A floating ground serves as the local zero-volt reference for an isolated section of a circuit, but it has no direct electrical connection to true earth ground. Designers use this isolation to break ground loops, reduce noise, and protect users from high-voltage shocks. Because the reference is floating, its actual voltage relative to earth can drift significantly, requiring careful insulation and sometimes high-value bleed resistors to discharge static.
The secondary side of that power supply uses a floating ground, so don't clip your standard scope lead to it or you'll blow the breaker.
When measuring a circuit with a floating ground, always use a differential probe or a battery-powered oscilloscope. Clipping a standard mains-earth scope probe to a floating ground bridges the isolation and can instantly vaporize the trace.
Related terms
Hot chassis
A piece of equipment where the metal frame is electrically connected directly to the mains power line.
PSU
A power supply unit converts utility AC power into the regulated DC voltages required by a system.
Return path
The route that electrical current takes to flow back to its source, completing a circuit.