 cdruGo ColtsPremium,MVM
join:2003-05-14
Fort Wayne, IN
kudos:7
1 edit | reply to fcisler
Re: Myth of the Neutral Wire I'm no EE, but I believe that's because the PEAK voltage in AC is Vrms*√2 volts. So that 208/240Vrms AC is really 294/339 Vpeak. It all just gets passed on to the output transformer as either a constant 330V DC or full-wave rectified 0-294/339Vpeak AC voltage. |
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 SparkChaserPremium
join:2000-06-06
Downingtown, PA
kudos:3
 ·Verizon FiOS
| said by cdru:I'm no EE, but I believe that's because the PEAK voltage in AC is Vrms*&sqrt2; volts. So that 208/240Vrms AC is really 294/339 Vpeak. It all just gets passed on to the output transformer as either a constant 330V DC or full-wave rectified 0-294/339Vpeak AC voltage. Passed to the power supply, not transformer. If you hook DC to a transformer you get to see lots of amps 
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cdruGo ColtsPremium,MVM
join:2003-05-14
Fort Wayne, IN
kudos:7 | said by SparkChaser:Passed to the power supply, not transformer. If you hook DC to a transformer you get to see lots of amps
Well I was close at least. |
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 leiboldPremium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:6
 ·SONIC.NET
| With a switch mode power supply using DC instead of AC as input is normally no problem since the AC is rectified as soon as it enters the power supply anyway.
There is a caveat however: with AC power each of the 4 rectifier diodes are used 1/2 of the time (one half of the sine wave) while with DC power two of the diodes remain completely unused and the other two diodes experience continuous current flow. While not very likely, it does allow for the possibility that those 2 diodes may get damaged.
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lutful... of ideasPremium
join:2005-06-16
Ottawa, ON Reviews:
·TekSavvy DSL
| reply to MatrixHDV
said by MatrixHDV:said by lutful:Post a reference and we will see who sings in that chorus and why. As for the reason it is quite simple an MGN system can present dangerous stray voltages and hazardous step voltages due to the nature of the way the MGN's operate. You can also find quite a bit of information on the subject on Mike Holts web page. I looked up just two threads. You should read them more thoroughly. » forums.mikeholt.com/showthread.p···4691c7d3» forums.mikeholt.com/showthread.p···4691c7d3Curiously I was born in a country where the "rural electrification board" used a single conductor strung on bamboo poles. There are many different power distribution methods but just one way to save lives ... a solid ground right at the home.  |
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3 edits | Not at all relative to this discussion as that is an example of non-MGN distribution, my guess is you cannot fathom the operation of nor the pitfalls of a 4 wire MGN system.
I'll give you a clue, it does not involve dual primary bushings and a phase to phase primary connection.
BTW the links you posted are of the same thread, not "two" threads. |
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lutful... of ideasPremium
join:2005-06-16
Ottawa, ON | You will be able to fathom my experience if I gave you some clues. But I won't ... because you made it personal.  |
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| Try this....
This is diagram of a typical four wire 3 phase distribution system.
Note we leave the substation with 4 wires (3 phases and an MGN) which at some point will drop to a single phase customer, at this customer we are delivering 240/120 service.
Do note the current flow from the primary drop through the (very common) single bushing transformer and then to the MGN and pole ground, so far all is well…
Then as the years pass and the corrosion and a lightning hits take their toll, the transformer can to the MGN conductor fails, you want to guess where the primary voltage is now finding a path back to the center tap at the substation?
The correct answer is “the customers ground system” and here’s the cool part, the primary current is quite often so low (10Kw at 1.3 amps) it goes unnoticed as the lights stay on.
That is unless the customer has a ground that has already failed out, a condition that very common in older residential construction and or flipped homes and can go un-noticed for years.
Where do you think the primary voltage is going to go now?.
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leiboldPremium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:6 Reviews:
·SONIC.NET
| I wouldn't want to be that single phase customer in your 3rd diagram 
I have seen utility poles (most distribution transformers here are pole mounted) where the ground wire was damaged but didn't fully realize all the consequences this may have.
Luckily there are usually multiple customers attached on a single distribution transformer and therefore there is more than one point where the neutral is (or should be) connected to a grounding electrode.
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garys_2kPremium
join:2004-05-07
Farmington, MI Reviews:
 ·callwithus
·Callcentric
| reply to MatrixHDV
The issue is the "Tank to neutral bushing bond." That makes the PoCo's MGN work a bit better (but if they maintained their own multi-grounds any such improvement would be vanishingly small) but can impose the risk of electrocution and fire on the poor homeowner served by that transformer. I know the power companies would piss and moan, but cutting that bushing bond and having one ground for the low voltage side and physically seperate grounds for the medium voltage side would keep the public safer. |
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1 edit | reply to leibold
said by leibold:I wouldn't want to be that single phase customer in your 3rd diagram And it all depends on the integrity of this single jumper.
And think about what can happen when an electrician is dispatched to that customer who is complaining about getting “tingled” (think step voltage) when he touches his outdoor main breaker box, an electrician who is probably not going to be gloved up for15Kv and who’s first step is not to go up in a bucket to make an up close examination of the transformer wiring, an electrician who somewhere in his TS process lifts that remaining grounding conductor from the grounding electrode…
Now he may AmpProbe the grounding conductor, but if the transformer is lightly loaded he may not even get a noticeable reading, however once opened a lethal potential becomes present.
About the only thing that might tip him off would be if he pounds in a ground rod a distance from the buildings grounding system and does a voltage measurement between the two.
I have seen utility poles (most distribution transformers here are pole mounted) where the ground wire was damaged but didn't fully realize all the consequences this may have.
Luckily there are usually multiple customers attached on a single distribution transformer and therefore there is more than one point where the neutral is (or should be) connected to a grounding electrode.
The solutions range from simple to expensive, none of which utilities want to hear about, the most basic one would be to eliminate the tank to neutral bond, but doing so would require the utilities to beef up their grounding system as they could no longer rely upon their customers ground systems to back up their *ground system.
Next up in cost would be to utilize dual primary bushing transformers and feed them phase to phase but this complicates load balancing and requires a transformer change out as they would be going from a phase to neutral voltage to a phase to phase voltage.
And the most expensive solution is to uni-ground the neutral and run 5 wire as they do in parts of California.
* Note the NESC only requires 4 grounds per mile on MGN. |
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1 edit | reply to garys_2k
said by garys_2k:I know the power companies would piss and moan, but cutting that bushing bond and having one ground for the low voltage side and physically seperate grounds for the medium voltage side would keep the public safer. The utilities claim removing the neutral busing bond will make their transformers more prone to lighting damage which I personally believe is a lot of BS. |
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| reply to tschmidt
said by tschmidt:The National Electrical Safety Code (NESC) controls what the utilities do and ends with the meter socket. The National Electrical Code (NEC) controls building wiring. Minor correction Tom, the NESC ends at the hand-hole or aerial drop, from there the NEC takes over which is why one can find this.
And this.
Note the differences in wire gauges where the NESC sized drop connects to the NEC sized conductors feeding the meter(s). |
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 AVDRespice, Adspice, ProspicePremium
join:2003-02-06
Onion, NJ
kudos:1 | Makes sense if you think about it, anything close to a structure should have a higher safety factor.
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garys_2kPremium
join:2004-05-07
Farmington, MI Reviews:
·callwithus
·Callcentric
| reply to MatrixHDV
said by MatrixHDV:said by garys_2k:I know the power companies would piss and moan, but cutting that bushing bond and having one ground for the low voltage side and physically seperate grounds for the medium voltage side would keep the public safer. The utilities claim removing the neutral busing bond will make their transformers more prone to lighting damage which I personally believe is a lot of BS. Wow, that's so ridiculous it's laughable.
They won't remove the bonds because they know their grounds aren't maintained well, so they like to piggyback on the customers' grounds to help their system continue to function regardless. |
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| said by garys_2k:They won't remove the bonds because they know their grounds aren't maintained well, so they like to piggyback on the customers' grounds to help their system continue to function regardless. Mike Holt gets to the point about utilities and their T&D engineers.
»youtu.be/pAs_FmdxXhQ?t=55m22s |
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 DarkLogixTexan and ProudPremium
join:2008-10-23
Baytown, TX
kudos:3 | reply to MatrixHDV
How do we know the larger diameter wires aren't just alu and the smaller are cu?
Not saying it is the case but just that from the pic I don't see any way to be sure of the metal used for the wire, and alu needs a thicker (lower gauge) wire for the same amp rating. |
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 mattmagPremium,ExMod 2000-03
join:2000-04-09
NW Illinois
kudos:3 | reply to sk1939
Actually, the smaller overhead wires vs. the service entrance wires is more about the rated ampacity of "conductors in free-air" vs. contained within conduit.
For example, a Type 90*C overhead conductor in free-air sized at 1/0 in aluminum or copper-clad aluminum is rated for 205 amps, but within conduit the same rating requires a 4/0 cable.
Yes, there are variations, but that is the general idea. |
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1 edit | reply to DarkLogix
said by DarkLogix:How do we know the larger diameter wires aren't just alu and the smaller are cu? Those particular drops are what is known as tri-plex which consists of a bare neutral ACSR (Aluminum Clad Steel Reinforced) conductor wrapped with two standard aluminum hot conductors, the reason for using aluminum is its light weight compared to copper which places less of a strain on the building it is mounted to and the pole it feeds from.
As to why copper is commonly run from the weatherhead to the meter pan is something you would have to bounce off an electrician, my guess is they do it to avoid the additional work needed to prepare aluminum for termination.
As for the shot with two weatherheads, I can assure you the wiring from the weatherhead down was copper as I was in that meter room taping the damage done from an electrical fire the building owner claimed the utility was responsible for. |
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| Aluminum may be more reliable than copper in some situations. Those situations are those where copper thieves cut copper drops to sell for scrap. This partly depends on the thieves being able to distinguish. |
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