standards and guidelines for communication sites - Radio And ...
standards and guidelines for communication sites - Radio And ... standards and guidelines for communication sites - Radio And ...
AC POWER CHAPTER 6: POWER SOURCES 6.2.2 LOCATION OF NEUTRAL-GROUND BOND It is strongly recommended for the proper performance of electronic equipment that the voltage between the neutral and ground conductors at the equipment be no more than 2V p-p (0.7 Vrms). Where the disconnecting means of the electrical service is remotely located to the site, the voltage developed between neutral and ground may cause equipment to malfunction, especially during a ground potential rise at the site. For the optimum performance of a Surge Protective Device (SPD) installed at the AC distribution panelboard, the Neutral-to-Ground bond must be near the SPD in order for the SPD to effectively redirect unwanted energies on the AC power cables safely to the site grounding (earthing) electrode system. Due to the physical separation between the remote service entry disconnecting means and the site disconnecting means, a steady state ground loop current could flow on the equipment grounding conductor bonding the two ground fields. An approach for eliminating the steady state ground loop current and lowering the voltage between the Neutral and Ground conductors is to bond the Neutral-to-Ground at the remote service entry disconnecting means, and at the communications site disconnecting means as allowed by NFPA 70-2005, Article 250.32(B)(2). In order to accomplish this, the equipment-grounding conductor between the remote disconnecting means and the site disconnecting means shall be disconnected at both ends and removed. Neutral-ground bonding requirements as allowed by NFPA 70-2005, Article 250.32 include: • The AC equipment grounding (ACEG) conductor shall not be run between the remote disconnecting means and the shelter’s disconnect. • There shall be no other metallic continuity established between the remote disconnecting means and the site. Examples of continuity that is not permitted between the remote disconnecting means and the site could include metallic conduit and the data/ telecommunications metallic shields installed between the remote electrical service area and the site. • Ground fault protection shall not be installed on the common AC service of the electrical disconnect NFPA 70-2005, Article 250.32(B)(2) allows the bonding of the neutral and ground conductors at the separate building or structure and also eliminates the creation of parallel paths for normal neutral current on grounding conductors, metal raceways, metal piping, and other metal structures. In previous editions of the Code, the grounding electrode conductor and equipment-grounding conductors were permitted to be connected to the grounded conductor at a separate building or structure. This multiplelocation grounding arrangement could provide parallel paths for neutral current along the equipment grounding conductors of the electrical system, metallic shields of cables, metallic structures that are continuous between, buildings, and along other continuous metallic piping and mechanical systems as well. Connection of the grounded conductor to a grounding electrode system at a separate building or structure is permitted only if these parallel paths are not created and if there is no common ground-fault protection of equipment provided at the service where the feeder or branch circuit originates. See Figure 6-1 on page 6-5. 6-4 68P81089E50-B 9/1/05
STANDARDS AND GUIDELINES FOR COMMUNICATION SITES AC POWER TOWER GROUND RING TOWER GROUNDING ELECTRODE CONDUCTOR AND GROUND ROD FEEDER IN PVC CONDUIT WITHOUT EQUIPMENT GROUND METALLIC FENCE EXTERNAL GROUND RING EGB 6 FT MINIMUM SEPARATION ON ALL SIDES FROM ANY METALLIC COMPONENT BONDED TO SITE COMMOM GROUNDING SYSTEM (NFPA 780-2004, SECTION 6.2.1) COMMUNICATION SITE PANEL WITH NEUTRAL-GROUND BOND, PER NFPA 70-2005, ARTICLE 250.32(B)(2) ELECTRICAL SERVICE ONLY (NO GROUND FAULT PROTECTION) FIGURE 6-1 REESTABLISHING THE NEUTRAL-GROUND BOND AT THE COMMUNICATION SITE 6.2.3 SEPARATELY DERIVED SYSTEMS USING TRANSFORMERS Communications site AC power can also be supplied by a transformer (also known as an isolation, stepdown, or step-up transformer). Separately derived transformers are typically used to step down threephase 277/480 VAC service to standard commercial 120/208 VAC service. To improve the effectiveness of line-to-neutral (normal mode) surge suppression, the transformer creates a separately derived system where the neutral and ground conductors shall be bonded together within the transformer or within the first disconnect after the transformer and bonded to the building’s common grounding electrode system. (see NFPA 70-2005, Article 250.30 for additional information.) See “Separately Derived AC Systems” on page 5-42. 68P81089E50-B 9/1/05 6-5 MGB M BONDED AT ALL CORNER FENCE POSTS NEUTRAL- GROUND BOND GROUNDING ELECTRODE CONDUCTOR AND ROD NFPA 70-2005, Article 250.32(B)(2)
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STANDARDS AND GUIDELINES FOR COMMUNICATION SITES AC POWER<br />
TOWER GROUND RING<br />
TOWER<br />
GROUNDING ELECTRODE<br />
CONDUCTOR AND GROUND ROD<br />
FEEDER IN PVC CONDUIT<br />
WITHOUT EQUIPMENT GROUND<br />
METALLIC FENCE<br />
EXTERNAL GROUND RING<br />
EGB<br />
6 FT MINIMUM SEPARATION ON ALL<br />
SIDES FROM ANY METALLIC<br />
COMPONENT BONDED TO SITE<br />
COMMOM GROUNDING SYSTEM<br />
(NFPA 780-2004, SECTION 6.2.1)<br />
COMMUNICATION SITE<br />
PANEL WITH<br />
NEUTRAL-GROUND<br />
BOND, PER<br />
NFPA 70-2005,<br />
ARTICLE 250.32(B)(2)<br />
ELECTRICAL<br />
SERVICE ONLY<br />
(NO GROUND FAULT<br />
PROTECTION)<br />
FIGURE 6-1 REESTABLISHING THE NEUTRAL-GROUND BOND AT THE COMMUNICATION SITE<br />
6.2.3 SEPARATELY DERIVED SYSTEMS USING TRANSFORMERS<br />
Communications site AC power can also be supplied by a trans<strong>for</strong>mer (also known as an isolation, stepdown,<br />
or step-up trans<strong>for</strong>mer). Separately derived trans<strong>for</strong>mers are typically used to step down threephase<br />
277/480 VAC service to st<strong>and</strong>ard commercial 120/208 VAC service.<br />
To improve the effectiveness of line-to-neutral (normal mode) surge suppression, the trans<strong>for</strong>mer<br />
creates a separately derived system where the neutral <strong>and</strong> ground conductors shall be bonded together<br />
within the trans<strong>for</strong>mer or within the first disconnect after the trans<strong>for</strong>mer <strong>and</strong> bonded to the building’s<br />
common grounding electrode system. (see NFPA 70-2005, Article 250.30 <strong>for</strong> additional in<strong>for</strong>mation.)<br />
See “Separately Derived AC Systems” on page 5-42.<br />
68P81089E50-B 9/1/05 6-5<br />
MGB<br />
M<br />
BONDED AT ALL<br />
CORNER FENCE POSTS<br />
NEUTRAL-<br />
GROUND<br />
BOND<br />
GROUNDING<br />
ELECTRODE<br />
CONDUCTOR<br />
AND ROD<br />
NFPA 70-2005,<br />
Article 250.32(B)(2)
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