then you could have touch potential issues all around you in some instances.

I certainly agree with that. Of course generally the voltage difference in that situation would be small, probably a few volts. V=IR so to get a higher voltage we need either a higher I or a higher R (or both) So can you give a specific example of a situation where we could have much higher voltage difference? I Assume we would need to have a ton of current flowing like during a fault or a high impedance part of the fault path like a untightened conduit fitting? I just like to have specific examples of scenarios rather than jjust "it lowers touch potential"

Thanks!

 

[petersonra]

along the same lines as a recent post that I missed, but thought I should let that die.

Q1. consider an ungrounded system that does not have non current carrying metallic parts earthed. Capacitance can/will develop between the metallic parts/raceways and earth and in extreme cases could be a shock and fire hazard. Is this correct?

It is possible. Perhaps a fairly remote possibility.

Q2. consider an ungrounded system that does not have non current carrying metallic parts earthed. Imbalances in current in conductors due to things such as improper grouping or multiple SAME Phase faults can/will lead to voltages on metallic parts/raceways due to electromagnetic induction and in extreme cases could be a shock and fire hazard. Is this correct?

I don't see how the balance of current in the conductors is likely to create a shock hazard or how grounding would mitigate it.

Q3. Other than providing a short circuit for a second and different phase fault, and the operation of ground detectors, does equipment bonding and earthing accomplish anything else not mentioned? What about lightning? Please give a specific hypothetical "step by step" example.

Thanks!

If you are working with an ungrounded system, and the conduit for instance is ungrounded, if you get a ground fault to the conduit, it will have a voltage there that exceeds that at ground leading to a potential hazard.

 

[kwired]

Certainly it does. Reread that statement again, I just want to discuss and make sure I understand the OTHER THINGS that bonding and earthing does, OTHER THAN the fault clearing process. These would be equipotential issues, during normal operation, and potentially DURING a fault event, although as I questioned last post I dont see how bonding can lower shock risk DURING a fault, unless the bond is very close to the touch point. Am I correct on that?



Sure but in terms of touch potential and human shock which require very little current, the earth can be considered a conductor like everything else, so we just want the earth AND all the metal stuff "bonded" together. But this is for low current events like capacitance, of course once we have significant current faulting through the earth, we would have to be just about standing on the electrode due to the high voltage drop of the dirt. Do you agree with this statement?



I certainly agree with that. Of course generally the voltage difference in that situation would be small, probably a few volts. V=IR so to get a higher voltage we need either a higher I or a higher R (or both) So can you give a specific example of a situation where we could have much higher voltage difference? I Assume we would need to have a ton of current flowing like during a fault or a high impedance part of the fault path like a untightened conduit fitting? I just like to have specific examples of scenarios rather than jjust "it lowers touch potential"

Thanks!

Correct human touch potential requires a pretty small current to do a lot of harm, and normal voltage drop in an otherwise good condition circuit can cause troubles here, that is a major reason why we run separate EGC's from the grounded circuit conductors, and I would assume why NEC has in more recent years minimized where you are allowed to use the grounded conductor for equipment grounding. With a swimming pool the resistance of the human body connection to earth is greatly reduced and the reason we need equipotential bonding - to put everything within reach of the user at same potential, some confuse this "bonding" with "grounding" but in reality that EBG could possibly be at 1000V above earth and the user still shouldn't be subjected to any touch potential, and it could happen if the POCO has a problem with their MGN.

The risk of having potential during a ground fault does increase, but there are two things here minimizing the number of injuries out there,
One is if the EGC is low impedance like it is supposed to be it allows high enough current flow for the overcurrent device to respond quickly which reduces the time where any risk of shock is present. Second is with a low impedance fault path and bonding of all non current carrying conductive components, the voltage rise happens on all of those components so there is not much voltage between them anyway. A conductive object that is not bonded and is not grounded is not as much of a threat as something that is well grounded but not bonded to the parts carrying the fault. I thought this was a general enough description you could come up with many scenarios that apply. If not look through some of Mike Holt's materials, he has better drawings then I would post here that show what can happen.

 

[Tony S]

1) Without moving your left foot, put your right foot in front of it.
2) Without moving your right foot, put your left foot in front of it.
3) Repeat steps 1 and 2.

Referred to by UK OH linesmen and fire services as ?the shuffle?.

Although the method I was taught your less likely to fall over as you?re not walking a tight rope. Alternately advance each foot by no more than ? your foot length.

 

[Smart $]

...

Although the method I was taught your less likely to fall over as you?re not walking a tight rope. Alternately advance each foot by no more than ? your foot length.

The method, as I wrote it, was not meant to be taken as one foot directly in front of the other, heel to toe.... but it can be, and also used as a gauge of one's disposition.

In my line of work your method is known as the "nuclear shuffle". Not meant to be literal, but rather figurative of the speed at which a project moves from concept to completion.

 

[gar]

150114-1620 EST

electrofelon:

An example that possibly is an illustration for one of your questions.

A CNC machine is fed from a 240 V delta wild leg source 3 phase panel. Neutral and EGC are bonded at this panel. An office computer's 120 V single phase is supplied from the same panel. Thus, the computer EGC is bonded to the CNC EGC at the panel, but these are two totally separate EGCs.

The CNC has no neutral connection, and what would neutral mean as applied to the CNC? So this is of no importance.

There is a direct RS232 connection from the CNC to the office computer. Even make the RS232 cable only 20 ft long. Assume Belden 8723 is used for the RS232 connection. Assume a 50 A breaker to the CNC machine.

Someone working on the CNC shorts one of the hot lines to the machine chassis. Assume the EGC wire size is the same as any one of the hot wires. What happens to the voltage on the machine chassis at the machine relative to the EGC-neutral bonding point in the panel (which is also the potential of the EGC at the computer)? What happens to the 8723 wire and the RS232 components at each end of the 8723, and possibly other components?

.