Backfeeding electric lines from a generator

Tom Dunlap

Tom Dunlap

Here from the beginning
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Retired in Minneapolis
There was a thread on a woodworking forum that I follow about full-house generators. The discussion moved to using portable gensets.

Someone asked what might happen if someone used a genset without an isolator/disconnect or pulling the main circuit breakers in the house.

Here is what someone with a curious mind found out:

The question posed was: "Given a non-isolated generator backfeeding the utility line, will the neighbors' lights and air conditioner drain the backfed electrical charge (and trip the generator's internal breakers) before it can reach the guy working on the lines two blocks away?"


The answer from the Jersey Central Power & Light technical rep is as follows: "No. Electricity is not water flowing in a pipe. When the backfeed occurs, the entire circuit past the transformer becomes energized and there is no appreciable time lag between that energizing event and the energy reaching the service person a mile away. The resulting charge may be quite sufficient to seriously injure or kill an unprotected person contacting the line even as the generator's trip would engage. The energy will discharge along the path of least resistance which in the situation described, would be the person in contact with the line rather than the neighbor's lights."

Besides gensets there are more and more alternative energy producers, wind/solar, that could also setup a dangerous backfeed.

Like Sgt. Phil Esterhaus would say, "HEY! Let's be careful out there...!"
 
I pull the main fuses and feed my generator in via the clothes dryer 220v plug. A basic understanding of electricity helps. I wait for the neighbors lights to come back on, kill and unplug the generator and then put the main fuses back in.
 
Not only does the circuit become energized but when the home generator back feeds through the transformer, the tranny steps up the voltage to whatever it can.

It's not going to give you the system voltage back down the circuit but it's going to be much more than the 110/220 that you're producing at home.

When we get to areas that have lost power, we always try to listen for the home genny screaming away. Never a bad idea to check. Don't know about you guys in the states but there is an approved load transfer switch here in Canada that integrates right at the meter base. Automatically senses load when the power comes back on and trips inside, isolating the generator.

Works great for those home setups but requires changing out the meter base. The generator plugs directly into the meter base, no hassle, reduces the chance of anyone getting hurt.
 
We encountered this situation a few months ago when doing some utility line clearance on a private lane. There are two residences at the end of the lane and both are equipped with automatic generators. Due to the specific nature of the work on that lane, we felt it would be faster and safer to shut the lines down during the course of operations. Before shutting down the line, both generators had to be set to 'Off' as a precaution, though if they are installed and wired properly, they should not back feed. The back feed issue would more likely be they guy powering his home with a portable generator and a custom extension cord with 2 male ends.

No matter the situation, it never hurts to double-check anything, but it can hurt not to.

Thanks for the post, Tom. Storm season is just around the corner...
 
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Not only does the circuit become energized but when the home generator back feeds through the transformer, the tranny steps up the voltage to whatever it can.

It's not going to give you the system voltage back down the circuit but it's going to be much more than the 110/220 that you're producing at home.



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Not true the voltage will be stepped up to the system voltage just not with as much power behind it. The voltage(V) on the primary(p) and secondary(s) are a function of the number of turns(N) on the windings of each: Vs/Vp = Ns/Np. It's a simple ratio (assuming no internal losses to resistance in the windings). What is different is the power or wattage (W or KW for kilowatts). Power is constant and determined by the load. For eg. your toaster draws 1.5 kW therefore the amount of power flowing through the transformer is just that on either side. Because Power is constant and voltage is variable then current is also variable but in the opposite or inverse. In a transformer, and this is the whole point of a transformer, when you step up voltage the current drops by the inverse ratio (eg. voltage goes up 3x, current goes down by 1/3).

With a portable generator or a back up generator we have much less power available so the current is going to be lower. However, it's plenty to kill you. Current is the killer not voltage.
 
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Not true the voltage will be stepped up to the system voltage just not with as much power behind it. The voltage(V) on the primary(p) and secondary(s) are a function of the number of turns(N) on the windings of each: Vs/Vp = Ns/Np. It's a simple ratio (assuming no internal losses to resistance in the windings). What is different is the power or wattage (W or KW for kilowatts). Power is constant and determined by the load.

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Not true. You are overlooking the fact that if the amperage and thereby the wattage of the system can not be supplied by the generator....then by definition the voltage would be dropped.

It is highly unlikely that the home generator would be able to backfeed the power line to 'system voltage'.

The only way system voltage could be obtained would be the home generator were able to supply the watts demanded by the rest of the high voltage system. To begin with...the generator would have to be huge....Second....the transformer that the house is on is not large enough to handle the wattage(voltage X amps) demanded by the rest of the high voltage system.

This of course depends on how much of a load the high voltage system represents after a line goes down but it is possible that tens of dozens or even hundreds of households would still be loading the system.

If the generator could maintain the voltage then it would be be supplying the wattage. You can not put 'voltage' across a load without the associated 'amperage' being developed. (Ohm's law)

<font color="red"> That being said, the voltage developed by backfeeding the power grid could easily be enough to kill a person or even dozens of people if they were in contact with the line.


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Remembering a quote from EHAP..."all conductors should be treated as if they are charged with fatal voltage". This would include utility lines such as phone, cable, etc...
 
We have a policy that we treat ALL lines as if they are energized. Everybody on our crew has a good understanding of electricity, but as this thread proves it can get complicated. There are plenty of dangers to be aware of in day to day operations, but storm work brings even more to the mix.
 
You guys need to understand the purpose of a main transfer panel feed's ability to isolate the house supply from the grid.

Backfeeding your system through house outlets of 110 or 220 is extremely dangerous.

Get a certified utility approved electrician to install a transfer panel at the main housefeed breaker that will isolate your generator feed entirely from the grid, at the main panel. It's the law here in CA, and a good one too.

http://members.rennlist.org/warren/generator.html

jomoco
 
Its the law everywhere.

But...some people get a genset and an extension cord. Then the make the extension cord into a Male/Male by changing ends. One end into the genset...the other into ANY outlet in the house..which then backfeeds.

Disregarding safe practices isn't confined to just treeworkers not wearing proper PPE and following industry standards.
 
Gensets are inherently dangerous made even more dangerous when used without a basic understanding of electricity.
 
Yeah, I one hand electric chainsaws too Tom.

Here I hook you up, answer your question, and get slapped for my sage advice on isolation panels?

Where's the neutral wire on this panel?

http://www.youtube.com/watch?v=dPuznSe7bmw&amp;feature=youtube_gdata_player

jomoco
grin.gif
 
Jomoco, neutral seems to be floating and carrying a charge at times indicating a bad connection between neutral and ground.

Had it happen in an apartment I owned where the aluminum ground wire from the grid had corroded and house ground was from plumbing pipes. Fridge would kick on and the kitchen lights would get brighter. If that wasn't bad enough I was working on the plumbing and broke some iron pipe, drew an arc.....and lost total house power at ten in the evening... in below freezing weather! A genset would have been no help other than to probably cause a house fire.
 
I stand corrected TreeCo. There would be line loses seen by the generator. However if you were close enough to it then the circuit and thus load represented by the line and equipment between you and the generator would be reduced and the potential significant enough to do harm. Damn electrickery is not so simple.
 
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I stand corrected TreeCo. There would be line loses seen by the generator. However if you were close enough to it then the circuit and thus load represented by the line and equipment between you and the generator would be reduced and the potential significant enough to do harm. Damn electrickery is not so simple.

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Yes sir, lots of potiential to do harm!

I grew up living next door to and 'sat' for a man while his wife went shopping that had worked for a power company and was paralyzed from the neck down from falling off a pole from getting shocked by a circuit that was supposed to be dead. It's been 40 years and I remember well giving him drinking water and always spilling some and him saying in a soft, soft voice and smiling......'it's only a little clean water'.

I read an article a few years ago about the number of electrocutions that are caused by underground wires....like old lights, etc. in cities, lawns, etc. that are 'capped off' underground but as the decades go by lose their integrity and charge the earth. JUst standing waiting for the buss, step in some moisture and it could happen to any of us.
 
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