[RE-wrenches] AC breakers used in DC applications

[Bill Loesch]

Thanks Guys,

Michael, your memory and/or your expert use of the archives clarified what I should have remembered as Windy's comments. Thank you.

In related matters, what are the benefits / disadvantages / preferred applications of a thermal-magnetic breaker over a thermal-hydraulic breaker.

Bill Loesch
Solar 1 - Saint Louis Solar

  ----- Original Message ----- 
  From: Allan Sindelar 
  To: RE-wrenches 
  Sent: Thursday, May 13, 2010 9:22 PM
  Subject: Re: [RE-wrenches] AC breakers used in DC applications


  Wow, Michael, look at the monster you have created, Dr. Frankensolarbozo! 
  Yes, that's the thread. And I was about to post that Windy left today for Costa Rica...just a trip, not a move.
  William, there are certain perfectly appropriate applications - such as a pressure switch controlling a DC pump, or an A/C disconnect (Square D CU200) 2-pole array disconnect for an array-direct pump. Historically there was nothing DC-rated available, and conservative use of available AC equiipment worked fine and lasted forever. Sometimes it just don' matter how it's Listed, ya just gotta use it...ya know?
  Allan


  <![endif]--> 
  Allan Sindelar
  Allan at positiveenergysolar.com
  NABCEP Certified Photovoltaic Installer
  EE98J Journeyman Electrician
  Positive Energy, Inc.
  3201 Calle Marie
  Santa Fe, New Mexico 87507
  505 424-1112
  www.positiveenergysolar.com



  Michael Welch wrote: 
    Is this conversation what you were refering to? 7/22/01

    Others offered caution to this approach, but clearly the subject was switches, not breakers.

    Allan at Positive Energy wrote:


      2. ...  I am using float switches to control 48V mercury displacement relays for these DC pumps. Can anyone suggest a source of SPST, SPDT, and DPDT bat (or any style) switches suitable for 48VDC nominal, low current loads? These would be for a custom relay control enclosure with override and remote low tank alarm signal circuits.
       

    My frequent observation on DC ratings for mechanical switch contacts is that it's amp rating at 125VAC equals its amp rating at 28VDC. This is consistent in the specifications of a great many switches, relays, etc. If there are higher voltage ratings, then if  voltage is doubled, amps is half.

    When 48V is not specifically listed (nor any higher DC voltage), I feel safe following that rule if I apply a good safety margin, as long as the switch has a rapid snap action like a toggle switch does. For the low current control and alarm loads you describe, I would feel forevever safe using a switch with a 15A rating at 28VDC. If you need to get official approval, ask the manufacturer of the switch you propose to use.

    Windy


    Bill Loesch wrote at 05:10 PM 5/13/2010:
     


      Fellow Wrenches,
       
      Some time ago, Windy Dankoff had a rule of thumb to substitute (conservatively) an AC breaker for use in a DC application. 
       
      Is anybody's memory, or perhaps even the old dog himself; if he is listening, better than mine?
       
      TIA,
       
      Bill Loesch
      Solar 1 - Saint Louis Solar
      314 631 1094
       
       
      _______________________________________________

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