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#171399 - 11/27/07 06:36 AM Backfeeding the grid
Mike Wescoatt Offline
Member
Registered: 06/17/01
Posts: 161
Loc: Cedar City, Utah
I've been thinking about this for a while and haven't quite figured it out, and I know y'all can help me understand...

Power goes out so some homeowner sticks his generator in his dryer plug and backfeeds the grid. Aside from all the safety no no's (we'll even say he put it in his indoor laundry room... and the CO detector battery is dead crazy) why doesn't the generator blow up, stop, or the breaker shut off? I'm assuming this 5000W genset is trying to power his house and the neighborhood amounting to much more than 5000W. Is it simply that the voltage drops so low (say 24V) that the power output hovers around 5000W?

Thanks!!!

Michael
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#171400 - 11/27/07 07:52 AM Re: Backfeeding the grid [Re: Mike Wescoatt]
gfretwell Offline


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Registered: 07/20/04
Posts: 9039
Loc: Estero,Fl,usa
I suspect that if he is actually getting to any significant load up stream he does just trip the overload on his generator. Where this might actually be dangerous is if the utility fault is right there at his transformer and he is only backfeeding his immediate neighbors. Even then I have to think the LRA of several refrigerators and the water heaters would quickly overcome all but the biggest residential generator.
This is more of the philosophical argument than a practical one.
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#171407 - 11/27/07 08:56 AM Re: Backfeeding the grid [Re: gfretwell]
JValdes Offline
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Registered: 06/03/07
Posts: 301
Loc: South Carolina
It's also a dangerous situation. But like you, I often think about things and how equipment would react to unorthodox applications.
It's okay to think and wonder, but I would not ever consider doing this. Theoretically, it would work, provided the upstream load was very low, below output FLA of the genset. The genset breaker would protect the genset from overload, but would not survive restoration of utilty power while in operation.
Just curious? Do you know someone who is doing this, or plans to try it? I sure hope not
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#171409 - 11/27/07 09:28 AM Re: Backfeeding the grid [Re: JValdes]
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9039
Loc: Estero,Fl,usa
I don't think anyone plans to backfeed the grid, It just happens when they do something dumb.
The fact remains that generators usually kill more people than the storm, either by electrocution or CO suffocation.
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#171410 - 11/27/07 10:09 AM Re: Backfeeding the grid [Re: gfretwell]
Alan Belson Offline
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Registered: 03/23/05
Posts: 1803
Loc: Mayenne N. France
Some poor linesman working up a pole is the usual victim - who else in a power outage will be handling conductors?
BTW: You don't suffocate with CO, you die of acute poisoning, if you're lucky. Survivors may take years to recover, if ever, from tiny doses.



Edited by Alan Belson (11/27/07 10:10 AM)
Edit Reason: removal of redundant indefinite article.
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#171411 - 11/27/07 10:18 AM Re: Backfeeding the grid [Re: gfretwell]
gibbonsseabee80 Offline
Member
Registered: 10/10/05
Posts: 40
Loc: Chesapeake, VA
Just curious? Do you know someone who is doing this, or plans to try it? I sure hope not


When I was stationed in Guam in 2000, there was a base wide power outage. Our generators only powered critical buildings while the rest of the base remained out. Our power on base remained off for a total of 11 hours because of emergency repair and then the outcome was someone back fed their (base housing) electrical panel with a generator. They (Ratheon) public works civilians had to check all of housing for a running generator because they backfed the grid. I do not know if the recloser at our station would not close or if they still detected power on the line after it was turned off. All I remember is being pissed off in the tropical heat on a Friday night with no air-conditioning.
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#171412 - 11/27/07 01:20 PM Re: Backfeeding the grid [Re: gibbonsseabee80]
WESTUPLACE Offline
Member
Registered: 03/29/04
Posts: 251
Loc: Kingwood, TX USA
Lets see, 24v into sec. of a 7200v to 120v transformer, still 1400v comming out, you hold this wire while I hook up the 24v, tell me if you feel it!Don't worry its low current just 5000 watts.
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#171414 - 11/27/07 02:09 PM Re: Backfeeding the grid [Re: WESTUPLACE]
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9039
Loc: Estero,Fl,usa
The linemen I know don't trust their own people enough to handle "dead" conductors without grounding them out first.
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#171432 - 11/28/07 03:05 AM Re: Backfeeding the grid [Re: gfretwell]
RODALCO Offline
Member
Registered: 12/08/05
Posts: 854
Loc: Titirangi, Akld, New Zealand
Standard practice on HT conductors in New Zealand is to earth the HT wires at both sides of the worksite, after the lines are proven de energised.
I take it that this standard is adapted by most POCO's around the world.


Even if someone with a 5 or 10 kVA generator tries to backfeed the grid, the existing load will absorb this amount and the generator will trip on overload or stall.

As per normal working procedures PPE is mandatory, harness, safety goggles, hardhat, full body cover overalls and insulating gloves rated at the HT voltage on the line during maintenance.

Of course care is always needed in case of backfeeding from an other source during line maintenance.
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#171437 - 11/28/07 05:30 AM Re: Backfeeding the grid [Re: RODALCO]
Jonno Offline
Member
Registered: 05/27/06
Posts: 20
The practise of grounding HV conductors is not one of trust but one of necessity. Except on distribution radials, there are typically parallel circuits, these parallel circuits can energize a nearby isolated conductor by mutual induction and capacitance. Capacitive coupling will impart a voltage on an isolated conductor. Grounding at one point corrects this. However when you have a conductor grounded at one point, the mutual inductance with parallel current carrying conductors creates an open loop (the ground is the other half of the loop). There will be a large potential at the gap that closes this loop (the gap is between the conductor and your hand if you are in a grounded bucket). This is why a ground is installed at each end of an isolated section of line.
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