# My new POCO vs. my old POCO



## guest (Feb 21, 2009)

Those of you who have been here a while (or new guys who do a search) know that I had no love for my old POCO in Southern California....between aged equipment and crews who don't give a damn about customer service they had a lot of problems. The latest one was the 36-hour outage right before I moved. Prior to that was the burned crossarm incident that had an 8-hour or so outage. 

By contrast, the POCO in my new home's region (Idaho Power) shows how it is supposed to be done. 

We had an outage a few days ago...system reclosed once then locked out. On my way to town on the "old road" (our power's ROW is there) found a phase laying on the burned crossarm. I had already called in the outage, but called again to let them know where this damage was at. They thanked me for the info and relayed the info to the crew. 

A mere few minutes after that call I saw a crew heading toward me, I flagged them down and relayed the info to them...they thanked me as well. 

I ran into town to run my errands, by the time I got done (about an hour) I was heading back up the road and saw the crew wrapping up the repair. :thumbup:

Spent some time chatting with them as the reclosed the line and got the power back up. :thumbup:

Total outage time? Under 3 hours start to finish. 

Cause: Failed new-school "plastic" insulator that tracked badly during the rain that day. (like one of the others on the same crossarm, to be fair it was most likely damaged by shotgun shot before it tracked. I have the third, undamaged insulator from that same arm.) They replaced the plastic ones with porcelain ones. 

Not bad for a "rural" POCO. 

What made the difference? Well:



Dispatchers who knew what an electrical power distribution system is, not some dumb betty who barely knows what electricity is;
Repair crews that actually carry such basics as spare crossarms and insulators ON THIER TRUCK, not crews who have to run back to the yard for every little thing;
Crews that assess the damage, have a quick tailboard and set to working on the repair, not crews who BS for an hour and finally lope into getting the repair done;
And an unrelated factor, but they also use wye distribution with a hard wire neutral and grounds on virtually every pole, not the ungrounded delta that SCE had such a hard-on for.
Bonus: my electric rates are cheaper here too.


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## That_Dude (Feb 13, 2012)

Rocky Mountain Power is the same way. Someone knocked a transformer out with a snow plow, called them up and a new one was installed in 4 hours. :thumbsup:


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## Bkessler (Feb 14, 2007)

I've lived in SoCal for 13 years in Fullerton and Anaheim with SoCal Edison as my utility and
Besides high rates I've never really lost power. Maybe once a year for 5 minutes. And I had my old a service drop replaced to a triplex with one phone call. Also most of the guys I see in the field seem to be alright. Now Detroit Edison crews are pos.


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## retiredsparktech (Mar 8, 2011)

Bkessler said:


> I've lived in SoCal for 13 years in Fullerton and Anaheim with SoCal Edison as my utility and
> Besides high rates I've never really lost power. Maybe once a year for 5 minutes. And I had my old a service drop replaced to a triplex with one phone call. Also most of the guys I see in the field seem to be alright. Now Detroit Edison crews are pos.


 Doesn't Detroit Edison furnish a bullet-proof vest, as part of their crew's PPE.


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## guest (Feb 21, 2009)

That_Dude said:


> Rocky Mountain Power is the same way. Someone knocked a transformer out with a snow plow, called them up and a new one was installed in 4 hours. :thumbsup:


Yeah I heard from others around here that they are really efficient too. (I am just outside of RMP's service area.)



Bkessler said:


> I've lived in SoCal for 13 years in Fullerton and Anaheim with SoCal Edison as my utility and
> Besides high rates I've never really lost power. Maybe once a year for 5 minutes. And I had my old a service drop replaced to a triplex with one phone call. Also most of the guys I see in the field seem to be alright. Now Detroit Edison crews are pos.


In Hawthorne I went over ten years without any issues, then suddenly all hell broke loose with several outages in just a few years, with the two major ones I mentioned only months apart. 

A fair number of the crews I spoke with were ok, but they definitely were not in any way motivated to work with even a slight sense of urgency. It may have been a regional thing within SCE, the Southbay crews may be of a different mindset than the crews in Orange County/Inland LA county.


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## TransientCW (Oct 26, 2012)

like a hawk, ive been keeping my eye out for apprentice opportunities with both idaho power and pacificorp.......i would LOVE to get onboard with either company =)


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## MTW (Aug 28, 2013)




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## guest (Feb 21, 2009)

MTW said:


>


What happened to the guy who posted this?:



> 12-20-2013, 02:03 PM #*50* MTW
> Senior Member
> 
> Join Date: Aug 2013
> ...


On this thread: http://www.electriciantalk.com/f2/oops-sce-did-again-63430/index3/


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## MTW (Aug 28, 2013)

mxslick said:


> What happened to the guy who posted this?:
> 
> On this thread: http://www.electriciantalk.com/f2/oops-sce-did-again-63430/index3/


He's still around. :whistling2:


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## guest (Feb 21, 2009)

MTW said:


> He's still around. :whistling2:


And ready to channel his own inner drama queen here:

http://www.electriciantalk.com/f2/national-grid-67005/

Seems like his POCO has some serious issues too. :whistling2::thumbup:


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## chicken steve (Mar 22, 2011)

In Vermont we've had a major acquisition by GMP (Green Mountain Power) that saw our major poco CVPS (Central Vermont Public Service) vanish

Inasmuch as it's the same 'ol REP era infrastructure, the dif in _customer service_ has been phenomenal

That's kinda nice if you've gotta deal with 'em day in/out:thumbsup:

~CS~


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## LARMGUY (Aug 22, 2010)

mxslick said:


> Those of you who have been here a while (or new guys who do a search) know that I had no love for my old POCO in Southern California....between aged equipment and crews who don't give a damn about customer service they had a lot of problems. The latest one was the 36-hour outage right before I moved. Prior to that was the burned crossarm incident that had an 8-hour or so outage.
> 
> By contrast, the POCO in my new home's region (Idaho Power) shows how it is supposed to be done.
> 
> ...


 Sounds like a match made truly in Heaven.


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## Meadow (Jan 14, 2011)

mxslick said:


> Those of you who have been here a while (or new guys who do a search) know that I had no love for my old POCO in Southern California....between aged equipment and crews who don't give a damn about customer service they had a lot of problems. The latest one was the 36-hour outage right before I moved. Prior to that was the burned crossarm incident that had an 8-hour or so outage.
> 
> By contrast, the POCO in my new home's region (Idaho Power) shows how it is supposed to be done.
> 
> ...


:thumbup:

You met a worthy poco! Its nice to see them listening to you and respecting you being a customer. :yes:

Tell them unless your using tree wire or spacer cable epoxy insulators are over rated. Never, ever use rubber tie downs on anything, they break like no tomorrow. Vise tops aren't the best for bare wire since the wire tends to slide through it when a pole or tree comes down. Good ole porcelain pole with steal tie downs still win hands down.


By the sounds of it they know protective relaying well. The cross arm was actually in one piece when the recloser locked out:laughing::laughing: I have seen conductors slide out of con ed insulators with the phase arcing for some time until it burns through the cross arm. Falls through hitting the neutral; line recloser will make 4 hard auto reclose attempts in that condition. Lights take their time staying dim, I swear they mustve been trying to clear the fault via buss bar burn through. 

In your case since your line is rural so don't worry about one auto reclose attempt, that's usually sufficient. A lot of distribution engineers fail to realize 4 attempt options are provided more for bare wire construction coordinating with trip count sectionalizers or fuse saving schemes. 



"And an unrelated factor, but they also use wye distribution with a hard wire neutral and grounds on virtually every pole, not the ungrounded delta that SCE had such a hard-on for."

I thought the 4.6kv system serving your apartment was an isolated neutral wye? At least it looked like that. Unless you referring to the 3 wire 16kv circuits. As for the SCEs phase to phase hard all California utilities are required to follow PUC 95 which has rules against using the grounding system as a neutral similar to the NEC rules. As a result California utilities are required to connect transformers phase to phase or to treat the neutral like a live phase keeping it isolated on insulators while keeping neutral ground bonds at a minimum after the supply substation. Sounds crazy but its something to be thankful for. Im not a fan of Multi grounded neutral distribution for reason I don't want to get into. 



"I have the third, undamaged insulator from that same arm.) They replaced the plastic ones with porcelain ones." Pics or it didn't happen:whistling2::jester: 



Best of luck MX. Sorry about the delay but was busy.


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## guest (Feb 21, 2009)

meadow said:


> :thumbup:
> 
> You met a worthy poco! Its nice to see them listening to you and respecting you being a customer. :yes:
> 
> ...


Thanks for the response, lots of good info there. :thumbup:

You're not mistaken, my old apt. was 4.6 wye. I was referring to the 16kv and 34.5 they use elsewhere. 

Can you elaborate on why the MGN is something you're not fond of? I do know that SWER (Single Wire Earth Return for those who have no idear what we're babbling about) was in use here for may years, but has been converted into a MGN system as evidenced by the construction visible. As for SWER, that is something I always thought was a bad idea. 

And I didn't take any pics of the damage or repair, I had a full schedule that day and probably shouldn't've wasted the time I did talking to the crews. I can get a pic of the insulator they gave me though, and if I can get back down the road I'll grab a pic of the new crossarm.


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## cdnelectrician (Mar 14, 2008)

I would also be interested to hear why an MGN system would not be preferred? SWER does seem like a bit of a dangerous idea lol.


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## MTW (Aug 28, 2013)

If I'm not mistaken, I think meadow is in favor of an isolated neutral rather than an MGN system.


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## MTW (Aug 28, 2013)

meadow said:


> I thought the 4.6kv system serving your apartment was an isolated neutral wye? At least it looked like that. Unless you referring to the 3 wire 16kv circuits. As for the SCEs phase to phase hard all California utilities are required to follow PUC 95 which has rules against using the grounding system as a neutral similar to the NEC rules. As a result California utilities are required to connect transformers phase to phase or to treat the neutral like a live phase keeping it isolated on insulators while keeping neutral ground bonds at a minimum after the supply substation. Sounds crazy but its something to be thankful for. Im not a fan of Multi grounded neutral distribution for reason I don't want to get into.


Is PUC 95 unique to California? I noticed the neutral conductors on insulators and always wondered why that was the case in my travels there.


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## guest (Feb 21, 2009)

MTW said:


> Is PUC 95 unique to California? I noticed the neutral conductors on insulators and always wondered why that was the case in my travels there.


I think they follow PUC 95 here as the MGN's are on insulators.


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## Meadow (Jan 14, 2011)

mxslick said:


> Thanks for the response, lots of good info there.
> 
> You're not mistaken, my old apt. was 4.6 wye. I was referring to the 16kv and 34.5 they use elsewhere.
> 
> ...



I meant the insulators, not the repair. My bad. 

I will try to keep it simple it, but any way, to the MGN aka TN-C earthing (combined neutral and ground). Its a step above SWER, but still has major, major issues. An MGN is no different then wiring a building without neutrals but with an egc system and using that as the neutral conductor as well as the EGC. Basically every load with a hot and the conduit used as both as a ground and neutral; or a hot and a bare wire at every load type deal. Bare wire goes to the N and jumps over to the frame as well. Will it work to some degree? yes. Will it save money? yes Safe? Not so much. 



Major problems exist with MGNs or using a buildings ground wires for as a neutral. 

1. Current will dived between the neutral and every metal object its grounded to more than once. Current will divide between the MGN and water pipes, telephone cable/cox shields, ground rods, gas lines, building steal and foundations, anywhere where a conductive path exits. This creates both EMFs (primarily magnetic) and stray voltage gradients which for some pocos are a major legal problem, which I will get to. Harmonic and none linear loads make the problem far worse since they drastically increase neutral current as well as unbalanced feeders from load growth. Years ago stray voltage was usually not a concern for pocos but its now becoming a huge and noticeable issue as the above increases as well as equipment ages. 




2. A broken neutral will go unnoticed until it sets something on fire or stray voltage problems arise. When an MGN breaks, current will just be pushed over what it can find which is ground rods and metal piping. Even though an MGN is close to earth potential (thanks to its repeated grounding) its actually at thousands of volts potential between the loads and the source transformer. Those thousands of volts show up in some way or another. If a neutral breaks where it has poor parallel paths such as a neighborhood with plastic mains and no gas lines it gets noticed fast . Thousands of volts will heat up ground rods quick as well as producing lethal step touch potentials. No device will notice this or stop it. It has happened before where less parallel paths were available. Even then broken neutrals show up as people getting shocked on pluming and swimming pools, but those cases have also been noted on systems where the neutral is under sized/overloaded. Theoretically, a person could loose the mgn at a pole or pad mount, and their water bond will take the whole thing over unnoticed. What happens to the person who has to lift it or there isn't on?

3. Downed conductors often go unnoticed in MGN systems. Line reclosers and substation feeder breakers have to be set with a high ground trip/ high phase differential pick up values (think GFCI or RCD protection in LV systems) to allow for normal neutral loads. Often the values are 1/3 to 2/3 of the phase pickup value but values as high as the phase setting are not uncommon. Pick up values of 250 to 800amps where the phases are set at 1000amps is fine for detecting bolted faults such as a phase crashing into the neutral, but faults on soil, cross wooden/ungrounded cross arms, concrete, tree branches and the like frequently go unnoticed. Not all downed conductor or tree contact will generate enough current to reach these values, protective devices just see it as a heavy single phase to neutral load. As the conductor burns usually the current will go down even further from soil drying out. I have seen countless times including up my street during storms where a phase will fall into the road arcing into the asphalt until a line crew shows up to trip the recloser. Last time it took over an hour for them to get there. I think we all know why that's dangerous. However, where all loads are connected phase to phase or an isolated neutral that can be ct, values can be set only to a few amps. Conditions that generate less than a few amps are extremely rare, so nearly all downed conductor contingencies including wires across a sidewalk are cleared, as well as minimal carnage from what would normally be sustained undetected arcing in an MGN. Of note, Recloser and breaker makers are now offering "sensitive earth fault logic" for protective microprocessor relays which is nothing more than highly sophisticated AFCI protection. Most high impedance downed conductor conditions will show an arcing signature. In an mgn system the arcing signature can help in detecting downed conductor conditions that fall out of the normally set current pick up values. However, its expensive, and 2 its not always perfect as the good ole low RCD method of clearing high impedance faults. I know what your thinking MXSlick, but its light years ahead of standard AFCIs. Its done with high bit oscilliography analysis that's then run through a slew of software equations. Some of them can even guestimate how far out on the line the fault is. And yup, as a result no nuisance tripping :yes: And yes, you know why the damn thing tripped in the first place:thumbup::laughing:

4. Legal issues. Because MGNs produce 60hz voltage gradients across the earth its getting noticed. POCOs are being flooded with lawsuits from dairy farmers and property owners over step voltage potentials. As loads increase and equipment ages more and more people are noticing it. Google would better describe the issue but countless dairy farmers have gone out of business because of it. A few volts may not get noticed by humans, but a 4 legged animal will feel it. Cows not drinking from the trough is one reason since the animal will feel it and associate the water trough as discomfort. Mike Holt actually some good videos on the subject. Property owners are also suing where its really bad because swimming pools and pipes are shocking. Up in Minnesota a utility actually had to change one of their rural systems over to a delta because of so many dairy farmers suing. Others all over the US are installing ronk blockers to mitigate the problem. Trust me some pocos are regretting the MGN system.


Another legal issue for pocos has been people getting killed in pools and lakes from deteriorated bare concentric neutral cables. Years ago when pocos started putting residential sub divisions under ground they had a thing for bare concentric neutral cables. EPR or XLPE with a bare concentric aluminum braid, its no different from the one around SEU cable, just it has no covering. Kind of like sticking SEU in the ground with no jacket. As these cables deteriorate, the HV neutral current ends up going through water pipes and the earth. As a result dangerous voltage gradients are occurring across the earth. Energized metal water pipes too as being noted. Such issues however in California are rare, and Dairy farmers suing pocos is unheard of. I wonder why? 

5. EMFs. MGN systems can and do give rise to major magnetic fields. Both power lines have higher EMFs as well as anywhere the neutral current is flowing. Power lines give off higher magnetic fields since current cant cancel. 50 amps flowing on a phase ends up as about 25 amps o the neutral and 25 amps over what else. The 25 amps that don't cancel show as excessive EMFs from the line as well as the metal water piping in the street, building steal or anywhere else that current is flowing. Believe it or not magnetic fields in residential neighborhoods are often much higher than main roads holding trunk lines since ground current wont cancel. 3 phase main lines hold predominantly 3 phase banks, and where single phase banks are present what phase they are connected to rotates down the line. Pole 1 has a 50kva pig pulling 10 amps on a phase, pole a few yards down with a pig of 11amp on b, pig pulling 12 on c phase again a few yards down. Net neutral current is about 1.7 amps up the line with 11 amps only between the pigs. However on a single phase lateral, current will add, 10+11+12 to 33 amps plus all the other pigs on the line. A single phase trunk can easily see 100 amps or more. Often over half that amount is being pushed into the earth via ground rods and water bonds. Add to the fact the HV neutral also doubles as the LV neutral and even more current winds up where it shouldn't. 

Yes its debatable what EMFs actually do, but considering the US has some of the highest cancer rates in the world (among other conditions) where this system is by far the most prevalent in use, shouldn't be ruled out as a potential contributing factor that needs real investigation. Even if it is proven magnetic fields are a potential risk (some already have) pocos will never admit their systems are an issue since overnight they would be opened up to potential lawsuits. Just trust me on the fact when I say its a good thing California has crazy laws in place.


MGN systems are extremely rare outside the US and Canada. 3 phase 3 wire is the norm through out the world. In some areas MGNs are even forbidden on the LV systems. 


Long info rant but I think you get the picture. Anyway, MX I wouldn't worry about your distribution system. Being a rural line I doubt its caring much load as is, and being a top poco I doubt its the system is in a neglected state. 



MTW said:


> If I'm not mistaken, I think meadow is in favor of an isolated neutral rather than an MGN system.


Yup. Or just connecting loads phase to phase, which ever gets the job done. Phase to phase tends to (but not all the time) be the preferred choice since it often comes out cheaper to buy a fully insulated transformer then to run the neutral, plus there is no concern of neutral shift or the infamous open neutral.




MTW said:


> Is PUC 95 unique to California? I noticed the neutral conductors on insulators and always wondered why that was the case in my travels there.


Puc is unique to California. California utilities will connect transformers phase to phase or to an isolated neutral. Its treated as a phase and in most case only grounded down at the substation. And when it opens up it gets noticed:thumbup: 






mxslick said:


> I think they follow PUC 95 here as the MGN's are on insulators.



I doubt it. If its an MGN its not following PUC:tt2: If the neutral has any connections to ground rods or jumpers over to the LV neutral its not an isolated neutral even if on insulators. Some pocos do actually put the MGNs on insulators (usually 600v rated spools) but not with the intention to electrically insulate but rather to prevent aluminum wiring from touching dissimilar metals.


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## Meadow (Jan 14, 2011)

What no response? :laughing: MX thinks Im nuts now:laughing::jester:


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## socalelect (Nov 14, 2011)

meadow said:


> What no response? :laughing: MX thinks Im nuts now:laughing::jester:


He's probably still reading the book u posted :thumbsup:


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## guest (Feb 21, 2009)

meadow said:


> What no response? :laughing: MX thinks Im nuts now:laughing::jester:


LOL you are nuts..but I have been busy with my other threads and dealing with the other loose nuts on here. 

Great read though and some solid ideas behind it. :thumbup:



socalelect said:


> He's probably still reading the book u posted :thumbsup:


May have been long but it made some really good points. :thumbsup:


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## socalelect (Nov 14, 2011)

mxslick said:


> LOL you are nuts..but I have been busy with my other threads and dealing with the other loose nuts on here.
> 
> Great read though and some solid ideas behind it. :thumbup:
> 
> May have been long but it made some really good points. :thumbsup:


Indeed on the good points


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## Meadow (Jan 14, 2011)

mxslick said:


> LOL you are nuts..but I have been busy with my other threads and dealing with the other loose nuts on here.
> 
> Great read though and some solid ideas behind it. :thumbup:
> 
> ...


:laughing: Its a fine line between knowledge and insanity:jester::laughing:

You will like this its an IEEE paper on MGNs. The guy acts like hes the first to think of it, but "Zipse law" is nothing new. International engineers have made similar laws long before hand. 


http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CCsQFjAB&url=http%3A%2F%2Fwww.ci.punta-gorda.fl.us%2FAgendaPublic%2FAttachmentViewer.aspx%3FAttachmentID%3D8905%26ItemID%3D6358&ei=s80sU5vOCeGqyAHDvYHwAg&usg=AFQjCNEymZrpKDxnE0yQvEat5Uerzdmg_Q&bvm=bv.62922401,d.aWc


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## Meadow (Jan 14, 2011)

socalelect said:


> Indeed on the good points


Almost, I was half asleep when I wrote that :jester:


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## Big John (May 23, 2010)

Interesting read, _meadow. _

I actually never really stopped to consider why they used MGNs. I know when we see rural residential distribution, they often send it out as two phases to help with voltage drop. There's no need to carry the neutral out there because of the spacing between houses so they just relay on phase-to-phase protection.


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## Meadow (Jan 14, 2011)

Big John said:


> Interesting read, _meadow. _
> 
> I actually never really stopped to consider why they used MGNs. I know when we see rural residential distribution, they often send it out as two phases to help with voltage drop. There's no need to carry the neutral out there because of the spacing between houses so they just relay on phase-to-phase protection.


Depends, if the voltage is already at the low end and no pick often voltage drop will force 2 phases to compensate for the long run. But when pocos have a choice 1 phase and an MGN is cheaper over 2phases. In super long runs SWER becomes the choice. Most sub transmission is 3 wire even for MGN pocos since the cost of a delta primary load side transformer is only slight compared to bringing the neutral. Sub transmission lines rarely have any pigs on them too.

In most urban settings running single phase lines from 3 phase trunks are status quo. Cheap way to serve resi neighborhoods along the commercial customers on the trunk.

Anyway, why an MGN is preferred cost wise:

Allows for single bushing pigs on both single and 3 phase banks. Economy scored for both customer types. 

Single phase banks require only 1 cutout, and 1 arrestor. 

Single phase lines only need 1 phase, insulator and arrestor. For most pocos the LV cable buss* neutral also doubles as the MV neutral, so 1 conductor is totally eliminated for the HV return (2nd phase on insulator eliminated)

Grounded wye primaries reduce ferroresonance risk. Single phase switching an unloaded 3 phase bank can cause this. Those who own delta primary trannys will actually load 3 phase pad mounts/pig banks when closing 1 cutout at a time to prevent it. Although with 15kv and below its far less of a concern.

Higher distribution capacity per cost. A poco that buys a major stock of 14400 volt 2 bushing pigs can either build and run their lines at 14.4kv or run their lines at 14.4/24.9 grY. If both have the same wire gauge and current capacity (800 amp rated reclosers and disconnects) the 25kv line can deliver about double the power (35MVA) over 20MVA @ 14.4kv, be run a greater distance or the phase size literally halved. Where long distances are involved or where an LV MGN will exist regardless, the cost savings are worth the 25kv insulators and suppressors. 

* LV "cable buss" is often strung on the lines in urban areas where pocos will feed it from a bank with customer service drops tapping from it. The messenger is used as both the LV and MV MGN:no::

http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet43

http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet38


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## socalelect (Nov 14, 2011)

Slick please define. 

Mgn
Swer
Pigs 
Ferroresonance
Thanx


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## MTW (Aug 28, 2013)

socalelect said:


> Slick please define.
> 
> Mgn
> Swer
> ...


MGN = multi grounded neutral. This is the neutral conductor system that is common throughout utility power systems in North America. It is neutral that is between the high voltage and low voltage systems and as the name implies, is connected to earth at regular intervals as prescribed by the NESC. I believe it's at least 4 times per mile. 

SWER = Single wire, earth return. Used primarily in extremely rural settings to cut down on line construction costs. It uses a high voltage primary wire only and the earth as the return path.


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## Meadow (Jan 14, 2011)

This isn't mine but helps explain why MGNs produce high magnetic fields. Its written at the 600volt and below level but the same concept applies to MV and HV as well. Lots of great points that also apply to utilities. 

http://www.mikeholt.com/technical.p...ed/EMIKarl&type=u&title=Power Quality Article

Quote excerpt: 

"Separation due to wiring error: How does modern wiring produce elevated magnetic fields? It will not if electricians follow the Code requirement that all conductors of a circuit be run together in the same cable, raceway, conduit, cord etc. This includes neutrals and equipment grounding conductors. (NEC 300-3(b)). The second requirement is that no neutral be connected to grounding paths on the load side of the service disconnect. (NEC 250-24(a)(5). Let’s take this situation first. 
Suppose an electrician has bonded the neutral bus in a subpanel to the box, or simply neglected to unscrew the bonding screw supplied with the box in case it was to be used as a service entrance box. Neutral current on the bus now has two or more paths to get back to the service point: through the neutral of the feed, through the grounding conductor of the feed, through any conduits connecting to the box as well as any metallic pipes, vents, building steel touching the conduits or the box, and to individual equipment grounding conductors of circuits which happen to serve appliances using water pipes, such as washers or sink disposal units.
Now every single path is carrying current with its magnetic field. And the panel feed is generating the strongest magnetic field because of the unbalance caused by missing neutral which is traveling in the other paths. When part of the current flow in a cable is missing, we refer to this as “net current”. The net current, which can be either missing current or excess current, acts the same way as a single conductor running alone and carrying that amount of current. For instance, if the hot of a feed is carrying 30 amps and the neutral is carrying only 20 amps due to the rest going on grounding paths, the net current is 10 amps. This 10 amps is usually flowing on a pipe or building steel, or it may have split up and is flowing on all of the above plus conduits. We use the term “net current” to refer to these currents also. 
10 amps will generate a magnetic field that may cause computer monitors to jitter within 12 feet of the circuit all along its run. An instrument like an electron microscope would be affected if within 60 feet of that circuit!




Relation of net current to magnetic field strength: The magnetic field at 1 meter (39”) from a net current source is twice the current. Four amps net current produces 8 mG at 1 meter. If you can’t directly measure current, such as in a building column, put your gaussmeter one meter away and divide by two to get the net current. If you want to know in feet, multiply the net current by 6.56 to get the magnetic field at one foot. At 5 feet it is 6.56 times the net current divided by 5."


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## guest (Feb 21, 2009)

socalelect said:


> Slick please define.
> 
> Mgn
> Swer
> ...





MTW said:


> MGN = multi grounded neutral. This is the neutral conductor system that is common throughout utility power systems in North America. It is neutral that is between the high voltage and low voltage systems and as the name implies, is connected to earth at regular intervals as prescribed by the NESC. I believe it's at least 4 times per mile.
> 
> SWER = Single wire, earth return. Used primarily in extremely rural settings to cut down on line construction costs. It uses a high voltage primary wire only and the earth as the return path.



Pete got the first two for ya.:thumbup:

Pigs: Slang term for pole-mount transformers. 

Ferroresonance: A tricky phenomenon to explain, but in a simplified nutshell:

Certain combinations of transformer connections, line capacitance and loads (Or lack thereof) will set up a circuit that will act like an oscillator at the 60hz powerline frequency. The result can be a situation where both currents and voltages will rise (or "runaway") to very dangerous levels, which can result in arc flash, equipment damage or explosions. 

This occurs mainly during single-phase switching of a three-phase transformer setup. It can be prevented or minimized by a few things:


Use only three-phase gang-operated air break switches (GOAB's);
Load down the transformers first (doesn't always work though according to some of the POCO guys I spoke with)
Install a cutout to intentionally ground one of the primarys on one transformer. That cutout is closed first, the other two transformer's primary cutouts are closed, then the grounding cutout is opened and finally the last primary cutout is closed. Very cumbersome and easy to make a mistake.
Or use wye connected primary only. :laughing:
I got to see an example of Ferroresonance (a luckily mild example) once a long time ago when I was in the Air Force. 



The Base Engineering distribution crew was energizing a new pad-mount transformer (I think it was around 150-250kvA). The first two cutouts closed in with barely a spark....the third one, as it was closed in drew an arc a few feet long, almost flashing over to the other phases. That padmount also made the most evil sound I'd ever heard any electrical equipment make. The fuse held though....so I asked wtf just happened. They explained the ferroresonance concept to me. :laughing:


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## MTW (Aug 28, 2013)

Thanks to meadow and the drama queen for the info. :thumbup:


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