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Offline charliebabb

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About ground for electricity
« on: 15/10/2012 04:05:55 »
How does putting the negative lead in to the ground make an application work? Whats the difference with negative and ground? Isn't the negative on a battery the same as ground? I don't get the whole picture of electricity. Can someone give me an explanation of all of this?

Thanks for the help


 

Offline yor_on

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Re: About ground for electricity
« Reply #1 on: 15/10/2012 08:13:16 »
We've had some discussions about electricity here before, somewhere :)
But to keep it 'simple' :) I would call it a unbalance where the 'ground' is equalizing that balance, until the battery is 'dead'. Without a difference you can't get a flow, and the ground would then be the definer of it.
 

Offline CliffordK

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Re: About ground for electricity
« Reply #2 on: 15/10/2012 12:33:25 »
Any electrical circuit must have a complete circuit from the source back to the source.

Or, on a battery, one typically thinks of current from the positive terminal to the negative terminal (although in fact one has actual electron flow from negative to positive).

If you disconnect everything from the negative terminal, then you can ground, touch, whatever the positive terminal, and nothing happens as long as you don't touch the negative.  In fact, for many auto mechanics tasks, one first removes the negative ground wire because one can't get shocked doing it, and it effectively shuts off all the power in the car.

Since our automobiles are typically made out of metal, it is convenient to attach one pole of the battery to the frame and engine block, and use the body/frame/engine as one conductor, and only need to run a single conductor to all the devices.  One can, in fact, ground either the positive, or the negative pole on the cars.  However, attaching the negative pole to the frame slightly inhibits rusting, while positive grounding slightly increases rusting, or so they say.

In your house, one doesn't truly have positive or negative since all the circuits are AC.  There are both benefits and costs of "grounding". 

In a grounded system, one generally can not get shocked by touching only "neutral".  However, touching "hot" + anything can generate a shock.
In an ungrounded system, touching any wire to ground does nothing.  However, touching the two poles together can generate a shock.

In the USA, with 120V/240V circuits, the ground is a little more complicated (sorry, I haven't ever quite figured out what Europeans do). 

Anyway, your residential power is supplied with two opposite phases, with the sum of these two phases being 240 VAC.  These two phases are split to form a 3rd wire, called "neutral" which is grounded.

Now, consider what happens of you wire up two 120V light bulbs using a 3-wire 220V circuit.

Circuit A: Wire the first bulb to Black (Hot) + White (Ground).
Circuit B: Wire the second bulb to Red (Hot) + White (Ground).

How much power does the white wire carry?
Does it get overloaded?

It turns out that circuit A has a loop of power travelling from the Black (hot) to White (neutral or ground).  And, likewise, circuit B also has a similar loop.  But, if the power is matched on the phases, then the two neutral wires exactly cancel themselves out, and one gets NO power on the neutral.  However, it is easy to get a circuit slightly out of balance.  For example, of both lights were on the black wire, and none on the red.  Then one gets the circuit from black to white, with power flow in both including the neutral.  I think part of the reason with bonding the neutral to the ground is that it actually helps stabilize the neutral circuit.
 

Offline evan_au

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Re: About ground for electricity
« Reply #3 on: 16/10/2012 11:43:11 »
In the days of vacuum-tube AM radios, you picked up a radio signal by stringing an antenna wire in the air, and an earth wire which literally went to a metal stake in the ground. The planet Earth acts as a large capacitor, allowing radio-frequency current to flow from the antenna through the radio receiver, significantly improving signal reception. Transmitters were configured the same way, with the Earth being one terminal of the transmitter.

In electronic circuitry, it became conventional to show a positive voltage supply which went to all parts of the circuit; the negative end of this supply also went to all parts of the circuit, denoted by an "earth" symbol. With some elaboration for equipment which had multiple supply voltages (positive or negative), this usage of "earth=negative" became a common drawing convention in semiconductor circuits.

However, all our portable electronics, from the transistor radio in the 1960s, through electronic watches up to our smartphones today work perfectly well without being attached to the real Earth. But if you look at the circuit diagram, you will still probably see the old "earth" symbol, usually on the negative DC supply conductor.
 

Offline evan_au

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Re: About ground for electricity
« Reply #4 on: 16/10/2012 12:05:32 »
Quote
I haven't ever quite figured out what Europeans do

Power distribution in Europe (and Australia) uses 3 phases and a neutral, with 4 wires running down the street.
  • If your house has low power consumption, you will get the neutral and one phase, supplying 230V AC (at 50Hz).
  • If your house has high power consumption, you will get the neutral and all three phases, supplying 230V AC between each phase and neutral. The three phases are separated by 120 degrees.

Depending on the electrical conductivity of the ground in your area, the neutral may be connected to a stake in the ground at your home, or in a more central location, such as the transformer in your street.

This is for electrical safety, as someone in bare feet has an electrical path to the earth; if they touch the metal case of some faulty electrical equipment, they could get a shock. However, if the metal case is wired to a nearby earth stake, there will be insufficient voltage between hands and feet to give an electrical shock.

However, European regulations have traditionally favoured a "double-insulated" approach to electrical safety, where there is no exposed metal attached to the circuitry, and the circuitry is not attached to Earth - it just uses a 2-pin plug.

For comparison: In the USA, a transformer typically only supplies a few houses with 2 phases, as resistance losses are higher at 110V AC. 60Hz is used in USA. The US regulations traditionally favoured earthed metal enclosures.

PS: Different parts of Japan run  at 50Hz or 60Hz, because they originally bought electrical generators from US or European suppliers. Unfortunately, this hindered their ability to recover from power shortages after the recent tsunami...
« Last Edit: 16/10/2012 12:31:48 by evan_au »
 

Offline evan_au

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Re: About ground for electricity
« Reply #5 on: 16/10/2012 12:34:42 »
Another industry ignoring the "negative earth" convention is the telephone service.

This typically runs off 48V batteries, with the positive terminal attached to earth.

This is to minimise corrosion of the copper wires buried in the ground.
 

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Re: About ground for electricity
« Reply #5 on: 16/10/2012 12:34:42 »

 

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