electrical tech question, non-valk

[Jeff K]

I get a kick out of this thread.
I really need to go back through the archives and see how many of the guys that are saying that this is a bad idea, and talk about "doing things the right way" "not taking chances" have car tires on their motorcycles. 

Every time I take off from a stop sign I have to take all precautions. I come to a complete stop, I look both ways, I double check guys that might be turning left, I look both ways again, then I proceed.

I'd have to say that going through an intersection is just about as dangerous, and could kill just as many innocent people.

If you have not been trained properly, and you don't know what you are doing... don't cross the intersection!!!
 

[FryeVRCCDS0067]

OK. I'll try to answer some of these questions and address some of the comments. I can speak to this topic as I was an electrical lineman for 37 years.
1. The initial post said he wanted to connect a 30 amp generator through his panel box. A generator that size will not run too many things but would be fine to run the essentials. ( frig. furnace blower if a gas furnace, a few lights. Anything with a motor uses whats called start up current which consider if your frig. uses 7 amps running current the start up may be 11 amps. That said you don't know when the motor on frig. or furnace will be starting or if they could start at the same time.
2. As to the double male cord the reason its called a death plug is the exposed prongs on the house end will be hot. If the cord worked loose and layed there and someone contacted it or fell against something metal a fire could be the result.
3. I could go on and on but sometimes too much info is bad. I have been up and down the eastern United States after all types of storms and there have been way too many FATALITIES caused by improper use of a generator. The cause of them is when connected to a house electrical sytem and is not isolated from the power grid it sends 120 volts onto the grid and transformers normaly lower the voltage from primary volts 23000 down to 120 volts but if a generator feeds 120 volts backwards through it then it sends 23000 volts onto the grid. So even though we test for backfeeds from generators we can't take for granted someone improperly connected a generator after the test.
4. Anytime I heard a generator running i approached the homeowner and asked to verify the connection many take exception to the inquiry. ( engineers are the worst).

 I hope I have shed some light ( no pun intended) on this topic but the best way is always the right way. Even though there are manys ways to connect to your house the right way is an automatic  transfer switch or use extension cords. Unless you have a decent sized generator stick with cords.

Hey Paps...

First a comment...excellent post...and very interesting information. 

I have a question...If you isolate the main breaker on the panel...does that stop the possibility of back-feeding the grid? 

Joe

Yep. Good info, it's easy to forget transformers work both ways. But, the real question is, does shutting off the main breaker isolate the home from the grid?

It seems that his reasoning has eluded you somewhere with what the retired lineman said.

The transformer is what isolates from the grid (substation). No power goes back out and past the transformer, it's one way in.

The whole point of the matter and where the danger lies at is when the lines are light up andhot because an illegally connected generator that causes a back-feed anywhere and up to and in between transformers. Not after that. It can be several city blocks served by those transformers. 

Ah, the little known diode enabled isolation transformer. 

[JP in SC]

So, what prevents a step-down transformer from working in reverse, becoming a step-up transformer?

Only the way you wire it. the transformer doesn't care whether it steps up or down.

I used a transformer to step up 208v 3 phase power to 480 volts to run a freezer at work for about three months. once we installed a 480 volt service into the facility i moved the transformer to the other end of the plant, hooked it up the other way(480v 3ph in / 208v 3ph out) and used it to feed a 208/120 distribution panel. The transformer is happy no matter which direction you wire it.

Yep. Good info, it's easy to forget transformers work both ways. But, the real question is, does shutting off the main breaker isolate the home from the grid?

Yes, shutting off the main breaker does isolate the home from the grid.

The problem is that if you ever forget to turn off the main there is a strong possibility off lethal consequences.

can anyone here say they've never forgotten anything? I can't.

[valky1500]

So, what prevents a step-down transformer from working in reverse, becoming a step-up transformer?

Only the way you wire it. the transformer doesn't care whether it steps up or down.

I used a transformer to step up 208v 3 phase power to 480 volts to run a freezer at work for about three months. once we installed a 480 volt service into the facility i moved the transformer to the other end of the plant, hooked it up the other way(480v 3ph in / 208v 3ph out) and used it to feed a 208/120 distribution panel. The transformer is happy no matter which direction you wire it.

Yep. Good info, it's easy to forget transformers work both ways. But, the real question is, does shutting off the main breaker isolate the home from the grid?

Yes, shutting off the main breaker does isolate the home from the grid.

The problem is that if you ever forget to turn off the main there is a strong possibility off lethal consequences.

can anyone here say they've never forgotten anything? I can't.

To put it all into perspective:
We're talking about connecting generators to circuits that are fed by another power source already, lines from a (split-phase) transformer in residential power areas.

The circuits that are fed by residential power transformers are not industrial (3phase) and by the same token you won't find 3phase transformers in residential areas either. 3phase is an entirely different animal that has no merit in this discussion.

Both the Voltage and Transmission type transformers are designed to go only one way and they do so by switches and link disconnects (fuses) added to them for that purpose by design. These switches and link disconnects are one-way only and they are at the transformer. Most power outages during storms and such are caused by these same switches and link disconnects.

Again, the danger that lies in back-feeding is when all the lines connected to it can become hot, from human error for example. If the main is closed thru human error then all the lines back to the transformer and in between can become hot. So, let's say when someone goes to work on it and they get zapped. Hit with that same power.

Where does the fault lie now?

Not the power company for sure.   

[sugerbear]

and pick up one of these

http://www.homedepot.com/h_d1/N-5yc1v/R-202019449/h_d2/ProductDisplay?catalogId=10053&langId=-1&keyword=generatorextension+cord&storeId=10051&superSkuId=202890992#.UMvJk2-kgwh

[paps350]

I tried to give some helpful info but i didn't want to get boring. Yes an open main breaker will isolate the house system from the utilities system which is what an automatic tranfer switch does. Where you get into trouble is not opening the breaker before connecting the generator and making sure to disconnect the generator first after the power is restored and then closing your main as that is how to destroy your generator. An automatic transfer switch will also open and connect back to the grid once power is restored.