Sub-panel grounding

[iwire]

Larry, I have no idea what you are trying to say.

If I lose the neutral between the source and say a standard home the service enclosure and all things connected to the EGC will be at a raised potential relative to the earth.

This is exactly the same situation as losing the neutral to a detached structure that uses the grounded conductor as the grounding means.

 

[Smart $]

Larry, I have no idea what you are trying to say.

If I lose the neutral between the source and say a standard home the service enclosure and all things connected to the EGC will be at a raised potential relative to the earth.

This is exactly the same situation as losing the neutral to a detached structure that uses the grounded conductor as the grounding means.

If you lose the service neutral connection, the result is essentially the same at the detached structure, but there would be slight differences because of the different grounding paths.

However, when using a grounding conductor and you lose the neutral between a detached structure and the service, all things grounded will still be at earth potential unless/until there is a line-to-ground fault.

 

[LarryFine]

Larry, I have no idea what you are trying to say.

If I lose the neutral between the source and say a standard home the service enclosure and all things connected to the EGC will be at a raised potential relative to the earth.

That is exactly what I am saying. In a home, those several once-grounded, now-energized surfaces will still be bonded to one another.

This is exactly the same situation as losing the neutral to a detached structure that uses the grounded conductor as the grounding means.

What makes this more dangerous is the still-grounded washer cabinet sitting right next to the now-energized dryer cabinet.

The jumpers I mentioned earlier (and the washer's EGC) carried neutral current as they bonded the two appliances together.

 

[iwire]

If you lose the service neutral connection, the result is essentially the same at the detached structure, but there would be slight differences because of the different grounding paths.

I don't see that you said anything there.

However, when using a grounding conductor and you lose the neutral between a detached structure and the service, all things grounded will still be at earth potential unless/until there is a line-to-ground fault.

Yes I am well aware of what happens with a four wire feeder.:roll:

My point was, and is, if a MGN is 'safe enough' on the supply side of the service disconnect I have hard time believing it is suddenly unsafe on the load side of the service disconnect.

 

[iwire]

That is exactly what I am saying. In a home, those several once-grounded, now-energized surfaces will still be bonded to one another.
What makes this more dangerous is the still-grounded washer cabinet sitting right next to the now-energized dryer cabinet.

The jumpers I mentioned earlier (and the washer's EGC) carried neutral current as they bonded the two appliances together.

Larry it is still going right over my head.

How is electricity behaving differently on one side of the service disconnect then the other?

All the things that could happen in the detached structure can happen in the main structure.

 

[LarryFine]

We all agree that, if a service neutral opens, everything that was at ground potential is now energized. But, everything is still bonded together. The major appliances and plumbing fixtures are all at the same potential.

But, if the neutral of a 3-wire major appliance circuit opens, only that appliance's cabinet (and anthing else on the load side of the break) is energized. That's especially dangerous around other, still-grounded appliances.


My whole point is that, as far as any given premises is concerned, the incoming service neutral, where we land electrodes, is the only point we have to use as a ground reference, regardless of its actual voltage to the earth.

It also explains why the pre-'08 allowance of the 3-wire detached-building feeder required no other conductive pathways between buildings. Such a path would either carry neutral current, increase shock hazards, or both.


This could be considered a great example of the grounding-vs-bonding discussion. The dryer with a bad neutral in its 3-wire supply sitting next to a washer with an intact EGC pathway really increases the likelihood of shock.

The same thing exists with a 3-wire-supplied range sitting near the kitchen sink. A customers of ours was severely shocked because the neutral conductor backed out of the crimped lug in the range's wiring compartment.


Had the service neutral had the open, instead of the range's neutral/grounding conductor, the range and the sink would have still been at the same potential. In a house, the meter, the hose bibs, and AC unit(s) are the dangers.

 

[iwire]

We all agree that, if a service neutral opens, everything that was at ground potential is now energized. But, everything is still bonded together. The major appliances and plumbing fixtures are all at the same potential.

Without a doubt, but far above earth potential which can still be a real hazard in the basement or outside.

But, if the neutral of a 3-wire major appliance circuit opens, only that appliance's cabinet (and anthing else on the load side of the break) is energized. That's especially dangerous around other, still-grounded appliances.

Agreed but I was stupidly staying on the original topic. :grin:

The subject had been feeders to subpanels in detached structures I missed the fact you moved on to branch circuits.

In the case of branch circuits I agree with you, dbuckley and likely Smart $ as well.

My whole point is that, as far as any given premises is concerned, the incoming service neutral, where we land electrodes, is the only point we have to use as a ground reference, regardless of its actual voltage to the earth.

Now you have lost me.

I would prefer to have the enclosure of my outdoor meter socket to be close to the same potential as the dirt I am standing on while touching it or any of my sill cocks.

It also explains why the pre-'08 allowance of the 3-wire detached-building feeder required no other conductive pathways between buildings. Such a path would either carry neutral current, increase shock hazards, or both.

Again, this is not a bit different then hundreds of thousands of homes with common metal water supplies.

This could be considered a great example of the grounding-vs-bonding discussion. The dryer with a bad neutral in its 3-wire supply sitting next to a washer with an intact EGC pathway really increases the likelihood of shock.

Yes, I agree.

The same thing exists with a 3-wire-supplied range sitting near the kitchen sink. A customers of ours was severely shocked because the neutral conductor backed out of the crimped lug in the range's wiring compartment.

Yes, I agree

Had the service neutral had the open, instead of the range's neutral/grounding conductor, the range and the sink would have still been at the same potential. In a house, the meter, the hose bibs, and AC unit(s) are the dangers.

Yes I agree.

 

[hurk27]

Install a double lug meter, run back into the earth with SE conductors, hit the main breaker in the out building and code will still allow a 3-wire connection.:grin: got to side with Bob and benny on this one

Until the POCOs are made to start running a 4-wire service from the transformer, it makes no sense to require a 4-wire to an outbuilding (in most cases) this will also stop most stray currents on pools and farms to boot!

 

[dbuckley]

The problem with a three wire feed to a detached structure is essentially one of perception.

If you're using your electric pressure washer with a GFCI protected plug to clean the windows and paintwork, you think the GFCI plug will save your life if anything bad happens.

What you dont expect is that is when you walk into your detached garage with pressure washer hose in hand is that you will die, and most annoyingly the GFCI you paid extra for wont save you, simply because the commissioning electrician used a three wire feed to the garage, which has a failed neutral, and the garage concrete (to which the garage supply is bonded) is 120V different to your grounded wand you're hanging onto.

You could argue the something similar if the service neutral failed, which is true, but we accept that should this happen then then (based on history of metallic plumbing) the other neutral-ish conduction paths would try to keep the voltage to somewhere near ground potential. In a PVC world that old assumption may no longer be true, for many structures...

 

[hurk27]

The problem with a three wire feed to a detached structure is essentially one of perception.

If you're using your electric pressure washer with a GFCI protected plug to clean the windows and paintwork, you think the GFCI plug will save your life if anything bad happens.

What you dont expect is that is when you walk into your detached garage with pressure washer hose in hand is that you will die, and most annoyingly the GFCI you paid extra for wont save you, simply because the commissioning electrician used a three wire feed to the garage, which has a failed neutral, and the garage concrete (to which the garage supply is bonded) is 120V different to your grounded wand you're hanging onto.

So what your saying is, it is ok for the POCO to do this but not a electrician? for safety reasons even on a service we must expect there is no other paths for neutral return, so if we loose a neutral ahead of the service we must assume that the grounding will go to a raised level, not all installations are the same, we have many houses here on wells with very sandy soil with ground rods as the only electrodes, and there is no way these ground rods will keep the grounding of these houses close to 0 volts to earth.

You are right about the GFCI's because the shocking current will not flow through them, as it is being supplied through the grounding, but then again even a shower on a concrete floor (slab type house) will be a hazard if the service neutral were to open?

 

[kwired]

The problem with a three wire feed to a detached structure is essentially one of perception.

If you're using your electric pressure washer with a GFCI protected plug to clean the windows and paintwork, you think the GFCI plug will save your life if anything bad happens.

What you dont expect is that is when you walk into your detached garage with pressure washer hose in hand is that you will die, and most annoyingly the GFCI you paid extra for wont save you, simply because the commissioning electrician used a three wire feed to the garage, which has a failed neutral, and the garage concrete (to which the garage supply is bonded) is 120V different to your grounded wand you're hanging onto.

You could argue the something similar if the service neutral failed, which is true, but we accept that should this happen then then (based on history of metallic plumbing) the other neutral-ish conduction paths would try to keep the voltage to somewhere near ground potential. In a PVC world that old assumption may no longer be true, for many structures...

The only way to prevent the shock hazard from the open neutral is to only have system bonding jumper at transformer and separate equipment ground and grounded neutral from there. And even that will have potential for failure.

High impedance ground systems with alarm systems for ground faults would probably be the safest thing for all installations. But there would probably be so many people that ignore or disable the alarm that it would not matter anyhow. Kind of like cutting the ground prong off of a cord, it still works, must be ok.

GFCI's should not be needed with this system, AFCI's would probably still detect some conditions that can happen no matter what the ground reference is.

 

[kwired]

If you lose the service neutral connection, the result is essentially the same at the detached structure, but there would be slight differences because of the different grounding paths.

However, when using a grounding conductor and you lose the neutral between a detached structure and the service, all things grounded will still be at earth potential unless/until there is a line-to-ground fault.

If you would lose the Grounding conductor at the detached structure and there are no other conductive paths back to source, and then have a ground fault from a phase conductor the entire grounding system at the structure is at the same potential as the faulted phase.

There will be no indication that anything is wrong until there is this fault condition.

With a grounded conductor only and bonding jumper at separate structure as previously allowed if you lose the grounded conductor and there are no other conductive paths back to the source the occupants will have noticeable voltage issues and will likely have things looked into.

Both ways of doing this have good and bad. I can't say one way is better than the other, if we knew what was going to fail when we installed something we would do it differently, but we don't know what is going to happen. Has been said you can't idiot proof anything - they will just make better idiots. Kind of a similar situation.

 

[dbuckley]

So what your saying is, it is ok for the POCO to do this but not a electrician?

Not quite, but I agree that's what it looks like. Why? Because PoCOs can. That fact on its own doesn't make it correct or safe, but thats the real life practice.

As you've probably noticed, ground vs ground hazards are my "thing", and thus yes, I do have a bee in my bonnet to make the world safer from these most terrible of hazards. I'm there, bashing the book, trying to change the world one reader as a time

I'm a big fan of TN-C-S power distribution, which is what all this is, and it essentially says that you can use a combined ground and neutral for the service entrance, and beyond service you must use separate neutral and ground.

If you do a three wire to an outbuilding you are doing it "like a PoCo".

Part of my problem with all this is that where I learned my electricity we had a special style of cable for combined N and G feeds, it was concentric. A thick hot in the middle, surrounded by many thinner neutrals. With this sort of cable, if the cable gets damaged, it is almost impossible for the hot to remain connected without there being at least some ground / neutral connection. This approach at least tries to make the combined N/G work safelyish... Of course, if the terminations fail...

Couple of stolen photos to illustrate:

 

[Smart $]

If you would lose the Grounding conductor at the detached structure and there are no other conductive paths back to source, and then have a ground fault from a phase conductor the entire grounding system at the structure is at the same potential as the faulted phase.

...and the fault will bring everyting effectively grounded, and try to bring nearby earth (wherever bonded via electrodes or grids), to the same [fault] potential... which isn't necessarily a bad thing... the only problem is....

There will be no indication that anything is wrong until there is this fault condition.

...and someone gets shocked or worse. The GES at the separate structure is required just for this purpose... to minimize the difference in potential of a line-to-ground fault and all things grounded thereabouts.

With a grounded conductor only and bonding jumper at separate structure as previously allowed if you lose the grounded conductor and there are no other conductive paths back to the source the occupants will have noticeable voltage issues and will likely have things looked into.

I agree. My previous statements were made with neither a pro or con stance on the matter. I would prefer to see the grounded conductor serve the purpose myself. For increased safety, perhaps require a GES GEC and ground rod bonded to the grounded conductor both where it leaves the feeding structure and where it enters the receiving structure, both exterior at grade level and not more than 6' away measured horizontally. Yes, I know it is already required that there be a GES at the separate structure. But a bonded ground rod within 6' horizontally at grade level where it enters the structure is not a requirement.

Both ways of doing this have good and bad. I can't say one way is better than the other, if we knew what was going to fail when we installed something we would do it differently, but we don't know what is going to happen. Has been said you can't idiot proof anything - they will just make better idiots. Kind of a similar situation.

Agreed... but I still prefer the grounded conductor method for the reason stated, and would prefer to see the bonding enhanced rather than separated. It doesn't hurt to mention that using one conductor to serve a dual purpose is my preference also... but it seems the CMP's want to get away from any such practice.

 

[kwired]

Not quite, but I agree that's what it looks like. Why? Because PoCOs can. That fact on its own doesn't make it correct or safe, but thats the real life practice.

As you've probably noticed, ground vs ground hazards are my "thing", and thus yes, I do have a bee in my bonnet to make the world safer from these most terrible of hazards. I'm there, bashing the book, trying to change the world one reader as a time

I'm a big fan of TN-C-S power distribution, which is what all this is, and it essentially says that you can use a combined ground and neutral for the service entrance, and beyond service you must use separate neutral and ground.

If you do a three wire to an outbuilding you are doing it "like a PoCo".

Part of my problem with all this is that where I learned my electricity we had a special style of cable for combined N and G feeds, it was concentric. A thick hot in the middle, surrounded by many thinner neutrals. With this sort of cable, if the cable gets damaged, it is almost impossible for the hot to remain connected without there being at least some ground / neutral connection. This approach at least tries to make the combined N/G work safelyish... Of course, if the terminations fail...

Couple of stolen photos to illustrate:

What type of cable is that?

I have seen old cloth covered type SE cable with single insulated conductor and the bare grounded conductor wrapped around it. Usually used for outbuildings with only 120 volt being supplied and usually 8 AWG.

Your photo appears like a more modern version of the same thing.

I still don't see it as being any better or worse than other wiring methods from a grounding reliability view. Failing connections do not care what type of cable or raceway you are using they are still failing. Metal jacketed cables or metal raceways can provide a safer situation when a connection fails but if things still appear to be working normally nobody knows the connection failed.

 

[petersonra]

I think there are several differences associated with services than with feeders that make it fairly safe to use a 3 way service connection but not to a detached building.

Someone brought up one of them. The typical service entrance cable has a bare neutral. If there is some kind of damage to a hot wire coming in, chances are it will cause the POCO transformer to melt, thus protecting the consumer.

Service cables are typically overhead or underground where they are away from people and well protected. Not perfectly, but pretty good. I suspect the number of injuries from services to consumers is very low, especially compared to injuries from feeders and branch circuits just because the average end user has far less access to the service entrance cables.

 

[iwire]

Someone brought up one of them. The typical service entrance cable has a bare neutral. If there is some kind of damage to a hot wire coming in, chances are it will cause the POCO transformer to melt, thus protecting the consumer.

Say what?????????

Service cables are typically overhead or underground where they are away from people and well protected. Not perfectly, but pretty good. I suspect the number of injuries from services to consumers is very low, especially compared to injuries from feeders and branch circuits just because the average end user has far less access to the service entrance cables.

The meter and service panel our at ground level and will be energized in relation to earth if the neutral is lost, that is a hazard.

 

[kwired]

Someone brought up one of them. The typical service entrance cable has a bare neutral. If there is some kind of damage to a hot wire coming in, chances are it will cause the POCO transformer to melt, thus protecting the consumer.

There are also many service conductors that are not a part of a SE cable assembly.

Have you ever seen a utility pole mounted transformer melt - especially single phase feeding residential units. Most of them have internal protector - if not they will have a fuse in the primary - these could have some significant damage when overloaded by a fault if the wrong fuse is installed.

 

[petersonra]

I think several of you may have missed the little smiley face after the melting transformer comment.