Technical Power System Definition - 640.3

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smalltime

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Roanoke, VA
640.3 defines a Technical Power System as:
"An electrical distribution system with grounding in accordance with 250.146(D), where the equipment grounding conductor is isolated from the premises grounded conductor except at a single grounded termination point within a branch-circuit panelboard, at the originating (main breaker) branch-circuit panelboard, or at the premises grounding electrode."

So it's an IG system. But what I'm struggling with is that it says, "...where the equipment grounding conductor is isolated from the premises grounded conductor..." Wait a minute. The regular EGC is insulated from the grounded conductor (neutral) until the single termination point. What's the difference?

Shouldn't it say, "...where the equipment grounding conductor is isolated from the premises grounding conductor..." In other words the IG grounding conductor is isolated (insulated) from the EGC until a single grounded termination point.

What am I missing?
 

fmtjfw

Senior Member
You are missing the fact that the receptacle is fed from a transformer secondary with the center-tap of the secondary grounded to the grounding conductor. The "hot" conductor is at 60V from ground and the "grounded" conductor is at 60V from ground. The voltage between the two is 120V.
 

GoldDigger

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You are missing the fact that the receptacle is fed from a transformer secondary with the center-tap of the secondary grounded to the grounding conductor. The "hot" conductor is at 60V from ground and the "grounded" conductor is at 60V from ground. The voltage between the two is 120V.
Or perhaps more precisely, the conductor connected to that terminal of the receptacle or equipment which is normally identified to receive the grounded conductor is not in fact a grounded conductor.
 

smalltime

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Roanoke, VA
You are missing the fact that the receptacle is fed from a transformer secondary with the center-tap of the secondary grounded to the grounding conductor. The "hot" conductor is at 60V from ground and the "grounded" conductor is at 60V from ground. The voltage between the two is 120V.

I believe what you're referring to comes out of 647.1. That's 120 V line to line and 60 V to ground - sometimes known as a "Balanced Power System". 647 is, as you know, "Sensitive Electronic Equipment".

"Technical Power System" is defined in 640, not 647, and 640 refers back to 250.146(D), an IG system.

The TPS definition in 640 is definitely an IG system. But even though with 640.7(B) and (C), I don't 640 as always being a 120 V line to line system. If it's 120 V line to line, it's certainly going to be a separately derived system but a TPS could also be 120 V line to neutral.

That's how I'm interpreting it. TPS is an IG system in 640. And it could also be a 120 V line to line. 647 is definitely 120 V line to line.
 

smalltime

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Roanoke, VA
I don't think you are missing anything. Should say premises grounding.

That's what I'm thinking exactly. Regardless of whether it's 120 V L-to-N or 120 V L-to-L, the grounded will be isolated from the EGC (or you would have a short - in essence a rogue Neutral/Ground connection). It doesn't make sense to say that here. The distinction of the IG system is that the IG grounding conductor is isolated until the point where the Neutral, EGC and IG all come together.
 

dereckbc

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It is called Balanced Power where there is NO GROUNDED CIRCUIT CONDUCTOR aka neutral. It is mainly used in recording studios where they have 120 VAC equipment. You can also find it in a few specialized data centers.

The problem with single phase 120 volt AC is it is unbalanced with respect to a reference point which is usually earth or some body acting in place of earth. Some would call it Ground. With filters, TVSS devices etc installed between L-G, and N-G induce current into the EGC and chassis. The effect is accumulative as more equipment is connected. This is where hum bars appear on video, and AC hum in audio circuits.

The noise problem is simply solved by changing the electrical architecture to a balanced power. By doing this all the noise injected in ground circuits is cancelled out. Basically it is the same thing as any residential single phase service. Th edifference is there is no grounded circuit conductor. Instead of L1 and L2 with 120 to G it is 60 volts.

The down side of balanced power systems just like any 240 volt circuit requires both polarities to have OCPD installed at the distribution panel. A lot of 120 volt equipment can work on balanced power but there are two issues.

1. Going back to the filters and TVSS installed N-G will require a modification because those devices are generally low voltage rated of 10 to 15 volts max. So they have to be either replaced with higher voltage components or removed.

2. Power switches need to be changed to 2 poles. Conventional switches only switch the hot conductor and not the neutral. In a balanced power circuit there is no neutral, and what was neutral is now line of 60 volts.
 

smalltime

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Location
Roanoke, VA
It is called Balanced Power where there is NO GROUNDED CIRCUIT CONDUCTOR aka neutral.

I'm not wishing to be contrary, but that's not how I read 647.3 General. Use of a separately derived 120-volt single phase 3-wire system with 60 volts on each of two ungrounded conductors to a grounded neutral conductor...

Even with 120 L-to-L, the neutral is grounded just like 240 single phase 3-wire L-to-L. Same architecture, different voltage.
 

dereckbc

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I'm not wishing to be contrary
No problem.

There is no grounded circuit conductor(neutral) in a balanced power system being it either single phase or 3 phase (6 phase wye). I have no idea why NEC used the term Grounded Circuit Conductor because there is none. My guess is the technical writer did not understand the context when written. WAG it is an error.

Basically it is the same are a 3 wire 240 AC circuit, two hots and a ground. Think of it like a 1:1 isolation transformer with a center tap on the secondary bonded to ground. From the transformer you only carry L1, L2, and G. There is no grounded circuit conductor aka neutral. The difference between a standard 120 volt circuit and a balanced circuit is the standard 120 circuit is asymmetrical, and balanced is symmetrical.

Asymmetrical circuits inject power frequency and RF noise current into ground circuit. Symmetrical circuit DO NOT inject power or RF into ground circuits. standard 120 VAC circuits is one major flaw we made 100 years ago. It causes all kinds of problems.

The system is grounded, just no grounded circuit conductors. Ground is used for 2 purposes in a Balanced Power System.

1. For fault clearing path and all the same reasons you run an EGC to any piece of equipment.
2. This is the biggie, provides a noise free reference point for signal circuits.
 
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fmtjfw

Senior Member
Now I understand

Now I understand

OK, 2008 and 2014 read:

640.3 Locations and Other Articles. Circuits and equipment
shall comply with 640.3(A) through (M), as applicable.

647.3 General. Use of a separately derived 120-volt
single-phase 3-wire system with 60 volts on each of two
ungrounded conductors to a grounded neutral conductor
shall be permitted for the purpose of reducing objectionable
noise in sensitive electronic equipment locations, provided
the following conditions apply:

Your quote is --
640.3 defines a Technical Power System as:
"An electrical distribution system with grounding in accordance with 250.146(D), where the equipment grounding conductor is isolated from the premises grounded conductor except at a single grounded termination point within a branch-circuit panelboard, at the originating (main breaker) branch-circuit panelboard, or at the premises grounding electrode."

which is actually in 640.2 -- the page headers display the LAST section number on the page -- not the FIRST.
 

smalltime

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Location
Roanoke, VA
There is no grounded circuit conductor(neutral) in a balanced power system...

Basically it is the same are a 3 wire 240 AC circuit, two hots and a ground.

The system is grounded, just no grounded circuit conductors.

Sorry, but I disagree. Whether single phase 3-wire (plus Ground) 120 V L-to-L or 3-wire (plus Ground) 240 V L-to-L, both have a grounded Neutral.

I do understand the whole asymmetrical/symmetrical (unbalanced/balanced) concepts and I didn't wish to get into debating the merits of a balanced power system.

I'm trying to figure out if there's an error in the definition of TPS in 640.2. I believe there is and that it should say, "An electrical distribution system with grounding in accordance with 250.146(D), where the equipment grounding is isolated from the premises grounding conductor...", not "premises grounded conductor..."

The grounded (current carrying return) is already isolated (insulated) from any grounding conductor by design. 250.146(D), Isolated Ground isolated (insulates) the normal EGC from the IG EGC until the bond at the power source.

Equitech, a name you're certain to be familiar with, shows the Neutral grounded in their illustrations of balanced power:
http://www.equitech.com/images/enigma7.gif
http://www.equitech.com/images/enigma8.gif

"Figure 7 and Figure 8 are illustrations of a balanced power application."

The Neutral is grounded, even in balanced power.
 

dereckbc

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Equitech, a name you're certain to be familiar with, shows the Neutral grounded in their illustrations of balanced power:
http://www.equitech.com/images/enigma7.gif
http://www.equitech.com/images/enigma8.gif

"Figure 7 and Figure 8 are illustrations of a balanced power application."

The Neutral is grounded, even in balanced power.

I am very familiar with Equitech. The two schematic drawings you link to DO NOT have a grounded circuit conductor period. What you think is a neutral is actually an Equipment Ground Conductor and does not carry any load currents. The ground is there to provide a fault path to operate an OCPD in the even of a fault, and as a noise free ground reference for signal circuits.
 

smalltime

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Location
Roanoke, VA
The two schematic drawings you link to DO NOT have a grounded circuit conductor period. What you think is a neutral is actually an Equipment Ground Conductor and does not carry any load currents.

Again, I'm not wishing to be contrary, but I disagree.

Both illustrations show the 3rd conductor tied to the midpoint of the secondary. That, by definition, makes it a Neutral.

Art. 100
Neutral. The conductor connected to the neutral point of a system that is intended to carry current under normal conditions.
Neutral Point. The common point on a wye-connection in a polyphase system or midpoint on a single-phase, 3-wire system...
 

smalltime

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Location
Roanoke, VA
You may be thinking that that one wire serves as both the Neutral and EGC but I would think that just as a 240 V circuit requires 3 wires plus a ground, so does the 120 V L-to-L require 3 wires plus a ground.
 

dereckbc

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Again, I'm not wishing to be contrary, but I disagree.

Both illustrations show the 3rd conductor tied to the midpoint of the secondary. That, by definition, makes it a Neutral.

Art. 100
Neutral. The conductor connected to the neutral point of a system that is intended to carry current under normal conditions.
Neutral Point. The common point on a wye-connection in a polyphase system or midpoint on a single-phase, 3-wire system...

Sorry but that is wrong and the Definition tells you that. By your logic then you must believe the EGC is a normal current carrying conductor in addition to the grounded circuit conductor (aka neutral). In a Balanced system there is no grounded circuit conductor, just an Equipment Grounding Conductor that carries no load current under normal conditions. Again pretty much the same thing as a 3 wire 240 volt circuit there is no neutral, just 2 hot wires (L1 & L2) and an EGC.
 

smalltime

Member
Location
Roanoke, VA
By your logic then you must believe the EGC is a normal current carrying conductor in addition to the grounded circuit conductor (aka neutral). In a Balanced system there is no grounded circuit conductor, just an Equipment Grounding Conductor that carries no load current under normal conditions. Again pretty much the same thing as a 3 wire 240 volt circuit there is no neutral, just 2 hot wires (L1 & L2) and an EGC.

I think we agree that the EGC normally never carries current unless there is a fault condition. (Except for "leakage" or "touch" current.)

I am saying that since the illustrations show that middle wire as connecting to the midpoint of the secondary, that it is by definition a Neutral as stated in Article 100. What I believe should be added to that illustration is an EGC. In other words, there should be no design difference between a 240 V 3-wire (plus ground) single phase and a 120 V 3-wire (plus ground).

Let me try this - in a normal 120 V branch circuit (2-wire plug Ground), current returns via the Neutral. In a 3-phase circuit (4-wire plus Ground), any unbalanced current returns via the Neutral. The Neutral is a normally current carrying conductor.

The same with 240 V single phase, 3-wire plug (plus Ground) for a Range or Dryer circuit. I suppose you could call the 240 V single phase a balanced circuit but I haven't heard anyone use that term before in the context of a range or dryer circuit. The current on L1 and L2 are in differential mode with one another - a difference circuit as the waveforms are in opposite polarity from one another. Any unbalance returns via the Neutral.

From Soares Book on Grounding and Bonding by the IAEI, 9th Edition, page 15:
"Grounded and Grounded Conductor
The grounded conductor (usually a neutral) is generally a system conductor intended to carry current during normal operation of the circuit. The grounding of the grounded (often a neutral) conductor of a system is accomplished by connection to ground through a grounding electrode conductor either at the service or at a separately derived system. Generally, it should be understood that the grounded conductor should not be used for grounding equipment on the load side of the grounding connection at the service or source of separately derived systems. This separation between grounded conductors and equipment grounding conductors keeps the return current on the neutral (grounded) conductor of the system, where it belongs when going back to its source."

This leads me right back to my original issue with the TPS definition in 640.2 where I believe it should say, " "...where the equipment grounding conductor is isolated from the premises grounding conductor..." rather than, ""...where the equipment grounding conductor is isolated from the premises grounded conductor..."

The grounded and grounding will be isolated (insulated) from one another. The TPS (IG System) isolates (insulates) the IG from the EGC.
 

don_resqcapt19

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...
I am saying that since the illustrations show that middle wire as connecting to the midpoint of the secondary, that it is by definition a Neutral as stated in Article 100. ...
Please read all of the words in the Article 100 definition. The conductor in question is clearly not intended to carry current under normal conditions.
Neutral Conductor. The conductor connected to the neutral point of a system that is intended to carry current under normal conditions.
I would agree that the point of connection for that conductor is a neutral point, but since the conductor is not normally a current carrying conductor, it cannot be a neutral conductor.
 
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