What Voltage?

[davedottcom]

I was discussing (Arguing!) about the dangers associated with people "Back feeding" their panels with generators that feed In through the dryer outlets without Any Transfer switch.

My question is:

When someone does this and does NOT shut off the main disconnect, what voltage is being sent through the Utility Lines?

The utility transformer is now being "Back fed " 240 Volts Into the original " Secondary " side... so what voltage would actually be present now on the original " Primary " side?

The person I was discussing this with insists the transformer can't work in "Reverse"! I said B.S. the voltage will Increase from 240 volts. but I don't know to what extent because of the limited power source.

A transformer will work "backwards" just not efficiently...right!?!?


Dave

 

[iwire]

Re: What Voltage?

Originally posted by davedottcom:
When someone does this and does NOT shut off the main disconnect, what voltage is being sent through the Utility Lines?

The utility transformer is now being "Back fed " 240 Volts Into the original " Secondary " side... so what voltage would actually be present now on the original " Primary " side?

If there where no loads on this transformer a small home generator could certainly produce 13.8 KV at the primary side of the transformer.

However that would be rare (still possible) for it to back feed into an open line, more likely this small generator would be trying to power at least some of the neighbors. This would most likely overload the generator and trips its breaker.

However before it trips the breaker it could still produce enough voltage in the primaries to really ruin the lineman's day.

Even if it only produces 5% of the 13.8 KV primary voltage you would have about 360 volts to ground off the primaries.

 

[iwire]

Re: What Voltage?

Originally posted by davedottcom:
A transformer will work "backwards" just not efficiently...right!?!?

A transformer will work just fine backward.

 

[physis]

Re: What Voltage?

I'm not so sure of these big voltages Bob. No regulation on the output. What manufacturer would want that kind of liability.

What will happen is the two sources will continually go in out of phase with each other. When in phase everything's cool and the grid gets free power.

When out of phase I hope the breaker works cause it's generator versus utility tranhsformer.

 

[jwelectric]

Re: What Voltage?

Originally posted by iwire:

Originally posted by davedottcom:
A transformer will work "backwards" just not efficiently...right!?!?

A transformer will work just fine backward.

I use a 120/24 volt connected to a 24 volt power supply to light a 100 watt light bulb in the class room just to show that a transformer will work as good one way as the other.


Edited to add;

Don?t know why I quoted Bob?s post for unless it was to say I agree with Bob completely.


[ July 09, 2005, 08:01 PM: Message edited by: jwelectric ]

 

[iwire]

Re: What Voltage?

Originally posted by physis:
I'm not so sure of these big voltages Bob. No regulation on the output. What manufacturer would want that kind of liability.

Sam I am talking about during a power failure, there will be no utility power to be in or out of sync with.

 

[jwelectric]

Re: What Voltage?

Now why I quoted Bob above, cause I want to be just like him!

 

[iwire]

Re: What Voltage?

Originally posted by jwelectric:
I use a 120/24 volt connected to a 24 volt power supply to light a 100 watt light bulb in the class room just to show that a transformer will work as good one way as the other.

Thats funny Mike I almost used a 24 VAC transformer as an example.

 

[physis]

Re: What Voltage?

By Bob:

If there where no loads on this transformer a small home generator could certainly produce 13.8 KV at the primary side of the transformer.

I'm sorry Bob. Rereading your post I see you're talking about those voltages appearing on the other side of the utility tranformer and we are in perfect agreement.

 

[davedottcom]

Re: What Voltage?

Originally posted by iwire:

Even if it only produces 5% of the 13.8 KV primary voltage you would have about 360 volts to ground off the primaries.

Thank you! and you answered my second question too... What is the Primary Voltage from the Utility Co.

Thanks again


Dave

 

[southernboys]

Re: What Voltage?

Guys I totally agree with the above post. I have a question as to if I pull the meter then hook the generator would this be considered safe? What if I unhook the feeder to my main then hook the generator to the main breaker would this be ok? Yes I do realize that an ATS would be the best route just dont have time. I plan to hook many ats up after the storm thanks

 

[iwire]

Re: What Voltage?

Originally posted by davedottcom:
Thank you! and you answered my second question too... What is the Primary Voltage from the Utility Co.

Thanks again


Dave

Dave glad I could help but as far as the primary voltage it may or may not be 13.8 KV in your area.

13.8 is very common in this area but I do work in an area that uses around 41 KV.

You would have to ask a utility guy in your area. I believe 15 KV and below is the most likely voltage. If I remember right 15 KV is the limit to work it live with gloves.

 

[jwelectric]

Re: What Voltage?

Originally posted by southernboys:
Guys I totally agree with the above post. I have a question as to if I pull the meter then hook the generator would this be considered safe? What if I unhook the feeder to my main then hook the generator to the main breaker would this be ok? Yes I do realize that an ATS would be the best route just dont have time. I plan to hook many ats up after the storm thanks

I made this same post in Electrical Contracting and Estimating/Management under what is the correct way to hook up a generator

The safest and fastest way I know of using a generator during a power outage with out a transfer switch is real easy. Buy some drop cords. Make sure to get #12 conductor cords.

 

[iwire]

Re: What Voltage?

Originally posted by southernboys:
Guys I totally agree with the above post. I have a question as to if I pull the meter then hook the generator would this be considered safe?

Southern, the problem is you are at a NEC web site looking for a short cut.

Kind of the wrong place for that I think.

If you pulled the meter yes it would be safe if you also installed a plastic meter blank to protect people from the live 240 on the bottom terminals from the generator and the the 240 on the top terminals when utility power is restored.

Also a pad lock with a LOTO tag locking the meter blank in place would prevent a power company guy from plugging a meter in.

 

[physis]

Re: What Voltage?

I misread the question too. Sorry

 

[peter d]

Re: What Voltage?

I don't think the primary voltage is predictable, depending on where the break is, and what load is connected to it.

Certainly, a generator backfeeding will cause the secondary to be live at 120/240 volts. That alone is lethal for any lineman or person that may come in contact with a downed secondary.

The primary is a different story. Let's say the primary is not broken. Then we have a small generator trying to drive an infinite bus. I don't think the generator will last long under those circumstances.

But, let's say the primary is broken right before the last transformer on the line. That transformer only serves 2 homes, and that other home has a properly installed transfer switch, and that home is "off line." Then I think it's conceivable that the downed primary could easily be energized to the full voltage.

Bottom line, any backfeeding into the utility could produce extremely dangerous voltages.

[ July 09, 2005, 08:33 PM: Message edited by: peter d ]

 

[jwelectric]

Re: What Voltage?

Dave and Bob

CP&lL or Progress Energy which ever they are going by changed us from 7.2 to 13kv about a year ago. Around the same time they were doing this one of their lineman was hurt by a home generator that was back feeding a line they were working on.

 

[southernboys]

Re: What Voltage?

Guys I know its a shortcut and no I dont like doing it one stinking bit. I will be taking these posts with me when I talk to customers after this crisis. I may let someone hook up the generators but I know theres at least one company out there doing it the shortcut way and this is my best opportunity to get my company jumpstarted. Hate to use a crisis to do this however Id rather me do it then some out of towner charging 500 to hook up a generator or 5000 to hook up a powerpack Hope I have not alienated myself in this forum as it has been a hugh help. Thanks for the strong advice

 

[peter d]

Re: What Voltage?

Southern, just remember you can't replace the life of a lineman with $500. If you can sleep at night with a jackleg hookup, well, I'm at a loss for words. I couldn't do it.

 

[southernboys]

Re: What Voltage?

Peter that is 100% what I am trying to avoid. That is why Im trying to find a safe way and I guess the only one is an ATS. so ill let these homeowners do it how thier going do it and fix problems after the storm. Its a catch 22 in a real bad way.