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Author Topic: How is the electrical circuit completed between the powerstation and my home?  (Read 22188 times)

Offline alancalverd

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There was an interesting incident a few years ago where the insulation of an underground  phase line was damaged. This set up a potential gradient to the nearby earthing point of the neutral line, such that the voltage drop across a few centimeters of fairly dry soil was too small for barefoot humans to detect, but across a couple of meters was enough to kill a horse. Which it did, twice, IIRC.
 

Offline wolfekeeper

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Interestingly the reason we have earths on the neutral is mostly for electrostatic reasons:

http://amasci.com/amateur/whygnd.html

If you don't have it, it will work fine nearly always, but occasionally you'll get huge sparks jumping out of equipment at you, because the L/N system (which is isolated) will have floated up to some high voltage relative to earth, and you're often connected to earth... zap!
« Last Edit: 03/09/2015 18:24:23 by wolfekeeper »
 

Offline syhprum

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American homes are normally supplied with single phase 220v with a grounded centre tap to supply low power devices with 110v.
What is the KVA rating of the local transformers and how many homes does it supply or does each home have its own transformer ? 
 

Offline jerrygg38

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I spent 5 years working for Con Edison in NY Distribution Engineering Dept. We had three phase "Y" systems for the low voltage end 120/208 volts. This had a common ground. We had 3 phase delta 13,200 volt systems which did not a common ground where current flowed. the lead cables were externally grounded to the ground for safety. When they worked on them they would spear them to make sure they were switched off. Sometimes we found private contractors trying to work them alive and we shut them down.
We had two phase systems where the power had 90 degree differences. For street lights we had an isolated series constant current circuit. Sadly during storms if the voltage at the break point would rise to 5000 volts and linemen picking up the normally safe wires would be killed. Thus the old constant current circuits can be dangerous. We also had DC circuits to homes (worked 1956-1961) slowly the homes were converted to AC and Con Ed would supply the people with new fans. We also provided substations for the DC subway lines. (About 500 volts DC- don't touch the third rail) Most country/ surburban areas have 120/240 volts in their local transformer to their house. In any event the advantage of the delta system is there is only three wires needed for the transmission but you must put in ground rods by the transformers for safety.
 

Offline jerrygg38

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Another interesting thing at con Edison was power to industrial welding companies. The spikes are brutal and they use series capacitors for the power line. Normally parallel capacitors help to keep spikes down but we had a least one company that needed the series capacitor circuit.
 

Offline evan_au

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Quote
how many homes does it supply or does each home have its own transformer?

I recently saw the following arrangement in Canada (which tends to follow US practice in many areas).
- The high-voltage wires are the 3 wires at the top of the pole.
- There is a single-phase transformer attached to each high-voltage wire, each producing single-phase "low" voltage.
- The 3 phases of low voltage are fed into a single large building (to the left), and also along the street to several houses (to the right).

I didn't look closely enough to see if there was a kVA rating stamped on the transformers.

In Australia, we tend to use three-phase transformers in suburban areas. Just as a "Delta" wiring system does not need an Earth (the currents in the three phases almost cancel), the magnetic flux in a three-phase transformer almost cancels, and you can use less iron overall than in three single-phase transformers.
 

Offline William McC

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No, Colin, that is EXACTLY what I was asking and your answer was spot on!

So, the "neutral" from my house to the substation is actually an Earthed line connected to one of the other two incoming phases (ie one of the other two points on the triangle as you put it)?
I had assumed you meant the first step down from generator to grid. You are meaning the consumer end.
The delta system is only used for transmission between the intermediate transformers. In the grid system. So in the UK the generators are outputting Star or Y configuration into the transformer to be converted to delta for long distance transmission (where the currents can be better balanced) and then back to star for local service. So your neutral goes back to the centre tap of the star/Y.
Your most likely earth arrangement, unless your property is very old, will be what is called Protective Multiple Earthing (coded TN-C-S) where the earth from your nearest transformer is carried on the neutral conductor to your home.
It can seem quite complicated, but I think we are there.

Some areas have the "high leg" other areas have balanced three phase. NYC still has balanced three phase, at least the last time I put a four channel scope to it at work, about a year ago. Some NYC buildings suffer from neutral to ground differentials as high as 24 volts though. That is reaching control voltage levels.

Out on the Island, Long Island some places still have balanced 208 other places have the 230 high leg.

When running 110 volt circuits you have to be careful which breaker (which leg) you hook up to, in the three phase box, that has a high leg, or you will get a surprise when you run your 110 volt equipment off that leg. I have seen voltages to ground from the high leg as high as 195 volts. In the panel there are usually two rows of breakers, the breakers run from top to bottom, alternating Line 1, Line 2, Line 3. If line 2 is the high leg usually marked by a piece of red electrical tape, sometimes blue tape, it will output high voltage to neutral and ground. Years ago different authorities claimed different color coding. Some authorities claimed black and red were standard color coding for single phase 220. Others claimed that blue and black were the standard color coding for 220 volt systems. It pays to check voltage to neutral and ground to make sure. I have worked on both color coded systems many times.

About ten years ago we had "RMS" (Root Mean Square) meters. But they were not "True RMS" meters. Haha. When we would hook up large Roof Top AC units, we of course check the power and we would read 217 to 219 volts. But in actuality that was 208 power. An analog meter would read much higher voltages. Balanced 208 reads 208 from my experience.

The primary taps on the low voltage control board transformer were usually set to 220-230 volts form the factory. But when we would fire up the unit, we would get a buzzing out of the 24 volt relay powered by the lower voltage transformer. It worked but was obviously not correct. The fix was to hook the 208 primary tap of the low voltage transformer up to the line voltage, which was actually 208 volts. The fans and compressors ran on either 208 or 230.

Today even with a Fluke True RMS meter, I see voltages over 240 volts from single phase 230 which used to be 220. Most equipment has a maximum voltage rating of 243 volts. In the same area 125 volts was coming from the 110 volt outlets. Which we now expect to see 120 volts from the 110 volt outlets.

It is funny but the old Simpson analog meters, when used on true balanced three phase AC, that is sometimes installed for gas stations to prevent an accidental high voltage leg from powering gas pumping equipment, read 110 volts from 110 volt outlets. Yet the same analog Simpson meter reads 145 volts from a high leg systems 110 volt receptacle. That is why guys stopped using the Simpson meters.

Things change a lot.

Sincerely,

William McCormick







 

Offline David Reichard

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I have seen a published engineer's study of ground current strengths or voltages actually measured in Columbus,Ohio circa the turn of the 19th century.I don't know if AC or DC.I think the publication was in the Columbus Metropolitan Library's main branch.The actual physical earth ground is a very large collection of parallel return conduction paths which enmasse would have a low resistance,even though any one of them might not have a low resistance.That is because all the small currents they carry can add up to quite a large current.It's complicated with AC because of capacitive and inductive effects.(phase)
 

Offline William McC

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I have seen a published engineer's study of ground current strengths or voltages actually measured in Columbus,Ohio circa the turn of the 19th century.I don't know if AC or DC.I think the publication was in the Columbus Metropolitan Library's main branch.The actual physical earth ground is a very large collection of parallel return conduction paths which enmasse would have a low resistance,even though any one of them might not have a low resistance.That is because all the small currents they carry can add up to quite a large current.It's complicated with AC because of capacitive and inductive effects.(phase)

www.youtube.com/watch?v=pFtoqm3ELLI

It is all about capacitors. The surface area, of a conductor connected to ground is very important. If you have ever welded on your back, while you are wet, you know for sure that cement conducts 40 volts of DC current very well. The formula for a capacitor uses three inputs the ohms or resistance of the dielectric, the thickness of the dielectric and the surface area of the dielectric connected to the conductor. As a welder, electrician, plumber, and server room tech as well as many other fields, you always have to keep in mind which part of the capacitor you are at any given time. There are air capacitors condensers, that you can become part of very easily.

Sincerely,

William McCormick
 

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