When maintenance personnel, during an installation or shutdown, want to work on existing switchgear or associated downstream devices protected by this switchgear, it is imperative to ensure that the line or load switchgear bus is completely grounded prior to any work being done. Working on a bus that’s not completely shut down can be deadly.
Using safety devices is important. The major manufactures of switchgear put primary disconnect devices (line and load stabs) behind shutter mechanisms so that maintenance personnel cannot inadvertently access them. When a circuit breaker is rolled into a switchgear compartment, rollers on the sides of the breaker lift the shutter mechanism inside the switchgear to rack the breaker in place and connect it to a given circuit.
Maintenance personnel who do not have a ground and test device have to manually climb into the switchgear, raise the shutter to access the primary disconnects, and attach grounds to the primary stabs and ground strap in the cell to ensure that the line or load stabs are not energized during maintenance. This procedure has resulted in many injuries.
To prevent injuries, plants have ground and test devices at each substation. Ground and test devices provide a safe way for maintenance personal to access switchgear primary stabs.
The six-stab ground and test device consists of a circuit breaker frame on which six primary disconnect stabs are mounted. The front of the device has two access doors — an upper and a lower. Behind each door are three mounted test ports. Each test port is connected to one of the six primary disconnect stabs in the switchgear. The upper test ports are connected to the upper disconnect stabs, and the lower test ports are connected to the lower disconnect stabs. Each group of test ports on the test device – the left, center, and right – are connected the left, center, and right disconnect stabs, respectively (ABC phase).
The ground and test device is equipped with a ground shoe that engages the ground bus in the switchgear cell. A ground bar extends from the ground shed to the lower front part of the device.
To use the ground and test device, rack the device into the switchgear cell exactly as you would a circuit breaker. Depending on whether you want to ground the line or load bus, open the upper or lower door. Finally, connect the cables from the ground and test stab to the ground bar.
The ground and test device provides a safe means for obtaining access to the primary disconnect devices of the switchgear cell for purposes of grounding the primary circuits or conducting certain high-voltage test procedures, such as phasing out circuits or running high-voltage withstand (Hi-Pot) tests. Using this safety device should be part of any routine maintenance procedure.