Why are the Outputs of many function generators earth grounded?

As far as I understand grounding the generator or Parts like the chassis can reduce shock hazards if something fails within the generator.

But it also produces ground loops when I try to measure things with my osci.

Ok but why are the BNC Outputs also ground referenced? Cant I simply cut the ground wires from the Outputs to ground so the Chassis is still earth grounded but my outputs are not to get rid of my ground loops? Or does this have any effect I am Not seeing?

Thank you all for your replies. What I did not understand was: the gnd wire is also connected to ne neutral wire in the transformer. I really did Not get how a current could actually Flow if I only have the signal wire (inner BNC) and only gnd aa the outer shell.

  • \$\begingroup\$ So, assuming the function generator generates a function that is a voltage over time, then, if you don't define a ground, how can you even have that function? \$\endgroup\$ Commented Nov 11, 2017 at 22:51
  • \$\begingroup\$ @MarcusMüller what i did until now is I floated my generator by putting tape on my plug (only the earth connecto Not hot neutral of course) which Powers the generator. And this works fine but now the whole generator is floating. Well I guess you are right when I cut the wire from the BNC connectors to ground There is no voltage reference. My idea was: let the chassis be grounded but the BNC connectors be floating \$\endgroup\$
    – user168605
    Commented Nov 11, 2017 at 23:23
  • \$\begingroup\$ @user168605: "... the gnd wire is also connected to ne neutral wire in the transformer." I'm not sure what you mean by this but never connect ground / GND / earth to neutral in a power supply or in your equipment. In certain fault conditions dangerously large currents will flow in your "modification". \$\endgroup\$
    – Transistor
    Commented Nov 12, 2017 at 22:48
  • \$\begingroup\$ Ok so here is my problem in thinking: The function generator has 3 connectors to the mains power (hot, neutral, earth). To get a compelte current return path a load needs somehow be connected to hot and neutral (via the generator). It is not enough to have a connection between hot and earth (common grounding error). So let's look at the BNC connector: Inner shell there is the signal. Outer shell there is GND. I attach a load (i.e a simple resistor) to the connector. How can a current float if the outer shell is not somehow connected to neutral? \$\endgroup\$
    – user168605
    Commented Nov 14, 2017 at 13:28

1 Answer 1


Because you can touch the BNC connector (outer shield), it is often treated in safety testing as part of the case. So, it is possible for a test protocol to make a 30 ampere test current run from that BNC to the AC ground pin on the power cord.

It is easier to justify grounding the shield, or using a CM choke on the signal connector and wiring the BNC shield through that to the case. Or, one can operate a function generator on battery power, with the AC cord and its ground wire completely detached.

To actually float the BNC shield as you want, might only meet safety standards with a class 2 power supply, or battery power.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.