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Say I used a 12v car inverter powered by a 12v brick connected to ac mains (no ground pin) to isolate a scope and bench dmm, the inverter has no earth ground. Is the equipment isolated? Is lack of earth ground an issue?

user33915
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  • [Why you should not float your scope.](https://electronics.stackexchange.com/questions/73991/why-do-we-need-an-isolation-transformer-to-connect-an-oscilloscope/) – JRE May 19 '18 at 14:14

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By regulation in most cases offline switchers have to be isolated. Inverters are typically isolated as well. However, a floating scope is a safety hazard. Suppose you put your ground clip on something at 500V potential. Then you pick up the scope probe for channel 2 while touching the case of your lab supply (for example) and come into contact with the barrel ground or ground clip. You probably wouldn't like the result. Better to use a real differential probe.

John D
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Since the problem of using oscilloscopes on mains circuits comes up frequently on EE.SE the following may be of help.

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Figure 1 and 2. Fluke Scopemeter and probe set. Note insulated "BNC" connector and leads including black plug on earth clip lead (which plugs into side of probe). The meter comes with a PSU jack that doesn't make contact with the internals until after the exposed metal has been inserted. An optical serial port is visible on the side of the scope.

Instruments such as the Fluke Scopemeter are fully insulated. As a result the scope ground can be connected to any point on the circuit under investigation including the rectified negative line of Figure 1. Even when on charge the device is fully isolated from mains earth. The only point to watch is that the earth clips of the supplied A and B channel probes are not connected to two different potentials.

It means investing in decent equipment but it could save a life. You might also try to find an isolated oscilloscope probe.

Ref: This is part of a more specific answer I gave to Help learning from a mistake connecting an oscilloscope.

See also @placeholder's answer to home brew isolated oscilloscope lead.

Transistor
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  • So bench scopes don't need earth ground to function too? – user33915 May 18 '18 at 22:09
  • WOAH! A bench scope will work with the ground disconnected but if you connect the earth clip to a live circuit the whole oscilloscope is live including the front panel and the BNC sockets. (This is very dangerous to you and others in the area. I strongly advise against this practice.) The same happens in the Fluke Scopemeter but it is fully isolated and the scope ground terminals are fingerproof. – Transistor May 18 '18 at 22:16
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The inverters are likely isolated, but to a degree of having 100 uA to 500 uA leakage between hot AC and ground. In a 110V environment your scope can register 55-70V AC voltage between "floating-isolated" scope and Earth ground.

Ale..chenski
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Even though your power brick does not have a ground, it depends on if the brick uses a step down transformer or not. A switching power supply may or may not be isolated, it's possible the neutral AC wire (which SHOULD be very close if not ground potential) is common to the input and output of the power brick. If you get a Non-Switching supply, much heavier than a switcher, you should be safe, but it is ALWAYS best to meter the negative lead to insure 100% isolation.

Sara Heart
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  • A switcher should NOT have its neutral (or its hot for that matter) connected directly to its output. A capacitive dropper does that, and those should only be used when there are no exposed connections - that means NEVER as a wall wart. – JRE May 19 '18 at 12:53
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    "*... it's possible the neutral AC wire (which SHOULD be very close if not ground potential) is common to the input and output of the power brick.*" Many countries don't use polarised mains plugs and sockets so phase and neutral can be swapped. For this reason it is most unlikely that a power bricks will have a "neutral" connection on the low voltage output. – Transistor May 19 '18 at 13:18
  • I have seen more than 1 step down circuit that is non-isolated at neutral. I had also added, "but it is ALWAYS best to meter the negative lead to insure 100% isolation" I always advocate for safety. So lets not split hairs. – Sara Heart May 19 '18 at 19:41