Page 2 of 2 FirstFirst 12
Results 11 to 16 of 16

Thread: 702.11 has me confused

  1. #11
    Join Date
    Dec 2011
    Location
    Ocala, Florida, USA
    Posts
    2,760
    Quote Originally Posted by JPinVA View Post
    I've read the section on generators probably twenty or more times, and still come away with a brain swirl. I try to look at the theory of what is being accomplished, and then back that into what they are trying to say.

    The primary purpose of the neutral/ground bond is to clear a ground fault involving the source. When the source is the generator, the difference between SDS and Non-SDS is where the EGC connects to the neutral to get back to the source.

    In an SDS, the bond occurs in the generator. The fault rides the EGC back to the SE ground bar, and from there rides the EGC of the cable connected to the generator (that is connected to the SE ground bar) all the way back to the generator, jumps to the neutral via the bond in the generator, and completes the low impedance path to clear the fault.

    In a non-SDS system, the fault rides the EGC back to the SE ground bar, and jumps to the neutral bonded at that point, riding this "un-switched" neutral of the cable connected to the generator back to the generator completing the low impedance path to clear the fault.

    Note that in both the SDS and the non-SDS connections described above, the EGC of the structure lands on the SE ground bar. And it is to this ground bar where the GEC is connected, and subsequently connected to the GE system. Pray tell me, why would the SDS system described above require any additional GEC connections than those that already exist at the SE ground bar? In both cases, any non-source current induced in the structure (from lightning or what not) are going to ride the EGC back to the SE ground bar....and from there to the GEC and to the GE. The fact it's an SDS on non-SDS is agnostic when it comes to non-source fault landing on the ground bus.

    I "think" all the mumbo jumbo in the NEC, when it's all laid out, is trying to tell us exactly what I said above, for a system as described by the OP. The reason it's worded all spaghetti-like is that it has to cover any and all SDS type configuration, including those where my assumptions about the bus bar loading won't pan out. But for an 8KW portable, I can't fathom any situation where the existing bus bar EGC/GEC/GES connections won't suffice for taking care of the non-source faults on the structure...be it SDS, non-SDS...or ahem....POCO itself!!!
    When you have a transfer switch that switches the neutral, the bond point between the neutral and the ground is also switched. IF the neutral and the ground are bonded together in the generator then you would need a 2500.66 grounding electrode conductor to be compliant. The path you describe through the equipment grounding conductor will surely provide a path, but it isn't what the code requires. But, why do you assume the neutral and the ground are bonded together in the generator? I would assume the exact opposite myself.


    I know what I don't know, and I know where to go to find it!

  2. #12
    Join Date
    Aug 2018
    Location
    Virginia
    Posts
    51
    Quote Originally Posted by Strathead View Post
    When you have a transfer switch that switches the neutral, the bond point between the neutral and the ground is also switched. IF the neutral and the ground are bonded together in the generator then you would need a 2500.66 grounding electrode conductor to be compliant. The path you describe through the equipment grounding conductor will surely provide a path, but it isn't what the code requires. But, why do you assume the neutral and the ground are bonded together in the generator? I would assume the exact opposite myself.
    Portable generators have two bonding methods. Bonded (neutral/ground bonded in generator) and Floating (neutral/ground NOT bonded in the generator). In the former, the neutral must be switched in the TS (resulting in the generator being a SDS). In the latter, the neutral is not switched at the TS (resulting in the generator being a non-SDS).

    But regardless of whether it's SDS or non-SDS, the structure's EGC lands on the SE ground bar. From a non-source fault standpoint, all the GEC and GES issues are taken care of at the same point and to the same degree as they would be normally.

    Now...if we're talking switching BOTH the Neutral AND the EGC...that's different. But that isn't necessary for a portable (and I'm not quite sure at what point it's necessary for bigger systems). We have a structure, usually with an elaborate code compliant GEC, and GES ready to drain off violent current imposed on a structure. And instead of letting all that get drained off normally, we shunt it over to a milk carton sized generator possibly 50 feet away (with resistance through the roof), and then back along a 250.66 sized conductor. Why? A 100 foot round trip for a high voltage high frequency pulse that could be purged normally? Why?

  3. #13
    Join Date
    Feb 2003
    Location
    Illinois
    Posts
    23,762
    Generator or other SDS source requires a grounding electrode. The fact that it is a portable generator does not change this. The rule in 250.34 does not apply where the loads are not directly supplied by cords connected to receptacles on the generator.
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

  4. #14
    Join Date
    Dec 2011
    Location
    Ocala, Florida, USA
    Posts
    2,760
    Quote Originally Posted by JPinVA View Post
    Portable generators have two bonding methods. Bonded (neutral/ground bonded in generator) and Floating (neutral/ground NOT bonded in the generator). In the former, the neutral must be switched in the TS (resulting in the generator being a SDS). In the latter, the neutral is not switched at the TS (resulting in the generator being a non-SDS).

    But regardless of whether it's SDS or non-SDS, the structure's EGC lands on the SE ground bar. From a non-source fault standpoint, all the GEC and GES issues are taken care of at the same point and to the same degree as they would be normally.

    Now...if we're talking switching BOTH the Neutral AND the EGC...that's different. But that isn't necessary for a portable (and I'm not quite sure at what point it's necessary for bigger systems). We have a structure, usually with an elaborate code compliant GEC, and GES ready to drain off violent current imposed on a structure. And instead of letting all that get drained off normally, we shunt it over to a milk carton sized generator possibly 50 feet away (with resistance through the roof), and then back along a 250.66 sized conductor. Why? A 100 foot round trip for a high voltage high frequency pulse that could be purged normally? Why?
    It sounds like you are saying regardless of the Transfer switch position, the ground to neutral bond is the same bonding jumper. That is not true. The Neutral is switched, so EACH neutral has to have its own independent bond to ground. If I misunderstand you, I apologize up front.


    I know what I don't know, and I know where to go to find it!

  5. #15
    Join Date
    Aug 2018
    Location
    Virginia
    Posts
    51
    Quote Originally Posted by Strathead View Post
    It sounds like you are saying regardless of the Transfer switch position, the ground to neutral bond is the same bonding jumper. That is not true. The Neutral is switched, so EACH neutral has to have its own independent bond to ground. If I misunderstand you, I apologize up front.
    I'm only discussing a single phase portable generator. These have a 14-30 receptacle (two phase conductors, one grounded conductor, one EGC) feeding a 14-30 receptacle. This receptacle is connected to a 14-30 power inlet box via a 14-30 cable (10 gauge for 30A). From the inlet box, the four wires go the the transfer switch.

    The TS switches whatever circuits have been chosen. The conductors from the chosen circuits are routed to the TS switch. If it's an SDS, the neutral of those circuits are also routed and switched for each circuit. In the non SDS case, the neutral from the generator is passed through the TS to the SE, and bounded to the SE neutral bar, consummating the non-SDS neutral/ground bond in the SE.

    In both the SDS and non-SDS configuration, the 14-30 EGC passes through the TS and is bounded to SE ground bar.

    Now let's follow what happens during a source ground fault. An equipment fault in the building while the generator is powering the building. In an SDS system, the EGC fault will land on the SE ground bar. Since the neutral is switched, there is no path from the EGC to the generator neutral at the SE. So the EGC fault travels to the TS...again...the neutral is not connected here either. So the EGC fault travels back down the 14-30 EGC wire to the generator where the EGC is connected to generator ground, which is bonded to the neutral (i.e., the generator has a bounded neutral). It hops to the neutral at the generator, and clears the fault.

    In a non-SDS system, the ground fault will, once again, land on the SE ground bar. But in this case, since the generator neutral is bonded to the SE neutral bar, with bonding jumper to the SE neutral bar, the EGC fault current will follow this jumper to the SE neutral bar and then ride the generator neutral back along the 14-30 cable to the generator, clearing the fault.

    Now, let's see what happens when the building is energized by a non-source entity (e.g., lightning). This energy pulse is looking for ground. It's looking for a GEC and a solid GES. Whether it's an SDS or non-SDS, the energy is going to flow through the building EGC to the ground bar in the SE. It doesn't care if there is an EGC back to the generator. It doesn't care if there is a bonded neutral in the SE back to the generator. The current doesn't need or want to go there. What this pulse sees is the GEC connected to the SE ground bar, connected to the GES. The GEC and GES is ALREADY THERE! It's already connected! What more does it need?
    Last edited by JPinVA; 10-11-18 at 05:15 PM.

  6. #16
    Join Date
    Aug 2018
    Location
    Virginia
    Posts
    51
    I've been thinking about this a LOT, and have come up with two reasons for the need of a GEC/GES on SDS.

    1. The SDS is of sufficient size that external energy imposed on it needs to be resolved.

    2. The external energy imposed on the neutral conductor in the structure served by the SDS needs to be resolved.

    One or both conditions can be valid depending on the configuration.

    For an 8KW single phase portable unit, I would venture that only number 2 is the concern. As for number 1, if the generator is small enough, it is...itself...little more than a piece of equipment from an EGC standpoint. So the EGC between the generator and the SE ground should be sufficient to bleed off any energy. The real issue is number 2. Any energy imposed on the structure, will connect to both EGC and the Neutral (and other conductors...but there isn't much we can do about the ungrounded ones). While the SE EGC connection will suffice for the EGC energy, the switched neutral doesn't have a SE route to ground. Hence the need for a GEC/GES connection.

    If I'm correct, this raises another conundrum. That connection needs to be as close as possible to the SE. Ideally, the TS will be next to the SE, and the bond can be made in the TS. The conundrum this raises is the neutral is also bonded in the generator. If we make the bond in the TS, there is a parallel path and the issue of objectionable current raises its ugly head. To solve the objectionable current, we'll need to un-bond the generator...but then we lose the UL listing. If we keep the genreator bonded and move the GEC/GES connection to the generator, we lose the "short path" benefit needed for a properly GEC/GES clear.

    Sigh. Makes my head hurt.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •