# Generator interlock for 3 phase breakers



## SWDweller (Dec 9, 2020)

You have done a fair job of describing the problems with bonded neutral generators.

Adding
If the people who are connecting the generator to a electrical service do not understand the dangers then I submit they should not be doing the job. Portables are made "bonded" so they can be used ALONE. They are NOT intended to be used on electrical services with an existing bonded neutral. If you intend to use it on an electrical service then you need to install the needed equipment. 

Using bonded neutral portable on a electrical service you have 2 grounds. One at the generator and one at the electrical service. You should understand that issue if not Soares Book on Grounding. Lots better than I can do and with pictures. 

connecting a portable generator (bonded neutral) into a sub panel is a problem. Sub panels have the neutral and ground separated. 

The utility conductors should NEVER be connected to another source of power. That is why their is a requirement for a interlock. The interlock keeps the main off and the electrical service separate from the new power source.

Transfer switches work really well. For some what less money you can get a On Off On 3 pole switch. I personally like switching the neutral for temp power connections. I have found that keeping the neutral and ground bonded to the electrical service when using a generator can create issues with electronics.

Electrical safety is one thing, cost of the equipment is another. The two are not balanced against one an other.

I would need specifics on the equipment both the generator and the electrical equipment.

Long ago I remember a building that had a 100 amp 4 pole 240v connection. It was for additional power when they were doing live shows. I have NO idea how it was connected as I never got into that. Might be time to look at the generators in stock and the locations where they would be used and have and have someone design something for you all.


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## splatz (May 23, 2015)

hornetd said:


> If we convert it to unbonded then it would be quite hazardous to use it elsewhere as a mobile generator because there would be no low impedance pathway back to the windings for fault current.


If the generator is unbonded, and you connect it to a regular transfer switch that doesn't switch the neutral, neutral is bonded to ground in just one place, the building's main bonding jumper. 

If the generator is unbonded, and you use it mobile, how hazardous is it? If you put an extension cord on the generator, and stuck a butter knife in the hot at the end of the cord, would you get zapped? I don't think so - no path through you / ground to the other winding.


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## Almost Retired (Sep 14, 2021)

this is a pic of the only style of inter lock Not mounted to the cover that i have seen
but it might be simpler to use security head type screws for the panel cover with a cover mounted style



https://www.geninterlock.com/wp-content/uploads/2016/04/faceatch100_pic1_i_lg.jpg


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## Almost Retired (Sep 14, 2021)

something that might help you visualize your scenario .....
when you put the gen on the trailer
start thinking about it like you would for your car's electrical system

if you take a jumper from the battery pos and put the other end in the dirt ... will current flow? no. the dirt is not part of the system. the dirt might as well be the air around the battery, the system is isolated from it
what if you add another jumper from the battery neg and put the other end in the dirt ...will current flow ? yes. the dirt has now been included in the system.

the same is true for that genny ....the genny is isolated..... until you start making connections to things not on that trailer, and begin to include them in the system


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## Almost Retired (Sep 14, 2021)

splatz said:


> If the generator is unbonded, and you connect it to a regular transfer switch that doesn't switch the neutral, neutral is bonded to ground in just one place, the building's main bonding jumper.
> 
> If the generator is unbonded, and you use it mobile, how hazardous is it? If you put an extension cord on the generator, and stuck a butter knife in the hot at the end of the cord, would you get zapped? I don't think so - no path through you / ground to the other winding.


since he mentioned a wet field ....

scene one) the trailer remains on the tow hitch of the truck, are there enough tall grass, weeds, bushes, etc. touching the truck body and trailer to give it a ground?

scene two) the field was recently mowed. but the trailer is unhooked from the truck and the jack stand is now on the ground, is the trailer frame still isolated from the genny frame?
wet wooden trailer bed, no;; metal trailer bed, no;; genny strapped against the trailer side rails, no

with a four wire feed leaving the genny there is a closed loop path back to power
yes the butter knife will hurt at least a little bit, depending on how wet your shoes and the grass are

edit :: i made the assumption that the genny was connected to a panel that was disconnected from poco in this scenario
however if it is used free standing with bonded neu and ground on the genny the same conditions would apply


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## splatz (May 23, 2015)

Almost Retired said:


> since he mentioned a wet field ....
> 
> scene one) the trailer remains on the tow hitch of the truck, are there enough tall grass, weeds, bushes, etc. touching the truck body and trailer to give it a ground?
> 
> ...


Let's say you have a generator frame welded to a one acre steel tub filled with an inch of saltwater, and you're standing barefoot in the tub. You run an extension cord to one of the receptacles on the generator and stick a butterknife in the hot slot on the end of the extension cord. 

What happens if the neutral is bonded to the frame of the generator? 

What happens if the neutral is not bonded to the frame of the generator?


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## Almost Retired (Sep 14, 2021)

splatz said:


> Let's say you have a generator frame welded to a one acre steel tub filled with an inch of saltwater, and you're standing barefoot in the tub. You run an extension cord to one of the receptacles on the generator and stick a butterknife in the hot slot on the end of the extension cord.
> 
> What happens if the neutral is bonded to the frame of the generator?
> 
> What happens if the neutral is not bonded to the frame of the generator?


A) you get bit
B) you dont
i agree,
but i did specify that if used free standing, the neutral should be bonded
if attached to a panel,,, then that panel and its grounding system with a neu/ground bond, will complete the fault path without the neutral bonded on the genny

i re-read your initial supposition and i think i understand your line of thought now.
if you purposely disconnect the neu/grd bond and use it freestanding , extension cords only
you now have a floating , ungrounded system, earth is not part of the system anymore,
current cannot flow through the earth because it is Not part of the system
like you, im not sure if that would be dangerous in some way or not, i cant think of a way right this minute
maybe a static charge if its on a trailer with the jack stand up


IIRC the initial electrical systems were not earth ground referenced/bonded
you could stand barefoot in a mud hole and pick up ONE end of a hi voltage line and be unharmed
which i understood to be the reason for using an open knife switch for lighting in a home 

i still havent heard a good reason why they changed it to grounded


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## splatz (May 23, 2015)

Almost Retired said:


> A) you get bit
> B) you dont
> ...
> i still havent heard a good reason why they changed it to grounded


Something about making lightning less destructive, never quite followed it. Maybe if the secondary side of the utility transformers are grounded then lightning strikes are less likely to blow them up. I think it's stated in 250.4 ...


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## Almost Retired (Sep 14, 2021)

splatz said:


> Something about making lightning less destructive, never quite followed it. Maybe if the secondary side of the utility transformers are grounded then lightning strikes are less likely to blow them up. I think it's stated in 250.4 ...


2014 nec is 250.4(A)1 to limit the voltage imposed by lightning, (and several other sources)

so ... im trying to visualize how grounding the neu and other metals etc. will _limit lightning voltage_ if it comes in on a hot leg
in my experience it doesnt
or if it does limit the voltage ,, it would be blowing up the entire house without it,, not just burning holes thru freezers, dryers, etc.


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## hornetd (Oct 30, 2014)

splatz said:


> Something about making lightning less destructive, never quite followed it. Maybe if the secondary side of the utility transformers are grounded then lightning strikes are less likely to blow them up. I think it's stated in 250.4 ...


Most of the North American Electrical Distribution Network has a Multi-Grounded Neutral. Look at the top of a utility pole that supports High Voltage distribution conductor/s and has a medium voltage service transformer (Pole Pig) attached to it. Regardless how many of the energized distribution conductors there are supported by that pole above the top of the service transformer you will not see a neutral up there. With most pole mounted transformers there will not be a second bushing on the top of the transformer either. That is because one end of the primary winding and the neutral point of the secondary winding are both connected to the utilities neutral on the pole. The transformers enclosure is also bonded to the utility's neutral by a "noodle" (strap conductor) which is bonded to the can (transformer enclosure) and the neutral bushing from the bonded points on both the primary and secondary windings. That will be the middle of the three terminals on the side of the can.

[The only exception is the transformers that are actually painted to resemble the hide of a cow. Those do have a second bushing on top and 2 phases powering the primary winding. The secondary neutral is not grounded at the pole but only at the Service Equipment mounted on a yard pole on the served premise property. That is done to isolate animal husbandry farm and ranch wiring systems from the utility's neutral to keep stray neutral current from injuring live stock.]

Poles which support transformers will also have a Grounding Conductor run down the pole to some kind of grounding electrode. That is also done at set intervals along the lines between widely separated transformers. In Utility Distribution Systems the grounding electrode used can take many forms such as a but plate; which is a copper sheet wrapped around and under the but end of the pole; the grounding conductor itself formed into a spiral on the but of the pole, a driven rod, and several others. Part of the imbalance current of all multi-grounded neutral power distribution systems returns to the the sub station through the earth. 

In most of Europe that is not done. European utilities put a lot more effort into balancing the loads over the phases of the system and use a neutral at least as large as the phase conductors to carry the remaining imbalance current back to it's source at the sub-station. Stray neutral current is as rare in European electric utility distribution systems as it is common in North American Multi-Grounded Electrical distribution systems. European electrical systems protect their systems from lightning; which is less common there; using lightning arrestors at intervals along the distribution lines which will connect the lightning current to earth instead of raising the voltage in the conductors to many thousands of volts above the normal distribution service voltage.

Tom Horne


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## Almost Retired (Sep 14, 2021)

hornetd said:


> Most of the North American Electrical Distribution Network has a Multi-Grounded Neutral. Look at the top of a utility pole that supports High Voltage distribution conductor/s and has a medium voltage service transformer (Pole Pig) attached to it. Regardless how many of the energized distribution conductors there are supported by that pole above the top of the service transformer you will not see a neutral up there. With most pole mounted transformers there will not be a second bushing on the top of the transformer either. That is because one end of the primary winding and the neutral point of the secondary winding are both connected to the utilities neutral on the pole. The transformers enclosure is also bonded to the utility's neutral by a "noodle" (strap conductor) which is bonded to the can (transformer enclosure) and the neutral bushing from the bonded points on both the primary and secondary windings. That will be the middle of the three terminals on the side of the can.
> 
> [The only exception is the transformers that are actually painted to resemble the hide of a cow. Those do have a second bushing on top and 2 phases powering the primary winding. The secondary neutral is not grounded at the pole but only at the Service Equipment mounted on a yard pole on the served premise property. That is done to isolate animal husbandry farm and ranch wiring systems from the utility's neutral to keep stray neutral current from injuring live stock.]
> 
> ...



what have you decided about your emergency generator ?


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## hornetd (Oct 30, 2014)

SWDweller said:


> If the people who are connecting the generator to a electrical service do not understand the dangers then I submit they should not be doing the job. Portables are made "bonded" so they can be used ALONE. They are NOT intended to be used on electrical services with an existing bonded neutral. If you intend to use it on an electrical service then you need to install the needed equipment.


There are quite a few generators, especially smaller units, that are shipped from the factory with an unbonded "neutral." That seems to be true of many of the inverter generators and especially the ones that produce only one voltage such as 120 volts. That said those are totally unsuited to powering building wiring systems, especially those which include multi-wire branch circuits


SWDweller said:


> Using bonded neutral portable on a electrical service you have 2 grounds. One at the generator and one at the electrical service. You should understand that issue if not Soares Book on Grounding. Lots better than I can do and with pictures.


Yes I know. What I'd like to know is why that is such a big concern when the conductors from the generator windings terminate to the same bus bars as the service conductors from the utility transformer secondary, inside the Service Equipment Enclosure, were both neutrals are terminated to the grounded current carrying conductor (Neutral) busbar. What's also interesting is that the conductors from the generator will include an Equipment Grounding (Bonding) Conductor (EGC) which connects the frame of the generator to the Equipment Grounding (Bonding) Conductor busbar of the service equipment. In a bonded generator the EGC and the neutral conductor are bonded together at both ends when both are connected each other by being terminated to the same busbar or by the Main Bonding Jumper when there is a separate EGC Busbar such as when the service equipment uses plug on neutral breakers.


SWDweller said:


> connecting a portable generator (bonded neutral) into a sub panel is a problem. Sub panels have the neutral and ground separated.


Yes that's true but what I'm asking about is when the conductors from the generator's windings terminate in the Service Equipment Enclosure.


SWDweller said:


> I would need specifics on the equipment both the generator and the electrical equipment.


Well you lost me there. Why would it make a difference as long as we are writing about 120/240 volt generators supplying a 120/240 volt premise wiring system.


SWDweller said:


> Might be time to look at the generators in stock and the locations where they would be used and have and have someone design something for you all.


That varies a lot. After a disaster our Office of Emergency Management (OEM) rents as many portable generators as they can lay their hands on. They inevitably include both bonded and unbonded units. They own some Generlink type meter base adapters which allow their volunteers; with each team including an electrician; to pull the meter, insert the Generlink type adapter with a insulated meter base cover permanently installed on its outer end, hook up the generator, and go on to the next electrically powered medical equipment dependent patient who must be defended in place to keep them from overwhelming the areas hospital emergency departments. [I'm sure that you can imagine how quickly they fill up with slips, falls, and fractured extremities during an ice storm.] The last detail is that most of the Generlink type adapters that the OEM has on hand do not have Meter socket jaws. The moment you install one you have executed a double block and break to the utility conductors. The adapter switch is the break and the lack of meter socket terminals is the double block. We installed over 40 of those during our last ice storm power outage which lasted for over 10 days. 

I realize that that technique is not representative of most portable or mobile generator connections to premises wiring systems. The point is that they do work. I have toned out, rung out, and even meggered some of the adapters and without exception they switch only the energized conductors. 

As a total aside, you should hear the bitching from some of the relatively healthy residents of the apartment houses about their not getting generator supplied power also. There aren't that many of them that complain about that but the ones that do sure get on my nerves. 

Tom Horne


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## hornetd (Oct 30, 2014)

Almost Retired said:


> what have you decided about your emergency generator ?


I'm still trying to figure that out. For one thing I'm thinking of installing a 30 ampere switch that will open and closed the bonding connection. The manufacturer suggested that I detach and attach the bonding strap to the stud on the generator winding housing because both are listed configurations and using a switch is not. [What else could they say without making the company vulnerable to a harmful reliance suit.] 

Tom Horne


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## gpop (May 14, 2018)

hornetd said:


> I'm still trying to figure that out. For one thing I'm thinking of installing a 30 ampere switch that will open and closed the bonding connection. The manufacturer suggested that I detach and attach the bonding strap to the stud on the generator winding housing because both are listed configurations and using a switch is not. [What else could they say without making the company vulnerable to a harmful reliance suit.]
> 
> Tom Horne


Does the generator have receptacles. If so make up a grounding plug using a 30 amp twist lock and connect G to N (they sell these pre-made). Un-bond the N/G at the generator and tie the plug to the generator key.

As long as something is plugged into the 30 amp twist lock then its either g/n by the house or g/n by the plug.


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## hornetd (Oct 30, 2014)

gpop said:


> Does the generator have receptacles. If so make up a grounding plug using a 30 amp twist lock and connect G to N (they sell these pre-made). Un-bond the N/G at the generator and tie the plug to the generator key.
> 
> As long as something is plugged into the 30 amp twist lock then its either g/n by the house or g/n by the plug.


*gpop*
The Generator does not presently have a key but I'd bet a dollar to the whole in a doughnut that the manufacturer can supply one. I'll get an hour meter for it while I'm at it. I will devise a way to tether the ground plug to the generator but there is one problem. When the generator is used in the field to support a temporary radio station's operation the various operating positions are supplied through 10/4 SOOW cord with Watertight L14/30 connectors to a junction box with 5-15 receptacle outlets protected by a Ground Fault Circuit Interrupter (GFCI) and a surge protector. From there the individual 5-15 cords co to each operating position. Since I could not afford one of the 5000 watt inverter generator the one I have is noisy. We locate it at least 50 feet away from the operators under the smallest popup canopy we could find together with a foam insulation board to reflect the sound away from the operating positions. What I'm likely to resort to is a 5-15P with a 10 AWG wire between the Neutral and Ground terminals. That should accomplish the same thing. Why 10 AWG? Because it will be the Main Bonding Jumper for a 30 Ampere supply.

Tom Horne


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## gpop (May 14, 2018)

hornetd said:


> *gpop*
> What I'm likely to resort to is a 5-15P with a 10 AWG wire between the Neutral and Ground terminals. That should accomplish the same thing. Why 10 AWG? Because it will be the Main Bonding Jumper for a 30 Ampere supply.
> 
> Tom Horne


That will work. We normally use the twist lock as no one wants to use that one other than to connect to the house.


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## paulengr (Oct 8, 2017)

hornetd said:


> *gpop*
> The Generator does not presently have a key but I'd bet a dollar to the whole in a doughnut that the manufacturer can supply one. I'll get an hour meter for it while I'm at it. I will devise a way to tether the ground plug to the generator but there is one problem. When the generator is used in the field to support a temporary radio station's operation the various operating positions are supplied through 10/4 SOOW cord with Watertight L14/30 connectors to a junction box with 5-15 receptacle outlets protected by a Ground Fault Circuit Interrupter (GFCI) and a surge protector. From there the individual 5-15 cords co to each operating position. Since I could not afford one of the 5000 watt inverter generator the one I have is noisy. We locate it at least 50 feet away from the operators under the smallest popup canopy we could find together with a foam insulation board to reflect the sound away from the operating positions. What I'm likely to resort to is a 5-15P with a 10 AWG wire between the Neutral and Ground terminals. That should accomplish the same thing. Why 10 AWG? Because it will be the Main Bonding Jumper for a 30 Ampere supply.
> 
> Tom Horne


You are attempting to use one piece of equipment with modifications to two different tasks.As a portable the generator IS the power distribution system in one case. It is part of the system in the other. Realistically barring something like removing/adding a bonding jumper as needed or installing a separate distribution panel with separate bonding on the trailer, it’s not solvable. None of it would be a Code violation if you left the generator bonded and set up the fIxed system assuming an unbonded generator hookup. You can’t have a “plug and go” solution. Either you run ungrounded or run current through your bonding:


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