ProSoundWeb Community

Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Thomas Metz on June 21, 2014, 12:18:09 pm

Title: Pulling power for ONE PA, from two different sources?
Post by: Thomas Metz on June 21, 2014, 12:18:09 pm
I am an AV professional, but in my travels I have never had a problem like this because I am always grabbing power from one structure. I am looking for input before I even think about hooking up.

I am doing an outdoor fireworks show in a neighborhood in about a week. I have been allowed to grab power from several houses on site.

Problem is:
I have one house, that will only let me have access to ONE 15 amp outlet / circuit. This is no where big enough.
I have another house about 100' away that will also give me ONE 15 amp outlet / circuit.

Current draw is going to force to to consider plugging two larger amps into house ONE, and plug the other two smaller ones, and my DSP gear into house TWO.

I know that I will have at least some ground differential between the two homes... will this cause me any problems other than "maybe some noise"?

Keep in mind all the amps are sharing earth in ONE rack, so the two houses will in theory have shared potential on ground.

Any thoughts for the newbie?

Thank you
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Kevin Graf on June 21, 2014, 12:31:00 pm
Will you be using a balanced analog interconnect system from one location to the other?
Do you have a DMM meter?
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Thomas Metz on June 21, 2014, 01:13:13 pm
Will you be using a balanced analog interconnect system from one location to the other?
Do you have a DMM meter?


I for speakers / amps, I will be running four JBL cabs from 2 Crown CTs1200's amps AND 2 sub cabs from a Crown XTi AND a set of 70v horns off a QSC CX class V series amp

I only have two sources, an AUX and a single Shure ULX wireless.

FOH,
all I/O's will be to/from a Biamp Flex DSP, with a wrls router so I can adjust sound in real time anywhere in the crowd via my laptop.
My back-up is a POD DI to a  Mackie VLZ P3 console straight to the amps obviously with a LOT less control (i would have to loose the 70V horns, because, no x-overs)

Yes, my amp racks will be sitting about 5 feet apart and be connected with analog balanced XLRs. I will be running ext cords to get the power to one general area (i have no choice)
I actually thought about putting the amps in separate racks, and then using a WRLs mic system to pass signal (zero physical connection) but I don't have enough tx/rx systems laying around.
I also have though about using cobra net, but again, I am missing one of the Biamp I/O parts that I would need to make that happen.

And yes... I have a fluke and I do understand how to use it :)

So, to make sure the picture is clear...

Two racks.
Rack ONE has : a few rack/service lights, DSP & wireless.  It will be fed from house ONE
Rack TWO has: all my amps. The two CTS1200,s will be fed from house TWO, where as the QSC and my XTI, will be fed from house ONE.
The two racks will be side-by-side, connected with a 6P snake, 3 pin XLR, Analog balanced.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: g'bye, Dick Rees on June 21, 2014, 01:21:09 pm
There are serious liability issues in this situation, both with the homes and long cable runs presenting tripping/access hazards.  If ANYTHING at all goes wrong, you're on the hook for a minimum of two properties. 
don't go there.

You're MUCH better off with a generator.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Tim McCulloch on June 21, 2014, 01:23:09 pm
There are serious liability issues in this situation, both with the homes and long cable runs presenting tripping/access hazards.  If ANYTHING at all goes wrong, you're on the hook for a minimum of two properties. 
don't go there.

You're MUCH better off with a generator.

This.  Right.  Here.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: g'bye, Dick Rees on June 21, 2014, 01:31:05 pm
Thanks, Tim.  I could sense you breathing down my neck as I was typing.

This makes me think of that great line, "You feel lucky, punk?".

DR
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Thomas Metz on June 21, 2014, 02:04:43 pm
Ok, It is a closed event in my neighborhood. I am doing the job for free, with zero connection to the company i work for....

Take a look at the picture.

The area inside the inner red circle is 100% off limits (barricaded off), that is where the large fireworks are set up. The fireworks crew is the only people allowed. The outer red circle is 100% roped off.  That is where they set up a few smaller fireworks. That is also where I set up. This large area is about over 150 yards and no one except me and the fireworks crew are let in the area.

The Fireworks guys have 6 staff who's only job is to "watch the line", along with several police officers... And the actual pyrotechnics guys once the show starts.  NO ONE can get to, trip on, knock over my gear. 100% safe. I have a better chance of one of my cabs catching on fire from a low deploying device, then anything else.....

MY FOH is the blue circle (in the red circle).  My speakers are represented with "half A$$" triangles (also in the red circle). The ONLY place I come anywhere near the public is where I cross the walking path with electricity.

I have done this event for several years (with a smaller rig). And the fireworks guys have a city officer to stand watch in the area on the right (where my cables cross).

For this event I run white jacket speaker cable and hiVis power cords,  so anyone can easily see it when it gets a little dark. And again, I use bright yellow cable trays, where my cords will cross the only space where pedestrians my come in contact with it.

The home owners that are supplying power are part of the home owners association, they have zero problems with me running cable in their yards (plugged in, tied up under their decks, short distance overhead to a fence, tied to the fence)... then it leaves the private property and is in tray, until it hits the "red zone"...



But, yes,... I understand there is SOME liability but my only concern is the way I am proposing to connect... not the wires or anything else.

Thank you for you consideration and input...

Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Thomas Metz on June 21, 2014, 02:06:39 pm
OH, and I am "as a person" insured for 2 million in personal liability.. not that it matters... but just an FYI....
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Tom Bourke on June 21, 2014, 02:13:13 pm
Another reason not to pull power from different houses in a neighborhood is the potential for VERY high voltages between houses.  A failure in the power company system could put thousands of volts between houses. Get one of the good inverter type of generators and bill them for the rental.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Chris Jensen on June 21, 2014, 02:23:25 pm
Surely someone in one of those houses has a 220V circuit for a dryer or range they can not use for a few hours?
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Thomas Metz on June 21, 2014, 02:43:28 pm
Another reason not to pull power from different houses in a neighborhood is the potential for VERY high voltages between houses.  A failure in the power company system could put thousands of volts between houses. Get one of the good inverter type of generators and bill them for the rental.

Can you elaborate on that?

I am not making a connection between the houses except they would be feeding equipment that share a copper signal line and a chassis ground.... How in the world could a 220V house service equal "thousands of volts between the houses"...?

Lets say one house somehow loses power... There is no way for the equipment to feed power from one house to another... well, not with out some sort of unrealistic scenario where I had a complete meltdown of a power supply at the exact same moment. And even if somehow, the equipment melt down and fused the hot to earth with out blowing a fuse, and it did connect... the other house would throw a breaker... no problem..

Back to reality, if, there was a ground differential, noise in the system and MY equipment should be the only thing at risk here... unless I am missing something... and I may be...

If you want to help me "please explain why it is a bad thing" don't just give me the cookie cutter response....

I am not opposed to renting a genset, but I am not going to shell out the money for that unless there is a "real danger" of damage of harm.

MY primary concern. House one has a crappy earth ground. One with a large level of resistance between earth and the ground buss. I connect all the equipment, and the ground in house two is fantastic. The crappy ground in house one may find the ground I am using to/from house two... aka, if my connections between the houses equals less resitance on ground, house ONE may try to find ground through my gear and IN house two....

That sounds so confusing... does anyone know what I am talking about?

I am concerned about my cord from house two, hits chassis ground in my rack along with the ground from house one. AND a potential of current dissipation between the houses on the GROUND.

Kind like, when you have a service in a home, and you want to add a sub panel... the way it grounds is very different depending on the relationship between ground and neutral in the main panel. If you start by sharing ground and neutral you can just add another ground rod for your sub panel... it would create two different impedance on the ground/neutral plane...


And Chris, You would think... but "obviously" I have not been able to make that happen.

Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Kevin Graf on June 21, 2014, 03:22:12 pm
I must be missing something. These appear to be nearby homes in a quite residential neighborhood. Most likely serviced by the same power company 240V output single phase transformer. Very little difference than powering your system from different circuits in the same building.

Tony Waldron writes about audio systems in large multiple buildings, multiple power source studio complexes.

"Tony Waldron's Audio & EMC ranting and ravings"
http://www.fragrantsword.com/twaudio/

Measurements from one home to the other with a long extension cord using a DMM and a Non Contact Probe could be made this weekend.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Tom Bourke on June 21, 2014, 03:37:07 pm
Can you elaborate on that?

I am not making a connection between the houses except they would be feeding equipment that share a copper signal line and a chassis ground.... How in the world could a 220V house service equal "thousands of volts between the houses"...?

If they happen to be on the same transformer then the voltage will stay around 220V max.  However the power company distributes power at much higher voltage.  Around 13KV at my last house.  There have also been instances of very high voltage buried cable (tens of KV) leaking stray voltage due to degradation of the insulation.  This is then picked up as a difference in ground potential.  If each house does not share a common electrical ground this can reach high voltages.
http://en.wikipedia.org/wiki/Electric_power_distribution  Also do a search on stray voltage.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Ray Aberle on June 21, 2014, 03:39:49 pm
The other advantage of using a small generator is that if a breaker trips in your distro, it's right there and easy to reset. If a breaker trips in one of the houses, you would have to run inside, find their panel, find the right breaker, and reset it… =\
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Tim McCulloch on June 21, 2014, 03:45:28 pm
I am not opposed to renting a genset, but I am not going to shell out the money for that unless there is a "real danger" of damage of harm.

Get the pryo guys to rent it and include it in their price.  Really.  The HOA is spending several thou for a shoot and want to get pissy over a genset rental when their members won't/can't up safe, suitable power?  "Intercourse" them.  They're penny wise and liability foolish.

Quote
MY primary concern. House one has a crappy earth ground. One with a large level of resistance between earth and the ground buss. I connect all the equipment, and the ground in house two is fantastic. The crappy ground in house one may find the ground I am using to/from house two... aka, if my connections between the houses equals less resitance on ground, house ONE may try to find ground through my gear and IN house two....

That sounds so confusing... does anyone know what I am talking about?

I am concerned about my cord from house two, hits chassis ground in my rack along with the ground from house one. AND a potential of current dissipation between the houses on the GROUND.

Kind like, when you have a service in a home, and you want to add a sub panel... the way it grounds is very different depending on the relationship between ground and neutral in the main panel. If you start by sharing ground and neutral you can just add another ground rod for your sub panel... it would create two different impedance on the ground/neutral plane...

And Chris, You would think... but "obviously" I have not been able to make that happen.

The bolded text illustrates the issue.  Go back and look through some of Mike Sokol's posts on "reversed polarity, bootleg ground" circuits and what that can mean for signal grounds.  While you're unlikely to encounter one of those, the various differences in power distribution and quality of wiring in general should make you very leery of the possibility of audio pin 1/shield becoming a common ground between houses on different transformers, with your gear in between.  Given that you know there are problems with household circuits/wiring you're expected to use, I'd decline and insist on getting "real power" from another house or generator(s).

I just read Kevin G's reply.  I'll agree that long extension cords, meter and other accessories can make for an educational afternoon.  I'd add that a load should be applied (flood light, toaster, something that draws 1000w) to each circuit and measurements taken to compare with unloaded measurements.  And I'd still want a generator, but maybe that's just me.

Thomas, I understand that we're pretty much dependent on our client's willingness to accept our recommendations, but this is one of the cases where potential safety decisions are being made based on being cheap asses.  Here's what I'd do:  I'd require the HOA to have me listed as an additional named insured on THEIR liability policy as part of my contract, and also require "absolute indemnification for any and all claims arising from any services provided by Thomas Metz."  If they won't do that you can say, "a generator rental is only $300."
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Thomas Metz on June 21, 2014, 03:54:58 pm
If they happen to be on the same transformer then the voltage will stay around 220V max.  However the power company distributes power at much higher voltage.  Around 13KV at my last house.  There have also been instances of very high voltage buried cable (tens of KV) leaking stray voltage due to degradation of the insulation.  This is then picked up as a difference in ground potential.  If each house does not share a common electrical ground this can reach high voltages.
http://en.wikipedia.org/wiki/Electric_power_distribution  Also do a search on stray voltage.


THIS... is exactly what I do for a living. LARGE scale production sound/ lighting install. It is not uncommon to have several parts of ONE system fed from different panels and or switch gear in a single building.

BUT.... (see next response)
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Thomas Metz on June 21, 2014, 03:57:29 pm

The bolded text illustrates the issue.  Go back and look through some of Mike Sokol's posts on "reversed polarity, bootleg ground" circuits and what that can mean for signal grounds.  While you're unlikely to encounter one of those, the various differences in power distribution and quality of wiring in general should make you very leery of the possibility of audio pin 1/shield becoming a common ground between houses on different transformers, with your gear in between.  Given that you know there are problems with household circuits/wiring you're expected to use, I'd decline and insist on getting "real power" from another house or generator(s).



THIS... is exactly what worries me and that's why I wrote it...  Thank you for adding some detail!
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Thomas Metz on June 21, 2014, 03:59:35 pm
However the power company distributes power at much higher voltage.  Around 13KV at my last house.  There have also been instances of very high voltage buried cable (tens of KV) leaking stray voltage due to degradation of the insulation.  This is then picked up as a difference in ground potential.  If each house does not share a common electrical ground this can reach high voltages.
http://en.wikipedia.org/wiki/Electric_power_distribution  Also do a search on stray voltage.

This I understand. Thank you.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Thomas Metz on June 21, 2014, 04:07:37 pm
Ok...

I was worried about it because i do understand that there is the potential for problems. With more insight added I feel that my concerns are warranted.

LOL, the professional pyro... he ALSO live in the neighborhood. lol, they are not getting paid either.... lol.

They project what "donations" will come in to pay for the fireworks and this year they ended up about $1500 short on funds.

Everything else... (labor, sound, food, etc...) is all free volunteer work. Cops and Fire department show up as part of the permit acquisition process.


Looks like I am renting a Genset...

Thanks guys...(or gals)



Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Jonathan Johnson on June 21, 2014, 04:12:34 pm
I'm with the others; I don't think this is a good idea. But if you're absolutely determined to go ahead with the plan, I would recommend that all signal lines between the equipment on different circuits be connected via isolation transformers with ground lift.

The potential for ground voltage differential between the systems is high; you don't want your signal line shields to be the balancing factor. To do so not only could introduce hum, but if a significant enough voltage is present, it could damage things and present a shock hazard.

And a warning: ensure that the equipment on different circuits is at least 10 feet physically separated. This will prevent you or anyone else from touching the chassis on two pieces of equipment that are at different ground voltage potentials.

You could also create a grounding grid that bonds the grounds of each house AND the chassis of each distro with a ground rod driven at the equipment location, but the cost of doing that would be more than the cost of renting a generator (which, by the way, should be bonded to a driven ground rod; look here for a discussion (http://forums.prosoundweb.com/index.php?topic=149818.0)).
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: g'bye, Dick Rees on June 21, 2014, 04:26:07 pm
Ok...

Looks like I am renting a Genset...

Thanks guys...(or gals)

Thomas...

You are to be commended for doing it right.  If all goes as it should, the association or some individuals might see their way to defraying your expenses.

Again, congratulations on taking responsibility and the high road.

DR

PS...

I carry liability insurance for my PA work, same amount as you...which is the minimum coverage required around here.  I would not expect it to cover jury rigging power as you proposed. 
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Jonathan Johnson on June 21, 2014, 05:01:37 pm
Looks like I am renting a Genset...

Just like the HOA is paying the actual costs of the purchase of the fireworks even though the pyros are donating their services (are they professional pyrotechnicians or just pyromaniacs?), so too they should pay your actual costs even though you are donating your services.

If they balk, explain that taking power from two different houses risks equipment damage and they WILL be liable for that. (For some reason, people are more likely to believe "equipment damage" than "electrocution" risk.)

But you DID say they were short on funds, do they plan on passing the hat to make up the difference?

P.S. -- a pyrotechnician is just a pyromaniac with a license. Most firefighters are also pyromaniacs. :)
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Ray Aberle on June 21, 2014, 05:52:58 pm
Just like the HOA is paying the actual costs of the purchase of the fireworks even though the pyros are donating their services (are they professional pyrotechnicians or just pyromaniacs?), so too they should pay your actual costs even though you are donating your services.

Can you source it, spec what's needed, give them the quote and tell them they need to take care of the actual rental? So you're not out money from your pocket, "hoping" that you'll get reimbursed.

You CAN get by with a Honda-series inverter (EU series); don't accept a Home-Depot/Costco styled construction generator. WhisperWatt/trailer-mounted diesel is ideal. 25kW would be more then enough, two spider boxes and 50A twist cabling. Don't forget grounding! (And any local permits that might be required…)

-Ray
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Thomas Metz on June 21, 2014, 06:45:41 pm
It is not the money... well, it is... but it's not....

Last year I spent about $1200 on equipment for my own collection so that I could do the gig W/O equipment from my employer...

I just don't want to start down the "hey Jim I took 4 amps, I will bring them back Monday" road... Just never ends well...

This year, I sold one amp, and purchased 3 more and some other random gear..... (used) down another $1200.

The gear I have purchased is an investment of sorts...

Where as renting a genset,... yep, that is me "throwing money away" OR... I guess now that my production rig has grown large enough... it is "extra insurance"

The pyros, yes, professional. At least the lead guy is. He has another guy or two from his work help out, the rest of his staff are his buddies.

I just called him, and told him the news, he was cool with it, said he would pay out of his pocket for half, and IF if they can not score enough donations between now and the event end to cover the fireworks "and more" he will cover the whole cost... But I doubt that will happen...  so... I only have to pay for half of it...



Ray, I am looking at the Honda EU, for like $100 for one night. I will throw it back in the trees and no one will ever know it is there. It will provide more power than the two 15 amp house circuits can...





Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Lyle Williams on June 21, 2014, 07:31:12 pm
Moving on from safety issues, where the cost of mitigation will probably outweigh the cost of a generator...

From an audio perspective, equipment powered from different sources should be expected to be at different ground and electrical potentials.  Audio cables running between different power zones should be transformer isolated and ground lifted.  Don't lift power ground on anything.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Mike Sokol on June 21, 2014, 08:01:42 pm
Ray, I am looking at the Honda EU, for like $100 for one night. I will throw it back in the trees and no one will ever know it is there. It will provide more power than the two 15 amp house circuits can...

I just saw this thread and agree with the consensus of the swarm. There's simply no upside for going cheap and running extension cords from multiple houses when YOU will be the one in the hotseat if something goes wrong. And the things that can go wrong are both expensive and potentially deadly. I guarantee that the HOA committee will scatter and leave you flapping in the wind if someone gets hurt or somethings blows up. A Honda EU3000 is quiet and makes very clean power.

Just make sure you add a Ground-Neutral bonding plug to the generator (these are floated neutral generators) and pound in a ground rod. But that's another thread...

See http://www.noshockzone.org/generator-ground-neutral-bonding/ (http://www.noshockzone.org/generator-ground-neutral-bonding/) for generator GN bonding for RVs. It's the same principal for sound systems except that you should also drive a ground rod and bond it to the generator's chassis/ground lug. 
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Stephen Swaffer on June 22, 2014, 12:15:41 am
Considering this is an annual event, another option-especially at the $300 price point-depending on the home and panel location you might me able to get a dedicated 20, 30 or 50 A 220 V 4 pole GFCI protected receptacle installed.  An upgrade for the homeowner for letting you use power and a convenience for you next year, too.

Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Steve M Smith on June 22, 2014, 06:01:57 am
From an audio perspective, equipment powered from different sources should be expected to be at different ground and electrical potentials.  Audio cables running between different power zones should be transformer isolated and ground lifted.  Don't lift power ground on anything.

Whilst I'm not suggesting anyone does this, theoretically, what would be the problem with bonding the two grounds together at the equipment end of the cables?  I mean joining the supply grounds, not a link via a signal cable.  Perhaps even with a ground stake at the equipment end too.


Steve.



Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Cailen Waddell on June 22, 2014, 07:24:42 am

Whilst I'm not suggesting anyone does this, theoretically, what would be the problem with bonding the two grounds together at the equipment end of the cables?  I mean joining the supply grounds, not a link via a signal cable.  Perhaps even with a ground stake at the equipment end too.


Steve.

I'll take a stab at this.....  And I could be wrong, so please correct me if any of this theory is wrong. 

In a perfect world, with perfect electrical service fed by a common poco transformer to both houses, nothing. It would work fine.

If the houses are fed off of different transformers, while each meters the same, they could be working at different ground/earth potentials with hot leg voltages relative to those.  Tying them together would result in a combined system with two ground neutral bonds, bring both the ground and the neutrals into one ground plane - but relative to which transformer? potentially bringing one house into a n over volt situation and one to an under situation. It could be small, like 110 and 125 or quite large I would suspect.   

Regardless of poco transformer, if a fault condition occurred in either house, and that house had a faulty ground, that fault current could flow up one extension cord and back down the other, at potentially unknown amperages....

Probably more domesday scenarios, but this is what I have thought of so far. 


Sent from my iPhone using Tapatalk
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Mike Sokol on June 22, 2014, 12:49:42 pm
I'll take a stab at this.....  And I could be wrong, so please correct me if any of this theory is wrong. 

In a perfect world, with perfect electrical service fed by a common poco transformer to both houses, nothing. It would work fine.

If the houses are fed off of different transformers, while each meters the same, they could be working at different ground/earth potentials with hot leg voltages relative to those.  Tying them together would result in a combined system with two ground neutral bonds, bring both the ground and the neutrals into one ground plane - but relative to which transformer? potentially bringing one house into a n over volt situation and one to an under situation. It could be small, like 110 and 125 or quite large I would suspect.   

Regardless of poco transformer, if a fault condition occurred in either house, and that house had a faulty ground, that fault current could flow up one extension cord and back down the other, at potentially unknown amperages....

Probably more domesday scenarios, but this is what I have thought of so far. 


Sent from my iPhone using Tapatalk

That's all correct. While there's unlikely to be a large voltage differential between the two houses, the current normalization can be staggering; into the hundreds of amperes. Connecting your extension cords between them will force anything in the middle to pass that amperage, and that's not good for the gear on a lot of levels.

If something really goes wrong at one of the houses (lost neutral to the pole, nearby lightning strike, etc) then your extension cords will be asked to normalize all that voltage as well, which in that case would be considerable and life threatening. And don't even consider bonding the neutrals together as that will cause any GFCI in the path to trip instantly.

A little genny is the way to go. I see those Honda EU3000 inverter generators at small portable stages all the time and they're great. Plus you can run them all weekend on a 5 gal container of gasoline (or petrol for those on the other side of the pond, I think). 
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Steve M Smith on June 22, 2014, 01:36:39 pm
Just to be clear, I also think the generator is the way forward.  I'm just thinking through the possibilities.

I suppose the most impractical but feasible way is to have the grounds bonded together at the houses (which they probably are already) and just run one of them to the equipment.

Again, not suggesting it is done this way, just thinking theoretically.


Steve.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Ray Aberle on June 22, 2014, 01:56:11 pm
And if one of the houses was *my* house, I'd just trench a conduit out to that back side and put a small post there with a range plug and a couple of GFCI 20A outlets. Cos that would be useful in the future for a multitude of things, not just the annual fireworks show.

(Or I'd drag out one of the 5 generators we have around the house. Haha.)
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: g'bye, Dick Rees on June 22, 2014, 02:12:08 pm
Just to be clear, I also think the generator is the way forward.  I'm just thinking through the possibilities.

I suppose the most impractical but feasible way is to have the grounds bonded together at the houses (which they probably are already) and just run one of them to the equipment.

Again, not suggesting it is done this way, just thinking theoretically.


Steve.

It matters not if it CAN be done.  The fact is it should NEVER be done short of a civic emergency.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Mike Sokol on June 22, 2014, 03:14:18 pm
It matters not if it CAN be done.  The fact is it should NEVER be done short of a civic emergency.

Or zombie apocalypse...  ;D
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Stephen Swaffer on June 22, 2014, 04:01:32 pm
I would go for either the genny or a dedicated circuit from one home.

That said, a couple of thoughts, I think too often we over think and ascribe almost vodoo powers to electrons.  On the POCO side, at the transformer the primary neutral and secondary neutral are bonded together to a grounding electrode of some sort-so if you draw the circuit out, the neutrals from the 2 homes are already connected together.  The potential difference will be the voltage drops determined by the neutral currents to the 2 homes and whatever neutral exists between transformers.  Likely to be several or even 10 s of volts-but not much more UNLESS there is a problem and ta should be creating an issue in one of the homes already.

Also, if this neighborhood has metallic water lines, likely both homes are grounded to the incoming water line (actually required to be by code) so their grounds are already connected together.  Of course, other posts have mentioned "objectionable" current flowing on plumbing lines-and tis could be the case as well-also another problem that needs to be fixed.

That said, I don't want to be the nice guy donating my services that discovers a problem by a meltdown!

I am actually working on a project right now where a customer wants to use an installed backup genny to provide emergency power for small crucial loads in 2 different buildings supplied by services on separate transformers.  This obviously requires me to connect the neutrals from each service.  No objections raised by the electrical inspector, but I will update this after I talk to the POCO this week.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Jonathan Johnson on June 22, 2014, 06:26:59 pm
That said, a couple of thoughts, I think too often we over think and ascribe almost vodoo powers to electrons.  On the POCO side, at the transformer the primary neutral and secondary neutral are bonded together to a grounding electrode of some sort-so if you draw the circuit out, the neutrals from the 2 homes are already connected together.  The potential difference will be the voltage drops determined by the neutral currents to the 2 homes and whatever neutral exists between transformers.  Likely to be several or even 10 s of volts-but not much more UNLESS there is a problem and ta should be creating an issue in one of the homes already.

I agree with that; it's true in most cases in the United States. However, if by some fluke of providence the houses were provided power from two different substations, this may not be the case as the shared bonding of neutrals and ground only exists on the secondary side of the substation. There is no neutral wire with the primary lines between substations. I have also seen some feeder distribution schemes where there is no common neutral with the feeder lines; the neutral is derived from the secondary side of a three phase transformer bank or even from earth ground.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Cailen Waddell on June 22, 2014, 08:35:23 pm
I'll also say that in my neighborhood, we have underground power, but it comes in from two sides of the neighborhood. My house is the last one on the 13kv feed from the east, my neighbor the last from the west.  Houses share the split phase step down transformers in pairs.  The neighborhood was built in 2 phases. 

Anyway - it's not out of the realm of possibility. 


Sent from my iPhone using Tapatalk
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: jason misterka on June 22, 2014, 09:55:42 pm
I am an AV professional, but in my travels I have never had a problem like this because I am always grabbing power from one structure. I am looking for input before I even think about hooking up.

I am doing an outdoor fireworks show in a neighborhood in about a week. I have been allowed to grab power from several houses on site.

Problem is:
I have one house, that will only let me have access to ONE 15 amp outlet / circuit. This is no where big enough.
I have another house about 100' away that will also give me ONE 15 amp outlet / circuit.

Current draw is going to force to to consider plugging two larger amps into house ONE, and plug the other two smaller ones, and my DSP gear into house TWO.

I know that I will have at least some ground differential between the two homes... will this cause me any problems other than "maybe some noise"?

Keep in mind all the amps are sharing earth in ONE rack, so the two houses will in theory have shared potential on ground.

Any thoughts for the newbie?

Thank you


Has anyone every though about offering a bit of money to the owner of the one house so you don't need to rent a generator?  Seems silly to me. Give the guy $50 and get a second circuit. Less money, less hassle, and more safe then a generator, that's for sure...

Jason
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: g'bye, Dick Rees on June 22, 2014, 10:02:47 pm

Has anyone every though about offering a bit of money to the owner of the one house so you don't need to rent a generator?  Seems silly to me. Give the guy $50 and get a second circuit. Less money, less hassle, and more safe then a generator, that's for sure...

Jason

Again, if anything should go wrong you're on the hook for damages, long cable runs lose voltage and present tripping hazards and as mentioned before, tripping a breaker means having to go into the house to reset.

Generator is a better bet all around.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Ray Aberle on June 22, 2014, 10:11:28 pm
Give the guy $50

And the OP says he found an EU3000is for $100.00 for the day-- twice the money you suggest, and *surprise surprise* a much better solution.

I think SOMEONE in the neighborhood should already have a small generator, though. And if not, they should buy one. :-p

Ray
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: jason misterka on June 22, 2014, 10:46:45 pm
Again, if anything should go wrong you're on the hook for damages, long cable runs lose voltage and present tripping hazards and as mentioned before, tripping a breaker means having to go into the house to reset.

Generator is a better bet all around.

He says the second house is 100 feet away. Not the first house.  And how close to your PA do you put the generator? I usually spec a generator 75' from the "stage" to keep the noise down.

Come on folks.  Using two circuits from two houses IS an issue. Using two circuits from one panel on one house shouldn't typically be.

Maybe I'm the only one that has had four or five problems with large generators over the last year.  I'd much rather be on shore power.

And on the hook for damages using two Edison circuits from someone's house? Probably, but the risk is low. But you are also on the hook if you set the woods on fire (saw a little generator do that a few years ago...)

I'm just saying,  he's running two amps on each circuit.  Unless they are Crest 9001 running full tilt boogie, he should be fine for a fireworks show.

Jason
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Ray Aberle on June 22, 2014, 11:15:49 pm
He says the second house is 100 feet away. Not the first house.  And how close to your PA do you put the generator? I usually spec a generator 75' from the "stage" to keep the noise down.

Come on folks.  Using two circuits from two houses IS an issue. Using two circuits from one panel on one house shouldn't typically be.

Maybe I'm the only one that has had four or five problems with large generators over the last year.  I'd much rather be on shore power.

And on the hook for damages using two Edison circuits from someone's house? Probably, but the risk is low. But you are also on the hook if you set the woods on fire (saw a little generator do that a few years ago...)

I'm just saying,  he's running two amps on each circuit.  Unless they are Crest 9001 running full tilt boogie, he should be fine for a fireworks show.

Jason
Yeah, BUT.... The OP stated that he cannot get 2 circuits from one house, but instead one from each of 2 houses. Hence the reason for the major discussion about it! And if you need to put your generators 75' away, you need to start working with the larger diesels, cos you can put one of those next to a stage and be OK noise-wise. In any case, the OP mentions he can get an EU3000is for $100; and that thing is gonna be quieter then anything else out there anyways.

And if the woods catch on fire, that's why you have a fire extinguisher there, any time you're on a generator... right? right?

No problems on large generators here, but I also always work with reputable suppliers, and meter my shit first. And if something's wrong, that gets fixed. Before the show.

Either way, in the end, I would rather see someone do the best practice because they know it's better, not because they're being forced to. I would rather someone say "it's gonna be a bit harder/more expensive to do it right, but I am not about to take a shortcut."

-Ray
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: g'bye, Dick Rees on June 22, 2014, 11:22:59 pm
He says the second house is 100 feet away. Not the first house.  And how close to your PA do you put the generator? I usually spec a generator 75' from the "stage" to keep the noise down.

Come on folks.  Using two circuits from two houses IS an issue. Using two circuits from one panel on one house shouldn't typically be.

Maybe I'm the only one that has had four or five problems with large generators over the last year.  I'd much rather be on shore power.

And on the hook for damages using two Edison circuits from someone's house? Probably, but the risk is low. But you are also on the hook if you set the woods on fire (saw a little generator do that a few years ago...)

I'm just saying,  he's running two amps on each circuit.  Unless they are Crest 9001 running full tilt boogie, he should be fine for a fireworks show.

Jason

I can set my EU3000i twenty feet from the stage and you can't really hear it running.  If I wanted to I could put it behind a half sheet of plywood and get even closer.

Title: Re: Pulling power for ONE PA, from two different sources?
Post by: jason misterka on June 22, 2014, 11:37:11 pm
Yeah, BUT.... The OP stated that he cannot get 2 circuits from one house, but instead one from each of 2 houses. Hence the reason for the major discussion about it! And if you need to put your generators 75' away, you need to start working with the larger diesels, cos you can put one of those next to a stage and be OK noise-wise. In any case, the OP mentions he can get an EU3000is for $100; and that thing is gonna be quieter then anything else out there anyways.

And if the woods catch on fire, that's why you have a fire extinguisher there, any time you're on a generator... right? right?

No problems on large generators here, but I also always work with reputable suppliers, and meter my shit first. And if something's wrong, that gets fixed. Before the show.

Either way, in the end, I would rather see someone do the best practice because they know it's better, not because they're being forced to. I would rather someone say "it's gonna be a bit harder/more expensive to do it right, but I am not about to take a shortcut."

-Ray

We are surely getting off topic here but I still disagree.

Show me a house that only has ONE circuit. The OP can probably only get one circuit because that is what is convenient to the home owner. $50 would probably change that.  Possibly even get a four wire range plug if needed.

I still don't feel like a generator is necessarily the "best way".  Yes it is better than running from two circuits in two houses.

I guess we work different shows. Most of my generator rentals are 60kw to 125kw.  Had a 175kw from Sunbelt do some crazy things on day two of a festival last year.

We don't put them near the stage because you CAN hear them and the get into the mics.  Even the modern "WhisperWatt" ones.  Not terribly, but enough that I still put them at 75',

Not that that probably matters one bit one a fireworks show.

I guess we will just have to agree to disagree.  This horse has been beaten.

Jason
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Ray Aberle on June 22, 2014, 11:45:56 pm
Show me a house that only has ONE circuit. The OP can probably only get one circuit because that is what is convenient to the home owner. $50 would probably change that.  Possibly even get a four wire range plug if needed.

You're missing the OP's situation. It's not that the house only has one circuit (that'd be stupid. I can't believe you suggested that was the case.) - it's just that the two houses only are willing to provide him with one each.

I guess we work different shows. Most of my generator rentals are 60kw to 125kw.  Had a 175kw from Sunbelt do some crazy things on day two of a festival last year.

Well I don't rent, I own five generators. So yeah. When the ball is in your court to ensure that they are working, all the time, without fail, you spend a bit more time making sure that you're not going to have problems. And I rent to others in my area, cos I will do it for them (as a cross rent) for cheaper then Sunbelt. And they know that my gensets are going to work for them cos my reputation is on the line each and every time. Regular, preventive maintenance is crucial, and you never know with the big houses where the unit was before your event. And what someone did to it.

-Ray
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Stephen Swaffer on June 22, 2014, 11:48:30 pm
There is no neutral wire with the primary lines between substations. I have also seen some feeder distribution schemes where there is no common neutral with the feeder lines; the neutral is derived from the secondary side of a three phase transformer bank or even from earth ground.

I could be wrong, but I don't think I have ever seen a transmission line that did not carry a grounded wire that is grounded at every pole.  The whole idea of grounding/bonding is to keep everything that should not be energized at nearly the same potential.  If that is not successful then electrically unintentional connections between homes-such as metallic water lines, gas lines, telco/cable lines, or even chain link fence that connects virtually every home on the block in some areas become hazards waiting to bite someone.

I don't say that to argue for pulling power from two homes.  There are too many ifs, and more controlled/reliable solutions are available at minor cost.  I am saying that you need to consider the whole system.  Drawn out in line diagram, other than the ground bond (a huge issue to be sure), this system is in reality probably simpler and smaller than the electrical system in a medium to  large venue-and if the venue has much age on it, the ground bond might not be a given or any better there either.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: jason misterka on June 23, 2014, 12:06:52 am
You're missing the OP's situation. It's not that the house only has one circuit (that'd be stupid. I can't believe you suggested that was the case.) - it's just that the two houses only are willing to provide him with one each.

-Ray

And my point was that $50 to the home owner may go a long way towards accessing a second circuit.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Ray Aberle on June 23, 2014, 12:15:51 am
And my point was that $50 to the home owner may go a long way towards accessing a second circuit.

That is always possible. However... I am going to go out on a limb, though, and surmise that since the OP is here asking the questions that he is asking, he's already exhausted all other options in getting more power from a single house. Just sayin' - I feel like he's done his due diligence already.

-Ray
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Mike Sokol on June 23, 2014, 12:21:39 am
Everyone.... Time to put this subject to bed.

Thx

Mike Sokol
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Ray Aberle on June 23, 2014, 12:24:11 am
Everyone.... Time to put this subject to bed.

Thx

Mike Sokol

sowwy   :-[
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Jonathan Johnson on June 23, 2014, 01:37:02 am
You're missing the OP's situation. It's not that the house only has one circuit (that'd be stupid. I can't believe you suggested that was the case.) - it's just that the two houses only are willing to provide him with one each.

Probably because each house has only ONE exterior receptacle circuit on the back deck. I'm guessing the owners aren't willing to have an extension cord going in the kitchen window.

Everyone.... Time to put this subject to bed.

Thx

Mike Sokol

Yes, I think this horse is dead. I'll stop beating it now.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Kevin Graf on June 23, 2014, 09:01:43 am
Now that it's to late to save the horse.  Why not just use a wireless audio link to the remote speaker system?
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: jasonfinnigan on June 23, 2014, 05:06:53 pm
I really don't get why there is so much discussion about this. I don't think it's a good idea to pull from two homes. I can get a 30Kw diesel Genny for $100-140 a day. and that includes them delivering it and driving the ground rods in (Yes, you do need ground rods for a Generator).
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Rob Spence on June 24, 2014, 12:00:51 am
Heck, see if there is an electrician in the area that could put a 30 amp dryer outlet in the close house basement. Plenty of power, no ground issues. $25 in parts.


Sent from my iPad using Tapatalk HD
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Tim McCulloch on June 24, 2014, 12:15:01 am
I really don't get why there is so much discussion about this. I don't think it's a good idea to pull from two homes. I can get a 30Kw diesel Genny for $100-140 a day. and that includes them delivering it and driving the ground rods in (Yes, you do need ground rods for a Generator).

Yeah.  I don't see why (other than the HOA proclaiming poverty) this is an issue.  Good generators don't have to be expensive unless you want holiday delivery and fuel service from the rental shop on that holiday.  The costs of delivery, fuel and return will be 3x the cost of the genset rental.

I can get a 25-40kW genset for about $125/day if we pick it up and return it.  The shop includes a ground rod and wire, and our preferred vendor will tech it in their shop before we pick it up.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Stephen Swaffer on June 24, 2014, 12:27:46 am
I really don't get why there is so much discussion about this.

2 reasons:

1.  Sometimes arguing is more fun than coming up with a simple solution.
2.  To check and see if Mike is doing is job. :)
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Mike Sokol on June 24, 2014, 06:53:43 am
2 reasons:

1.  Sometimes arguing is more fun than coming up with a simple solution.
2.  To check and see if Mike is doing is job. :)

Yeah, yeah, yeah...  :o
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Stephen Swaffer on June 24, 2014, 11:27:13 pm
I am actually working on a project right now where a customer wants to use an installed backup genny to provide emergency power for small crucial loads in 2 different buildings supplied by services on separate transformers.  This obviously requires me to connect the neutrals from each service.  No objections raised by the electrical inspector, but I will update this after I talk to the POCO this week.

At the risk of raising a dead horse, I got my answer today.

One genny can serve 2 separate services.  Grounds would be bonded together.  One of the transfer switches would be a normal 2 pole switch.  The second must be a 3 pole switch-the neutral on one of the services must be switched so that the neutrals from the 2 services are never connected.  Also, the second service must be wired with grounds and neutrals separated to have the proper configuration (one ground/neutral bond,period) when running off genny.

Too much of a headache IMO for the benefits to my customer at this point, but I suppose if the check is big enough?

For purposes of this thread, the POCO is obviously against using two services in a cooperative manner.  Though the have no problem bonding the grounds together.   
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Steve M Smith on June 25, 2014, 01:36:40 am
2 reasons:

1.  Sometimes arguing is more fun than coming up with a simple solution.
2.  To check and see if Mike is doing is job. :)

Or to quote something I read a few days ago:

"I'm not arguing with you, I'm explaining why you're wrong!".


Steve.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: David Buckley on July 30, 2015, 11:40:50 pm
(Late response to a old thread)

There's some hints in the thread about high voltage bad things happening, but no clear explanations as to why.

The USA has adopted a system of distribution called Multi Grounded neutral  -MGN.  This is a system very different to that used over most of the rest of the world, and brings its own issues.  In theory, it is a superior system, saving money, and providing better lightning protection for distribution assets.  In practice, however, it provides a mechanism of fault current propagation, and has its own set of special dangers.

The big difference between what everyone else uses (which has no name) and MGN is to do with the way that single phase transformers are wired, the type of transformers that serve homes and smaller businesses.  Ordinarily, the final, single phase transformer serving a home(s) or building has its primary connected between two high voltage phase lines, but under MGN the transformer is connected between one phase line and a neutral cable, often called the messenger wire.  So a transformer here in New Zealand, typical of the rest of the world, has two high voltage terminals on it.  A USA transformer has one.

So in the USA, neutral is distributed via a wire.  And... the neutral to your home is also connected to that same neutral wire, so your home neutral is connected to the high voltage distribution network.  And that neutral distribution network is deliberately connected as far and as wide as possible.  It is also connected to the ground through plates and rods, at a substation, at the pole where the transformer is, and at your home.  The idea is that it is one big equipotential zone.

Having introduced all that, there are two problems that are seen with MGN.

The first is that there will be potential differences between the neutral "points" of each home, caused by voltage drop in the neutral wiring.  This is not just from the neutral current between the service entrance and the pole, but from current flowing along the messenger neutral from out-of-balance (and harmonic!) neutral return currents along the messenger back to the substation.  This can vary between millivolts and quite a lot of volts, depending on loads and distance.  At the service entrance of each house, neutral and ground are joined to form the earth point of the home.  Thus it follows that ordinarily, normally, and expectedly, there will be potential difference between the ground points of different homes.

If audio stuff is plugged in this situation, coming between two different ground potentials, then there will be circulating currents, which could potentially cause hum like a ground loop.  The current flowing could be quite significant, if the impedance of the system between different points is low, and this could leac to heating effects across audio cabling, and potentially across the PCBs of electronics.  Not good.

The more terrible situation is that of faults.

One fault mode is that many houses have poor - or missing - neutral connections, and thus their local ground potential is way out.  A house can function with a missing neutral using the water pipes as a neutral return circuit, or even sometimes a ground rod.  This can cause significant voltage difference between a house with a fault and other houses.

A bigger problem is a fault in the neutral messenger wire.  If this wire has an imperfect connection somewhere along its length then a voltage will appear across that imperfection, and that voltage will be reflected through the messenger neutrals to all the grounds of all the hoses, and the potential difference here could be quite large.  The edge case is total failure of the messenger neutral at some point.  The power system will continue to operate as the return currents will make their way through the ground system (including water pipes), but the potential difference between houses on either side of the fault could be hundreds or even thousands of volts.

Finally, if a transient event occurs, such as a lighting hit, or a fault such as a line touching another line (wind, car crash into pole) then there can be thousands of volts transiently across the messenger neutral.  This normally isn't an issue because nothing is connected "along" the messenger to be hit by that voltage potential, but if one has a audio rig plugged into two homes a distance apart, then these hundreds or thousands of volts could transiently appear across audio equipment, with bad effect.  Or it could appear across someone holding a microphone.

So, in summary, MGN brings a whole class of dangers associated with violating individual equipotenital zones, which could lead to damage and injury.  Just don't do it.

(Note to electrically savvy people - I have chosen to misuse the term "high voltage")
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Frank DeWitt on July 31, 2015, 09:21:02 am
(Late response to a old thread)
 So a transformer here in New Zealand, typical of the rest of the world, has two high voltage terminals on it.  A USA transformer has one.

So in the USA, neutral is distributed via a wire.  And... the neutral to your home is also connected to that same neutral wire, so your home neutral is connected to the high voltage distribution network.  And that neutral distribution network is deliberately connected as far and as wide as possible.  It is also connected to the ground through plates and rods, at a substation, at the pole where the transformer is, and at your home.  The idea is that it is one big equipotential zone.

(Note to electrically savvy people - I have chosen to misuse the term "high voltage")

One of the wonderful things about the USA is that nothing is everywhere.  Or as someone said, Standards are a wonderful thing.  Everyone can have there own.

The transformer in front of my house has two wires connected to the input.  (Two large isolated posts on top)

Each post is connected to one of the two wires going from pole to pole

I don't know how common MGN is but it isn't used in my neighborhood.
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: David Buckley on July 31, 2015, 06:41:13 pm
One of the wonderful things about the USA is that nothing is everywhere.  Or as someone said, Standards are a wonderful thing.  Everyone can have there own.
USA's a big place, so yeah, I'm sure there's a bit of everything there.

The transformer in front of my house has two wires connected to the input.  (Two large isolated posts on top)
Th wires between the poles, are the insulators for the wires the same, or is one smaller than the other(s)?  Is there another wire going between poles lower down?
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Stephen Swaffer on July 31, 2015, 11:30:44 pm
I'm curious.  With no common return neutral-the main purpose of which is to trip protective devices-what happens when one of the phase wires shorts to ground-or to a metal object like a power pole or fence?
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: Frank DeWitt on August 01, 2015, 09:24:31 am
USA's a big place, so yeah, I'm sure there's a bit of everything there.
Th wires between the poles, are the insulators for the wires the same, or is one smaller than the other(s)?  Is there another wire going between poles lower down?

Both wires use the same size and number isolators.  One runs on top, the second one is about 2 ft down,  about 8 ft below that is a telephone cable.  No others wires for about a mile where the wires on my street tie into others.  All installed in the 80s
Title: Re: Pulling power for ONE PA, from two different sources?
Post by: David Buckley on August 01, 2015, 09:25:56 am
I'm curious.  With no common return neutral-the main purpose of which is to trip protective devices-what happens when one of the phase wires shorts to ground-or to a metal object like a power pole or fence?

I take it this is in relation to high voltage wires?  What happens is breakers open.  There is a return path through a plate under the substation, and the breakers are operated by gizmos called "protection relays" which are much more sophisticated than a simple circuit breaker.  Where pure three wire (no neutral) distribution is used with a "wye/wye" transformer, the return path is often artificial, created by a small zigzag transformer (https://en.wikipedia.org/wiki/Zigzag_transformer) to generate a neutral especially for fault detection, and there is often a resistance in the earth line to limit fault current.

The cute and really danagerous bit is that faults do occur on high voltage lines, and particularly with overhead lines, they can be for non-bad reasons, such as wind blowing the lines too close together, or a bird getting its wings between the wires or landing on a transformer, or indeed lightning hitting the lines.  These are transitory, annoying faults.  But they're a fault, and they cause the line to drop out of service.  So, there's a thing called a recloser.  A few seconds after the breakers open, the recloser commands them to shut again, and nine times out of ten, power is restored without incident.  The tenth time the fault is permanent, such as a real short or lines on the ground, perhaps from a vehicle knocking over a power pole.

Once you know about this its quite scary.  A few years back on the TV news there was a story about a hot air balloon that flew into overhead transmission lines.  Big flash, balloon and basket sits there trapped in wires.  I knew exactly what was going to happen a few seconds later, but morbid curiosity prevented me from not watching, as the reclosers did what reclosers do, the arcing destroyed whatever was holding the basket up, and the basket and occupants fell to their death.