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The Inspector's Journal

Grounds/Neutrals Not Isolated


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Would you be OK with the neutral feeder between the panels being bare or un-insulated wire or using metal conduit as the neutral conductor? 'Cos that's what you can be effectively creating. The exact scenario where that neutral current could be fatal is a bit fuzzy in my old head (and may involve standing in a bucket of salty water), but I'm comfortable that there's sufficient reason for the insulation on neutrals, and the need to keep that current from traveling along the grounds.

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1) With the neutral and grounding conductor on a feeder connected in two places, neutral current is divided between the two instead of carried solely by the neutral as planned. The neutral is sized for the load but the grounding conductor is smaller and cannot carry near as much. It may overheat under a heavy but otherwise safe neutral load. We all know what can happen when conductors become overheated.

2) With neutral and grounding conductors alike connected to the same bus-bar in a sub panel, a break in the neutral connectivity between the sub and the main causes the grounded conductor to carry the full current load. If that overheats or otherwise causes a connection to fail, the entire neutral/grounding bus in that sub can become energized with up to 120 volts and so will the sub enclosure and every appliance, fixture, electrical box and device connected to it.

There are more hazardous scenarios than these that I can't recall.

Marc

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When neutral and grounds are connected in more than one place, it causes electricity to flow in places where it was not intended, such as through other grounded metal parts (water pipes, shielding on coax, etc).

"Electricity, like teenagers, is not known for making good decisions."

(I wish I could take credit for that line, but I'm not that clever)

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At the main panel, the return path back to the source thru the service conductors is kept short as possible, and most of the current follows the path of least resistance.

if the neutral is bonded at the sub panel, the grounding conductors will carry some current whenever the neutral is energized.

The bare copper conductor between the sub panel and the main panel will therefore be a shock hazard.

If copper plumbing pipes are bonded to the sub panel, then bonding the neutral at that point puts a potential shock hazard on the plumbing fixtures. A fraction of one amp is enough to cause a human heart to stop pumping.

Wet skin conducts electricity better than dry skin.

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Please describe a couple real-life scenarios where one can be harmed by having the ground wires and the neutral wires connected on the same bus bar in a panel that's beyond the service equipment.

As long as the service neutral and the feeder neutral are in good condition, there's pretty much no chance of physical harm.

However, if, for some reason, the resistance of the neutral were to increase or, most dramatically, if it were to break, then having the ground wires connected to the neutral terminal is a sub panel could kill someone.

It's like a seat belt. As long as nothing else goes wrong, it doesn't do much.

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Please describe a couple real-life scenarios where one can be harmed by having the ground wires and the neutral wires connected on the same bus bar in a panel that's beyond the service equipment.

As long as the service neutral and the feeder neutral are in good condition, there's pretty much no chance of physical harm.

However, if, for some reason, the resistance of the neutral were to increase or, most dramatically, if it were to break, then having the ground wires connected to the neutral terminal is a sub panel could kill someone.

It's like a seat belt. As long as nothing else goes wrong, it doesn't do much.

I respectfully disagree a little bit. [:)]

It doesn't do much only if the load on the sub is balanced perfectly or if nobody comes in contact with the bare wire, with is usually covered with a thin outer sheath.

At the main panel, the grounding conductor presents a high resistance compared to the neutral, so very little current will flow there. At the subpanel, the neutral and the ground in a 4 wire cable offer a similar resistance, being of similar or equal gauge and length. Juice will flow thru both wires when one side of the panel is loaded, if neutral is bonded to the subpanel.

If a 120 volt space heater is plugged in to that subpanel, about 1/2 of the current would be carried by that grounding conductor.

Edit: Or in Chicago, by the conduit.

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These pictures are from an apartment where someone was being shocked in their kitchen. The genius that "repaired" it decided to bond every neutral to the panel enclosures. The wiring was all in EMT.

The picture with my meter shows a half amp on one of the pieces of EMT. That's the same amount of current a 60Watt bulb would draw, except in this case that energy is just being expended to heat up the conduit.

In the kitchen, the client who hired us had experienced a mild shock when touching the stove and refrigerator at the same time, after the "repair." UL refers to these low level shocks as "startle current." They really can't have more voltage behind them than the voltage drop of the neutral, yet it is enough to get your attention.

The inductive reactance heating of those conduits is racking up the electric bill. It will also create odd magnetic fields. If you have a high-end stereo, you will get a hum on your speakers.

Even though the consequences are usually not severe, it is the first step on the slippery slope. Most electrical accidents are the result of more than one thing going wrong, and this one should be fixed.

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These pictures are from an apartment where someone was being shocked in their kitchen. The genius that "repaired" it decided to bond every neutral to the panel enclosures. The wiring was all in EMT.

The picture with my meter shows a half amp on one of the pieces of EMT. That's the same amount of current a 60Watt bulb would draw, except in this case that energy is just being expended to heat up the conduit.

In the kitchen, the client who hired us had experienced a mild shock when touching the stove and refrigerator at the same time, after the "repair." UL refers to these low level shocks as "startle current." They really can't have more voltage behind them than the voltage drop of the neutral, yet it is enough to get your attention.

The inductive reactance heating of those conduits is racking up the electric bill. It will also create odd magnetic fields. If you have a high-end stereo, you will get a hum on your speakers.

Even though the consequences are usually not severe, it is the first step on the slippery slope. Most electrical accidents are the result of more than one thing going wrong, and this one should be fixed.

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tn_20121023215834_BondedNeutralApartment1.jpg

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tn_20121023215853_BondedNeutralApartment2.jpg

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Maybe I should begin lugging that clamp-on around after all.

Marc

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Please describe a couple real-life scenarios where one can be harmed by having the ground wires and the neutral wires connected on the same bus bar in a panel that's beyond the service equipment.

As long as the service neutral and the feeder neutral are in good condition, there's pretty much no chance of physical harm.

However, if, for some reason, the resistance of the neutral were to increase or, most dramatically, if it were to break, then having the ground wires connected to the neutral terminal is a sub panel could kill someone.

It's like a seat belt. As long as nothing else goes wrong, it doesn't do much.

I respectfully disagree a little bit. [:)]

It doesn't do much only if the load on the sub is balanced perfectly or if nobody comes in contact with the bare wire, with is usually covered with a thin outer sheath.

At the main panel, the grounding conductor presents a high resistance compared to the neutral, so very little current will flow there. At the subpanel, the neutral and the ground in a 4 wire cable offer a similar resistance, being of similar or equal gauge and length. Juice will flow thru both wires when one side of the panel is loaded, if neutral is bonded to the subpanel.

If a 120 volt space heater is plugged in to that subpanel, about 1/2 of the current would be carried by that grounding conductor.

Edit: Or in Chicago, by the conduit.

You've explained why current will flow in the grounding conductor, not why that would cause harm if everything else in the system were working properly.

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Please describe a couple real-life scenarios where one can be harmed by having the ground wires and the neutral wires connected on the same bus bar in a panel that's beyond the service equipment.

As long as the service neutral and the feeder neutral are in good condition, there's pretty much no chance of physical harm.

However, if, for some reason, the resistance of the neutral were to increase or, most dramatically, if it were to break, then having the ground wires connected to the neutral terminal is a sub panel could kill someone.

It's like a seat belt. As long as nothing else goes wrong, it doesn't do much.

I respectfully disagree a little bit. [:)]

It doesn't do much only if the load on the sub is balanced perfectly or if nobody comes in contact with the bare wire, with is usually covered with a thin outer sheath.

At the main panel, the grounding conductor presents a high resistance compared to the neutral, so very little current will flow there. At the subpanel, the neutral and the ground in a 4 wire cable offer a similar resistance, being of similar or equal gauge and length. Juice will flow thru both wires when one side of the panel is loaded, if neutral is bonded to the subpanel.

If a 120 volt space heater is plugged in to that subpanel, about 1/2 of the current would be carried by that grounding conductor.

Edit: Or in Chicago, by the conduit.

You've explained why current will flow in the grounding conductor, not why that would cause harm if everything else in the system were working properly.

I'm pretty sure you could get a shock from any bare wire or conduit carrying 120 volts, even if the amperage is low. Especially in bare feet on a basement floor.

What do you feel is the reasoning behind the code rule, if not for the sake of safety?

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Please describe a couple real-life scenarios where one can be harmed by having the ground wires and the neutral wires connected on the same bus bar in a panel that's beyond the service equipment.

As long as the service neutral and the feeder neutral are in good condition, there's pretty much no chance of physical harm.

However, if, for some reason, the resistance of the neutral were to increase or, most dramatically, if it were to break, then having the ground wires connected to the neutral terminal is a sub panel could kill someone.

It's like a seat belt. As long as nothing else goes wrong, it doesn't do much.

I respectfully disagree a little bit. [:)]

It doesn't do much only if the load on the sub is balanced perfectly or if nobody comes in contact with the bare wire, with is usually covered with a thin outer sheath.

At the main panel, the grounding conductor presents a high resistance compared to the neutral, so very little current will flow there. At the subpanel, the neutral and the ground in a 4 wire cable offer a similar resistance, being of similar or equal gauge and length. Juice will flow thru both wires when one side of the panel is loaded, if neutral is bonded to the subpanel.

If a 120 volt space heater is plugged in to that subpanel, about 1/2 of the current would be carried by that grounding conductor.

Edit: Or in Chicago, by the conduit.

You've explained why current will flow in the grounding conductor, not why that would cause harm if everything else in the system were working properly.

I'm pretty sure you could get a shock from any bare wire or conduit carrying 120 volts, even if the amperage is low. Especially in bare feet on a basement floor.

What do you feel is the reasoning behind the code rule, if not for the sake of safety?

The couduit wouldn't be energized at anything like 120 volts. Not even close. In order for that to happen, something else would have go wrong.

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At the main panel, the grounding conductor presents a high resistance compared to the neutral, so very little current will flow there.

Since the neutral is bonded to the ground in a service panel the ground would not show a high resistance compared to the neutral.

I mean the path back to the source, the transformer on the pole. The neutral presents a direct connection with minimal R while the grounding wire connects to the transformer by way of a ground rod, a bunch of dirt and another rod.

But you are right, the potential is equal on both because they are connected together. I just know that it works for the main service panel, but when you go to a 4 wire feeder, you have to keep the neutral isolated or there will be a shock potential.

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The couduit wouldn't be energized at anything like 120 volts. Not even close. In order for that to happen, something else would have go wrong.

Example: The only load on the subpanel is a 120 volt space heater, drawing 7 amps. Neutral and ground are erroneously connected together, and are equal gauge copper wire in a 4 wire feeder cable. The voltage is equal on both conductors. Current will be split, 3.5 amps on each. Correct me if I got it backwards.

Edit: I had it backwards. Current is equal for all parts of the circuit and voltage varies with the resistance. The high resistance portions drop the most voltage.

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I think you've got it wrong. The current will take it's easiest path back to earth - through the grounded conductor, because via the ground path via the equipment-grounding conductor is a higher-resistance path. It's why you can touch a grounded (neutral) bus in a panel and not get knocked on your keester unless you disconnect the grounded conductor from that bus and grab the end of it to turn yourself into the only available path.

ONE TEAM - ONE FIGHT!!!

Mike

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The couduit wouldn't be energized at anything like 120 volts. Not even close. In order for that to happen, something else would have go wrong.

Example: The only load on the subpanel is a 120 volt space heater, drawing 7 amps. Neutral and ground are erroneously connected together, and are equal gauge copper wire in a 4 wire feeder cable. The voltage is equal on both conductors. Current will be split, 3.5 amps on each. Correct me if I got it backwards.

In theory, yes, the current will be split. But there will be not be much voltage on either one. If everything else in the system is in proper working condition, you will not be shocked if you touch the grounding wire or the neutral wire. There is no significant voltage on either one. Connecting them together at a sub panel doesn't change this.

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I mean the path back to the source, the transformer on the pole. The neutral presents a direct connection with minimal R while the grounding wire connects to the transformer by way of a ground rod, a bunch of dirt and another rod.

But you are right, the potential is equal on both because they are connected together. I just know that it works for the main service panel, but when you go to a 4 wire feeder, you have to keep the neutral isolated or there will be a shock potential.

John, there is no 'grounding wire' on the electric utility lines, only a neutral and the hot wires. 'Grounding wire' begins in the consumer load centers. It's the neutral that the utility runs into the earth with ground rods.

Marc

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I mean the path back to the source, the transformer on the pole. The neutral presents a direct connection with minimal R while the grounding wire connects to the transformer by way of a ground rod, a bunch of dirt and another rod.

But you are right, the potential is equal on both because they are connected together. I just know that it works for the main service panel, but when you go to a 4 wire feeder, you have to keep the neutral isolated or there will be a shock potential.

John, there is no 'grounding wire' on the electric utility lines, only a neutral and the hot wires. 'Grounding wire' begins in the consumer load centers. It's the neutral that the utility runs into the earth with ground rods.

Marc

Thanks, Marc. I know I'm arguing both ends here so I'll go back to what I know.

Here is a picture I took last spring when we were discussing clamp meters.

That wire being measured is a power cord plugged into a 120 volt outlet. The meter shows 6 amps on the neutral, marked with white tape. Is there a shock potential here? You bet.

My argument is that if you attached another wire here running back to the panel, it too would carry enough voltage and current to do some serious shocking.

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I mean the path back to the source, the transformer on the pole. The neutral presents a direct connection with minimal R while the grounding wire connects to the transformer by way of a ground rod, a bunch of dirt and another rod.

But you are right, the potential is equal on both because they are connected together. I just know that it works for the main service panel, but when you go to a 4 wire feeder, you have to keep the neutral isolated or there will be a shock potential.

John, there is no 'grounding wire' on the electric utility lines, only a neutral and the hot wires. 'Grounding wire' begins in the consumer load centers. It's the neutral that the utility runs into the earth with ground rods.

Marc

Thanks, Marc. I know I'm arguing both ends here so I'll go back to what I know.

Here is a picture I took last spring when we were discussing clamp meters.

That wire being measured is a power cord plugged into a 120 volt outlet. The meter shows 6 amps on the neutral, marked with white tape. Is there a shock potential here? You bet.

My argument is that if you attached another wire here running back to the panel, it too would carry enough voltage and current to do some serious shocking.

John,

Neutral wires nearly always have current on them but they very rarely have much voltage.

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The neutral in that picture is carrying 6 amps back to the receptacle. I was saying for example, a severely unbalanced load where only one leg of the subpanel is used, the neutral carries a lot of current. It's not a malfunction, but there is a shock potential, no?

OK, I'll have to measure the voltage.

Jim, you are correct, of course.

Thanks, Douglas, I will let you have the final say. I measured the neutral voltage on my zip cord with the space heater energized and it was a mere 0.2 volts at 6 amps.

Then when I clipped a 330 ohm ceramic resistor in series into the neutral wire, I measured 109 volts across that resistor and considerable heat was generated! I cut power in less than a minute and that little resistor was smoking hot.

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If you get in series with the neutral, you could indeed be electrocuted. When you are in parallel with the neutral, even assuming you are standing barefoot in a puddle of water and you are making a great connection to the earth where the utility drove their ground rod under their transformer, the only voltage you experience is approximately the voltage drop of the neutral. It can be enough to measure on the ammeter without being at a voltage level that will even produce a startle current.

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