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Old 11-28-2018, 07:05 AM   #39
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Default Mechanical interlock issues

Originally Posted by Rusty View Post
Could you give me the "jurisdictions" and "utilities" that don't allow an interlock on the panel cover. Also do you know of any cases where someone removed the panel cover and caused damage to anything while operating a portable generator.

I do agree that an interlock that is under the cover is better and real easy to put on.
Well, read the link at the bottom, all the way to the end. A hot topic among electricians. Typically disallowed in northern NJ, apparently. These have come up in informal discussion before and the understanding of the PE's I was talking with was that there were numerous locations that disallowed them.

In my mind any device that only works with the panel cover installed is unsatisfactory. Homeowners and even some lazy electricians leave covers off of panelboards all too often, either for a period while rewiring or for good.

I would be suspicious of any "third party" device, not manufactured by the panelboard/breaker manufacturer. The panelboards must be listed by UL or another listing agency, to be installed in compliance with the NEC, and quite likely (unless your jurisdiction has exempted the listing requirement) to be compliant in your jurisdiction. There is a significant question whether the panelboard listing, obtained by its manufacturer, is still valid (or would be considered to be valid by the manufacturer or the AHJ) if someone, licensed or not, modifies that panelboard with a third party device, particularly since the modification in this case requires drilling holes in the cover and putting a plastic wire tie around the circuit breakers (Yes, read the instructions provided!) The "kits" sold by the panelboard manufacturer itself would more likely be complaint with the listing.

Also note that the obtained listing for that product is for panelboards, not transfer switches, and the UL description for UL 67 specifically says it is not for transfer devices.

Additionally, if you check the listing restrictions on small portable generators, the article below says that the listing of the generator requires any transfer switch used with the generator to switch the neutral as well as the "hot" conductors. Circuit breaker interlocks can't do that. The NEC section on ground bonding limits most electrical systems to a single bond between the neutral and the safety ground, typically at or near the service disconnect. I think you'll find small portable generators have the neutral tied to the ground connections on the outlets (so that a ground fault can trip the breaker or GCFI) and probably also the frame of the generator. The NEC requires the portable generator ground to be tied to the building ground, and without a transfer switch that switches the neutral you will have a second bond between the neutral and the safety ground, which can cause stray currents in your system. If your portable generator has GCFI's, these stray currents may possibly cause tripping of the GCFI's. Since I don't have copies of the listing requirements (I do have a current copy of the NEC), I can't comment on the listing issues beyond what I've mentioned here.

Regarding people being injured or killed by backfeeds, Google "Lineman injuries from backfeeds". There are hundreds, maybe thousands, of links.

Beyond that, it is up to the owner and/or the electrician (or a Professional Engineer hired by the owner) to verify what your AHJ (Authority having Jurisdiction) and utility will permit. You might want to read your utilitiy's terms of service. They do have the right, (and under OSHA the requirement) to disconnect you if they find your installation presents a safety hazard to their employees.

I personally would not consider any "interlock" that would be inoperative if a cover were removed, or that required plastic cable ties around breakers to assure operation to be safe, but I'm not the AHJ or the utility safety engineer. Of course I (or anyone else) could file a request for a change of the NEC in the next (2020) edition of the code to add language to the code requiring the interlock not be defeated by removing a cover.

Do read the following:
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