use of the mass as in-circuit conductor

Question

In some applications, such as dynamo-powered bicycle lights or torches (the old-fashioned ones with a metal barrel) the mass is truely integrated in the circuit. That is, the wiring does not form a closed circuit with reference to the mass, but the metal of the housing is used as conductor for considerable lengths. See also the circuit in the right below (taken from fig. 2b in the answer to this question) where the volt-meter could have been a lamp or anything else.

Is this good practice, or advisable only in such exceedingly simple circuits? Is it safe to have stronge currents freely floating through the mass? What considerations exist not to do this?

Answer 1

The illustration you posted is mine and (a) was for a different purpose but (b) suits your question.

Is this good practice, or advisable only in such exceedingly simple circuits?

Yes. Most of the worlds cars, vans, trucks and buses use the metal chassis of the vehicle as the common return conductor for the DC circuits.

Is it safe to have strong currents freely floating through the mass? What considerations exist not to do this?

At low voltages it is generally quite safe. The amount of metal available in the chassis of a car provides adequately low resistance despite steel's higher resistance when compared to copper. At higher voltages you need to follow local regulations and perform a risk analysis under possible fault conditions.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. A simple car lamp circuit using the chassis as a return conductor.

Note that the chassis ground symbol is preferred in this case to avoid confusion with the earth symbol (shown in your question) as the latter implies a connection with Earth which normally is not the case on a vehicle.