The National Electrical Safety Code


 
Part 1 of the National Electrical Safety Code Applies to equipment inside the fences for substations and switch yards.  Energized Equipment operating above 150 volts to ground without an adequate insulating covering is required to be guarded either by elevation or with appropriate shields or enclosures.   For the above that would  include the entire bushing or insulator since any part of the bushing is considered live.
The tall upside down cone shaped devices are current transformers used on 500,000 volt lines near the Hanford Nuclear Power Plant.

Guarding Live Parts Inside Electric Supply Stations, Substations, and Switch Yards

Rule 124

Where Required

Guards shall be provided around all live parts operating above 150 volts to ground that do not have adequate insulating covering, unless located to give sufficient horizontal or vertical or a combination of these clearances to limit the likelihood of accidental human contact.  Clearances from live parts to any permanent supporting surface that workers may use shall equal or exceed either of those shown in Table 124-1 and illustrated in Figure 124-1 of the NESC.
 

Discussion:

The clearances given in Table 124-1 are for authorized persons.  The inside of substations, electrical switch yards and electric supply stations are not accessible to the general public and this is made mandatory with a NESC rule 110 A1.  "Rooms and spaces in which electric supply conductors or equipment are installed shall be so arranged with fences, screens, partitions, or walls to form an enclosure as to limit the likelihood of entrance of unauthorized persons or interference by them with equipment inside.  Entrances not under observation of an authorized attendant shall be kept locked."    An additional new requirement for the 1997 NESC requires that exposed live parts inside a fence be located outside of a clearance zone per table 110-1.  This rule requires that live equipment be at least 10 feet from the inside of a fence to keep persons from trying to contact equipment through the fence mesh using a 10 foot piece of conduit or other object.  The safe clearance distances range from 10 feet for 151 volts between phases to 17.3 feet for 500,000 volts between phases.

The first step in learning how to read Table 124-1 is to know that the guard zone subtracted from the vertical clearance is always 8 ft. 6 inches.  In other words, the distance from a surface where a worker can stand to the guard zone is always 8 ft. 6 inches.  Each live equipment has a guard zone around it varying form 2 inches for 151 to 600 volts to 30 ft. 3 inches for 800,000 volts with the greatest switching surge factor.  For 550,000 volts with the lowest switching surge factor the guard zone is 10 ft. 4 inches.  Then from the table we can add 8 ft. 6 inches to 10 ft. 4 inches to find that the vertical clearance to 550,000 volt live conductors or bushings.   This is 18 ft. 10 inches, and indeed, this is the value given in Column 3 for 550 kv.  For voltages not in the Table the clearance distances must be Interpolated.

Figure 124-1 Clearance from live parts Figure 124-1

The horizontal Clearance is given in Column 3, Table 124-1
The Vertical Clearance is given in Column 2, Table 124-1
The Guard Zone Radius is given in Column 4, Table 124-1
 

Now what happens if the vertical clearance cannot be met?  Then we need a guard around the live equipment or a railing as illustrated in figure 124-2.  The guard around the equipment with one exception must be no closer to the equipment than the guard zone distance.  A railing if used shall be located at a horizontal distance of at least 3 to 4 feet from the nearest point of the guard zone that is less than 8 ft. 6 inches above the floor.  This is illustrated by figure 124-2 in the NESC and below.

Figure 124-2
 

There is one more little tidbit that has to be known to complete this discussion.  That is the "taut string measurement" for measuring the vertical distance to bushings.  This is not in the NESC but is illustrated by the experts in their handbook.  Remember the NESC tells you what must be done, but does not tell you how to do it.

Figure 124-3

The distance to the bottom of the bushing as measured with the taut string shall be 8 ft. 6 in. or a guard or railing is required.  The vertical distance to the top of the bushing is based the voltage at the conductor and on Table 124-1.  The vertical distance to the remaining part of the bushing is based on the voltage gradient across the bushing using Table 124-1 as illustrated in Figure 124-4.

Figure 124-4

It is probably fairly accurate to state that if a person can touch any live equipment including the bottom of bushings in a substation, electric supply station, or switch yard the clearances required by the NESC probably are not being met.  For higher voltages the distances required are substantial.  In such cases a worker should not be able to reach into the guard zone about equipment which for 550,000 volts is 10 ft. 4 inches to 19 ft. 10 in. depending on the Switching Surge factor.
 

The worker inside the substation in Figure 124-5 is clearly in an area that should be guarded.  By Table 124-1 a guard zone of 3 ft. 8 in is required for 242,000 volts phase to phase with a BIL of 550 kv.  If the distance from the ground to the guard zone were 8 ft. 6 in. guards would not be required.   There are provisions in Part 4 of the NESC to allow qualified workers to approach the overhead lines at closer distances if they use the necessary precautions stated in Rule 441.

Figure 124-5

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