electrical system "dropped a leg"

I won't go into all of the details, and the problem is fixed now. But this morning when my daughter's alarm clock went off, the electricity was off in some rooms of the house (including the master bedroom). some rooms had plenty of juice. One room had 2 computers running. They are on UPSs, but the UPSs weren't beeping.
Nothing on 240V that I tested was working, except that the clothes dryer would spin (I don't know if it was heating). Our water pump (on 120V, I think) was not working. I checked the circuit breakers, and they were OK. After I did that, the lights came back on in 3 rooms that had not had electricity. A TV started repeatedly trying to turn itself on, but it couldn't, and each time it caused the lights to blink elsewhere. I had to unplug it.
Upstairs there was a UPS, but the computer was off. The UPS had almost no load on it, just two wall warts, I think. I noticed that if I turned on the overhead light in the adjoining room, the UPS would start beeping.
Some of the electrical outlets showed their usual 120V and some of them showed only 110V.
I called the electric company and reported a partial outage. They found the problem, saying something about "dropping a leg", and dug up the cable in the yard and fixed the problem.
What is "dropping a leg"?
As I understand it, you have 3 wires, two of them carry 240V and the third is a ground back to the power company. You get 120V between either of the hot wires and the ground. Is dropping a leg related to that?
Why would some rooms have electricity and others not, in a situation like this?
--
Replace you know what by j to email

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I wouldn't describe it quite that way. You have a 240V circuit, with a centre tap (normally called neutral, the fact that it's also grounded isn't relevant in this context). You get 120V between either hot and neutral. Each 120V circuit is the neutral plus a connection to one or the other of the two hots.

Yup. You lost one of the hot legs. 120V circuits "on that side" failed. Since you only had one working conductor in your 240V circuits, they didn't work either. Your dryer spun because the motor is 120V (and it was on the working side), but the heating element didn't, because one end of the element was disconnected.

Simply depends on which 120V "leg" they were attached to.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 21 Sep 2004 19:30:43 GMT, snipped-for-privacy@nortelnetworks.com (Chris Lewis) wrote:

OK, so part of the 120V in the house could be on one leg and part on the other, and that is why part of the house was working and the other wasn't.
A related question: what kind of wave is on the 240V line - sine, square, or other? IS the 120V the same, or is it only one side of the 240V wave?
--
Replace you know what by j to email

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

http://encyclopedia.thefreedictionary.com/alternating%20current
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload



Right.

Both 120V and 240V circuits are sine wave when you compare one lead to the other. Or when you compare any hot lead to neutral or ground.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
(Chris Lewis)

Each leg of the 240V service is a sine wave. They are 180 deg out of phase with each other.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 21 Sep 2004 19:30:43 GMT, snipped-for-privacy@nortelnetworks.com (Chris Lewis) wrote:

If it was going bad (i.e. a bad connection), could that have made my electric bill higher? The last two months it has been somewhat higher than expected.
--
Replace you know what by j to email

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Is your meter before or after the damaged section?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 22 Sep 2004 22:14:08 GMT, snipped-for-privacy@aol.com (HA HA Budys Here) wrote:

The damaged part of the cable was between the house and the transformer.
--
Replace you know what by j to email

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload



It's unlikely to have affected your electrical bill, except when it was actually not connected at all.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Proclaimed on Tue, 21 Sep 2004 15:25:20 -0400,

A leg is 120V feeding your house, you have two. To get 220V (your dryer) both are used. Usually half of the house is run on one (phase) and the other is split. There is no method set in stone to distribute the legs.
As to your voltage readings, that will fluctuate throughout the day 110 is the lowest it should go, but testing the circuit again at 3:00 AM would probably yield a reading of over 120 VAC. It is dependant on usage in the area. The one constant is time, measured in Hertz, which is 60 cycles per second, that is VERY accurate and the electric company works hard to maintain (your analog clocks rely on it).
-Graham
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Interesting problem. First it sounds as if one of the hot legs is open. Then you say the lights come on and turning one other devices causes lights or other devices to be effected. That sounds like neutral problem - or the open hot somehow reconnected, but in a high resistance state causing the odd behavior. John

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

That's what it was. I'd never heard of that until this morning. I thought it was either a brown out or a poltergeist :-)
--
Replace you know what by j to email

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

All the symptoms are explainable by a loose main feed hot in the panel (or upstream) making intermittent (and poor) connection. Consistent with "dropped a leg" terminology.
If the neutral comes loose, 120V circuits on _both_ legs start behaving "funny", but 240V circuits are unaffected. By "funny", I mean that some 120V circuits show low voltage (brownout) and some show high voltage (lights overbright).
The former isn't very pleasant at all, but unless you're unlucky (as per the intermittent connection), no damage will be done and it doesn't present a shock hazard.
The latter can be _extremely_ dangerous when 120V devices try to cope with anywhere up to 240V. Boom. Flames. Etc.
Not only that, under some circumstances, the entire grounding system in the house can go "hot".
Whenever you see something this drastic happening, it's time to kill the main breaker and get help ASAP.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 22 Sep 2004 04:28:17 GMT, snipped-for-privacy@nortelnetworks.com (Chris Lewis) wrote:

The power company dug up part of the cable running to my house and fixed it there. They said that the cable TV company had cut the wire and water got in it. (We've had a lot of rain lately.) I assume that either the cable TV company repaired the cut, but didn't do it properly to seal out the water, or partially cut it.

That's why I got on it right away. I thought it could possibly be something wrong in my house. I called the electrician, and they asked if the stove would come on. It wouldn't. They said to have the power company come out and check it first, and that's what I did.

I didn't kill the main breaker (in retrospect I should have). I did call the electrician ASAP.
--
Replace you know what by j to email

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.