Incorrect Generator Neutral-Grounding Connection

[cirelo]

I am an electrical contractor, and I have a client who told me they plan on connecting their generator to the dryer plug in their house.

Now, besides the obvious dangers of backfeeding the panel, the generator also has the neutral and ground connected together, which means it should be a separately derived system, and the neutral in the panel should be diconnected along with the two hot conductors. Of course, the homeowner is only going to switch off the Main breaker, so the neutral will not be disconnected.

I told the homeowner this was dangerous, but when he asked me what was the worst that could happen, I drew a blank. From what I understand, this situation will not result in any extra shock danger, because any fault current will be trying to return to the source, and not to the ground.

Help me out here. What's the possible dangers of not switching the neutral in the panel?

 

[davidr43229]

Help me out here. What's the possible dangers of not switching the neutral in the panel

Better yet, explain to the HO that the mains when power is restored are not rated nor been tested to have 220V on the line side and and a different 220V on the loadside, that is what an ATS is for.
Just my $.02

 

[coulter]

Well let's see. Should be a lot of threads on this one. Just off the top of my head:

1. Suicide cords aren't a real good idea - even on a bad day.

2. A bum neutral connection can energize the generator frame. (I'll have to sketch this one out and verify - there are lots of nuances here)

3. Forgeting to shut off the main can either cause a hot line (utility break near the transformer - no other houses fed) or, drive the generator into a dead short. (lots of other houses on the same xfmr or break is far from the house)

4. The utility doesn't like you. Which may not, in of itself, be against the law, but if someone gets hurt, there are likely reckless endangerment laws. It likely won't be a lineman that gets hurt - they are pretty good at working between grounds. It will be a close neighbor doing the same thing you are when your generator comes on-line.

5. If you touched it, it will be your fault no matter what went wrong.

6. It offends my professional philosophy (that is code phrase that means, "Likely no one else gives a RA") :grin:

carl

 

[realolman]

Electrically, I don't see any great dangers in not switching the neutral... the dryer plug business... now that's another story.

 

[cirelo]

Well, I think I have convinced the homeowner not to do it, and I made him sign a big warning letter saying that I informed him of the dangers, etc. I think the letter actually was part of what convinced him, in fact. Everyone is afraid of being sued now a days.

Still though, I am a little foggy on the whole neutral connection thing. I guess this actually applys anywhere....Why does the NEC prohibit having a neutral-ground connection anywhere but the panel? If current wants to return to the source, the only thing I can see wrong is that if there is a neutral-ground fault after the load, there will be two return paths for the current. But even then, with one of the return paths being unshielded, how is this dangerous? The direct copper connection is always going to be a better return path for the current than the path through you, through the ground and back to the panel... Is it only dangerous in the case of there being a open neutral?

 

[haskindm]

There have been many discussion on improper ground/neutral connections. Basically it comes down to grounded (Neutral) conductors are intended to carry current, grounding conductors are not intended to carry current except when clearing a fault. If they are connected together, both will carry current since electricity takes alll available paths back to the source. Also remember that it takes hundreds or thousands of times more current to trip a breaker than it takes to kill or injure a person.

 

[dbuckley]

Why does the NEC prohibit having a neutral-ground connection anywhere but the panel?

Common sense dictates not having neutral / ground bonds downstream of the panel, not just the NEC, but the very fact that the NEC continues to recognise that N/G in dryers are acceptable shows that the NEC is fallible.

As to why? Draw out a dryer circuit with the three pin receptacle (H/H/G), with a dryer (complete with it's internal N/G bond, and the 120V motor) all plugged in, and then (using an eraser), cause the grounded wire to the dryer receptacle to fail. For bonus points, draw in the homeowner with one hand on the dryer and then other on a grounded object, eg the washing machine.

 

[elvis_931]

When you connect neut. to ground in this manner, you create circulating ground current. Basically what everyone else is saying. In a normal situation, if you break the ground wire and your body gets between the wires you would be fine. With the circulating ground current, if you were to get between the ground you would have the unbalanced current that is supposed to be going through the neutral going through your body. This would be bad

 

[busman]

there are likely reckless endangerment laws. It likely won't be a lineman that gets hurt - they are pretty good at working between grounds.
carl

Don't forget that transformers work in both directions. If there is a lineman working on a de-energized section to repair it and someone cranks up a 240V generator without opening the main, that will place the full line voltage up on the utility high-side. Of course the generator will trip off quickly due to the HUGE load, but the lineman probably won't survive.

Mark

 

[coulter]

Don't forget that transformers work in both directions. If there is a lineman working on a de-energized section to repair it and someone cranks up a 240V generator without opening the main, that will place the full line voltage up on the utility high-side. Of course the generator will trip off quickly due to the HUGE load, but the lineman probably won't survive.

Mark

Mark -
I certainly thank you for the lesson in physics, safety, and lineman work philosophy.

However, I am somewhat surprised you think linemen are not aware of the inherent dangers of idiot home owners - not to mention possible cogeneration backfeed and utility induced backfeed. In the forty years I've been around this business, I've never seen a line crew work other than between grounds, and only after doing a dead-line check. As far as I know this was standard practice far before OSHA demanded it.

Then again, I am not a lineman. However there are some in the forum. Perhaps they could pitch in and give us the standard safety practices.

carl

 

[coulter]

...someone cranks up a 240V generator without opening the main, that will place the full line voltage up on the utility high-side. ...

Full voltage -Hummm. 5kW HO gen driving into 10 houses (break at the end of the cul-de-sac) What do you think the voltage would be on the customer side of the break?

My guessing ~ 1500V on a 7200V system. Nothing I would want to tie into, but not 7200V.

carl

 

[busman]

Carl,

I didn't mean to imply that lineman don't behave in a safe manner, but it seems like every few years, I see another article about a lineman killed by backfeed (see below). Maybe this is just urban myth. I'm not a lineman either, but I know that safety procedures aren't always followed. Sorry if I told you something you already know.

Mark

"July 13, 2005
South Carolina Lineman Killed by Generator Back Feed
Victim Helping Restore Power in Alabama

Sumter Electric Cooperative (SECO) officials report that a South Carolina lineman helping to restore power in Alabama after the damage caused by Hurricane Dennis was killed late Tuesday, reportedly by an improperly installed customer generator.

According to SECO Director of Public Affairs Barry Bowman, the lineman was helping to restore power lost during Hurricane Dennis to customers in Alabama. The report Bowman received indicated the lineman was working on a power line that was supposed to be dead. It was not.

?Tragically, the line this technician was working on was not dead,? said Bowman, ?The line he was trying to repair had been re-energized by a customer who had improperly hooked up a generator and created a back feed of electricity from the generator into the supposedly dead line. The death of a lineman who was there to help the victims of Hurricane Dennis was the horrible result.?

Bowman noted that the lineman?s name was not immediately released, but he was from South Carolina and worked for Pike Electric, Inc. in North Carolina.

Alabama authorities are looking for the person responsible and indicate that charges are pending.

?This could happen to any lineman, anywhere, if generators are improperly used or hooked up during a power outage. That?s why we caution homeowners and businesses about the proper use of generators. And, why we continually stress generator safety awareness with our SECO employees. I don?t think anybody wants to be responsible for the death of another human being,? said Bowman."

 

[coulter]

...Maybe this is just urban myth. ...

I don't know. I hear about the same articles - and I still don't know. There was a thread a month or so ago where the poster was musing about similar doubts.

The area I'm in, I'd guess 75% of the homes have a small gen. And if one is outside of the city limits, there are no permits and no inspection. I can only shudder at how many are likely installed not to code. We get a few outages per year lasting longer than an hour. And if it's -40F, plenty are cranking up their gens. This has to be a high risk area for HO gen back feeds. And yet the linemen deal with it.

I was going to say I'd never heard of a lineman killed up here by a back feed, but I don't really know. I work with the local utility some, and I'll call and ask if this has happened. I'll let you know what they say.

carl

 

[don_resqcapt19]

There are a number of cases where linemen have been hurt or killed by back feeds, but these incidents are not all that common. I think there is two reasons for that...the safety rules that are followed by lineman almost always required that the circuit be shorted and grounded before they work on it. The second reason is with small home generators, the generator breaker will trip or the prime mover will stall because of the large load that the generator is trying to feed. If the load between the generator and the utility broken wire is small, then you have the back feed problem.
That being said there is no excuse for creating the possiblility that someone may be hurt or killed...the generator must be installed so that back feeds are not possible.
Don

 

[coulter]

... there is no excuse for creating the possiblility that someone may be hurt or killed...the generator must be installed so that back feeds are not possible...

Absolutely

...the safety rules that are followed by lineman almost always required that the circuit be shorted and grounded before they work on it.

I carefully reread the posts. The discussion appears to be limited to the high side of the customer xfmr and does not include bare-handed live-line work. From that point of view. I would disagree with this statement. It's not "almost always", it's "always" verified dead, shorted, grounded.

There are a number of cases where linemen have been hurt or killed by back feeds, but these incidents are not all that common. ...

Just an opinion, while the cops are hunting down the idiot HO, why isn't someone packing the lineman's instructor outback. The fact that a lineman thinks it's okay to do this work without following safety practices is disgusting.

Of course, as I said, I'm not a lineman. Again, maybe some of our linemen could pitch in and educate us.

carl

 

[Bmadmax40]

Quote:
Don't forget that transformers work in both directions. If there is a lineman working on a de-energized section to repair it and someone cranks up a 240V generator without opening the main, that will place the full line voltage up on the utility high-side. Of course the generator will trip off quickly due to the HUGE load, but the lineman probably won't survive.

The generator will not trip (assuming that there is not much load on the same secondary). All the "huge" load on the primary side of the transformer has higher resistance. Am I wrong?

Also, if a backfeed can kill a lineman on the other side of the transformer, then a few volts of some kind of backfeed can kill the same person. Grounding and shorting all the wires make sense while working on high voltage lines.

 

[coulter]

...The generator will not trip (assuming that there is not much load on the same secondary). All the "huge" load on the primary side of the transformer has higher resistance. Am I wrong?...

I think so. My reasoning is: Every transformer on the customers' side of the line break represents one or more houses. Figure every one at 100A at 240V or 2.4 ohms So if the break is at the end of a cul-de-sac(sp?) with ten houses, that represents .24 ohms on a 240V base. That reflects to a much higher impedance on the 7200V side, Z(high side) = Z(low side) X N^2. The impedance then reflects back down to .24 ohms on the HO gen side of its utility xfmr. The HO gen, still sees the impedance of ten houses. Not accurate science - but hopefully close enough for this purpose.

carl

 

[got_nailed]

I had a power outage a few years back where a tree fell on the line and popped a fuse / disconnect on the high side of the line. It left 5 blocks in the dark.

I think it was about 5 hours before POCO showed up. I think there were about 20 generators running at the time. I did drive out to see what he was doing and after he did some tests in the bucket truck he came down and stared to cuss. He went door to door pulling meters at anyone who had a generator going. When he was done and the power was back on I think had 5 meters in his truck and had cut the wires to those houses.

I do remember there being a big stink and a few pissed off families. I was told that the house that he had pulled the meters out of did not get there power back a week to 3 months and they were fined out of there as_.

I am one of the few who have a large propane genset with an automatic transfer switch feeding a sub panel. My POCO knows I have a genset and the meter can, there feeder wirers and the transformer are marked to identify the use of a genset. When I added the genset I had to get a special letter during the inspection and a POCO rep did come out and did a quick look over before marking the meter can, wires, and the transformer.

The tag the used on my feeder is a red and green tag and on my meter can and transformer have a big red and green sticker. If I get my camera out this week I will grad some pics. I don’t know what they do other places but this is what they did to me.

*** I’m not an electrician or a journeyman. I work in a huge automotive plant and work on 96 volt DC and down, 120 AC, deal with a lot of relay controls, air controls, and the equipments computer systems. I do work with other electrician any time I enter anything over 48 volts dc or 110 ac. When working in hot DC control panels I have to have an electrician present.

 

[don_resqcapt19]

Carl,
Why won't the generator try to supply all of the connected load, but on the same secondary and on the primary? I don't think the generator breaker will trip because the prime mover will be overloaded and stall out before the breaker trips.
Don

 

[coulter]

Don -
I wasn't ignoring you, I just had alot of work last week. Go figure - just cause they give me a pay check, they expect work in exchange

The disclaimer: First I'm not promising this is all exactly right. Second, my thinking is based on a set of assumptions - change them and the thinking changes.

The assumptions:
1. Only one HO has a gen that is setup to backfeed. This would have to be - only one backfeeding - no way to parallel HO gens.

2. Others houses may have gens, but they are using a transfer switch and are separated from the utility. These have no effect on this issue.

3. Several (pick 10) other houses on the customer side of the utility line break, have no gen, and did nothing to separate themselves from the utility. I'll pick a nominal 100A (25kW) load for each connected house.

4. The home owner with the backfeeding gen has a nominal 8kW gen and has shutoff enough load to be under the 8kW.

5. I'm going to neglect the xfmr impedance for the this first approximation.

Carl,
Why won't the generator try to supply all of the connected load, but on the same secondary and on the primary? ...Don

If I'm understanding your question, I think the backfeeding gen will try to pick up all of the 10 connected houses. If the HO had a 250kW gen, they could possibly do it. The 240V from the back feeding gen transforms up to 7200V, then back down to 240V at each of the other connected xfmrs. Since I am neglecting the xfmr impedance, the other 10 houses could be represented as 250kW of 240V load connected right at the backfeeding HO gen. And I'm thinking that looks essentially like a dead short to an 8kW gen.

...I don't think the generator breaker will trip because the prime mover will be overloaded and stall out before the breaker trips...

I didn't think of that. I figured the HO gen CB would just trip. You could be right - I have never tried this, nor laid out a 40A (8kW gen cb) inverse time trip curve against short circuit current capability of an 8kW gen driving into a .24ohm load.

Just a guess based on a bunch of assumptions.

carl