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The 2007 NESC – Part 3

Posted By David Young, Saturday, July 01, 2006
Updated: Sunday, February 10, 2013

This article is a continuation of what I see as significant changes that are coming in the 2007 revision of the National Electrical Safety Code.®

Direct embedded metal poles as ground electrodes

New Rule 094B7 addresses the conditions under which a direct embedded metal pole is considered an acceptable ground electrode.

Maximum ground resistance of ground rods on single-grounded (ungrounded or delta) systems

The present Rule 096D requires the ground resistance of individual ground rods on a single-grounded system to be not more than 25Ω where practical. It also states that if the ground resistance of an individual rod exceeds 25Ω a second rod must be installed in parallel with the first one. The 2007 wording requires the ground resistance of individual ground rods of single-grounded systems to meet the requirements of Rule 096A and not exceed 25Ω. Rule 096A states that "Grounding systems shall be designed to minimize hazard to personnel and shall have resistances to ground low enough to permit prompt operation of circuit protective devices.” It also states that if the resistance of a single rod cannot meet these requirements, other methods of grounding as described in Rule 094B shall be used to meet the requirements. Rule 094B lists the various types of grounding electrodes including driven rods, buried wire, strips, or plates, pole-butt plates and wire wraps, concentric neutral cable, concrete-encased electrodes and direct embedded metal poles.

Guarding of the secondary grounding conductor when primary and secondary neutrals are interconnected with a spark gap

One way of reducing stray voltages or currents on multi-grounded systems is to isolate the primary and secondary neutrals at the source transformer with the use of a spark gap or a device that performs an equivalent function. These devices are commonly called neutral isolators. Rule 097D addresses such applications. The present rule requires separate grounding conductors, one for the primary neutral and one for the secondary neutral, extending to the respective ground rods which must be separated at least six feet apart. One or both of the grounding conductors must be insulated for 600 volts. The 2007 edition now requires the secondary grounding conductor on a pole to be guarded in accordance with Rule 093D2.

Bonding of communications systems to electric supply systems

When electric supply systems and communications systems are grounded on a joint use structure, Rule 097G presently recommends, with the word "should,” either a single grounding conductor be used for both systems or the separate grounding conductors should be bonded together. The 2007 wording now requires this with the word "shall.”

Access to electric supply stations by unqualified persons

Rule 110B2 now allows access to electric supply stations by unqualified persons as long as they are escorted by qualified personnel. The present rule only allows access by qualified personnel.

Grounding of anchor and span guys

The present Rule 215C2 requires all uninsulated anchor and span guys to be effectively grounded if they are attached to a supporting structure carrying any supply conductors energized at more than 300V, or if vulnerable to accidental energization by such conductors due to a slack conductor or guy. The new wording requires all uninsulated anchor and span guys to be effectively grounded.

Grounding of span wires carrying luminaires, traffic signals, trolley or electric railway contact conductors

New Rules 215C3 and 4 require all span wires carrying luminaires, traffic signals, trolley or electric railway contact conductors to be effectively grounded unless they have insulators meeting the requirements of Rules 279B and 215C 5 and 6 respectively.

Insulators in anchor guys, span guys, and span wires

New Rules 215C5 and 6 address the use of insulators in anchor guys, span guys, and span wires in lieu of grounding.

Insulators used to limit galvanic corrosion

New Rule 215C7 addresses the use of insulators in anchor guys to limit galvanic corrosion. By the way, when was the last time you checked anchors for corrosion? The NESC in Rule 214A2 requires inspection "at intervals experience has shown to be necessary.” If you have never inspected, you don’t have experience. More and more utilities are reporting failures of anchors due to corrosion.

Rounding of calculation results

There are many locations in the NESC where calculations must be made to determine clearances. New rule 230A4 requires the resultant of the calculation to be rounded up to the same level of decimal places as the basic value shown in the rule or table, regardless of the number of significant digits of individual values required to be used in the calculation.

Water slides and other fixed pool-related structures

Table 234-3 now addresses the minimum clearances of water slides and other pool-related structures in the same category with diving platforms.

Installation of electric supply and communications cables installed in the same riser duct or u-guard on a pole

New Rule 239A2 states that electric supply and communications cables cannot be installed in the same riser duct or u-guard on a pole unless all of the cables are operated and maintained by the same utility.


Read more by David Young

Tags:  July-August 2006  Other Code 

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