2.5 Temporary Protective Grounds & Induced Voltage

Why Temporary Protective Grounds Are Used

When a circuit is de-energized and LOTO is applied, two residual hazards remain: accidental re-energization (a worker re-closes a breaker, an automatic transfer scheme operates, a generator backfeeds) and induced voltage from adjacent energized circuits. Temporary protective grounds (TPGs), also called safety grounds or working grounds, address both:

• They provide a low-impedance fault path so that any accidental energization trips the source protective device immediately, clearing the fault before it can reach the worker.
• They create an equipotential zone at the work area so that the worker, the conductor, and the surrounding metal are all at the same potential — no current can flow through the worker's body.
• They drain induced current from capacitive and magnetic coupling with adjacent energized lines, keeping the work conductor at ground potential.

Placement: Work-Area Grounding vs Bracket Grounds

There are two common placements:

• Work-area grounds — installed at the work location itself (or as close as physically possible). These create the equipotential zone that protects the worker. This is the preferred and required method per OSHA 1910.269 and NFPA 70E.
• Bracket grounds — installed on both sides of the work section but not at the work point. Bracket grounds protect against re-energization (they trip the source breaker) but do not create an equipotential zone at the worker, and they do not protect against induced voltage at the work point. Bracket grounds alone are inadequate for worker protection.

The equipotential zone is the key concept: the worker and the conductor being worked on must be bonded together so there is no voltage difference between them. A bracket ground 50 feet away does not bond the worker to the conductor.

Connection Sequence: Ground-End First, Remove Last

The cardinal rule of TPG installation is ground-end connection first, conductor-end last; removal is the reverse — conductor-end first, ground-end last. The reason: if the conductor is unexpectedly energized while the ground is being installed, the fault current flows from the conductor through the ground cable to the grounding point — but only if the ground end is already connected. If the worker connects the conductor end first and the conductor is live, the clamp and the worker can become the path until the ground end is connected.

The worker uses a live-line tool (hot stick) to install and remove the conductor-end clamp, stands clear of the ground cable's potential fall radius, and wears appropriate PPE (rubber gloves with leather protectors, arc-rated clothing, face shield). The ground cable should be routed so that it will not whip into the worker if fault current flows.

Sizing Ground Cables

TPG cables are sized for the available fault current and the clearing time at the installation point. The cable must carry the maximum fault current without fusing until the source protective device clears. A typical substation ground set is 2/0 or 4/0 extra-flexible copper with a high-current clamp rated for the available fault current; the ferrules and clamps are rated to match. The set must be tested and certified, and the clamps must make clean metal-to-metal contact (paint and corrosion must be removed from the connection point).

A common trap: assuming a #2 AWG ground set is adequate because "it's a ground." A #2 set may be rated for only a few thousand amps for a few cycles — insufficient for a substation where the available fault current is 20 kA or more. Always verify the set's fault-current rating against the engineering study for the location.

Induced Voltage and Mitigation

A de-energized circuit that runs parallel to an energized circuit for any distance will have voltage induced on it by capacitive coupling (the energized conductor and the de-energized conductor form a capacitor) and magnetic coupling (the energized conductor's magnetic field induces a voltage in the de-energized conductor, like a transformer). On long parallel runs at transmission voltage, the induced voltage can reach hundreds of volts and the induced current can be lethal.

Drain grounds are TPGs installed at both ends of the work section to drain the capacitive and magnetically induced current to ground, holding the de-energized conductor at ground potential. Combined with a work-area ground at the work point, they create the equipotential zone. The worker must ground the conductor before contacting it and maintain the ground until the work is complete.

Exam trap: "Open only one end" is a common wrong answer for induced-voltage mitigation. Opening one end leaves the conductor floating at one end and does not drain the induced current; it can actually increase the voltage at the open end due to capacitive voltage division. Both ends must be grounded.

Equipotential Grounding for Worker Protection

The equipotential zone is the foundation of modern grounding practice. The worker, the conductor, and any nearby grounded metal (the structure, the switchgear frame) are bonded together so that even if the conductor is energized, the worker is at the same potential as the conductor and no current flows through the body. This is why work-area grounds are required in addition to bracket grounds — the bracket ground trips the breaker, but the work-area ground protects the worker during the fault.