Ground fault- Why doesn't anyone get shocked?

mivey

Senior Member
Its not malfunctioning. Its been documented in event data. Even in stability reports its assumed 2 extra cycles for a 3 cycle breaker and 1.5 extra cycles for a 2 cycle breaker. PJM, ISO-NE, Ercot, ect assume these extra few cycles for a breaker in stability reports involving critical clearing time. Yes these are transmission breakers, but it still holds true for distribution breakers.
We keep our transmission and distribution breakers maintenanced and tested and don't allow the stickiness you evidently are seeing.

If the relay is set to trip instantaneously for this type of fault. Add a few reclosers in series and you are "stacking" curves. Fuse blowing in fuse saving schemes will also increase the time.
Depends on reach. You would normally want your relay to look further and catch those bolted faults. I wil normally set the instantaneous to see 80% of the main line. I will stop at a recloser if it is catching the remainder.

Still more then 5 cycles for that old oil dinosaur.
Sounds like it is time to tweak the budget.

If that was the case, then we could safely eliminate all breaker failure. And use normal clearing times in substation worker protection calcs. Not going to happen.
We use alternate (faster and no reclosing) settings when working on the line. It may cause a lockout for what would normally be a blink but so be it.

Now that is what I would consider a rare enough scenario mitigated by correct maintenance and design- not a stuck breaker.
That contractor was fired.

Yes, if your breaker fail time is less then 72 cycles have at it. But you can't ignore it, nor assume normal high speed clearing.
We assume our breakers will operate during a fault the same way as they did during testing.

True- unless you insulate everything phase to phase and use only delta loads. :p
Hey, it is only money.

Already linked it:
But I'm being lazy and want a spoon-fed summary.

You can hook everything phase to phase and just keep the MGN.
Like California.

But you should know that graph. Its the foundation for which nearly every step potential, clearing time, loop impedance, bonding and grounding mandate is based upon across most of the globe and soon to be for North America. Its already being used by UL to set the basis for industrial GFCIs.
OK. Haven't ran across it yet.

Ok, for the sake of the argument lets agree on 5 cycle clearing. After all it is very doable. Where in the NESC does it say a bolted L-N fault (on a distribution line) must clear within 5 cycles? That is what I'm trying to get at.
It doesn't and I have not said it did.
 

mivey

Senior Member
3 cycles is reasonable for a typical distribution breaker whether air or vacuum. I see oil breakers so rarely these days it's hard to say. I've encountered just two in 25 years and both went out of service before it was time to test. If we stack a 50G we have a 1 cycle delay on even old GE and ABB mechanical relays plus another cycle for an Electroswitch pistol grip 86 relay gets us to a maximum 5 cycles. This is for a typical pre-2000 design. If we used a multifunction relay such as a 351 the way it's intended relay delay is 1 cycle worst case for a total cycle time of 4 cycles minimum in a modern design. So for most equipment using 50G 5 cycles is very reasonable. It is a common error to loom at breaker test reports and ignore relay delay times.

Since this is safety, NFPA 70E and a dearth of other documents makes it very clear that none of the standards apply to equipment that is not properly maintained. I will concede that a breaker might trip slowly if not maintained but using 6-12 cycles is pure fantasy. And when testing it is well known that trip performance improves as you "work" a malfunctioning relay. Spray a little WD40 in there and it easily passes testing fir a couple days until the lubricant evaporates! Then it is even worse since the residual wax and clays and maybe a little oil are dissolved and cleaned out, leaving metal on metal contact. It is just as likely to trip in 30-60 cycles or simply fail to trip when a breaker malfunctions. Or the 50G can malfunction and trip early or nuisance trip. We don't have design data for this. Backup protection is there but might be pure guess work and might end up being when the fault is consumed by the arc. Standards do not exist for improperly maintained equipment because it's all a guess. So trying to engineer for improper maintenance is not good practice, and violates engineering ethics and professional engineering standards.


Sent from my SM-T350 using Tapatalk
I agree and those have been my experiences as well.
 

mbrooke

Senior Member
We keep our transmission and distribution breakers maintenanced and tested and don't allow the stickiness you evidently are seeing.
This is good, but its difficult to do with tens of thousands of breakers every few years. Especially when for some you need to schedule an outage (straight bus).

Depends on reach. You would normally want your relay to look further and catch those bolted faults. I wil normally set the instantaneous to see 80% of the main line. I will stop at a recloser if it is catching the remainder.
With multiple reclosers in series it gets complicated, ie recloser loops. Parts of the line loose instantaneous clearing.


Sounds like it is time to tweak the budget.
I'd agree, but most EEs aren't the bean counters.


We use alternate (faster and no reclosing) settings when working on the line. It may cause a lockout for what would normally be a blink but so be it.

Yup- work ALWAYS gets a hot line tag.

We assume our breakers will operate during a fault the same way as they did during testing.

Never, ever assume this. Its asking for a blackouts or injury. Thats why despite everything the relaying is duplicated as well as the DC system and communications. Everything besides a casual SCADA trip gets BF relaying initiated. Over reaching zone 2/3, over lapping 50/51, transformer neutral over current, ect as a last resort. MHO reaching through some transformers or even a dedicated 311C for a transformer. Over seas Substations without dedicated busbar protection typically have not just zone 2 from the first supply substation in the chain but zone 3 from the stations before that despite each breaker having a revere zone for BB protection.

Granted every POCO has their own philosophy- but for me and to a lesser degree NERC assume a breaker will fail when called to trip.




But I'm being lazy and want a spoon-fed summary.
If the MV neutral is capable of significant voltages (to remote earth) during any fault condition its isolated from the LV neutral. The LV neutral is then grounded at least 8 feet away where the sphere of influence from the MV rod does not reach the LV ground rod.

Your number of 2.4kv will require that in certain parts of the system the customer neutral must be disconnected and permanently isolated from the MGN.








Like California.

:happyyes::D I like what Cali does. Though so I've heard that on 3 wire systems they do not ground the can :eek:

OK. Haven't ran across it yet.

You will soon enough if you haven't already without knowing. :thumbsup:


It doesn't and I have not said it did.

And thats what I'm getting at. I can legally set my relaying so that a line to neutral fault takes say 60 cycles to clear. 60 cycles of 2.4kv is going to harm someone. Even 5 cycles has risk.


I encourage you to run the numbers on an adult person assuming a resistance from hand to two feet and then compare to the IEC graph.
 

mivey

Senior Member
This is good, but its difficult to do with tens of thousands of breakers every few years. Especially when for some you need to schedule an outage (straight bus).
Either be in the utility business or don't. Difficult or not, it is just part of the job.

Seems odd to stress over NEV during a fault if you are perfectly willing to skip routine maintenance to ensure your equipment is functioning in the first place.

I'd agree, but most EEs aren't the bean counters.
So true. The Dilbert factor.

Granted every POCO has their own philosophy- but for me and to a lesser degree NERC assume a breaker will fail when called to trip.
I assume, and it proves to be true for a well-maintained system, a breaker will rarely fail when called upon, but do have backups in case.

If the MV neutral is capable of significant voltages (to remote earth) during any fault condition its isolated from the LV neutral. The LV neutral is then grounded at least 8 feet away where the sphere of influence from the MV rod does not reach the LV ground rod.
That would work. How fast does it react and what is the trigger mechanism?
 

mbrooke

Senior Member
Either be in the utility business or don't. Difficult or not, it is just part of the job.

Seems odd to stress over NEV during a fault if you are perfectly willing to skip routine maintenance to ensure your equipment is functioning in the first place.

Prove to me that even with functioning equipment 5 cycles of 2.4kv will not hurt someone. Calculate the body resistance, the current that will flow and check in relation to the IEC graph.



So true. The Dilbert factor.
Thats business I guess.

I assume, and it proves to be true for a well-maintained system, a breaker will rarely fail when called upon, but do have backups in case.
Not rare enough. Regardless of equipment you assume worst reasonable clearing times.


That would work. How fast does it react and what is the trigger mechanism?

MV breaker- it could be set so it trips in several cycles.


Tony would know the exact details however.
 

mivey

Senior Member
Prove to me that even with functioning equipment 5 cycles of 2.4kv will not hurt someone. Calculate the body resistance, the current that will flow and check in relation to the IEC graph.
Pick whatever cycle, kV, ohm, amps you want. If you fail to maintain the equipment what have you got? Kinda defeats the purpose of worrying about all the finer details doesn't it?

Kind of like saying you have the linemen use insulated gloves and buckets then don't test the gloves and buckets because there are too many gloves and buckets and/or they are too difficult to test.

Not rare enough. Regardless of equipment you assume worst reasonable clearing times.
One way to do it. Not the best way but certainly one way.

MV breaker- it could be set so it trips in several cycles.
Full circle?

You were just arguing assuming the worst and that operating in several cycles was not a safe assumption. Now it is for this scheme?

Tony would know the exact details however.
No details for a scheme you have concluded is the answer? Sounds odd.
 

mbrooke

Senior Member
Pick whatever cycle, kV, ohm, amps you want. If you fail to maintain the equipment what have you got? Kinda defeats the purpose of worrying about all the finer details doesn't it?

Kind of like saying you have the linemen use insulated gloves and buckets then don't test the gloves and buckets because there are too many gloves and buckets and/or they are too difficult to test.

Page 4:

https://www.scientificbulletin.upb.ro/rev_docs_arhiva/full628_442581.pdf

We have a voltage of 2.4kv

5 cycles 10 cycles 15 cycles.

Where do the values land on the physiology graph?

One way to do it. Not the best way but certainly one way.

Full circle?
Yes, from fault to trip.

Why not the best way? Voltage will not get onto the LV noodle.


You were just arguing assuming the worst and that operating in several cycles was not a safe assumption. Now it is for this scheme?

No details for a scheme you have concluded is the answer? Sounds odd.

Tony would know the relaying as its not my drawing.

But anytime you have the potential for harmful voltage on the LV neutral you isolate it from the MV noodle.
 

mivey

Senior Member
Page 4:

https://www.scientificbulletin.upb.ro/rev_docs_arhiva/full628_442581.pdf

We have a voltage of 2.4kv

5 cycles 10 cycles 15 cycles.

Where do the values land on the physiology graph?
Why does it matter? You won't meet the safe area on that graph because you are counting on your protective device to not work correctly.

Why not the best way? Voltage will not get onto the LV noodle.
You don't know that. You don't know how that device is triggered nor how fast it reacts. Even so, you will assume the protective device is going to fail anyway.

Tony would know the relaying as its not my drawing.

But anytime you have the potential for harmful voltage on the LV neutral you isolate it from the MV noodle.
That seems to be the idea but we have no details.
 

mbrooke

Senior Member
Why does it matter? You won't meet the safe area on that graph because you are counting on your protective device to not work correctly.
Remember, I just said you can show me the results at 5 cycles. Lets first determine if 5 cycles is ok or not ok.

You don't know that. You don't know how that device is triggered nor how fast it reacts. Even so, you will assume the protective device is going to fail anyway.
I have always been taught to take BF into account. I normally bite my tongue, but you are going to have to tell a half dozen ISOs, NERC, FERC, and countless utilities they are doing it wrong.

That seems to be the idea but we have no details.

True- I'll let Tony fill us in.
 

mivey

Senior Member
Remember, I just said you can show me the results at 5 cycles. Lets first determine if 5 cycles is ok or not ok.
Let's say no for the chart that goes along with the neutral switching scheme. Now what?

If you install the device you propose, it is a single point of failure. You claim a single point of protection is not good enough for one scheme but is good enough for another scheme.

You can't have it both ways.

I have always been taught to take BF into account. I normally bite my tongue, but you are going to have to tell a half dozen ISOs, NERC, FERC, and countless utilities they are doing it wrong.
You are missing the point.

Also, since you are name-dropping: Tell me how many of those do not maintenance their breakers because there are too many of them and/or because some of them are difficult to switch around?

True- I'll let Tony fill us in.
Have you contacted him?
 

mbrooke

Senior Member
Let's say no for the chart that goes along with the neutral switching scheme. Now what?
What neutral switching scheme :?


I'm asking you to use this chart on a 5 cycle clearing MGN system:









60479-1 is the basis of what is considered safe vs unsafe and plays a role in just about every aspect of an electrical system.





If you install the device you propose, it is a single point of failure. You claim a single point of protection is not good enough for one scheme but is good enough for another scheme.

What device? :blink: The drawing I'm showing you has no device other then a human removable link in the underground feed version. The LV neutral is separate from the MV neutral and only a person can connect them together.


You can't have it both ways.

You are missing the point.

Also, since you are name-dropping: Tell me how many of those do not maintenance their breakers because there are too many of them and/or because some of them are difficult to switch around?
Thats my point all along. You may have small utility that has time to test every breaker on a 3 year (or the like cycle) but try being something the size ComEd, National Grid or Dominion. In such a case these guys must assume breaker failure for distribution breakers.



Have you contacted him?

Yup- got this reply back:

Tony S
Senior Member


Join DateJul 2013LocationResting under the Major Oak UKPosts1,346Mentioned0 Post(s)Tagged0 Thread(s)

[h=2]Re: Request[/h]
For the LV and MV earths to be linked the LV side earth nest on its own has to be <1Ω.
Between 1Ω and a maximum of 10Ω the link is removed and the 8 metre rule comes in to play.
>10Ω more work is needed on the earth nest to get it <10Ω.

BTW, is there a site I can download the NEC from?​

The reports of my death are greatly exaggerated.
 

mbrooke

Senior Member
This being what determines the two being bonded together vs not bonded together:


For the LV and MV earths to be linked the LV side earth nest on its own has to be <1Ω.
Between 1Ω and a maximum of 10Ω the link is removed and the 8 metre rule comes in to play.
>10Ω more work is needed on the earth nest to get it <10Ω
.
 

mivey

Senior Member
What neutral switching scheme :?
The switchgear you proposed as a switching scheme. You tell me.

I'm asking you to use this chart on a 5 cycle clearing MGN system.
The clearing times are in conjunction with IEEE std 80. There we look at the corner mesh voltage to reduce exposure in and around the substation.

IEC may play some role in a new standard here but I haven't seen it.

What device? :blink: The drawing I'm showing you has no device other then a human removable link in the underground feed version. The LV neutral is separate from the MV neutral and only a person can connect them together.
The device you proposed in an earlier post before you got your new information.


Thats my point all along. You may have small utility that has time to test every breaker on a 3 year (or the like cycle) but try being something the size ComEd, National Grid or Dominion. In such a case these guys must assume breaker failure for distribution breakers.
Multi-billion dollar companies test on regular cycles as well. Been there, done that. In my experience, bigger companies tend to be better at it.

Just part of doing business.
 

mbrooke

Senior Member
The switchgear you proposed as a switching scheme. You tell me.

Not switched.

The MV neutral is simply not connected to the LV neutral.


The clearing times are in conjunction with IEEE std 80. There we look at the corner mesh voltage to reduce exposure in and around the substation.

IEC may play some role in a new standard here but I haven't seen it.
But I'm talking about a multi grounded neutral system and a user barefoot in his backyard touching a metal electric grill plugged into his exterior receptacle.



The device you proposed in an earlier post before you got your new information.

Your voltage of 2.4kv actually supports having the primary and secondary neutrals not connected.



Multi-billion dollar companies test on regular cycles as well. Been there, done that. In my experience, bigger companies tend to be better at it.

Just part of doing business.

Yes- but you still have 3 cycles.
 

mivey

Senior Member
But I'm talking about a multi grounded neutral system and a user barefoot in his backyard touching a metal electric grill plugged into his exterior receptacle.
and what standard, criteria, probability tables, etc. are you using along with that scenario?

Your voltage of 2.4kv actually supports having the primary and secondary neutrals not connected.
Could be. By what applicable standard?

Yes- but you still have 3 cycles.
Yes. But not an assumption of failure and SOP of not testing because it is too difficult.
 

mbrooke

Senior Member
and what standard, criteria, probability tables, etc. are you using along with that scenario?

Could be. By what applicable standard?
This:





Yes. But not an assumption of failure and SOP of not testing because it is too difficult.


Even if we assume the breaker will be 100% reliable tripping under 3 cycles, the NESC does not stipulate a maximum allowable clearing time.
 

mivey

Senior Member
This:


Even if we assume the breaker will be 100% reliable tripping under 3 cycles, the NESC does not stipulate a maximum allowable clearing time.
APPLICABLE standard. This IEC standard is not adopted here.

Nothing is perfect. Prioritize one thing and you may lessen the focus on another. Standards committees spend time debating conflicting priorities, standards, weighing statistical probabilities of things happening, etc.

For example: At first, secondary systems were isolated here. Tying vs. isolating was debated for quite a few years. Experts on both sides. Over time consensus decided it was safer to tie primary and secondary systems together. While it is not perfect, it was decided the benefits of tying outweighed the benefits of not tying.

We do have standards for neutral isolators in agricultural applications to deal with NEV. But they tie during a fault (IIRC).

I don't have data on the standards debate here and it appears neither do you. Until a different approach is adopted here, we live with the currently accepted standards here.
 

mbrooke

Senior Member
APPLICABLE standard. This IEC standard is not adopted here.

But the graph is:

https://www.csemag.com/articles/uls-new-gfci-classes/


https://www.littelfuse.com/~/media/protection-relays/white-papers/littelfuse_industrial_shockblock_gfci_whitepaper.pdf


UL felt it good enough to be taken seriously here using it as a foundation on which industrial GFCI protection is implemented.




Nothing is perfect. Prioritize one thing and you may lessen the focus on another. Standards committees spend time debating conflicting priorities, standards, weighing statistical probabilities of things happening, etc.

For example: At first, secondary systems were isolated here. Tying vs. isolating was debated for quite a few years. Experts on both sides. Over time consensus decided it was safer to tie primary and secondary systems together. While it is not perfect, it was decided the benefits of tying outweighed the benefits of not tying.

We do have standards for neutral isolators in agricultural applications to deal with NEV. But they tie during a fault (IIRC).

I don't have data on the standards debate here and it appears neither do you. Until a different approach is adopted here, we live with the currently accepted standards here.

But I am theorizing that in some cases it does more harm then good.

2.4kv can be brought into the structure.

There is nothing in the NESC which says this voltage must be cleared under 5 cycles.


I'd like my theory challenged and discussed because its something I'm genuinely curious about.
 
MGN

MGN

Ok, say you have a 7,200 volt single phase line running down your street. A a tree falls on the primary and it comes down on the neutral conductor.


According to electrical theory 3,600 volts will drop across the phase conductor and 3,600 volts across the neutral going back to the substation.


My question is, why doesn't 3,600 volts appear on the service neutral and thus everything in the home thats grounded?
Simple answer is utilities, excepting California and a few others, utilize multi grounded neutral (MGN), which under the NESC requires the utility neutral to be grounded a set number of grounds per mile of neutral.

Then the NEC requires the structures neutral to be bonded to the structures grounding system, so in the worst case, if everything is properly grounded and bonded during a cross fault, everything rises to the same voltage level with little difference in potential, therefore making it safe and minimizing damage and hopefully pulling enough amperage to either blow a lateral cutout, or run a recloser through it's cycles and until it locks out.

Now that's the simple answer, in the real world there are many variables, some of which can burn a home to the ground or kill someone.

But luckily between the NEC and the NESC the risk is greatly minimized.

Wayne
 

mivey

Senior Member
But the graph is:
...
UL felt it good enough to be taken seriously here using it as a foundation on which industrial GFCI protection is implemented.
LV GFCI and MV are not the same application of course.

But I am theorizing that in some cases it does more harm then good.

2.4kv can be brought into the structure.

There is nothing in the NESC which says this voltage must be cleared under 5 cycles.


I'd like my theory challenged and discussed because its something I'm genuinely curious about.
The NESC does say that the primary and secondary neutral should be connected when you have a MGN system.
 

mbrooke

Senior Member
LV GFCI and MV are not the same application of course.
True- but the physiological response to any given shock does not change.


The NESC does say that the primary and secondary neutral should be connected when you have a MGN system.

True- but nothing on how to limit the duration of the voltage imposed on the LV neutral.
 

mbrooke

Senior Member
LV GFCI and MV are not the same application of course.


The NESC does say that the primary and secondary neutral should be connected when you have a MGN system.
Food for thought. What do you think? Is this a legit way of determining if they should be interconnected?
 

Attachments

Top