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No Chassis ground causes O scope to short out the circuit

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ParkerMike

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Why does the O-scope short out when probing a circuit when the Test fixture to the circuit board under test doesn't have a Chassis ground?

When probing the circuit under test , the Oscope's probe causes a short or a dead signal

What else causes the Oscopes probe to short out when there is a grounding issue with the circuit under test or test fixture applied to the circuit under test?

How would you find out which ground issue it is causing this? a logic ground, a signal ground, a power ground, a chassis ground, etc.

How would you find out which ground it causes this issue of the O scope probe shorting out or causing a dead output of the circuit under test?
 

The probe tip is connected to the CRO via a 10 M resistor, it will not cause any problems. Your circuit under test must have its common rail either earthed or properly floating. So get a DVM, switched to AC volts first, connect one probe to a point you can designate as earth. The earthed pin of a power socket would be a good place. With the other probe,, check the chassis voltages on every thing on your bench including the test fixture. You should get zero volts every where. If the earth is missing on a piece of test gear then the chassis will float to 1/2 mains voltage, connecting this to a piece of sensitive equipment can blow it up.
frank
 

You should get zero volts every where. If the earth is missing on a piece of test gear then the chassis will float to 1/2 mains voltage, connecting this to a piece of sensitive equipment can blow it up.
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So how can I test to verify the missing earth or chassis ground if My DVM meter is going to blow up the circuit under test or sensitive equipment?

Is there any other ways of testing for missing earth, missing chassis ground, missing neutral? or if the grounds are at different potentials

The test fixture can be at a different ground potential compared to the circuit boards ground an the O-scopes ground potential

All 3 grounds can be at different potential , so when probing around can cause a potential short and sparks from the Oscope probe to the circuit under test

Anyway to test and check the ground potential between all 3 grounds?
 

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