Spark from ground to neutral???

[OTT2]

The impedance would roughly be the same since we are talking about EGC that would run parallel with the Neutral.

Why do you think the EGC wiould have higher impedance than the neutral?

Like I said, just my theory. A typical 200 amp feeder might likely have a 3/0 grounded conductor and a #6 AWG EGC, Cu on both. Table 8 tells me that the DC resistance of a #6 AWG is about 639% more than that of a 3/0 Cu. grounded conductor. The EGC of course is required to have an Effective Ground-Fault Current Path.

Just my theory though!

 

[weressl]

Like I said, just my theory. A typical 200 amp feeder might likely have a 3/0 grounded conductor and a #6 AWG EGC, Cu on both. Table 8 tells me that the DC resistance of a #6 AWG is about 639% more than that of a 3/0 Cu. grounded conductor. The EGC of course is required to have an Effective Ground-Fault Current Path.

Just my theory though!

I was just thinking of a single 15/20A circuit with same neu/ground sizing. Now I understand your thinking.

 

[a.bisnath]

grounding

grounding

met this at a cement plant 440volts 3 phase,220v to earth, check the protective multiple earthing grid, in fact used to get a voltage and a little shock from neutral to earth

 

[DWZipse]

Spark from ground to neutral???

You may have been measuring “stray current” originating from the primary neutral of the distribution system. Bear in mind little know fact: That the primary neutral of the service transformer is directly electrically connected to the secondary neutral in the service transformer of the utility. The secondary neutral is connected to the equipment grounding conductor which is connected to all metal enclosures. Some of these grounded metallic enclosures may be connected to earth beyond the service entrance.
Primary neutral current can and does flow into a site on its way back to the substation.

In order to determine the amount of stray current flowing into a home all one has to do is to place a clamp-on ammeter around ALL the service drop conductors and read the ammeter. Any reading of current will be the amount of stray current flowing into the home.
Also remember that the lightning arrestor is connected to the distribution neutral which is connected to the service entrance panel and to ALL receptacles and appliances in the home. Thus any lightning strike to or near the service transformer will be directed into the home and will probably destroy sensitive electronic equipment. This has been happening since 1932 and has been kept a secret.

 

[kwired]

Since this circuit is fed from a sub-panel, any load on this sub-panel can add or take away from the neutral load, this is because the sub-panel is fed via a multi-wire circuit that shares the neutral, and also the fact that the grounding and neutral are kept separate at this sub-panel, this allows the voltage drop of the neutral between the sub-panel and the main service panel or disconnect (the location of the main bonding jumper) to have a difference of potential, because there is no load on the grounding it will not have a voltage drop, so you are reading just like you are connecting one lead to the neutral at the main panel and the other lead to the neutral at the end of the circuit.
The voltage you are reading is the voltage drop in the neutral.
when you touch the neutral to the EGC at the load end, you have just added the resistance of the grounding path to the neutral thus lowering the over all resistance, this is the spark you see, but this allows dangerous current to flow through the grounding and can cause a shock or fire hazard, this is why the NEC does not allow the grounding to carry current.

I think you have the right idea but wrong wording. You would actually be adding more conductor which would result in less resistance. Connecting a conductor that had a higher resistance than the neutral would not cause a spark because there would be virtually no current flowing on the added conductor

 

[mivey]

I think you have the right idea but wrong wording. You would actually be adding more conductor which would result in less resistance. Connecting a conductor that had a higher resistance than the neutral would not cause a spark because there would be virtually no current flowing on the added conductor

As for wording:
"...Connecting a conductor that had a significantly higher resistance than the neutral..."

 

[gar]

100410-0905 EST

To see a spark there is enough voltage to cause a breakdown of the air between two conductors. Under standard conditions - air pressure, etc - air breakdown is about
http://en.wikipedia.org/wiki/High_voltage
See "Sparks in Air"
33 kV/CM or 83 kV/inch for large round balls for the electrodes. Lower for sharp points. A spark plug with a 0.03" gap at 8 atmospheres may breakdown at about 10 kV, a guesstimate, I pulled values out of my hat.

When you have current in an inductor and open the circuit a large voltage is generated across the inductor.

When the two wires contact there is some contact bounce and thus a large voltage generated.

An experiment:
A 1 VDC source, HP power supply, set to current limit at 225 MA.
A small 120 V input transformer used as an inductor. With this current is 40 MA. DC resistance is 25 ohms.
In the dark I can see a spark on connection, but it is really the contact bounce and opening that produces the spark. At 5 V the spark is visible under room light conditions.

So with a small voltage difference between neutral and EGC you can get a visible spark because of the bounce that occurs when the wires contact, and the breaking of an inductive circuit with current flowing producing a large voltage. The intensity is a function of the energy available. In the experiment above and clip leads only I could not see a spark at 225 MA.

.

 

[KP2]

WOW!!! There is alot more to this than I realized.
Thanks for all your input.

 

[ggunn]

WOW!!! There is alot more to this than I realized.
Thanks for all your input.

Aren't we making this more complicated than it needs to be? In a split phase 240/120 system, the neutral carries the difference between the return currents of the 120 loads if they are not exactly balanced. Since no wire has zero resistance at reasonable temperature, the current flow in the neutral creates a voltage drop, so at any point in the neutral conductor displaced from the neutral-ECG bonding point, the voltage on the neutral is non-zero. The ECG is non current carrying, so its voltage is zero. Short the neutral to the ECG somewhere other than the bonding point, current flows into the ECG from nonzero potential to zero potential. Spark. Am I missing something?

 

[mivey]

Aren't we making this more complicated than it needs to be?

SOP

In a split phase 240/120 system, the neutral carries the difference between the return currents of the 120 loads if they are not exactly balanced. Since no wire has zero resistance at reasonable temperature, the current flow in the neutral creates a voltage drop, so at any point in the neutral conductor displaced from the neutral-ECG bonding point, the voltage on the neutral is non-zero. The ECG is non current carrying, so its voltage is zero. Short the neutral to the ECG somewhere other than the bonding point, current flows into the ECG from nonzero potential to zero potential. Spark. Am I missing something?

Not much. Brian John covered this on page 1.

You could have currents from something other than loads. Also, an open neutral would spark when grounded.

 

[ggunn]

You could have currents from something other than loads. Also, an open neutral would spark when grounded.

Quite significantly, I'd think, if there was much difference in loading on the legs.

 

[mivey]

Quite significantly, I'd think, if there was much difference in loading on the legs.

And the open neutral on a two-wire is one that can sneak up on you if you are not careful. Another good reason to not go around touching neutrals.

 

[ggunn]

And the open neutral on a two-wire is one that can sneak up on you if you are not careful. Another good reason to not go around touching neutrals.

Wouldn't an open/floating neutral screw around with the voltages between the legs and the neutral pretty badly? Depending on which 120 loads you had turned on, it seems to me that you could have (for instance) 10 VAC on one leg and 230 on the other. The current on both legs has to be the same and the resistances of the loads can't change. Ohm sez the voltage has to move.

 

[mivey]

Wouldn't an open/floating neutral screw around with the voltages between the legs and the neutral pretty badly? Depending on which 120 loads you had turned on, it seems to me that you could have (for instance) 10 VAC on one leg and 230 on the other. The current on both legs has to be the same and the resistances of the loads can't change. Ohm sez the voltage has to move.

Absolutely on a 3-wire with the load example you gave. If the loads are closely balanced, it might not cause much of a spark, of course.

On a 2-wire, the full load causes the spark (and I can tell you it hurts if you fill the gap).