Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Marty McCann]

The theory behind "three phase" is entirely different than the theory behind "two hots".    If your terminology is wrong, people won't be able to give you meaningful help.

Yes correct terminology is very important when it comes to something that if misunderstood by those that are too Cavalier about about this subject can KILL people.

How many of us have even heard a local electrician refer to U.S. household power as 2-Phase?  A lot more times than I should have.

The concept of a Neutral is an issue NOT understood by many (including some registered Electricians).

A so called Neutral is only really Neutral in a 3-Phase system that is absolutely balanced equally on all three legs.  If this condition is met, then the Neutral conductor is just that. 

However as soon as you derive 120 VAC from the 208 3-Phase system, the Neutral no longer resembles that designation as there will indeed be current present in that perhaps formerly neutral conductor.

[David Buckley]

I disagree your definition, as it suggests you cant have a neutral in a single phase supply.

In the single phase world, a neutral only comes into existance when an isolated source of power (for example, the secondary of a transformer) has one of its (general term) wires connected to ground.  At that moment then the wire connected to ground becomes the neutral, and other wires from the same source of power become hots.  Note that if there are two or more other connections besides the neutral, then they do not have to be at the same potential from neutral, though in a three phase, and domestic 120V centre tapped supply they generally are.

A transformer that has none of its secondary terminals connected to ground cannot have a neutral.  I'm not aware of a general term that is used to describe the outputs of a transformer in this situation.  A true isolating transformer with no connection from any point on the secondary to ground would fit this description, as well as violating the NEC.

It is generally accepted that neutral and ground start out at the same potential, as they are bonded together at a point, but as you note, all real wires have impedence and thus current flowing in the nuetral will cause it's potential to differ from that of ground.

[Marty McCann]

I disagree your definition, as it suggests you cant have a neutral in a single phase supply.

In the single phase world, a neutral only comes into existance when an isolated source of power (for example, the secondary of a transformer) has one of its (general term) wires connected to ground.  At that moment then the wire connected to ground becomes the neutral, and other wires from the same source of power become hots.  Note that if there are two or more other connections besides the neutral, then they do not have to be at the same potential from neutral, though in a three phase, and domestic 120V centre tapped supply they generally are.

A transformer that has none of its secondary terminals connected to ground cannot have a neutral.  I'm not aware of a general term that is used to describe the outputs of a transformer in this situation.  A true isolating transformer with no connection from any point on the secondary to ground would fit this description, as well as violating the NEC.

It is generally accepted that neutral and ground start out at the same potential, as they are bonded together at a point, but as you note, all real wires have impedence and thus current flowing in the nuetral will cause it's potential to differ from that of ground.

Here is what punters who may "think" they understand this discussion do not get . . . .

Scenario:  Outdoor Festival
Service Panel 100+ feet from stage distro.  Yes everything is to code and the Neutral (White wire) is bonded correctly to ground (Green wire) in the main service panel.

Now a punter, tech, or muso is in wet grass and comes in contact with the neutral, he can be lit up.  Hopefully only getting a mild shock.   But, it is also possible that he can pass enough current thru him to the wet grass to kill him (0.015 Amperes) long before (as much as 19.985 amps) that 20 Ampere breaker trips.

My point, even a properly NEC met power distribution scheme can kill people.

[Rob Spence]

Here is what punters who may "think" they understand this discussion do not get . . . .

Scenario:  Outdoor Festival
Service Panel 100+ feet from stage distro.  Yes everything is to code and the Neutral (White wire) is bonded correctly to ground (Green wire) in the main service panel.

Now a punter, tech, or muso is in wet grass and comes in contact with the neutral, he can be lit up.  Hopefully only getting a mild shock.   But, it is also possible that he can pass enough current thru him to the wet grass to kill him (0.015 Amperes) long before (as much as 19.985 amps) that 20 Ampere breaker trips.

My point, even a properly NEC met power distribution scheme can kill people.

And to explain further, the reason for the danger is that the neutral, several hundred feet out, isn't at ground anymore because it is carrying current.

For the previous poster... A single phase residential feed is a center tapped transformer. There is 120v between the hots and the neutral (center). There does not need any reference to ground at the transformer for it to work. General practice may be to ground it there - I don't know. I don't climb poles anymore :-)

However, when that neutral gets into the house service panel, it is bonded to a local ground at that point and that point only. Please note that the NEC does not concern itself with what the utility company does.. the demark is where the service cable ends.

[Jonathan Johnson]

And to explain further, the reason for the danger is that the neutral, several hundred feet out, isn't at ground anymore because it is carrying current.

The term "neutral" is a misnomer; to avoid confusion the term should not be used in formal discussion. The grounded, current-carrying conductor we frequently refer to in informal discussions as "neutral" should never be assumed to be at the same potential as the safety ground (earth) conductor, except where the two conductors are bonded according to accepted safe installation practices. Even the safety ground, at some point a significant distance away from its connection to a ground rod, can exhibit a voltage potential between it and the localized environment.

There is a saying among utility linemen: "if it ain't grounded, it ain't dead."

According to this article, "Formally the neutral is called the "grounded conductor"; as of the 2008 NEC, the terms "neutral conductor" and "neutral point" have been defined in the Code to record what had been common usage."

[David Buckley]

Here is what punters who may "think" they understand this discussion do not get . . . .

I agree your entire scenario, and further agree that an NEC compliant installation is unsafe in these situations.  Worse than that, the NEC mandates you use the TN-C-S arrangement that is the cause of this unsafeness; the NEC does not allow the selection of earthing arrangements to match the scenario, as a competent person routinely does in other parts of the world.

Outside of NEC-land, there are a few schools of thought about how to deal with this issue of the stage in the middle of a field being fed as you describe, but the school I would shout for would adopt a TT system with RCD protection as the least dangerous course of action.  But not in NEC-land, where a TT system in the middle of a field is not a recognised arrangement.

As a general principle, TN-C-S works acceptably well whilst the person and the stuff are within what everyone (except those subject the the NEC) calls the equipotential zone.  As soon as you extend the ground wire outside the equipotential zone then you are looking for trouble.  (The NEC has a definition for equipotential zone, but it it much narrower and specific in usage than the non-NEC common usage)

This unsafeness crops up in many other situations than stages in the middle of fields; milking sheds, swimming pools, and boat jetties are just a few that spring to mind.

[David Buckley]

A single phase residential feed is a center tapped transformer. There is 120v between the hots and the neutral (center). There does not need any reference to ground at the transformer for it to work.

It is true there is 120V between each of the outer connections and the centre tap, but the centre tap does not become a neutral until it is grounded, which makes this conductor the reference to which all measurements are made.

By way of explanation: Instead of describing that transformer as 120-ct-120, you coud equally describe as a 0-120-240 transformer.  Its just a question of where you hold one probe of the meter, as opposed to the two places you place the other probe.

Whereever you attach the ground wire to, thats the neutral.  I agreee it would be unheard of to ground an end of a residential supply transformer, but just because it isn't done tha doesn't invalidate the definitions.

For the previous poster...   General practice may be to ground it there - I don't know. I don't climb poles anymore :-)

Oh, its worse than that.  Standard American urban utility practice is to have four wire MV distribution, so there is a distribution neutral that goes out with the three phases.  This is so the single phase step-down transformers can be built more cheaply.  Elsewhere in the world there is three wire MV distribution.

The four wire distribution system is not good, but was done years ago for cost reasons.  You can read all about why it is bad in a paper entitled "The hazardous multigrounded neutral distribution system and dangerous stray currents", but it seems sadly that this paper is no longer publicly available.

[David Buckley]

According to this article, "Formally the neutral is called the "grounded conductor"; as of the 2008 NEC, the terms "neutral conductor" and "neutral point" have been defined in the Code to record what had been common usage."

This is the case of the NEC being a century behind on its terminology; everyone around the world knows what a neutral is, except the NEC.  They've finally joined the party by having a neutral.  Unfortunately, the definition they've adopted isn't a good one, and the FPN makes it worse by adding constraints that are only seen within the NEC.

However, having the term "neutral" I think this is good; the terms "grounding conductor" and "grounded conductor" have confused far too many people for far too long.

The grounded, current-carrying conductor we frequently refer to in informal discussions as "neutral" should never be assumed to be at the same potential as the safety ground (earth) conductor...

This is another failing of the NEC prior to 2008; calling the neutral conductor "grounded" makes matters worse, by suggesting it is similar to "ground".

In other jurisdictions, regulations require that all circuit condutors except ground are to be treated as live conductors at all times, this includes the neutral.

Contrast this with the once NEC permitted (albeit now banned, but still grandfathered, and still apparently often seen) 3 prong dryer sockets which use a common conductor for both the protective grounding function, and carrying current.

[Othmane Alaoui]

Hello !

I beleive there is a lot to speak about in this tread but I also beleive that this topic is very dangerous topic for newbies.
Short story about me : My primary job is Application Engineer / Product Manager PQS for a compagny named ABB. I do work in what we call the low Voltage ( MCC', Switchgear, cap. banks ... ) and Power Quality 1000V and less. I primarly design and size active filters for harmonic compensation. That being said, I highly recommend not to transform or try to transform any single phase to  tree phase network. I also recommend the use of proffessionnal people working in the electricity with good knoledge of the NEC, UL508A, UL845 and NFPA70E. theese are regulations and norms that we need to respect when we work with power.

I also suspect that a lots of us do not fully master differences between single phase loads with 2 and 3 phase loads.

Because of all the above, I think we should end this discussion before someone gets heart.

Regards,

Othmane Alaoui

[ProSoundWeb Community]