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Thread: 240V across mains but not across bus bars?

  1. #16
    Moderator & Master Plumber hj's Avatar
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    Measure voltage from the top/incoming of each breaker to the bus below it. IF the breaker is good the voltage will be zero, if bad it will be something other than zero. forget about the water heater or anything else. If you have 240 to the main breaker and anything other than 240 AFTER it, the problem is in the breaker or its connections. You are making your problem WAY too difficult. All you have to do is make FOUR measurements and you will know where the problem is. Feed to feed, bus to bus, feed to bus1 and feed to bus2.
    Last edited by hj; 02-18-2012 at 03:55 PM.
    Licensed residential and commercial plumber

  2. #17
    Test, Don't Guess! cacher_chick's Avatar
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    Quote Originally Posted by hj View Post
    Measure voltage from the top/incoming of each breaker to the bus below it. IF the breaker is good the voltage will be zero, if bad it will be something other than zero.
    Just a note that some loads should be on when you do this test. If there is no load on the side of the panel being tested, you will not get an accurate reading of the voltage drop.

  3. #18
    DIY Junior Member ColdShower's Avatar
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    Quote Originally Posted by hj View Post
    Measure voltage from the top/incoming of each breaker to the bus below it. IF the breaker is good the voltage will be zero, if bad it will be something other than zero. forget about the water heater or anything else. If you have 240 to the main breaker and anything other than 240 AFTER it, the problem is in the breaker or its connections. You are making your problem WAY too difficult. All you have to do is make FOUR measurements and you will know where the problem is. Feed to feed, bus to bus, feed to bus1 and feed to bus2.
    Well I still don't understand...maybe I should have a picture of my panel...or take a look at a standard one. My breakers connect directly to the bus bars...thats the "incoming"...and there are two contacts on each breaker (two breakers really on each unit)...and each one makes contact with a different bus. So there's like two buses above and below each breaker and two leads coming off.

    I'm not sure if this house was ever getting more than ~200V combined.

  4. #19
    Retired Defense Industry Engineer jadnashua's Avatar
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    Look at it this way...the CB are just smart disconnect switches. A switch connects the input to the output. There should be effectively a direct short between the input and the output. If you were to measure with your meter on one end of the bus bar to the other end of the same bus bar, with the meter you likely have, you'd measure zero volts, since the bus bar is acting like a short connecting the two ends. A CB is similar. So, if you measure from the inlet to the outlet across the CB, it should read zero volts since the switch inside should be closed (or shorting the inlet to the outlet). If you measure some voltage, that means that the switch isn't working properly and is acting like a load - dropping some voltage inbetween.

    It can get a little messy depending on the load, if any, and sometimes you need a load to get an accurate reading.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #20
    Test, Don't Guess! cacher_chick's Avatar
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    Is is a Federal Pacific panel?

  6. #21
    DIY Junior Member ColdShower's Avatar
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    ok I think I see...but I'm pretty sure the two suspect breakers are working ok since I tried those circuits on other breakers with the same result...albeit I did do a lot of different measurements putting the panel back together and may have got confused over what I was doing. I'll measure that way and see though.

    Apparently the panel is made by Zinsco...the breakers look like this:

    http://www.amazon.com/Connecticut-El.../dp/B000H5R6CI

  7. #22
    Test, Don't Guess! cacher_chick's Avatar
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    The Zinsco panels are well known to have problems with degradation of the buss and the breaker to buss connections.

    If it were my panel, I would have the meter pulled and remove all the breakers including the main to inspect for any sign of corrosion or arcing at the buss to breaker contact points.

    Many electricians recommend replacing these panels due to their reputation alone.

  8. #23
    DIY Junior Member ColdShower's Avatar
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    Quote Originally Posted by cacher_chick View Post
    The Zinsco panels are well known to have problems with degradation of the buss and the breaker to buss connections.

    If it were my panel, I would have the meter pulled and remove all the breakers including the main to inspect for any sign of corrosion or arcing at the buss to breaker contact points.

    Many electricians recommend replacing these panels due to their reputation alone.
    yeah it's a crap design...constant maintenance and expensive breakers. I didn't think I had a shut-off at the meter but I was wrong. Pried off the main and the connection on that one bus bar was "tight" but all like carbonized. Filed it off and adjusted the contacts and it's working...for now. I'll probably need a new main breaker before long...but first a hot shower! :-)

    thankya

  9. #24
    General Engineering Contractor ballvalve's Avatar
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    A good case made for better metals on the bars and the use of anti-oxidants. I had nearly the same problem, had to change the hot main, [bolted in, so not a fun bit of work] didnt want to go thru the hassle with the smart meter pull. One leg of the main gave partial readings relative to the meter bars.

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