Floating an o-scope ground

Discussion in 'General Electronics Chat' started by badboydano, Sep 27, 2011.

  1. badboydano

    Thread Starter New Member

    Mar 4, 2011
    When I am using my oscope to measure duty cycle and pulse width from my phone line, I must float my oscope to see the signal.
    I'll explain... if I connect the oscope chassis ground to earth ground i can not trigger the scope to see the sgnal.
    If I float the scope's ground then I can.
    Does anyone know why?
  2. #12


    Nov 30, 2010
    The phone company does not use earth ground. It is poison to their lines. Causes terrible hum problems. Do not allow connection from phone line to earth ground.
  3. oldtech33709

    New Member

    Sep 24, 2011
    The voltage should be -48V, make sure you use a negative trigger level.
  4. colinb

    Active Member

    Jun 15, 2011
    Oh, the joys of grounding systems! “Ground” is all relative... your home electrical power has a star grounding system originating at the ground stake by the service panel. Only at the point is the “neutral” conductor and the “protective ground” conductor at the same potential (voltage). When the neutral conductor is carrying current, its nonzero resistance results in a voltage difference from earth ground. This is why you generally don't want to touch the neutral conductor in a wall outlet even though it is wired directly to the earth grounding stake at your service panel.

    Similar things for the phone lines: “your earth ground” is not the same as “the earth ground at the phone company telephone switch”.

    Read about stray voltage, the occurrence of electrical potential between two objects that ideally should not have any voltage difference between them.