Grounding Cluster
How does a grounding cluster reduce shock hazard if the system becomes energized?
A grounding cluster creates an equipotential zone by bonding all conductive objects the worker can contact to a common cluster bar, which is bonded to the system neutral or ground grid. If energized, fault current flows through the low-impedance jumpers to ground, not through the worker, keeping touch voltages low.
What standards govern grounding cluster design in Canada and the US?
ASTM F855 governs performance requirements for temporary protective grounding equipment. OSHA 1910.269(n) requires grounds to withstand maximum fault current and create equipotential zones. Canada OHS Reg 8.19–8.21 mandates testing before grounding, proper attachment sequence, and sufficient current-carrying capacity.
Where should the cluster bar be located relative to the worker?
On overhead lines, mount the cluster bar on the pole below the worker's feet, with jumpers to phases and neutral. In switchgear, place it near the work location on the bus. The worker's entire work envelope should be within the equipotential zone created by the cluster.
A grounding cluster (or grounding cluster bar) is a temporary personal protective grounding assembly that mechanically and electrically connects multiple conductors, such as phase conductors and system neutral, at a work location. It creates an equipotential work zone where workers and all reachable conductive parts are at the same electrical potential during live-line or de-energized work, preventing shock hazards.
In industrial plants, grounding clusters are used during maintenance of medium-voltage switchgear, motors, bus duct, and overhead feeders. They are applied after isolation and voltage testing, connecting a cluster bar to the plant ground grid and phase conductors. This ensures an equipotential zone, protecting workers from induced voltages or backfeed. Typical steps include isolating equipment, verifying absence of voltage, attaching the cluster to the ground point first, then to conductors, and removing in reverse order. This practice aligns with OSHA 1910.269 and Canadian OHS regulations for safety grounding.
Incorrect placement: Grounds applied only at remote locations instead of at the work site, failing to create a true equipotential zone and exposing workers to voltage differences.
Undersized components: Using cables, clamps, or cluster bars not rated for maximum short-circuit current, risking melting or vaporization and leaving workers ungrounded.
Improper sequence: Attaching cluster to phases before ground connection or removing ground first, exposing workers to full system voltage during connection/disconnection.