Electronics > Beginners

Should I float a scope on a car

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Ian.M:
There's a considerable difference between floating a scope to break a ground loop that would otherwise cause a large circulating current through the probe ground, but with only a volt or two of difference between the ground points in question, and floating a scope to attempt to make a differential measurement with a probe ground clip on a non-ground circuit node. The latter is *ALWAYS* unsafe and if mains or high voltages are involved, is potentially lethal.   

However  using an isolation transformer to power the scope with output Neutral strapped to output Ground, and an adequately sized grounding connector bonding its output ground to the engine block, steel or aluminum hull or sacrificial anode on a boat is just about the only way to avoid the risk of high circulating currents in the ground if the boat doesn't have a permanent shorepower installation with properly bonded ground.  Use a RCD on the output of the isolating transformer, connected *AFTER* the Neutral to Ground strap.  N.B. the isolating transformer output is *NOT* floating, as it has been re-referenced to ground.  Its Line terminal is 'hot' and although its Neutral is grounded, from a safety viewpoint it should also be handled as if it was 'hot'.

analogo:

--- Quote from: Benta on August 03, 2018, 09:44:39 pm ---NEVER float test equipment. Float the device under test (DUT). And your car is already floated.

--- End quote ---

I understand the general reasoning, but what about handheld testing equipment that is by its nature is floating?

tggzzz:

--- Quote from: Ian.M on August 04, 2018, 07:17:23 am ---There's a considerable difference between floating a scope to break a ground loop that would otherwise cause a large circulating current through the probe ground, but with only a volt or two of difference between the ground points in question, and floating a scope to attempt to make a differential measurement with a probe ground clip on a non-ground circuit node. The latter is *ALWAYS* unsafe and if mains or high voltages are involved, is potentially lethal. 

--- End quote ---

The latter is true.

The former is true unless the UUT is faulty (which might be the reason you are probing it!), or it is left floating and the next person doesn't notice it.......

tggzzz:

--- Quote from: analogo on August 04, 2018, 08:03:49 am ---
--- Quote from: Benta on August 03, 2018, 09:44:39 pm ---NEVER float test equipment. Float the device under test (DUT). And your car is already floated.

--- End quote ---

I understand the general reasoning, but what about handheld testing equipment that is by its nature is floating?

--- End quote ---

If it is designed to measure high voltages relative to earth, then OK. But if it has, say, a metal case to reduce EMI/RFI then that could be at an "interesting/entertaining" voltage.

Seamus16:
Thanks to all who have replied and the warnings are well noted.

I suppose to satisfy yourself that you are in a safe environment would be to connect a multimeter between the scope chassis and the car earth.
 

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