mbrooke
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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.
Ground fault- Why doesn't anyone get shocked?
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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
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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
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Of course, but electrical theory tells us that the more current you have the more voltage drop you have.
mivey
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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.
mivey
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Why thank you Capt.
mbrooke
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mbrooke
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Why thank you Capt.
So it is not out of bounds of the discussion we are having and the Peterson coil RJ brought up.
mbrooke
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Not if all loads are connected in delta and your zero sequence is set very low.
romex jockey
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you mean other earthing systems ?
https://en.wikipedia.org/wiki/Earthing_system
I always felt it somewhat a challenge getting my head outta the 'nec' box.....:happyyes:
~RJ~
mivey
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Perhaps you have some setting or breaker selection issues. 2-8 cycles is normal total clearing with 5 pretty common.
mivey
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You don't use a Petersen coil on a MGN system so it is not really relevant.
mivey
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Tell me how you propose to set it below shock levels on MV system.
mivey
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Yes. There are several other systems around the world and more than one in the US but MGN is the one most used in the US for distribution.
mbrooke
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Show me what part of the NESC mandates that an MGN system or any particular system must clear in 2-8 cycles.
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.
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
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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.
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.
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, 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
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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
Peterson coils are used in distribution systems.
As are remote LV grounds to prevent MV neutral voltage from entering the building.
Depends on many things, anywhere from 5 amps to 560 amps for a typical feeder.
With wye primary distribution transformers, of course!
Its not a speculation when it clearly violates IEC 60479-1:
mivey
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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.
We don't want to depend on broken equipment in our prtection scheme. That is why we perform routine testing and maintenance.
yes
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 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.
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.
very dependable when maintained.
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 .
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."
But not in a MGN distribution system.
Please clarify
Kinda need a wye distribution system to go along with your neutral scenario don't you think?
Not my area of expertise as I don't work under that code.
mbrooke
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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.
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.
Still more then 5 cycles for that old oil dinosaur.
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.
Now that is what I would consider a rare enough scenario mitigated by correct maintenance and design- not a stuck breaker.
Of course, but slower then MP relays.
For you, yes.
Ok, good.
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.
True- unless you insulate everything phase to phase and use only delta loads.
Already linked it:
You can hook everything phase to phase and just keep the MGN.
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.
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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.
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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.
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mbrooke
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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:
https://www.eng-tips.com/viewthread.cfm?qid=414455
Through out its assumed 3 cycles as the fastest clearing, not the slowest.
Here is a relaying engineer saying the same thing toward the end of a thread:
https://www.eng-tips.com/viewthread.cfm?qid=414455
Through out its assumed 3 cycles as the fastest clearing, not the slowest.
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