ungrounded

An ungrounded circuit or component exists at some potential relative to earth or a reference ground, but has no direct conductive path to that reference point. In industrial practice, this means the circuit can drift in voltage, since there is no fixed anchor tying it to a known electrical baseline. An ungrounded power supply, for example, may have its negative terminal sitting at plus 120 volts relative to earth, while the positive terminal sits at plus 240 volts, even though the voltage difference between them is still 120 volts.

Ungrounded circuits create several practical hazards. A person touching both the ungrounded conductor and earth, say, standing on wet concrete while touching an ungrounded power line, completes a path to ground and receives a shock. In three-phase systems, an ungrounded delta-connected transformer presents particular risk because all three phases float; a single fault to the frame can go undetected until a second fault occurs, at which point phase-to-phase voltage appears across the ground path. High-impedance voltmeters measuring between an ungrounded circuit and earth can show wildly unstable readings due to capacitive coupling to nearby powered conductors.

When ungrounded systems are intentional

Some industrial equipment deliberately uses ungrounded supplies for safety reasons. A Class II isolated transformer with a 1:1 turns ratio creates an ungrounded secondary winding that, if properly shielded and insulated, allows work on the secondary side with reduced shock risk; touching one conductor while isolated from ground cannot form a complete shock path. Medical-grade isolated power systems in operating theaters operate on this principle. Control circuits in hazardous locations sometimes use ungrounded supplies rated for the application.

The term 'ungrounded' must be distinguished from 'floating' (which emphasizes lack of reference) and from 'isolated' (which implies deliberate decoupling from ground for safety). North American electrical codes generally require systems to be grounded through a path of impedance low enough to permit protective devices to operate in fault conditions. An ungrounded system that meets code will have protective measures appropriate to its hazard level; an ungrounded system that does not meet code is simply dangerous.