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Old 02-28-2010, 11:44 AM   #1
 
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"Hot" polarity compared to coax/other circuits


When we recently plugged in our new TV and coax, it tripped the circuit with what was clearly a short. When plugging the same TV into another receptacle, everything works fine. So, when using a three-prong tester, we found the circuit polarity on the first receptacle was reversed. After switching, we get the same problem. In addition, the entire branch that receptacle belongs to has reversed polarity (more on that in a minute).

When checking voltages, the "bad" circuit now reads fine:
H-N: 119
H-G: 119
N-G: 0

But, when comparing the "bad" circuit to the coax ground, it is up 119V:
H-CoaxGnd: 0
N-CoaxGnd: 119
G-CoaxGnd: 119

By comparison, a known "good" circuit is correct:
H-CoaxGnd: 119
N-CoaxGnd: 0
G-CoaxGnd: 0

Finally, when comparing the "bad" circuit with a known "good" circuit, I get:
Bad Good Voltage
G G 119
G N 119
G H 239
N G 119
N N 119
N H 239
H G 0
H N 0
H H 119

One more thing: the "bad" circuit is the only remaining old circuit in a 1920s-era house, and it supplies way more than it should (I will redo it and split it up sometime...) using BX/armored cable. The circuit is fine from the breaker to the first receptacle, but sometime after the polarity gets reversed - haven't pinned that down yet. The switches all switch the white wire, but that seems to be hot. They also use a red tied to black as a pass-through even without a 3-way switch in most cases. This would make sense to me if red/black were hot, but they seem to be neutral on this circuit.

So, correct me if I'm wrong, but it seems like the neutral on the "bad" circuit was at some point tied hot, and now it is setup like a 240V circuit with respect to the rest of the house. How would this happen without tripping a breaker? It seems like maybe the shielding/ground on the BX circuits might be tied to the hot used for the rest of the house - could just be a frayed wire or incidental contact in the box? Any other ideas how this might happen?

No problems with anything else in the house, but the potential dangers of this setup are clear. Thanks.
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Old 02-28-2010, 12:06 PM   #2
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You have two problems:

1. Neutral and hot reversied i.e. white wire was hot,

2. Ground tied/bonded or shorted/faulted to neutral; it should not be except at the panel.

Since you mentioned old wiring, you need to also check if the previous owner of the house decided to not even have a separate ground wire and connect the ground pin of the receptacle to the white wire (also incorrect).

In a standard 120/240 volt service there are two hots (call them A and B; one may or may not be red) which measure 240 volts between them. If your hot and neutral were reversed on a circuit on side A and the known good outlet were on side B then you would measure 240 volts from white on the bad circuit to black on the known good circuit.

Very old wiring did have switches in the neutral if it was more convenient to do it that way. Modern wiring has switches only on the hot side.

Red can be used as a hot wire anywhere. Most commonly red is used when (1) switched hot and unswitched hot and neutral are carried in the same cable, (2) in a 3 way switch setup, or (3) when both sides of the 240 volt system and also the neutral are carried in the same cable. When unswitched hot and switched hot are in the same prefabricated cable with no neutral, white may be the unswitched hot if there is no red.

Half of the regular receptacles are on side A and the other half on side B. You would never encounter 240 volts and may well never discover the bad circuit if you just used ordinary 120 volt equipment and no equipment required a coax cable connected between them.
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Last edited by AllanJ; 02-28-2010 at 12:24 PM.
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Old 03-02-2010, 01:22 AM   #3
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Quote:
Originally Posted by reader4 View Post
No problems with anything else in the house, but the potential dangers of this setup are clear. Thanks.
The other poster explained the likely problem (reversed hot and neutral, with neutral and ground bonded downstream of the reversal) - but I think the degree of danger here bears reiteration. This can kill you and your family or burn your house down at any moment. This is not a condition to be left on, even for a minute. All "grounded" objects connected to that circuit are actually hot. Any metal appliance you have plugged in, any metal boxes or switchplates, all metal conduit and cable, and any ductwork or other metal in the house that touches any of these things is energized. It's a disaster waiting to happen at any moment.
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Old 03-02-2010, 01:03 PM   #4
 
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Okay, this is what I suspected. I need to edit the data I provided in the first post a bit, though. The "bad" outlet was reversed at the receptacle with respect to white/black in this circuit (to "fix" the reversal noted using a 3-prong tester in the first place), so any other outlet on this circuit compared to a "good" circuit would read:
Bad Good Voltage
G... G..... 119
G... N..... 119
G... H..... 239
H... G..... 119
H... N..... 119
H... H..... 239
N... G..... 0
N... N..... 0
N... H..... 119

Which seems to imply still that the white is shorted to ground (armor) somewhere in the circuit, but that the white is actually "hot". Correct? If the breaker to the house is turned off, I should read an infinite impedance between HOT and NEU or GND in any circuit, and continuity between NEU/GND and any other NEU/GND, correct? If the white (HOT) side of this circuit is indeed tied to the BX armor (GND), shouldn't this trip the breaker continuously? I don't think the armor is floating, but I can check. Interestingly, the only grounded equipment in the whole house is the TV on that circuit and a computer monitor on the same circuit. The monitor, however, is on a branch separate from the BX and does not have this voltage differential problem. Sounds like once I track down the short and get the polarity switched this should be resolved until we can replace the wiring entirely... easier said than done, I'm sure.

Also, the two circuits I checked were indeed on opposite sides of the breaker box, as expected for a 240V circuit. You are right - if these two circuits weren't next to each other in the room, and the coax wasn't used with the one circuit, we would never have known about this condition.

GND is setup properly in the breaker and the rest of the work in this house has passed inspection for other remodeling projects.

Last edited by reader4; 03-02-2010 at 01:36 PM.
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Old 03-02-2010, 01:28 PM   #5
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The ground is definitely not grounded or it would be tripping the breaker. What type of meter are you using? If you are using a digital meter some of those readings could be false phantom voltages.

What happens if short the good and bad ground together? Does a circuit trip?
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Old 03-02-2010, 02:08 PM   #6
 
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Both digital and galvanometer types.

Yes, the breaker trips when the grounds are tied together... the BX armor must be floating. We first discovered this problem after installing the new digital TV, which uses a grounded plug. The same ground is shared between the plug and the coax jack, so the TV shorts the two grounds and trips the breaker unless it is plugged in to a different circuit.
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