GFCI's and current imbalance

[Eddy Current]

I'm studying residential services and in the book i have they are talking about how a GFCI work's. I know the concept but what im not understanding is current imbalances and neutrals, which i know the neutral carries the unbalanced load but i thought that between two phases it would always be the a number divisible by two 240/120 and so on.


In the GFCI chapter they say "If the current in the neutral wire becomes less than load in the hot wire, then a ground fault exists" but earlier in one of the other chapters they are quoted as saying "In a single phase feeder circuit with one phase carrying 50A and the other carrying 40A the neutral conductor would carry 10A.

 

[ptonsparky]

The current transformer for a GFCI goes around all the normally current carrying conductors. Hot, neutral for a 120 volt and hot, hot, neutral for the 240/120. In your example you would not have an imbalance. 50, 40, and 8 would cause a trip. As would 50, 50 and 2.

 

[Eddy Current]

I think its in the way they phrased their statement. Isn't the current in the neutral wire always less than the load in the hot wire?

 

[ptonsparky]

I think its in the way they phrased their statement. Isn't the current in the neutral wire always less than the load in the hot wire?

In a 2 wire circuit the neutral current should be equal to the hot, in a multi wire circuit the neutral current should be the difference of the 2 hot conductors, anything else gives you a net imbalance and trips the GFCI. Remember the GFCI is looking for about 5ma imbalance before tripping. An imbalance means the current is finding another path back to the transformer. A Ground Fault, which could be on either the hot or the neutral.

 

[Eddy Current]

So a single phase system will have the same current in the neutral as the hot wires?

 

[ptonsparky]

So a single phase system will have the same current in the neutral as the hot wires?

You tell me:

L1=5.5, N=__

N=___, L1=4, L2=6.5

L1=8, L2=___, N=45

 

[gar]

130728-0942 EDT

Eddy Current:

I believe that the array of questions you ask are a result of the courses or study materials you are exposed to not being sufficiently directed at basic concepts.

Looking at your question on GFCI. Basically in a two terminal load type of problem you have a source of energy, one wire from the source to the load, and a second wire from the load back to the source. This can be described as a single series loop conductive path. With some common assumptions it can be said that at an instant of time the current magnitude is the same at any point in the loop. Note: that statement says nothing about direction.

If those two wires form straight parallel paths, and you pick a point somewhere along the path, then at that point at an instant of time the current in one wire is moving in a direction opposite that of the current in the other wire.

Because the current directions are opposite the individual magnetic fields oppose each other. A current transformer that encompasses both wires will see a zero current. Somehow change the current in one conductor relative to the other, and now the current transformer sees a net current equal to the difference in current between the two conductors. This unbalance can be created experimentally by a resistor from one load terminal to one source terminal by a path that is not thru the current transformer.

Next open a GFCI receptacle and study how it is designed.

Once you get some of these concepts clear, then figure out the meaning of neutral as used in most electrical distribution systems. Also try to relate this to the statement that --- the sum of all the currents at a point is zero.

A mathematical equation related to some physical process is simply a model that approximately describes the process. A well tested model with known assumptions, and definitions, can provide a means to predict what will happen in some circuit. The accuracy of the end result will depend upon how accurately the assumptions and definitions apply.

.

 

[kwired]

What goes out has to come back, if goes out but doesn't come back on one of the monitored conductors then it is taking an unintended path, the whole concept of GFCI is based on this, and that unintended path may be a person.

Monitoring is done by placing all monitored conductors through a current transformer, if what goes out also comes back the net current in the CT is zero and the GFCI holds. Once there is an imbalance of 4-6 mA through the CT then the trip circuitry kicks in and trips the device.

In a two wire circuit what goes out on one conductor must come back on the other conductor or the GFCI trips.

In a three wire multiwire branch circuit it still monitors what goes out and what comes back. Current leaving from line 1 can return on line 2 or N and current leaving from line 2 can also return via L1 or N but if it finds some alternate path then current imbalance on the sensing coil will happen and it will trip.

In either case it would also trip if more comes back than what went out - which could happen if current from another circuit somehow got mixed into this circuit.

Bottom line is the GFCI effectively is like placing a clamp on meter around all the conductors of the circuit, if what goes out comes back the meter reading is zero. If current finds a path outside the intended circuit or is introduced from outside the circuit then the clamp on meter will read other than zero and the device is designed to trip whenever this level is more than 4-6 mA.

 

[Eddy Current]

You tell me:

L1=5.5, N=__

N=___, L1=4, L2=6.5

L1=8, L2=___, N=45

N=2.5

L2=37 Is this right?

I believe that the array of questions you ask are a result of the courses or study materials you are exposed to not being sufficiently directed at basic concepts.

You're right i need to get the basic concepts of line phases understood first.

So incoming service feeds say for instance 120/240 aren't necessarily an even amount of voltage across each leg as long as they both add up to 240? What about say 120/208 half of 208 is 104 would each leg read 104?


The book goes on to say talking about single pole and double pole GFCI breakers now

A single pole GFCI breaker has two lugs and a white wire pigtail in addition to the line side plug on or bolt on connectors

Two lugs? I thought single pole was just like a normal single pole breaker why would there be two lugs is this a misprint? Unless i guess it could be talking about where the line neutral goes.

 

[kwired]

N=2.5

L2=37 Is this right?



You're right i need to get the basic concepts of line phases understood first.

So incoming service feeds say for instance 120/240 aren't necessarily an even amount of voltage across each leg as long as they both add up to 240? What about say 120/208 half of 208 is 104 would each leg read 104?


The book goes on to say talking about single pole and double pole GFCI breakers now


Two lugs? I thought single pole was just like a normal single pole breaker why would there be two lugs is this a misprint? Unless i guess it could be talking about where the line neutral goes.

GFCI breakers have neutral connections on them, as for them to function the neutral must also pass through the internal CT that monitors for current imbalance. So a single pole will have a Line side plug on jaw, a line side neutral conductor that lands on the neutral bus of the panel, and load side lugs for both hot and neutral. A two pole GFCI will be exactly the same plus another plug on jaw and one more load side hot lug.

 

[ptonsparky]

N=2.5

L2=37 Is this right?



You're right i need to get the basic concepts of line phases understood first.

So incoming service feeds say for instance 120/240 aren't necessarily an even amount of voltage across each leg as long as they both add up to 240? What about say 120/208 half of 208 is 104 would each leg read 104?


The book goes on to say talking about single pole and double pole GFCI breakers now


Two lugs? I thought single pole was just like a normal single pole breaker why would there be two lugs is this a misprint? Unless i guess it could be talking about where the line neutral goes.

For now, assume a single phase service, 240/120 and that the voltages are constant. Get this clear in your mind before you add variations.

I gave you three problems. One of your answers is correct, maybe.

 

[ggunn]

I'm studying residential services and in the book i have they are talking about how a GFCI work's. I know the concept but what im not understanding is current imbalances and neutrals, which i know the neutral carries the unbalanced load but i thought that between two phases it would always be the a number divisible by two 240/120 and so on.


In the GFCI chapter they say "If the current in the neutral wire becomes less than load in the hot wire, then a ground fault exists" but earlier in one of the other chapters they are quoted as saying "In a single phase feeder circuit with one phase carrying 50A and the other carrying 40A the neutral conductor would carry 10A.

The difference is that a GFCI circuit typically operates on a single phase two wire (120V) branch circuit where the hot and neutral have the same current and balance is not an issue.

 

[gar]

130729-1322 EDT

In the theoretical world I can place linear loads on a single phase center tapped transformer system that will cause the neutral current to equal the sum of the magnitudes of the two hot line currents. In the real world I can come relatively close to the magnitude sum.

In the question presented in a previous post the assumption, but not stated, was that the loads on the two phases were resistive. Then the neutral current is the difference of the magnitudes of the two phase currents.

Eddy Current:

You need to study, trig, algebra, vector arithmetic, and possibly calculus as well as electrical circuits from the basics and up.

Two sine waves of identical frequency and arbitrary phase difference sum to a sine wave which can include zero magnitude.

Two sine waves of different frequencies when multiplied together produce sine waves of sum and difference frequencies. When the two source frequencies are equal, then one output is of 0 frequency (DC).

.

 

[Eddy Current]

What im having a problem understanding is feeder line voltage and their single phases and what each leg would read. I know that across them it should add up to 240 but what about each individual leg, would each one theoretically be 120 or could they be different voltages as long as across both it is 240?

 

[Volta]

Well, ignoring for a moment your earlier 120/208 question, which is another few steps away, you can consider that 120/240 will be balanced voltage, and each leg will be 120 volts, and together would be 240. But we can ignore that for a minute too.

The balance mentioned in earlier posts is only about the current.

Starting with single phase, as a two wire circuit, the flow of current will be the same everywhere in the circuit. Therefore, it doesn't matter if it is 120 or 240, wire 'A' has the same current as wire 'B'.

If the GFCI electronics see that they are not the same, it knows that another (third) path has carried some current, and then stops the flow.

 

[Eddy Current]

Well, ignoring for a moment your earlier 120/208 question, which is another few steps away, you can consider that 120/240 will be balanced voltage, and each leg will be 120 volts, and together would be 240. But we can ignore that for a minute too.

The balance mentioned in earlier posts is only about the current.

Starting with single phase, as a two wire circuit, the flow of current will be the same everywhere in the circuit. Therefore, it doesn't matter if it is 120 or 240, wire 'A' has the same current as wire 'B'.

If the GFCI electronics see that they are not the same, it knows that another (third) path has carried some current, and then stops the flow.

I think i've got it now thanks!

 

[ggunn]

What im having a problem understanding is feeder line voltage and their single phases and what each leg would read. I know that across them it should add up to 240 but what about each individual leg, would each one theoretically be 120 or could they be different voltages as long as across both it is 240?

They had better be the same (120V), or two wire 120V loads are going to either be starved for voltage or exposed to overvoltage. This happens, BTW, if and when the neutral gets separated from ground at the service; in that case the leg voltages do sum to 240 but can vary according to what loads are on which leg(s). Bonding the neutral to ground keeps it from happening.

 

[K8MHZ]

They had better be the same (120V), or two wire 120V loads are going to either be starved for voltage or exposed to overvoltage. This happens, BTW, if and when the neutral gets separated from ground at the service; in that case the leg voltages do sum to 240 but can vary according to what loads are on which leg(s). Bonding the neutral to ground keeps it from happening.

Are you sure about that?

An open neutral is one that is not connected to the POCO's neutral. Connection to the earth has nothing to do with preventing an open neutral. It may provide some path back, but that's not what the intention of the bonding of neutral to ground is.

 

[gar]

130730-1300 EDT

On a well constructed transformer with good core material and coil design the open circuit voltage on each side of center tap can be quite close.

A small Stancor P-8668 117 V primary, 28 V 2 A secondary had open circuit output voltages of 16.52 and 16.52 with 124.3 V input. This balance under no load looks very good.

The actual turns ratio is about 3.762 to 1. The rated voltage ratio is about 4.18 to 1. The disparity in turns and voltage ratios is because the transformer is designed to provide 28 at 2 A with 117 V input, and transformer internal impedance causes substantial voltage drop at the output with 2 A load.

At my main panel a few minutes ago my phase voltages were 124.0 and 124.4 V. The voltage read at this main panel point will be a function the power company transformer internal impedance, the supply lines, my neighbor's present loading, and my loading.

Under normal conditions the difference between my two phases is usually less than 1 to 2 V. But I don't apply large unbalanced loads.

At a main panel you can generally expect some difference between the two phase. It is when this difference gets large that you question whether there is a neutral problem or not.

A test at my main panel just now with a 10.7 A resistive load on one phase resulted in the following:
The change in voltage on the load phase was -0.8 V, and on the other phase +0.4 V. This means each wire from my panel to the transformer has a resistance of about 0.4/10.7 = 0.037 ohms.

This test also makes the neutral connection look good, because the voltage drop on the hot wire is the same as the neutral wire.

.

 

[ggunn]

Are you sure about that?

An open neutral is one that is not connected to the POCO's neutral. Connection to the earth has nothing to do with preventing an open neutral. It may provide some path back, but that's not what the intention of the bonding of neutral to ground is.

You are correct of course; if the neutral were to get separated from the POCO neutral and still be connected to ground the branch voltages would still wander. Usually when the neutral gets disconnected it gets separated from both the POCO neutral and the ground, though it is the separation from POCO neutral that causes the problem. I don't know what I was thinking.