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I have been taught that current flows in a closed loop back to the source. And with electrical distribution, current flows to ground because the whole grid is effectively grounded back to the source. Therefore if you were to touch a hot lead, current will flow from your hand through you body and out your foot to ground and back to the source.

My question is what if the source is not grounded to earth? For example, let's say you have a mobile generator that runs off a diesel engine and powers an alternator. The whole generator is insulated from earth because the rubber tires and the alternator is not grounded. In theory, you cannot get shocked and current will not go to ground if you touch one of the leads because the ground is not connected to the source which is the alternator. Is this logic correct? Please Help!

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  • \$\begingroup\$ Read this answer: electronics.stackexchange.com/a/334119/27943 \$\endgroup\$ – nidhin Oct 28 '17 at 5:34
  • \$\begingroup\$ Grounding (or not grounding) is not at all a simple, one size fits all thing. In general, keeping household power systems "close to the nearby ground" helps keep high voltages from accumulating. But there are many cases where galvanic isolation is desired anyway (shipyards, hospitals, etc) because a galvanic connection, while protective of excessive accumulations, also provides dangerous paths in accidental or unintended ways, too. So each situation has to be weighed and choices made to pick the "least bad" of the alternatives. \$\endgroup\$ – jonk Oct 28 '17 at 5:52
  • \$\begingroup\$ You logic about the generator is correct, as long as the generator is not grounded. Usually, though, generators ARE grounded, so please don't test this out. For example, if the generator is connected to a transfer switch, it will be grounded by way of the transfer switch. The only time you might see an ungrounded generator is when it is a portable generator directly connected to a tool or appliance. This is perfectly safe. Also note that "grounded" may mean different things depending on context. I mean connected to earth in this case. \$\endgroup\$ – mkeith Oct 28 '17 at 6:09
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In theory, you cannot get shocked and current will not go to ground if you touch one of the leads because the ground is not connected to the source which is the alternator.

That's correct but both in theory and in practice we have to remember about capacitance between the generator and ground and between the human body and ground.

enter image description here

Figure 1. Capacitance can close the loop in an isolated system.

In Figure 1 we can see that the effect of capacitance between the generator and ground could lead to a significant (mA) current flow.

In Europe we use isolation transformers for portable tools in the workplace.

enter image description here

Figure 2. A 110 V tool safety transformer with centre-tapped secondary.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 3. In the tool transformer the 110 V secondary centre-tap is deliberately grounded to provide a 55-0-55 output voltage.

  • The tool is powered from the two outer pins. This limits the output voltage to 55 V AC maximum in the event of a fault.
  • Having a definite ground point means that in the event of a single fault one of the output fuses will blow and this will alert the operator to the fault. Without this earthing feature one side could become grounded and if the operator were to come in contact with the other wire s/he would receive a 110 V AC shock.

You will find that many portable generators feature a 55-0-55 winding or transformer built in to provide this safety function. It's safety will depend on a proper grounding of the winding centre-tap.

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  • \$\begingroup\$ Thank you! I understand now that electrostatic electricity can close the loop. Like lighting or a capacitor that static charge can cause current to flow. \$\endgroup\$ – bittersweet Oct 28 '17 at 19:31
  • \$\begingroup\$ Not quite right. It's not electrostatic. It's real - although small - AC current flowing through capacitance. Lightning tracks through ionised air so it's not that. Static charge will jump an airgap when the gap is small enough for the voltage available. It's not that either. See if youtube.com/watch?v=aoY1n2U9-AA helps. \$\endgroup\$ – Transistor Oct 28 '17 at 19:48
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Yes, your logic is correct. If a complete circuit is missing, then no current will flow.

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