Residential Loss of Phase 2

[jfpe]

Has anyone seen the loss of a single phase in a 120/240V residence? I expect it would be unlikely, since center tapped 120/240V transformer is usually connected to a single phase of the 3-phase distribution system,so the loss of a single phase of the distribution would lead to the loss of both phases in the residence. If it did occur, what are the possibly causes? Partial transformer failure? Loss of one conductor from the service drop?

This question came up because we have an electrician trying to install a single phase grid tied inverter in a residence and the inspector wants is to shut down if the phase it is not connected to is lost. I'm trying to understand when this would happen and what are the impacts if the inverter did not shut down, given that the phase it is connected to is still fed from the utility.

Thanks,
John

 

[rbulsara]

The inspector is right.

The good news is that all inverters listed for tying to the grid have that feature built-in. Check the specs.

A phase can be lost for a variety of reason such a break in the conductor, opening of fuse opening, etc. Any reason that makes the power go away to a residential service.

The very purpose to shut down the inverter. when the utility power is lost (single or more phase..does not matter.) is to prevent the back feed to the utility system. This is to ensure safety of their personnel if working on a de-energized line.


Rafiq Bulsara

http://www.srengineersct.com

 

[jfpe]

Rafiq,

No, most inverters don't do this. This is a situation where the 120V inverter is connected only to "A" phase and "B" phase is lost. The inverter will only shut down if "A" phase (i.e. the phase it is connected to) is lost

I don't see the hazard to the utlity, since the phase the inverter is connected to is still fed by the utility. Even if the inverter shuts down, it will still be live.

I've handled this before by adding a relay between the inverter and the point of interconnection. The the coil connected to whichever phase the inverter is not connected to, so if that phase is lost, the inverter is disconnected.

Thanks,
John

 

[resqcapt19]

Rafiq,
If the inverter output is 120 volts and the service is 120/240, how would the inverter know that the ungrounded leg that it is not connected to was lost? The only time I have seen this was with a failed connection at the utility transformer secondary or storm damge that broke one of the ungrounded conductors. It appears that the inspector is wanting to have some method of monitoring the other ungrounded leg.

That being said, I don't see a saftey issue if the inverter continues to feed the leg that is still live.

 

[rbulsara]

OK, that (losing B when connected to A) was not very clear. Perhpas you need to have a discussion with the inspector.



Rafiq Bulsara

http://www.srengineersct.com

 

[jfpe]

Thanks. I've talked to the inspector and he isn't moving on this one. I can't see the danger, but if you read the letter of the electric code, it does appear to require this.

At least thinking of the possibility of a failed connection at the utility transformer secondary or storm damage breaking one of the ungrounded conductors makes is easier for me.

-John

 

[jghrist]

One thing to consider is that if one leg of the 120/240V service is open, it may be energized form the good leg through connected 240V loads.

This happened to me during an ice storm. My service drop had one leg open. My hot water heater is 240V and would operate at reduced voltage as long as I had lights connected to the open leg. It got power from the good leg, through the water heater to the open leg and then to neutral through lights connected on the other leg. There was a major outage in the area, with thousands without any power, and I had to live with this situation for several days. It was strange. I'd turn the bathroom lights on and eventually, I'd get enough hot water to take a shower. When the water got hot enough, the water heater shut off and the bathroom lights went out. After showering long enough to get the water heater to turn back on, the lights would come back on.

 

[itsmoked]

Just so you know I saw three different residential groups lose one 'phase' of their 240V power in the same year. I even caused one of them.

In my case I had a large worm drive skill saw. As I turned it on I noticed all the lights dimmed far more than I expected. So.. I clicked it on and off rapidly about 3 times. The third time was the charm. The lights went out in the shed. About the time I was looking at the sub panel I was informed, "a bunch of lights have gone off". No breakers were tripped, yet half the house outlets, and lights were off. Inspection of my main panel showed one phase missing. Inspection of the neighbors on my transformer group also showed the problem. The splice from the transformer to the runner on one of the legs was open.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

So as to the danger, consider that the load center has a single split bus: the two "sides" of the 240V service (let's not use the term "phase", it's incorrect) L1 and L2. The 120V inverter is connected to only L2. The electric dryer is connected to both sides and there is a 120V tap to the neutral to run the dryer controls. So now lets say that the dryer control tap connects it to L2, the inverter fed supply. Power goes out on L1 due to a storm but the inverter allows Joe Blow Homeowner to turn his dryer on to dry his socks. When the dryer energizes the 240V heater coils, only one side is powered by the inverter. That means that if a line worker repairing the downed line is grounded and touching L1, thinking it was dead because he opened the cut-out switch on the transformer, he would complete the circuit. Although it completion would be through the series resistance of the heater coil, it's still enough to kill him...


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[jghrist]

That means that if a line worker repairing the downed line is grounded and touching L1, thinking it was dead because he opened the cut-out switch on the transformer, he would complete the circuit.

But if he opened the cutout on the transformer, it would kill L1 and L2 and the inverter would sense a loss of voltage and disconnect.

 

[jfpe]

Jraef,
Check the OP. A grid tied inverter will only operate in paralle with the utility grid. L2, the "side" fed by the inverter is also still fed by the utility. Only L1, the "side" without the inverter experiences an outage. I don't see any additional danger posed by the inverter. The line worker won't think that L2 is dead because the utility is still feeding it
-John

 

[jraef]

Hmmm... yes good point, assuming of course it was a grid tie inverter that has that detection circuit.

jfpe, is it a "Grid Tie Inverter"? If not, it would not be allowed to be on the "live" circuit anyway, you would need a transfer / isolation switch.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[jfpe]

jraef,
The inverter is a grid tied inverter, it has all the requied under/over voltage, under/over frequency, and anti-islanding protections required by UL1741. The problem is it's a single phase unit, and the inspector wants it to detect loss of the phase it isn't connected to.
-john

 

[LionelHutz]

The fact that the unit is UL1741 listed doesn't matter to the inspector???

I also can't think of a situation where losing one of the 2 live conductors would create a hazard. Utility workers tend to treat all line work as if it were live unless the lines are ground clamped.

Maybe you could ask the inspector if he would go to the local utility yard with you and discuss the concerns with the line workers forman or whoever else is appropriate.

 

[alehman]

The inspector's concern is really not justified. UL 1741 is the only certification available. If he's hard-nosed about it and the utility won't help, you could probably take it up with the utility commission.

I guess you could install a voltage relay on the "B" phase and arrange it to disconnect the inverter on the "A" phase when the "B" phase voltage gets too low. That would obviously not be a UL Listed arrangement.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[stevenal]

jraef,
It's a new world, but the old rules still work. Before the line worker can consider it isolated, he must disconnect all possible sources, including any customer side generation. And I would not consider an unseen, inaccessible, unopenable inverter to provide isolation no matter what UL says. If there is no utility accessible and lockable disconnect available, the meter will need to come out (assuming self contained metering.)

 

[jraef]

The entire Solar PV installation industry is, in my opinion, a lot like the old Wild West; lots of local regulation with not enough consistency across jurisdictions. Yet. I was told there is another major revision to the NEC coming in this regard, and a lot of manufacturers aren't going to like it.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[alehman]

I agree that a disconnect, operable by the utility, would be appropriate. I don't agree with requiring automatic shutdown of the inverter upon loss of power on the unused phase.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[Skogsgurra]

Don't know what rules apply in the US, but 'our' electricians are supposed to disconnect AND ground clamp before working on any circuit.

Most skip on the ground clamping and just put a screwdriver between line(s) and ground to make sure no sparks there.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jfpe]

Thanks for all the input.

In the end I wrote a nice letter to the inspector explaining my posistion. I'll post his response here when I get it.

-John