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York has a 3amp circuit breaker they use on their rooftop units. I bought a bunch and put clips on them. Now all my men have a tool to trouble shoot a low voltage short without going through a box of fuses. I'm sure most other brands have a simular breaker. And yes after they fix the short they put a regular fuse on the job. Mick


  • newbie_3
    newbie_3 Member Posts: 1

    If a low volt transformer is not marked, how do you tell which side of the low volt is hot and which is common?
  • Newbie

    You ground one lead of your multi meter and then touch each terminal individualy. From the Hot (often labeled "R" nowdays) you should read 24 volts. From the common (Often labeled "C") you will read Zero volts.

    Just make sure that the primary is connected corretly as reverse polarity will cause a reverse on the secondary.

    I have a brochure for (FREE) I will send you on this just e-mail me at [email protected]
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Well Put, Tim

    Short, sweet and accurate. Great reply, Tim.

    By the way, Tim, which brochure are you referring to?
  • It is something I use in

    my classes to teach transformers/polarity/phasing. I have not actually put it into one of my troubleshooting guides. I can send a copy to you if you want just e-mail me your address and I will get it off to you.
  • thp_8
    thp_8 Member Posts: 122
    Well thats one way of looking at it

    About 99% of todays hvac transformers are isolated. So for the transformer sake you can us either side as a hot or ground. The common is which side is shared or in most cases grounded to the equipment. So as the question was stated a transformer with no makings has no dedicated hot or common unless it is installed in a system. Then at that point you can check power to common.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Another Issue

    Since you raise the transformer issue, you may be interested in this.

    When working on systems, consider installing an inline fuse in the control circuit. I personally required on all installations we did.

    In the long run, a 50-cent fuse was a lot cheaper than replacing transformers.

    All too often, a technician will go on a job and determine that the control transformer has gone bad. Solution? replace the transformer, of course!

    When I train my guys, I insist that they locate the cause of the problem, not the effect. By replacing the transformer, they are alleviating the end result, but not the initial cause for failure.

    In the early stages of learning to troubleshoot HVAC systems, you may feel the need to replace the transformer in order to find out why the first one went bad. Well, depending on the problem, the new transformer may burn out before you are able to locate the problem.

    How many times can you do this? Depends on how many transformers you have on your truck! ALl kidding aside, the installation of an inline fuse will help you troubleshoot the system as well as protect the transformer in the event of future control circuit issues.

    Keep up the good work.
  • Robertshaw, Honeywell and

    White Rodgers all offer transformers with a built in circuit breaker which takes are of the issue of damaging the replacement transformer when replacing a transformer in a cicuit that may have a short of the secondary side.
  • thp_8
    thp_8 Member Posts: 122
    Every job gets a little

  • thp_8
    thp_8 Member Posts: 122
    Every job gets a little

This discussion has been closed.