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#6124 12/25/01 02:16 PM
Joined: Aug 2001
Posts: 7,520
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Yes, that's pretty much the size of it. If we go into STATIC discharges (e.g. lightning strikes) then it gets a bit more complicated, but as far as normal power distribution systems are concerned no current can flow to ground so long as there is only one point of the system connected to ground.

And yes, if you have a generator which has no ground connection you could take a wire and ground either either conductor and nothing would happen -- No sparks, no flash, no current to ground (except perhaps a very tiny leakage current due to the aforementioned capacitance effect).

#6125 12/31/01 07:10 AM
Joined: Dec 2001
Posts: 1
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Junior Member
cindy
so much has been said about the topic, that there isn't much left to say. but i'd like to pitch in the way i see it.

to understand the concept of grounding, first of all you have to understand the concept of the star point, or more common neutral. why isn't there a short circuit when all 3 phases are shorted together. the answer is that in a balanced three phase system (same amount of current flowing through all three phases, equal voltages, 120 degrees apart) then at one given moment the summ of the three voltages will be zero, thus if the three wires are shorted, a zero potential point is created which is called the neutral. So no current flows out of a zero potential point (neutral).

but there will be curent flowing from one phase to another.

i don't want to make this too long, if u have understood what i've tried to explain, tell me so and i will try to explain the rest of it to u.

happy new year [Linked Image]

#6126 01/01/02 11:25 PM
Joined: Oct 2001
Posts: 19
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Member
This is a bit off topic, but think of what happens to maintenance guy's example if you use standard 120 single phase and a scope instead of a volt meter.

I've never tried it, but I assume that If you use the neutral/ground as 0 and the look at the two hots, you will see two sine waves seperated by a phase shift of 180 degrees, so you have two phase current (but not real two phase).

But if you use one of the hots as 0, and look at the neutral and the other hot, you will see 2 sine waves in phase 1 with twice the amplitude of the other, so you only have single phase current.

I guess it just depends on your point of view.

#6127 01/05/02 02:11 AM
Joined: Oct 2000
Posts: 2,722
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rbiro,

I would imagine that if you scope'd out a 1 phase 3 wire system and used a "common" probe on the center tapped grounded neutral conductor - with probes also on the 2 ungrounded conductors, the results would be sine waves of equal intensity and equal time, rather than any time offset.

Seems likely that the wave displayed for "Line A - Center Tap" circuit [let's call this one "Ckt. 1"], the wave would begin at the zero line off of the ungrounded conductor Line "A".

The wave displayed for "Center Tap - Line B" circuit [let's name this one "Ckt. 2"] would begin at the zero line on the center tapped common netutral, and in-time with the wave for Ckt. 1 [no time offset or other type of lag/lead - except those of Reactance per wave].

I have never scope'd the described connections, so if anyone has conflicting information, please chime in!!!

Would like to perform this test ASAP. If I can get hold of a scope for a day or so, definitely will run this analysis!

Scott SET.

BTW, has this topic's original question[s] been covered to everyone's liking, or are there still some pending Q's???
If there are, feel free to toss them in!
As said many times before - "There's no Stupid Questions in this forum, just occasional Silly answers".
No one needs to feel intimidated if they are unsure of ANY topic in which this forum may help with. Throw it in and let everyone chime in.

As far as the tech stuff goes, don't feel bad if it doesn't make 100% sense right away!
Electrophysics is by far one of the most in-depth and complex theories in the Scientific Community, but with a little time and patients, ANYONE interested can grasp the principles.

S.E.T.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#6128 01/05/02 07:37 AM
Joined: Aug 2001
Posts: 7,520
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If you connect a dual-beam 'scope with its common to the neutral, channel 1 to one hot leg and channel 2 to the other hot leg, then you will see two sinewaves of equal amplitude but 180 deg. out of phase (i.e. the zero crossings will coincide but the positive peak of one will coincide with the negative peak of the other).

A similar center-tapped arrangement is common for obtaining DC supplies in much electronic equipment, because full-wave rectification can then be achieved with just two diodes.

Connecting the scope common to one hot leg with channel 1 on the neutral and channel 2 on the other hot changes the reference point. In this case you would see two sinewaves in phase, but ch. 2 with twice the amplitude of ch. 1.

Notes if anyone tries these experiments:
1. Many 'scopes have the chassis and common point grounded via the cord. Obviously, connecting the common of such a unit to a hot leg will result in a flash and a tripped breaker. Use an isolated scope or a separate isolation xfmr, and remember that the case of the scope could end up at 120V to ground, so take appropriate precautions.

2. Modern dual-beam scopes usually have different trigger modes for the horizontal timebase. To get a true comparison of the phase of the two inputs, make sure the triggering is set to one channel only, otherwise it will synchronize to each sinewave individually and you won't get a true phase comparison.

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