When first connecting scope ground clip to a battery does the positive V change now that it's not floating?

Question

I'm thinking of a scope ground clip being attached to a battery, let's say 12 V. The battery is just floating to being with. When the scope ground clip is attached to the negative terminal of the battery, this voltage is changed to ground potential. Does this mean that the postive voltage swings up or down (whatever is needed) in order to be exactly 12V higher than the negative terminal? How would one be able to monitor this? What if the battery was floating thousands of volts higher than the ground, would there be an arc when the ground clip is first connected?

Thanks!

Answer 1

Good question, though the answer should be obvious. The scope reads potential between the probe tip and the ground clip. It doesn't matter what potential the ground floats at, other than it must be within the limits of insulation capability of the safety ground on the scope.

It is a good question because you have realized that the ground potential can cause problems when making multiple readings (say with dual probes). Then the difference in voltage can cause errors and may cause the test circuit to malfunction. When this happens, you will need to isolate the grounds so that this difference does not cause problems.

Answer 2

The positive voltage will indeed swing up or down with respect to ground in order to remain exactly 12V with respect to the negative terminal. You could monitor that by clipping the probe to ground and putting the probe on the positive terminal. Then close a connection between the negative terminal and ground while observing the scope trace.

There could be an arc when the negative terminal is grounded. If the circuit is truely "floating", it is a high impedance connection between the circuit and a grounded power system that is causing the high potential. The high impedance connection could be capacitive or inductive coupling between the floating circuit and the grounded power supply. If the impedance is high, the arc should be small. If there is any doubt, connect a high impedance load between circuit common and ground and measure the current. The impedance could be low enough to make it dangerous to ground the circuit while it is energized. That is why circuits shouldn't ordinarily be allowed to float.