Page 1 of 2 12 LastLast
Results 1 to 13 of 21
  1. #1
    Join Date
    Nov 2008
    Location
    Northern California, foothills.
    Posts
    212

    Tripping breakers & blowing disconnect fuses. (Condensers)

    This is just a general troubleshooting question for condenser units
    or package units in cooling, not a specific service call.

    Regarding tripped circuit breakers and blown disconnect fuses, is it true
    they can only be caused by shorted windings or shorts to ground?

    .....because I'm thinking I've seen compressors that have failed to start,
    either because the run capacitor was shot or the compressor needed
    a hard-start kit, and despite the locked-rotor amps being sky high,
    circuit breakers and disconnect fuses were not affected.

    So if a circuit breaker is tripping very intermittently and resistance checks
    of the motors look fine, is it safe to place blame on the circuit breaker
    or the wiring between the panel and the unit?

    All input is appreciated. Thank you.

  2. #2
    Join Date
    Aug 2010
    Location
    Northern Cali
    Posts
    72
    If the compressor is short to ground the fuse or breaker will blow or trip instantly. If the problem is intermittent , to me that would mean sometimes , then you should take amp draw on compressor and see what you get . But in regards to your question: Regarding tripped circuit breakers and blown disconnect fuses, is it true
    they can only be caused by shorted windings or shorts to ground? I would say the answer is NO.

  3. #3
    Join Date
    Nov 2004
    Location
    Austin, TX
    Posts
    876
    I've had old 600v fuses blow and breakers trip for no reason except possibly age. I've had disconnects with high internal resistance (loose lugs and overheating wires) trip the breaker or overheat the compressor. I've added a manufacturer provided and tech support suggested hard start kit under warranty and discovered the disconnect was overheating behind the switch (looked like a breaker) after a callback. I'd check the lugs and make sure they're tight and the breaker has a good connection to the panel. Breakers are originally sized to protect the house wiring. The locked rotor amps may trip the internal overload (sized to protect the device) before it blows a slow blow fuse or breaker.

    As far as I know, these protection devices trip on heat, not on current. The current will produce heat and that is what trips overloads, breakers and fuses.
    High resistance in a circuit (bad connection or contact) decreases voltage. A motor will increase the current draw when a voltage drop happens. This makes the motor hot and may trip other circuit protections. I cold be wrong, this perspective has served me well up to now.
    I am for doing good to the poor, but...I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it. I observed...that the more public provisions were made for the poor, the less they provided for themselves, and of course became poorer. And, on the contrary, the less was done for them, the more they did for themselves, and became richer.
    ― Benjamin Franklin

  4. #4
    Join Date
    Jul 2009
    Location
    South Carolina
    Posts
    3,189
    A bad breaker that trips intermittently is usually pretty hard to diagnose. Only after checking everything I can think of & can't find a problem will i change a breaker. Unless its a square D outside panel, I always change the panel instead of the breaker. The most common cause I've found for intermittent breaker trips is a bad internal overload on the compressor & no delay on break on the system. Trane's old climatuff compressors have given me the most problems through the years. Someone jiggles the t-stat & instead of the compressor not starting & cutting out on overload it trips the breaker. Today with the majority having digital t-stats or condensers with time delays I don't see it nearly as much as I used to but it still happens.
    Gary
    -----------
    http://www.oceanhvac.com
    An engineer designs what he would never work on.
    A technician works on what he would never design.

  5. #5
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    If you have an intermittent problem with a breaker tripping, or fuse blowing, check the voltage drop through it, the whole assembly, while the compressor is running.
    If the voltage drop is more than 0.05v or so, it may be the problem.
    Just a 0.1v drop through a breaker or fuse assembly will cause it to get hot pretty quickly, which will cause it to trip/blow, even though its current rating was never exceeded.

    Also, if the breaker or fuse is sized for the minimum circuit ampacity of the unit, but lower than the max fuse/breaker rating, you can get nuisance trips, even though nothing is wrong with the equipment, especially if the system is short cycled.

    Sometimes breakers just get weak and trip at much lower than rated amps, even though there is no voltage drop through them, over heating, or other direct indication that it is going bad.

    Always install the a breaker/fuse that is as close to the max fuse/breaker size rating as you can. As long as the wiring is sized per the minimum circuit ampacity of the unit, it is perfectly acceptable to install a larger fuse/breaker to match the MCA rating of the unit. AC equipment wire and fuse/breaker sizing follows NEC440, which has different rules than the general wiring codes most people are simifamiliar with.

    With the correct size and type of breaker or fuse, a compressor not starting should always trip on its overload protection before the breaker trips or fuses blow.
    If that is not the case, something is either failing, or not correctly sized.
    If more government is the answer, then it's a really stupid question.

  6. #6
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    18,327
    Quote Originally Posted by georgelass View Post
    This is just a general troubleshooting question for condenser units
    or package units in cooling, not a specific service call.

    Regarding tripped circuit breakers and blown disconnect fuses, is it true
    they can only be caused by shorted windings or shorts to ground?

    No, it is not true.

    .....because I'm thinking I've seen compressors that have failed to start,
    either because the run capacitor was shot or the compressor needed
    a hard-start kit, and despite the locked-rotor amps being sky high,
    circuit breakers and disconnect fuses were not affected.

    Let us be clear, that when you say the current was "sky high," you mean it was high when you were measuring it.

    Let's say you have a dual pole breaker with a 100% dead short. Within milliseconds after the short is imposed, the current will be VERY high, on the oder of thousands of amps. The breakers used in panels are "inverse time" breakers, which means the faster the current rise time, the faster they open (or "clear the fault.") This happens so fast you may not be able to register that current rise on any field-used test equipment. The breaker opens "instantly" as far as we can tell.

    A breaker can take up to a minute or two to open at its rated value. For locked rotor current, it can take many, many seconds, which is far longer than most techs expect to see, so they question the breaker.

    Breakers are designed to open on current flow, using magnetism.


    So if a circuit breaker is tripping very intermittently and resistance checks
    of the motors look fine, is it safe to place blame on the circuit breaker
    or the wiring between the panel and the unit?

    No. As stated above, the breaker or fuses ("overcurrent device") need to be sized to the Maximum Overcurrent Device rating, as put forth in NEC 440, as referenced above. This article allows the breaker size to be increased to a point where the breaker does NOT trip in a nuisance manner. Have a qualified electrical person show you this article and explain the process used to determine the MOD when none is given on the equipment label. Using the sizing adjustment provisions of the article, you can go larger than the MOD value, which is calculated as 125% of RLA.

    All input is appreciated. Thank you.
    I hope that helps.

    A note:

    In an HVAC branch circuit installation, the breaker in the panel is used as a short circuit/ground fault protection device, rather than the typical function of protection of the building wiring as in general lighting circuits. The internal overloads in the equipment compressors and motors are protecting the wiring.
    Last edited by timebuilder; 09-21-2012 at 07:49 AM.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







  7. #7
    Join Date
    May 2005
    Location
    South
    Posts
    580
    I was on a call once because of an intermittent tripped breaker. It had a square d disconnect with a 50 amp breaker, the clips were loose and the lugs were burnt. I showed the homeowner this and he said he's been an electrical engineer for XX years and this type of breaker won't trip because of heat. It was burnt up anyway so replaced it with the same type disconnect box. He's never called back and we still do his maintenance.

    So is this true?
    Seems like I work at a lot of engineers homes and they think they know more than anyone, yet they always seem to be wrong.

  8. #8
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Quote Originally Posted by truck12 View Post
    I was on a call once because of an intermittent tripped breaker. It had a square d disconnect with a 50 amp breaker, the clips were loose and the lugs were burnt. I showed the homeowner this and he said he's been an electrical engineer for XX years and this type of breaker won't trip because of heat. It was burnt up anyway so replaced it with the same type disconnect box. He's never called back and we still do his maintenance.

    So is this true?
    Seems like I work at a lot of engineers homes and they think they know more than anyone, yet they always seem to be wrong.
    Well, he may have thought it had the disconnect switch the box came with, which won't ever "trip" under any conditions. If someone installed a 50A breaker in it, over heating due to bad electrical connections will cause it to trip just like any other breaker.
    If more government is the answer, then it's a really stupid question.

  9. #9
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    18,327
    Okay.

  10. #10
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    18,327
    A key concept for breakers is the integrity of the buss and conductor connections, which can act just like heating elements, causing the thermal element to open the breaker. Normally, this is not the operating mode.

  11. #11
    Join Date
    May 2005
    Location
    South
    Posts
    580
    He knew it was a 50 amp breaker and insisted that it will not trip because of heat.
    Thats what he said, not my opinion.

  12. #12
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    18,327
    Quote Originally Posted by truck12 View Post
    He knew it was a 50 amp breaker and insisted that it will not trip because of heat.
    Thats what he said, not my opinion.
    You can share with him this Mike Holt article:

    http://www.mikeholt.com/mojonewsarch...s~20030621.htm

    He is a sample of the description of a circuit breaker:

    The five basic components of a circuit breaker are:

    • Frame or case made of metal or some type of electrical insulation
    • Electrical contacts
    • Arc extinguishing assembly
    • Operating mechanism
    • Trip unit, containing either a thermal element, a magnetic element or both

    While the typical action to open is based on current alone, many breakers have the thermal element as an additional part.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







  13. #13
    Join Date
    Jan 2010
    Posts
    79
    Quote Originally Posted by truck12 View Post
    I was on a call once because of an intermittent tripped breaker. It had a square d disconnect with a 50 amp breaker, the clips were loose and the lugs were burnt. I showed the homeowner this and he said he's been an electrical engineer for XX years and this type of breaker won't trip because of heat. It was burnt up anyway so replaced it with the same type disconnect box. He's never called back and we still do his maintenance.

    So is this true?
    Seems like I work at a lot of engineers homes and they think they know more than anyone, yet they always seem to be wrong.
    When a homeowner tells me he's an engineer I ask him what kind of train he drives, that usually gets them to back off and let me do my job.

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event