Why Are Most Scopes Grounded?

Gina BoniniQuestion: I am curious why A/C powered oscilloscopes do not have the ground lead isolated from the A/C chassis ground. Digital multimeters that plug in the wall do. Aside from hand held or portable scopes, shouldn’t that be the norm now by now, or are they, and my scope is just old??

Comments?

Answer: Interesting observation.  There’s actually a few different reasons:

  1. Digital Multimeters (DMMs) don’t draw a lot of power, so it’s pretty easy to power them with an AC/DC power supply that has low-power isolation transformers.  These transformer isolate the product from ground. The power requirements of many scopes are beyond what can be served by this design. That said, Tektronix does offer a unique oscilloscope with isolated channels, the Tektronix TPS2000 series.  This basic oscilloscope is specially designed with isolation transformers.
  2. DMMs and their leads are carefully designed so it’s very hard for you to touch any metal on them, so it’s hard to get shocked if they’re not grounded. In general, that’s not at all true of oscilloscopes. There are a few exceptions, like the Tektronix TPS2000 series and the Fluke handheld scopes. In these cases, the individual inputs are floating.
  3. DMMs are only capable of measuring low frequencies, so the stray capacitance between the chassis and earth ground doesn’t affect the measurements much. In addition to the safety issue (in #2 above), the large capacitance of a floating scope would serious affect the quality of most oscilloscope measurements.
  4. The dielectric in the transformers in most power transformers are not designed to have large voltages applied across them. Over time, high voltage differences across the transformer’s isolation barriers will cause them to fail, creating a reliability and a safety issue.

 For all of these practical and safety reasons, most oscilloscopes will continue to be grounded.

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Explore posts in the same categories: Ask Scope Guru Q/A, Oscilloscope Fundamentals, Uncategorized

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