Isolation Transformer Grounding question

Discussion in 'General Electronics Chat' started by BFreemanNH, Feb 3, 2013.

  1. BFreemanNH

    Thread Starter New Member

    Jan 6, 2013
    Hi Folks,

    I purchased a BK precision 1655 AC Power Supply. It is a variable isolation transformer for working on older electronics. I noticed that the house ground is being passed through this box to the device I am working on.

    I was under the assumption that the device I am working on should not be connected to house ground.

    Can someone clarify this for me?
  2. MrChips


    Oct 2, 2009
    The earth ground is connected to the ground pin of the 3-pin socket. This is correct.
    The two supply pins are isolated from ground because the internal transformer is isolated, i.e. not electrically connected to ground or power lines.

    To remove the ground connection to your 3-prong plug, you need an adapter that accepts the 3-prong plug but only has two prongs going to the BK Precision 1655. The ground connection comes out of the adapter on a green wire (blue wire in the picture).

    Last edited: Feb 3, 2013
  3. BFreemanNH

    Thread Starter New Member

    Jan 6, 2013
    I just thought it strange that an isolation device made for bench testing, would pass earth's ground. From what I have read, older radios and TV's could have the neutral connected to the frame and you could blow your scope if the device has an earth ground plug.
    Could someone explain why I would ever want to ground a device that I am working on?
  4. russ_hensel

    Distinguished Member

    Jan 11, 2009
    Consider that older radios normally do not have a three prong plug so......
  5. ramancini8

    Active Member

    Jul 18, 2012
    Prior to polarized plugs the hot and netural wires were often interchanged causing a hot chassis. An isolation transformer has a floating secondary, so either lead can be connected to safety (earth) ground if desired. If safety ground is used either isolation transformer lead (or neither) can be connected to safety ground. Safety ground protects from a short to chassis.