Multiple Clearance/Step and Touch Potential (115kV)

Revised: August 2010

Topic: Analysis to determine step potential levels due to mutual coupling of a de-energized District 115kV circuit with parallel energized circuits

Background

The Clearance Committee has asked about induced voltages in a de-energized 115kV overhead line due to mutual coupling with parallel 115kV, 230kV or 500kV lines of the District or other utilities in a corridor. The most commonly encountered levels on the District’s system would be generated by a District double 115kV circuit with one circuit carrying load and the other de-energized and grounded. Analysis was conducted to determine these mutually induced voltage levels in a double circuit configuration during high load conditions on a 115kV line that is parallel to a de-energized grounded line.

Induced voltages are dependent upon current levels, inter-phase spacing, coupling distance and intercircuit spacing. Because of the number of variables and the wide range of values within each variable, this analysis did not attempt to cover all possibilities. In order to assess other field conditions, two extreme examples were evaluated in addition to the double circuit configuration.

The committee was concerned about the level of ground potential rise (GPR) on ground rods that is created as a result of linemen installing single-point grounds at two separate points within a clearance. There is a concern that this could lead to dangerous step potential levels for personnel on the ground.

Assumptions

An analysis was completed to determine the step potential due to GPR in the zone between ground rods tied to single-point grounds. The analysis considered a de-energized 115kV line on the same pole line with another 115kV line during high load conditions. During this situation, a voltage is induced upon the de-energized 115kV conductors due to the flux generated by the load current and the mutual coupling between the two 115kV lines.

The induced voltage level on the de-energized conductors is dependent upon several factors. These include separation between conductors and circuits, magnitude of load current, and line length of parallel operation. For separation between conductors and circuits, this analysis assumed the worst case – maximum spacing between individual energized 115kV phase conductors and one single conductor of the de-energized 115kV circuit. A maximum load current on our 115kV system was used to again obtain the worst case results. The table below shows voltage levels for parallel distances of one, two and four miles between ground rods.

Earth resistivity (ρ) is a factor that affects the effective resistance to earth of a ground rod and the actual circulating current between ground rods. The value of ρ can vary significantly from place to place and is rather difficult to ascertain. The common range of values is from 10 to 1000 meter-ohms with most studies using a value of 100 meter-ohms. This analysis used 10, 100 and 1000 meter-ohms with 1000 meter-ohms yielding the highest calculated ground path resistance and lowest ground circulating current, yet not affecting the step potential. Resistivity has more effect on induced voltages in the case of imbalanced currents on the energized circuit.

Results

Table 1 lists the results of the step potential analysis on a de-energized 115kV line that runs parallel to another District 115kV line:

Table 1: Step Potential Levels for Double Circuit 115kV w/ Multiple Single Point Grounds
Load Current Earth Resistivity (ρ) Distance Between Grounds Maximum Step Potential
1000 amps 10 meter-ohms 2 miles 65.0 V
1000 amps 100 meter-ohms 2 miles 65.0 V
1000 amps 1000 meter-ohms 2 miles 65.0 V
1000 amps 10 meter-ohms 3 miles 97.6 V
1000 amps 100 meter-ohms 3 miles 97.6 V
1000 amps 1000 meter-ohms 3 miles 97.6 V
1000 amps 10 meter-ohms 4 miles 130.1 V
1000 amps 100 meter-ohms 4 miles 130.1 V
1000 amps 1000 meter-ohms 4 miles 130.1 V

Notes:

  1. The step potential levels assume a step distance of 1 meter (39.37 inches).
  2. The maximum step potential was from the ground rod to the earth one meter away.

A similar analysis was completed to determine the step potential at a ground rod due to mutual coupling voltages induced on a de-energized circuit under the following conditions: two miles between ground rods of parallel construction along a 500kV line (using BPA construction standard M17 with a GMD of 41.92 feet) with 3000 amps flowing and 48 feet of space between the nearest energized and deenergized conductors. The step potential voltage calculated under these conditions is 364 volts. When the parallel distance is reduced to ½ mile, the step potential level is reduced to 91V.

Another analysis was completed under the following conditions: two miles between ground rods of parallel construction along a 230kV (using BPA construction standard J9 with a GMD of 33.14 feet) line with 2000 amps flowing and 48 feet of space between the nearest energized and de-energized conductors. The step potential voltage calculated under these conditions is 322 volts. When the parallel distance is reduced to ½ mile, the step potential level is reduced to 81V.

Discussion

At some of the ground rod separations listed above, the resultant step potentials exceed the acceptable level and may present a danger to District personnel or the public. The District’s acceptable level for step potential voltage has been adopted from BPA standards and is 100V. Based upon the cases analyzed above, it is recommended that the following rules be implemented by the District:

  • Multiple clearances shall not be issued on a District 115kV line when the section of line between personal protective grounds is running parallel (in a transmission corridor) from another District energized 115kV line for more than 15,840 feet (3 miles).
  • Multiple clearances shall not be issued on a District 115kV line when the section of line between personal protective grounds is running parallel (in a transmission corridor) to another utility’s line energized at 115kV or above for more than 2,640 feet (1/2 mile).

These rules have been developed using extreme loading on energized lines. In all other cases, the actual voltages encountered will be less than those listed above.

Guidelines for Responding to Downed Power Lines
Revised on June, 2007

The following are guidelines for District qualified electrical workers when responding to downed power lines. Because each situation may be different, these are only general guidelines. The District will rely on the discretion of our qualified electrical workers at the site to take the necessary precautions to make the site safe from being re-energized from the District’s electrical system. These guidelines also provide direction for trained District assessors when needed to perform a field assessment during major emergency/ storm conditions.

Guidelines for a qualified electrical worker responding to an accident site, such as a carpole accident or a person making accidental contact with District power lines.

  1. Notify ECC of situation.
  2. Advise lead emergency personnel and the public if on site, of the status of the line and request them to stay clear until told it has been made safe.
  3. If the line has not been de-energized by the District’s automatic protection system, (i.e. fuses, switches or breakers) obtain authorization from ECC to open and de-energize line.
  4. If no one is on site and the employee needs to leave to de-energize line, leave a visual warning (i.e. flares, cones or danger tape) in place to alert public of the hazard.
  5. If the situation at the scene is life threatening, the worker should cut down the wire to allow emergency access to the site of the accident, if it can be done safely. If it is not safe to cut wire and the energized line needs to be de-energized immediately, contact ECC and request the line to be de-energized via SCADA. Then clear the downed wires to allow emergency personnel to reach the injured.

Guidelines for a qualified electrical worker responding to or discovering a downed wire under emergency/storm conditions.

  1. Notify ECC of situation.
  2. If the line has not been de-energized by the District’s automatic protection system, (i.e. fuses, switches, or breakers) obtain authorization from ECC to open and de-energize line.
  3. Advise lead emergency personnel and the public if on site of the status of the line and request them to stay clear until told it has been made safe.
  4. If no one is on site and the electrical worker needs to leave to de-energize line, leave a visual warning (i.e. flares, cones or danger tape) in place to alert public of the hazard.
  5. After the line has been de-energized, cut the line(s) at a point that it will remove hazards to the public from the source of the District’s electrical system. If the load side of the line is down in roads, driveways, or areas likely to have exposure to the public cut the wire down if the work can be done safely.
  6. If the line cannot be cut down safely and the location has a reasonable chance of exposure to the public, call and report to ECC. ECC will issue a tag at the open cutout or device that protects the downed section of line from being re-energized by the District. After completing the above, if emergency services are on site, give them the status so they can determine if it is OK for them to leave and call ECC/Storm Center and report situation.

Guidelines for a qualified electrical worker responding to or discovering a downed wire during routine work or an isolated call out.

  1. Follow steps 1 through 5 in the above section.
  2. If the line can not be cut down safely and the location has a reasonable chance of exposure to the public, call and report to ECC. ECC will issue a tag at the open cutout or device that protects the downed section of line from being re-energized by the District. After completing the above steps, if emergency services are on site, go ahead and give them OK to leave and call ECC and report situation. The qualified worker will then provide a visual warning (i.e. flares, cones, or danger tape) around the wire as reasonably possible before leaving the site.

Guidelines for a trained District assessor discovering a downed power line while performing a field assessment of system damage under emergency/storm conditions.

  1. If you encounter downed wires of ANY type, stay a minimum of 15 feet away. Consider ALL WIRES ARE HOT.
  2. Do not Drive over wires!
  3. Report back to the Storm Center if you discover wires are arcing or if the switch(s) is closed, and the Storm Center manager will inform the ECC superintendent. You will be required to stay and protect the site until ECC can dispatch a qualified electrical worker to de-energize the line.
  4. If the switch(s) is open, and the line in NOT on the road blocking traffic and does not appear to have high exposure to the public, report to storm center and then keep moving on to complete the assessment. On two and three phase lines, all switches feeding the lines must be OPEN.
  5. If police or fire personnel are encountered standing by downed wires, report the situation back to the Storm Center and standby. The Storm Center will determine if you must stay and will ask you to advise the police or fire personnel of the situation so they can determine if they can leave. ONLY police and fire department personnel can decide if they stay or leave the site. If you are asked to stay, a qualified electrical worker will be sent to the location to clear the problem.

Communication/Work Guidelines for Crews Working Together During Emergencies

A tree crew working under the direction/clearance of the line crew foreman may perform work on the same line section, remote from the line crew foreman, under the following conditions:

  1. The line crew foreman and tree crew foreman/lead shall patrol the areas to be worked together. The line crew foreman shall take the necessary steps to ensure the area(s) is safe for the tree crew to work. The line crew may leave the tree crew and work at a different location on the same line section once the area is determined safe for the tree crew to perform their work.
    • If there is structural damage to the system that makes it unsafe for the tree crew to perform the work, the line crew shall cut the line(s) down to isolate the work area from the system before the tree crew can proceed.
    • If the position of the trees is such that the tree crew can safely perform the work as if the line is energized, they can proceed after setting up a communication link with line foreman.
  2. The two foremen shall have a method of communication between crews (i.e. radio/cell phone) and shall contact each other to ensure they have a communication link before starting work at their respective work sites. The two crews can then proceed with the work. One of the crew guides can take a radio and go with the tree crew or both can stay with the line crew.

Super crews consisting of two or more line crews may perform work under the following conditions:

  1. The crews shall be assigned work in the same line section and one of the crew foremen shall serve as the lead foreman. That foreman shall be responsible for all switching and clearances needed for the work and for providing the overall job briefing to the crews. The lead foreman shall have communication established with each foreman before work begins on the lines or equipment. The means for communication can be radios or cell phones. The other crew foreman can take a clearance on the same line section as the lead foreman if needed.

Crew Guides shall be issued two District radios to allow a guide the ability to leave the work site for short periods of time when a personal break is needed. One crew guide and their radio shall stay with the lead crew foreman unless all work is stopped.

Revised 7/02/07