A ground tree consists of four metal clamps connected by three thick copper cables. Three of these clamps are on the ends of the three cables, and the fourth clamp connects the other end of the three cables together. The cable itself is made of an unusually high number of very thin strands of copper. This gives the cable adequate thickness to handle high levels of current while still being extremely flexible. The thin strands themselves, however, are delicate so the cables are wrapped in a protective layer of plastic.
The purpose of the ground tree is to connect all three poles of an electrical circuit breaker, disconnect switch, or other power disconnecting means to ground, in order to safely work on the equipment. This ensures that, in the event that the equipment becomes unexpectedly energized, the circuit will immediately short circuit to ground, tripping any circuit breakers or fuses upstream and protecting the workers. Additionally, the direct connection to ground ensures that the equipment will not be elevated to hazardous voltage levels.
Although the circuit should be locked out as per OSHA regulations if anyone is doing work on it, the ground tree provides a second level of protection if something goes wrong. Although not required to do so by OSHA, some electrical workers use ground trees in addition to lock outs when working on more dangerous equipment. MSHA, on the other hand, strongly encourages their use in most applications due to the more dangerous conditions found in underground mines.
The ground tree should be sized as per the grounding system installed on the equipment. That is, the ground tree cables should be as thick as or thicker than the grounding cables to which the equipment is connected. Since the grounding cables should be sized appropriately for the system to handle ground faults, the ground tree needs to be just as large because its purpose is to create ground faults in the event of an accident.