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kingdaddy

05:02AM | 02/05/07
Member Since: 02/04/07
3 lifetime posts
Bvelectrical
A friend of mine recently had a scary situation where her power went out in her home and all of her electrical appliances started smoking. I had her shut off the main at the box and call the electrical company. They came out and repaired an open ground. All of the TV's, VCR's, computers, etc were damaged beyond repair. The washer, dryer, dishwasher and refrigerators were not damaged. I am just wandering what causes this type of open ground condition???

househelper

05:27AM | 02/05/07
Member Since: 03/31/05
265 lifetime posts
Correctly called an open neutral. Biggest offenders are squirrels. They like to chew on the uninsulated aluminum. Many jurisdictions are going to copper neutrals for the service drop to prevent this problem.

Billhart

05:41AM | 02/05/07
Member Since: 04/25/05
1915 lifetime posts
Residential power comes from a 240v transformer with a center tap. From that center tap to either hot wire you get 120 volts. Or between the two hots you get 240 volts.

The wire connecting the center tap to the house is called the Neutral. The reason is that it only cares the difference in currents between the what is being drawn on the 2 120 volt legs.

The neutral is connected to the "ground" at the pole and at the house and the technical term for the neutral is the Grounded Conductor. So it can be confused with the "ground".

The "ground" consists of connection to ground rod(s), metal water lines, and other metal that is burried in the ground. However, the ground is a poor conductor and won't carry the currents needed if the neutral is cut.

Without the neutral you have two sets of 120 volt loads across 240. That will be OK as long as they are identical.

But say that you have a 50 watt VCR on one leg and a 1500 watt toaster on the other. Without the neutral you have a "tug of war" between the two. And the toaster tries to take all of the power and can't get it so that the VCR is essentially across the 240.

kingdaddy

06:13AM | 02/05/07
Member Since: 02/04/07
3 lifetime posts
A couple more questions, I assume that damage is happening on a pole some where outside the house? What causes the appliances to superheat, is the system trying to establish ground through these appliances? What dangers are their to the occupants when this happens...electrical shock..?

Billhart

07:12AM | 02/05/07
Member Since: 04/25/05
1915 lifetime posts
I gave some more details below.

But to your specific questions the break in the neutral can be at the trasnformer, the arial or underground cable, where it is spliced to the feeder wires at the weather head, at the connections to the meter, the meter to service disconnect (usually the main panel), or from the serice disconnect to subpanels, if any. Of courese if the problem is feeder to a sub-panel then it only affects those loads that are on the sub-panel.

But the connection to the transformer, and the weahter head are all exposed to weather and are most likely source of problems.

One of the hots could have also failed and they power would be lost to 1/2 of the 120 loads and the 240 loads won't work correctly. Or if turn on something like an electric stove then it might back beed the other 120 v loads, but the stove would barely get warm.

As to a shock hazzard. There really should not be one, ALL OTHER THINGS BEING correct. At the serivce disconnect (most often the main panel) the neutral is bonded to the Ground Electrode wiring (what goes to ground rods, underground metal water pipe), bonds to the water pipe if it is metal inside, but not outside, and the Equipment Grounding Conductors (the EGC or what is common called "the ground").

The whole idea is that inside the house everything be at the same potential. It really does not matter what that is, but if everything in the house is the same potential there is no shock hazard.

But also note that older house sometimes have defective grounding and bonding.

One example of this is that it used to be allowed to attach EGC's to local coldwater pipes. Now if a section of that pipe is replaced with plastic pipe you lose that ground connection. That is why it is no longer allowed.

But, as you found out, there is danager to the equipment in use and that can start a fire.

Also the problem indicates a bad connection and if it is in the panel or meter or internal wiring that bad connection can heat up enough to cause a fire.

kingdaddy

08:20AM | 02/05/07
Member Since: 02/04/07
3 lifetime posts
So what actually causes the damage to the appliances, in this case the TV's, VCR's, DVD players, Stereos, computers were all destroyed, the other "non solid state" appliances are fine (dishwasher, washer and dryer). In this case, the electrical utility takes no responsibility for the damage. Thanks so much for the help with this one.

jackofallmasterof1

10:49AM | 02/22/07
Member Since: 08/03/05
20 lifetime posts
daddy, your friend needs to push the power co. to replace her damaged items AND have her electrical system checked by a licensed electrician if the problem was caused by their equipment or cables. In laymans terms the neutral is the balance, or pivot point for a transformer. What happens is that when a neutral connection fails, the voltage on one leg can exceed 250 volts while the other leg can fall to almost zero. Any equipment that is being fed by the leg that rises can be literally smoked. I work as a subcontractor for my local power company checking systems, repairing appliances, and replacing damaged items (flourescent ballasts, doorbell transformers, HVAC transformers, etc.)and I can tell you that their claims / loss dept. paid out over $16,000 to my company in the second half of '06. Push it.

Jack

househelper

05:32AM | 02/23/07
Member Since: 03/31/05
265 lifetime posts
"the voltage on one leg can exceed 250 volts while the other leg can fall to almost zero."

For a loose neutral connection, that is not true. The voltage can never exceed 240V since that is all the transformer can supply. Typical unbalance is 150/90. That 150V is more than enough to fry the power supply in electronic devices.

Most POCOs may pay a portion of the damage if it can be attributed to their problem, but it is usually only up to the deductible of your homeowners insurance.
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