Author Topic: What the heck is negative voltages?  (Read 20186 times)

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Offline james_s

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Re: What the heck is negative voltages?
« Reply #50 on: January 24, 2017, 06:02:20 am »
Look at failures to really learn - as an example , if the neutral were missing coming into the house , you not have enough circut / loop to do much work , some light bulbs would light up , but any motors [ like in your furnace would not run ] .
And if you measured the hot leg , depending on the grounding system - including the makeup of the earth / dirt , you would get a varying voltage of around 50 - 90 volts .
The ground is not part of the circuit , its a safety backup - in this case if you lost neutral hopefully your ground would be enough to trip a breaker / GFI , when needed , also to bring potential down .

My grandmother's house had precisely this happen when I was a kid back in the early 80s. Lightning struck on or near the pole out in the field and vaporized the neutral lug in the meter base which was on another pole in the yard near the house. Some of her lights were very bright and others were very dim. The 240V loads worked fine but turning on any light caused the other lights to change brightness. This is of course because the two banks of 120V circuits were now in series, having no neutral/ground reference. At the time it was freaky but looking back I understand exactly why everything behaved as it did.
 

Offline james_s

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Re: What the heck is negative voltages?
« Reply #51 on: January 24, 2017, 06:12:41 am »
Kind of off topic but if you had 80 AA batteries (120 volts without an awesome batterizer bateroo) in series, you could connect the + side of the battery bank to a meters red (positive lead for example),and connect the black lead to a grounding rod and see +120 volts? Or what if you connected up the the negative of the batt to the red lead you would see -120 volts? I thought that AC power could do that because ultimately the negative of the generator at the power station was tied into the plug(forget that it three phase keep it simple at = and +)? Now after reading this thread I don't understand the negative voltage.

With an AC generator (alternator) there is no "negative output", rather there are two outputs that have a rapidly reversing potential across them. On one half of the rotation (assuming a simple 2 pole alternator) one output will be positive and the other negative, then the potential will flip through the other half of the cycle. You can tie either one of these two outputs to the earth and call it "ground", consider that point to be 0V, and then from that perspective the remaining output will cycle above and below ground. It doesn't matter which of the two outputs you tie to ground, the result is the same. When you look at a circuit that is rated at 120V, that is the nominal voltage. That is an incandescent lamp connected to 120V nominal AC will glow with the same brightness it would connected to 120V DC. In reality the voltage is constantly changing, it peaks at around 170V peak to peak, so 85V above "ground" and 85V below.
 

Online tggzzz

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Re: What the heck is negative voltages?
« Reply #52 on: January 24, 2017, 09:02:11 am »
The commonest UK one is TN-S, where separate  protective and neutral conductors run all the way to the substation and are grounded there.

Every house in my road, and many in my village have their supply taken from 4 wires on poles outside the house. Every house is connected to the lowest wire, and every house is connected to one of the three other wires. Yes, that's neutral at the bottom and the other three are the three phases. I'm not exactly in the sticks; I'm 4 miles from a major city centre!

I do not intend to have the lead water supply pipe replaced - although I do run the water for 1 minute before drawing any for drinking :)

Quote
Never assume that neutral is at ground potential and never assume that it will stay there (even if you've measured it at ground) unless you know the particulars of the supply in question. Failure to observe this rule can result in "Bang! You're dead!" type outcomes.

As you are aware...

Even if you "measure it at ground" to within, say 10mV, it can supply a lot of current at 10mV. That could cause "bang you have a heart attack or are blind" or "bang you are on fire" situations.

The actual potential of the neutral wire depends on the degree of imbalance in the currents in each of the three phases. That's why in my road every third house is connected to the same phase; hopefully no one house takes "too much/little" current.

Only if perfectly balanced will it be at 0V (except during lightning strikes and gross distribution problems!). ISTR a couple of volts being not uncommon.
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Offline Cerebus

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Re: What the heck is negative voltages?
« Reply #53 on: January 24, 2017, 02:46:14 pm »
The commonest UK one is TN-S, where separate  protective and neutral conductors run all the way to the substation and are grounded there.

Every house in my road, and many in my village have their supply taken from 4 wires on poles outside the house. Every house is connected to the lowest wire, and every house is connected to one of the three other wires. Yes, that's neutral at the bottom and the other three are the three phases. I'm not exactly in the sticks; I'm 4 miles from a major city centre!

I do not intend to have the lead water supply pipe replaced - although I do run the water for 1 minute before drawing any for drinking :)

Out of curiosity, what is the grounding arrangement TN-C-S, or TT? (The latter in my mind standing for not Terre-Terre but Truly-Terrifying)?

For those not in the know geographically, overhead (low voltage*) supplies are pretty rare in the UK. Not unknown, as the above proves, but I'd guess that the proportion of the total premises that are delivered overhead to the customer is in the low single figures percent. (I just tried to find actual figures but failed.)

Correction, I found figures. These are just based on total length of low voltage supply lines above or below ground, and one would expect rural supplies (where one finds most overhead delivery) to be physically longer and so use more length of cable per premise served. It takes 10 metres of cable to get to my (urban) next door neighbour; if that was rural it would be possibly hundreds of metres. So, 14.6% of UK low voltage distribution cable is above ground (European average 42.4%) - which I think is consistent with my guess of single digit percent premises served.

Quote
Only if perfectly balanced will it [neutral] be at 0V (except during lightning strikes and gross distribution problems!). ISTR a couple of volts being not uncommon.

The worst place for floating (relative to local ground) neutrals are light industrial trading estates where the demand balancing across phases can be all over the place, especially when there's big-ish machinery. Heavy industry tends to get individual three phase supplies and some thought put into designing the loading of them. Light industry shares one supply between many premises and there is little or no planning goes into load balancing. Add a few load dumps, some crappy power factors and you've got an 'interesting' supply to deal with.


* In the electricity supply world the term low voltage gets used for everything below about 1kV, and the term extra low voltage for what most of us would think of as low voltage - 50V and down.
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Offline IanB

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Re: What the heck is negative voltages?
« Reply #54 on: January 24, 2017, 03:09:51 pm »
Here's an example of the 4 wires on poles. We have three phase 11 kV coming into the transformer, and three phase 415 V/240 V leaving.

https://goo.gl/maps/s9cLaNfVciG2

If we look closely at the transformer it appears to be grounded at the foot of its mounting pole, but there is no evident ground connection carried along to the houses. At a guess each house will have it's earth tied to the neutral wire where the supply enters the premises?
 

Online tggzzz

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Re: What the heck is negative voltages?
« Reply #55 on: January 24, 2017, 03:44:59 pm »
The commonest UK one is TN-S, where separate  protective and neutral conductors run all the way to the substation and are grounded there.

Every house in my road, and many in my village have their supply taken from 4 wires on poles outside the house. Every house is connected to the lowest wire, and every house is connected to one of the three other wires. Yes, that's neutral at the bottom and the other three are the three phases. I'm not exactly in the sticks; I'm 4 miles from a major city centre!

I do not intend to have the lead water supply pipe replaced - although I do run the water for 1 minute before drawing any for drinking :)

Out of curiosity, what is the grounding arrangement TN-C-S, or TT? (The latter in my mind standing for not Terre-Terre but Truly-Terrifying)?

There's nothing on the pole, as I said; see attachment from google earth. Curiously it appears that some of the distribution around here has 5 wires, with a thin one on the top. I don't know what that is, and can't make it out on the google earth pictures; conceivably it is some form of lightning conductor.

As far as I can see, I confirm that in/around the fuseboxes. From memory there's a strap on the lead cold water supply, but I'm not going to move appliances to check that! These are 1930s houses, with modern internal wiring.
« Last Edit: January 24, 2017, 03:58:17 pm by tggzzz »
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
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Offline james_s

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Re: What the heck is negative voltages?
« Reply #56 on: January 25, 2017, 03:03:40 am »
In the USA power is normally distributed from the substation as 13,500/7200V 3 phase. That will be either overhead or underground and run along main roads. Suburban neighborhoods generally get just one phase feeding off the main line. Then there is a transformer either on the pole for overhead or in a box on the ground feeding anywhere from 1-10 homes nearby with a center-tapped 240V secondary. The center tap is bonded to earth at the transformer, and also to the neutral wire that runs to each house along with a pair of hots. I've never seen a low voltage feed that was more than a few hundred feet long.
 


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