tl;dr: To facilitate using any generator with the house inlet, regardless of transfer switch type or generator neutral-ground-bond status, may one leave the generator ground wire unconnected in the inlet?
longer version: The going requirements for generator usage as house power backup require no parallel neutral-ground path between the generator and the house, so current can't flow back to the generator through the ground wire, thus energizing the ground system. The two common solutions center on:
- Removing the neutral-ground bond from the generator. or:
- Using a switched-neutral transfer switch.
Another thing is that NEC doesn't like checklists. During a power outage, one must be able to connect everything regardless of procedure and have it all just work safely.
If one goes the 'remove-the-bond' route, one can use a cheaper transfer switch or even head-to-head circuit breakers with a interlock plate between, and have common neutrals. However, one must remember to put back the bond when taking the generator to a jobsite or campsite etc. Even if one has a permanently-mounted generator, if that generator should fail to start and one has to borrow a portable, the generator's bond must be removed if the house is not switched-neutral. It is a checklist item, see above. (Not to mention the owner of the generator may not like the borrower disassembling it....)
The alternative is to use a more expensive switched neutral transfer switch. However, there is apparently a possible problem with switched neutrals: If by chance the power lines connect before the neutral, then dangerous imbalances in the 120V legs can develop depending on what loads are connected at the time, with resulting damage to devices. One could remember to disconnect all loads before switching the transfer switch, but that's a checklist item again.
A thought on a third method appears when considering the standard North American utility-to-house connection:
The power from the utility comes through a transformer to a center-tapped secondary, with the center tap grounded. The power lines and neutral go to the house, where the neutral is again connected to ground in the main panel. Note that the Utility power source neutral is bonded to ground on both ends, but only delivers two powers and neutral. There is no ground wire connection in the utility feed.
In theory, if we remove the utility transformer from the house's main three-wire utility feed and substitute the typical neutral-ground-bonded 120/240 portable generator, with the case grounded to a ground rod, we have the same setup as the utility transformer: Power source neutral bonded to ground, two powers and neutral but no ground wire connection to the house, where ground is bonded to neutral again. Lack of a fourth wire for ground seems to achieve the same setup as the utility provides.
Since NEC may want the ground wire still in place in the cable if the cable gets cut, we might not connect the ground wire in the house inlet. With no ground connection in the inlet, a ground loop cannot form with the generator. Yet with the ground-neutral bond in the main panel still connected because we are not using a switched neutral transfer switch, the generator power is still referenced to the ground system in the house.
This concept of not connecting the generator ground wire in the inlet was mentioned in this answer https://diy.stackexchange.com/a/8715/140350 by @gregmac, referencing a no-longer-online Generac webpage:
If the neutral bond cannot be removed, you have two choices. The easiest solution is to lift the ground wire coming from the generator inside the transfer switch, and secure it with a wire nut, by itself. This eliminates the loop. Your other choice is to install a Switched Neutral Kit
Not connecting the ground in the inlet could theoretically make a house compatible with any generator of sufficient size one wanted or had to use, without having to modify the generator's electric setup. (My generator's neutral-ground bond is buried in the alternator, requires tools to remove or replace, and is under cover out of sight out of mind if I forget to put it back or forget to remove it depending on circumstances, which is a checklist thing again.)
Is not connecting the ground in the inlet a sufficient way to avoid complications of switched neutral and make my house compatible with any 120/240 generator? If code strikes this down, is it only because it hasn't been considered, or is it dangerous?