802/803 Neutral bonding

[Jeepadict]

I'm studying the TM wiring diagram to answer a theory question that stems from my brother's commercial genset.

It is understood that the Neutral bond must be disconnected at the lugs (in most cases) to feed a structure service as NEC requires the Neutral to be bonded "at the first disconnect means"...unless there is a reason that NEC would necessitate the bond required at the set if there is no other "disconnect means". Ergo, if bonded at the panel, then must be un-bonded at the set.

It is understood that the Neutral needs to bonded at the lugs if the set is to be used in-field or off-grid as it would therefore be the "first disconnect means".

Now my query: if the set is un-bonded at the lugs and for whatever reason is left to run by itself, then is the convenience outlet in a "floating Neutral" configuration or is there a Neutral bond dedicated and isolated to the outlet? My theory concedes that if the bond is open at the terminal lugs then there would be no way internally to bond the outlet without bonding the lugs as a consequence since everything is intended to resort back to the source and the ground is intended to be an emergency return path should the Neutral fail.

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[Scoobyshep]

In theroy no self bond (unless it is run through a transformer and then is a separately derived system)

The ground is "a low impedance path back to the source to facilitate the operation of the over current protective device" the neutral can do the same thing (all depends on whats shorted)


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[HORNETD]

I'm studying the TM wiring diagram to answer a theory question that stems from my brother's commercial genset.

It is understood that the Neutral bond must be disconnected at the lugs (in most cases) to feed a structure service as NEC requires the Neutral to be bonded "at the first disconnect means"...unless there is a reason that NEC would necessitate the bond required at the set if there is no other "disconnecting means". Ergo, if bonded at the panel, then must be unbonded at the set.

This is going to take some explaining so bear with me. In any electrical system were one of the current carrying conductors is to be bonded to Ground, and thus to all of the Equipment Grounding Conductors, it must only be bonded at a single point within the structure. Since we are talking about portable and mobile generators we can expect that only a fool would run them inside the structure without taking elaborate precautions well beyond the scope of this discussion. That means that what I'm about to write does not apply to generators which are purposely setup to run inside the structure.

The US NEC does not forbid bonding the grounded point of the generator's windings to the frame when used to power a structure. What it does instead is to require that a bonded neutral generator be connected to a transfer mechanism which transfers the "neutral" conductor along with the energized conductors. That requires that whichever transfer mechanism you use it must have an additional pole to switch the Grounded Current Carrying Conductor when it is switching the energized conductors. The concern that this is intended to address is that if the Neutral is grounded in more than one place normal operating current will be flowing on non current carrying conductive parts of the electrical system. In the event that the neutral failed open and all of the operating current were then flowing over that unintended pathway the voltage drop of that pathway may elevate the touch potential, meaning the voltage to grounded floors and other non energized conductive surfaces to a dangerous level.

It is understood that the Neutral needs to bonded at the lugs if the set is to be used in-field or off-grid as it would therefore be the "first disconnect means".

It needs to be bonded to the frame of the generator set and to any vehicle, including a trailer, on which it is mounted and from which it will be operated. It does not particularly matter were on the frame that bonding occurs as long as there are no other connections between the frame and the neutral point of the generator windings.

Now my query: if the set is unbonded at the lugs and for whatever reason is left to run by itself, then is the convenience outlet in a "floating Neutral" configuration or is there a Neutral bond dedicated and isolated to the outlet? My theory concedes that if the bond is open at the terminal lugs then there would be no way internally to bond the outlet without bonding the lugs as a consequence since everything is intended to resort back to the source and the ground is intended to be an emergency return path should the Neutral fail.

Not quite. The purpose of the Equipment Grounding Conductor is to provide a low impedance path for fault current to return to the source at a high enough current to cause the OCPD of the faulted circuit to open and clear the fault. The EGCs are all connected to a single point inside the enclosure of the main switch or Service Disconnecting Means. In grounded electrical systems that point is connected to the neutral point of the source winding by the Main Bonding Jumper. What the Main Bonding Jumper, no matter what physical form it takes, is installed to do is to connect the normally non current carrying conductive parts of an electrical system to the neutral point of the source's windings. The Main Bonding Jumper is the last portion of the low impedance fault current pathway to the source of supply in the windings.

The language in the US NEC is:
"Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded ystems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path."

 

[Jeepadict]

@HORNETD thank you for taking the time to write such detail to answer my question. It's all a bit over my head as I'm just an airplane fixr, but I'll read it over a few more times and I'm sure more will make sense. I sure wish you had the same intimate knowledge with the 802/803 sets as you do with the NEC, as your information seems to be in general application and not unit specific...I guess I might say I was asking more of a "nuts n bolts" question and not so much on electrical theory. Cheers!

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