The setting for this demonstration is a transmission line work site where a wooden structure is being replaced. The de-energized line parallels an energized line such that induced current flows through the temporary ground cable from the phase conductors to the temporary ground rod creating ground potential rise at the work site. As a result of the ground potential rise, shock hazards (step potential, touch potential, and transfer touch potential) exist at the work site. Figure 1 depicts the work site arrangement.
During the removal of a structure, the work site arrangement is dynamic. Depending upon the process used, at least one but possibly two bucket trucks are on the work site along with a digger truck and possibly a crane. The process begins with the equipment staged adjacent to the old structure. The work site grounding system is installed. Work progresses until the old structure is removed. Equipment is re-staged for digging the holes for the new structure. The work site grounding system is adapted to the new configuration. After the holes are dug, equipment is once again re-staged for installing the new structure. The work site grounding system is adapted once more to this new configuration. The arrangement in Figure 1 depicts the situation just after the old structure has been removed (the structure is shown in the figure, but the grounding is consistent for the structure having been removed). The work site ground consists of a single temporary ground rod. The bucket truck and phase conductors are grounded to the temporary ground rod. A digger truck has been positioned on the work site but has not been bonded to the work site grounding system yet. The digger truck has a reel mounted vehicle ground with a t-handle clamp flat-jaw clamp. The ground cable has been spooled out, but has not been connected to the temporary ground rod.
- Demonstrate electric shock hazards produced by ground potential rise created resulting from induced currents flowing into a temporary ground rod.
- Step and touch potential at location A (temporary ground rod)
- Transfer touch potential at location B (tool box on bucket truck)
- Demonstrate the GPR Monitor’s ability to alert the linemen to the presence of GPR before conditions become hazardous.
- Work site with wooden structure and overhead phase conductors.
- Two ground cables for bonding phase conductors together.
- One temporary ground rod. The resistance between the ground rod and remote earth should be at least 100 ohms. This can usually be achieved by monitoring the resistance while installing the rod.
- One long ground cable to connect one of the phase conductors to the ground rod.
- Controllable current source connected to one of the phase conductors. This current source will provide the “induced” current needed to produce the GPR at the work site. During the demonstration the current is ramped up slowing causing the GPR at the work site to gradually increase. If the resistance of the ground rod is 100 ohms or greater, no more than 10 amps will be needed.
- Two vehicles to represent the bucket truck and digger truck.
- Two ground cables for bonding the vehicles to the ground rod.
- Two ground cables for connecting the GPR Monitor Sensor Unit to the temporary ground rod and reference probe.
The demonstration consists of two parts:
- Part 1 – With the GPR Monitor Sensor turned off, slowly ramp up the current into the ground rod and measure the exposure voltages (the step potential, touch potential, and transfer touch potential) at locations A and B. This should be done at least twice. The first time with low GPR. After the first measurements have been taken, the current should be ramped up. The presenter informs the spectators that the ground cable for the digger truck needs to be connected to the ground rod. Before connecting the clamp, the touch voltage is measured again. This time the GPR level results in a serious shock hazard. The presenter also measures the step voltage at the ground rod and the transfer touch voltage at the tool box at location B. The objective of this part of the demonstration is to highlight that the shock hazards associated with GPR are dynamic. And there are no indications when the GPR levels have reached hazardous levels.
- Part 2 – Repeat Part 1 with the GPR Monitor Sensor unit turned on. Describe the operation of the GPR Monitor System (sensor unit, hand-held remote etc). Part 2 of the demonstration begins with the same initial measurements of the exposure voltages as done in Part 1. These voltages should be approximately the same as those in Part 1. After the voltage measurements are taken, the presenter again prepares to connect the ground cable clamp to the ground rod. The current is ramping up, and the GPR Monitor alarms alerting the presenter to a change in the work site conditions. The presenter explains the function of the hand-held remote and the alarm levels.
The demonstration benefits spectators best if they gather very close to the work site. They must be insulated/isolated from the soil to protect them from step potential. A buffer zone should be established around them so that they can’t touch any grounded objects.
Without a GroundHound, the shock hazards could also be demonstrated by measuring the voltage with a volt meter or rigging up a light bulb arrangement that lights up when the exposure voltages reach hazardous levels would give a better idea. A clamp-on ammeter on the ground cable between the phase conductors and the ground rod could be used so that the injected current could be monitored if desired.
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