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Thread: Ground fault- Why doesn't anyone get shocked?

  1. #101
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    Quote Originally Posted by mbrooke View Post
    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.

    Quote Originally Posted by mbrooke View Post
    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.

    Quote Originally Posted by mbrooke View Post
    Still more then 5 cycles for that old oil dinosaur.
    Sounds like it is time to tweak the budget.

    Quote Originally Posted by mbrooke View Post
    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.

    Quote Originally Posted by mbrooke View Post
    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.

    Quote Originally Posted by mbrooke View Post
    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.

    Quote Originally Posted by mbrooke View Post
    True- unless you insulate everything phase to phase and use only delta loads.
    Hey, it is only money.

    Quote Originally Posted by mbrooke View Post
    Already linked it:
    But I'm being lazy and want a spoon-fed summary.

    Quote Originally Posted by mbrooke View Post
    You can hook everything phase to phase and just keep the MGN.
    Like California.

    Quote Originally Posted by mbrooke View Post
    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.

    Quote Originally Posted by mbrooke View Post
    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.
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  2. #102
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    Quote Originally Posted by paulengr View Post
    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.
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  3. #103
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    Quote Originally Posted by mbrooke View Post
    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.
    But scuffed to operate slower after sitting then operate faster once cycled. If it is sticking it needs work.
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  4. #104
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    Quote Originally Posted by mivey View Post
    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.

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

    OK. Haven't ran across it yet.

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


    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.

  5. #105
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    Quote Originally Posted by mbrooke View Post
    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.

    Quote Originally Posted by mbrooke View Post
    I'd agree, but most EEs aren't the bean counters.
    So true. The Dilbert factor.

    Quote Originally Posted by mbrooke View Post
    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.

    Quote Originally Posted by mbrooke View Post
    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?
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  6. #106
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    Quote Originally Posted by mivey View Post
    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.

  7. #107
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    Quote Originally Posted by mbrooke View Post
    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.

    Quote Originally Posted by mbrooke View Post
    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.

    Quote Originally Posted by mbrooke View Post
    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?

    Quote Originally Posted by mbrooke View Post
    Tony would know the exact details however.
    No details for a scheme you have concluded is the answer? Sounds odd.
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  8. #108
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    Quote Originally Posted by mivey View Post
    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.r...628_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.

  9. #109
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    Quote Originally Posted by mbrooke View Post
    Page 4:

    https://www.scientificbulletin.upb.r...628_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.

    Quote Originally Posted by mbrooke View Post
    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.

    Quote Originally Posted by mbrooke View Post
    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.
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  10. #110
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    Quote Originally Posted by mivey View Post
    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.

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