Ground fault- Why doesn't anyone get shocked?

mbrooke

Batteries Not Included
Location
United States
Occupation
*
There is a difference between touching a 4 kV line while standing on the ground vs. touching a grounded line in contact with a 4 kV line.
Or rather there is no difference touching a 4kv line vs touching the MGN of a 34.5kv line during a phase to MGN fault.
 

mivey

Senior Member
Yes, because the massive amount of current in an MGN produces tremendous neutral to remote earth voltage, where as in an ungrounded system the voltage to remote earth is small during a fault.
If you want to change over to discussion of faults on ungrounded systems, that is a whole different discussion. Designs for the two are not the same of course.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
If you want to change over to discussion of faults on ungrounded systems, that is a whole different discussion. Designs for the two are not the same of course.


Of course, but electrical theory tells us that the more current you have the more voltage drop you have.
 

mivey

Senior Member
Or rather there is no difference touching a 4kv line vs touching the MGN of a 34.5kv line during a phase to MGN fault.
Of course there is. One occurs during a fault while protective devices are reacting and the other occurs with no protective devices reacting.

The kV present during a L-N fault on a MGN should normally clear on instantaneous in about 5 cycles or less.

The kV present for just touching a line will be there all day and all night.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
Of course there is. One occurs during a fault while protective devices are reacting and the other occurs with no protective devices reacting.

The kV present during a L-N fault on a MGN should normally clear on instantaneous in about 5 cycles or less.


5 cycles? Real world often takes longer then that.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
Perhaps you have some setting or breaker selection issues. 2-8 cycles is normal total clearing with 5 pretty common.
Show me what part of the NESC mandates that an MGN system or any particular system must clear in 2-8 cycles.



You don't use a Petersen coil on a MGN system so it is not really relevant.

You know the theory which I am referring to. Limit the neutral current, you limit the remote earth voltage which is what RJ was getting at.


Tell me how you propose to set it below shock levels on MV system.

Definite time ground trip set below the anticipated body current clearing in 5 cycles.



Honestly, you don't want to admit that tieing the primary neutral into the secondary neutral can get someone hurt or killed.
 

mivey

Senior Member
Show me what part of the NESC mandates that an MGN system or any particular system must clear in 2-8 cycles.
Show me where I said it did. I'm just telling you what I see in switchgear and fault data.

Where did your get your number of often more than 5 cycles from? My 2-8 cycles comes from many years of experience.

You know the theory which I am referring to. Limit the neutral current, you limit the remote earth voltage which is what RJ was getting at.
Throwing out unrelated applications as a solution to a different application hints at a willingness to throw stuff together because some of the words and terms are used in both applications but not because it is applicable to the situation at hand.

Definite time ground trip set below the anticipated body current clearing in 5 cycles.
and what do you use for ground trip instantaneous settings in a distribution feeder?

What if I told you for a lot of systems I work on, I set the 50G pickup to ~140-400 amps?

Honestly, you don't want to admit that tieing the primary neutral into the secondary neutral can get someone hurt or killed.
Honestly, speculation with wild assumptions don't fit an engineer mindset. We deal with data. We love data. You can't give an engineer too much data. Do you have any data showing probability studies, mortality rates, etc. or are you just throwing more stuff out there?
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
Show me where I said it did. I'm just telling you what I see in switchgear and fault data.

Where did your get your number of often more than 5 cycles from? My 2-8 cycles comes from many years of experience.
An SPS2 breaker on its best day clears in 3 cycles page 15:

https://w3.usa.siemens.com/smartgri.../IEEE 2016/HP_SiemensCircuitBreakerFamily.pdf


A breaker that hasn't opened in a while can take a few extra cycles.


It takes about 1/2 1 cycle for a microprocessor relay to make a trip decision.


Of course that is best case scenario tripping on definite time or zone 1 MHO (none delayed).


Come real world you have oil breakers that just start at 8-10 cycles.

Breaker failure that can take 35 cycle to complete.

Eletromechanical relays.

Inverse time over current tripping if coordinating with a string of other devices.

All these add time, 5 cycles is best case in a new system, often a pie dream in old ones. Which is why its never assumed when it comes to life safety.

IE, page 23 assumes 72 cycles:


https://www.eversource.com/content/...tive-grounding---td-703.pdf?sfvrsn=f340c162_2



Throwing out unrelated applications as a solution to a different application hints at a willingness to throw stuff together because some of the words and terms are used in both applications but not because it is applicable to the situation at hand.
Peterson coils are used in distribution systems.

As are remote LV grounds to prevent MV neutral voltage from entering the building.


and what do you use for ground trip instantaneous settings in a distribution feeder?

Depends on many things, anywhere from 5 amps to 560 amps for a typical feeder.

What if I told you for a lot of systems I work on, I set the 50G pickup to ~140-400 amps?
With wye primary distribution transformers, of course!



Honestly, speculation with wild assumptions don't fit an engineer mindset. We deal with data. We love data. You can't give an engineer too much data. Do you have any data showing probability studies, mortality rates, etc. or are you just throwing more stuff out there?

Its not a speculation when it clearly violates IEC 60479-1:


 

mivey

Senior Member
An SPS2 breaker on its best day clears in 3 cycles page 15:
would you believe a breaker rated to open in 3 cycles will open in 3 cycles or less? I know because we routinely test them.

A breaker that hasn't opened in a while can take a few extra cycles.
Then it is malfunctioning and needs repair.

We don't want to depend on broken equipment in our prtection scheme. That is why we perform routine testing and maintenance.

It takes about 1/2 1 cycle for a microprocessor relay to make a trip decision.
yes

Of course that is best case scenario tripping on definite time or zone 1 MHO (none delayed).
and for your scenario of a phase dropping into a neutral, that is what we expect to happen. In fact, that is what I am most likely to see when reviewing the fault data.

Come real world you have oil breakers that just start at 8-10 cycles.
come real world most that I see are rated faster than that and operate within specs. If you are having breakers operate outside specs then you need to re-work your maintenance schedules and procedures.

Breaker failure that can take 35 cycle to complete.
Backup schemes. We use them as a safety net but also recognize the probabilities are very low that we will have to use them (maintenance and testing, remember?).

You can have an entire station in fault for a very long time. Minutes.

We had that after a contractor melted a lockout relay during testing and did not reveal their mistake. We found out later when a fault occurred and just sat there cooking everything. The DO called and said "hey, your station has some extremely high currents, would you like us to trip it?" It was dragging down the transmission network. Those are the odd scenarios where equipment and people fail but not our planned protection scheme that we consider to be working as it should.


Eletromechanical relays.
very dependable when maintained.

Inverse time over current tripping if coordinating with a string of other devices.
not our scenario. A bolted L-N fault assumes no fault impedance so we should be hitting 50G and it will win the race (tie at worst) against the 51 relays .

All these add time, 5 cycles is best case in a new system, often a pie dream in old ones.
Quite frankly, you don't know what you are talking about. I work on systems that have been around for 80-100 years and with proper maintenance and testing they don't slow down with age. Where do you get this nonsense?

We tested breakers a few weeks ago that are 60+ years old and 3-cycle rated. The mechanism operated in about 2.5 cycles and the instantaneous relays in about 1/2 cycle.

read the fine print:
"The fault-clearing times designated above are based on broad-application worst-case relay and/or breaker-failure situations. If other fault-clearing times are substantiated, maximum current single cable ratings may be adjusted by calculation on a case-by-case basis."

Peterson coils are used in distribution systems.
But not in a MGN distribution system.

As are remote LV grounds to prevent MV neutral voltage from entering the building.
Please clarify

iwth wye primary distribution transformers, of course!
Kinda need a wye distribution system to go along with your neutral scenario don't you think?


Its not a speculation when it clearly violates IEC 60479-1:
Not my area of expertise as I don't work under that code.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
would you believe a breaker rated to open in 3 cycles will open in 3 cycles or less? I know because we routinely test them.

Then it is malfunctioning and needs repair.
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.





and for your scenario of a phase dropping into a neutral, that is what we expect to happen. In fact, that is what I am most likely to see when reviewing the fault data.

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.



come real world most that I see are rated faster than that and operate within specs. If you are having breakers operate outside specs then you need to re-work your maintenance schedules and procedures.
Still more then 5 cycles for that old oil dinosaur.

Backup schemes. We use them as a safety net but also recognize the probabilities are very low that we will have to use them (maintenance and testing, remember?).
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.



You can have an entire station in fault for a very long time. Minutes.

We had that after a contractor melted a lockout relay during testing and did not reveal their mistake. We found out later when a fault occurred and just sat there cooking everything. The DO called and said "hey, your station has some extremely high currents, would you like us to trip it?" It was dragging down the transmission network. Those are the odd scenarios where equipment and people fail but not our planned protection scheme that we consider to be working as it should.
Now that is what I would consider a rare enough scenario mitigated by correct maintenance and design- not a stuck breaker.


very dependable when maintained.
Of course, but slower then MP relays.

not our scenario. A bolted L-N fault assumes no fault impedance so we should be hitting 50G and it will win the race (tie at worst) against the 51 relays .
For you, yes.

Quite frankly, you don't know what you are talking about. I work on systems that have been around for 80-100 years and with proper maintenance and testing they don't slow down with age. Where do you get this nonsense?

We tested breakers a few weeks ago that are 60+ years old and 3-cycle rated. The mechanism operated in about 2.5 cycles and the instantaneous relays in about 1/2 cycle.

Ok, good.



read the fine print:
"The fault-clearing times designated above are based on broad-application worst-case relay and/or breaker-failure situations. If other fault-clearing times are substantiated, maximum current single cable ratings may be adjusted by calculation on a case-by-case basis."


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.


But not in a MGN distribution system.

True- unless you insulate everything phase to phase and use only delta loads. :p

Please clarify

Already linked it:









Kinda need a wye distribution system to go along with your neutral scenario don't you think?

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

Not my area of expertise as I don't work under that code.

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.


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




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.
 
Last edited:

paulengr

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
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
I still assert that its "normal" for a 3 cycle breaker to clock in at 5 cycles. Let alone an oil unit.

Here is a relaying engineer saying the same thing toward the end of a thread:

davidbeach (Electrical) 26 Sep 16 12:38
A cycle or so for the relay and 2-3 cycles for breaker time plus, maybe, a bit of slop. I like to list the longer end of the time range but I do see the occasional fault that is cleared in 3 cycles (relay + breakers) with a 3 cycle breaker. But I also see a 3 cycle breaker sometimes take an extra cycle or two if it's been closed for a very long time and the go just 3 cycles on the second trip.

https://www.eng-tips.com/viewthread.cfm?qid=414455


Through out its assumed 3 cycles as the fastest clearing, not the slowest.
 
Top