Mike's PBX Cookbook

Assuring Power Quality for
Meridian 1 Installations

 

Effective grounding (NEC 250-2): Establishes a zero voltage reference for logic circuits in electronic equipment. The fault path shall be permanent and electrically continuous, shall be capable of safely carrying the maximum fault likely to be imposed on it, and shall have sufficiently low impedance to facilitate the operation of the over-current protective devices under fault conditions. All of the grounding conductors (except the equipment ground to the MFA-150) associated with the Meridian 1 system (LRE, Battery Return, and #6 Safety Ground) are part of the UTILIZATION EQUIPMENT, not the PREMISES WIRING SYSTEM and, as such, do NOT fall under the requirements of the National Electrical Code.

References:

1999 edition of the National Electric Code (NEC)
Power Tek Services, Inc., Ed Cantwell, 99 Mayer Avenue, Wheeling Il, 60090 - Tel: 847-808-1555 - Email: eccpowrtek@aol.com

 

Installation

Grounding Electrodes:

Can be a grounded steel structure, reinforced concrete, or grounding rods. Each time you double the number of electrodes or double the depth, resistance is reduced approximately 40%. All earth grounds for the same building shall be bonded to prevent a potential between grounds. It may be necessary to bond separate buildings if metallic data cable (ex. RS232) is used between the buildings (or use nonconductive fiber optic cable).

Single Point Ground:

Nortel requires all grounding conductors from the telephone system to terminate at a single point including the following:

  1. All AC branch circuit equipment grounds supplying Meridian 1 equipment.
  2. The personal hazard or Safety grounding conductor.
  3. The Battery Return Equalizer (BRE) ground for DC systems.
  4. The Logic Return Equalizer (LRE) grounding conductor.

Auxiliary equipment not on the same isolated ground, shall be isolated with fiber optic cables.

Bonding of Neutral and Ground in Isolated Ground Systems:

• Neutral and Ground are only bonded at the Main Service Panel and at all separately derived systems (ex. secondary of an Isolation Transformer or a Generator or Inverter). Each bond is then connected to a grounding electrode, either a grounded steel structure, or grounding rod. Do not bond at sub-panels or at receptacles. Improper grounding of Neutral and Ground can cause a voltage potential between grounds thereby causing current flow on grounds and disrupting Meridian 1 performance.

1999 National Electrical Code (NEC) reference:

250-24-5: Requires that no grounding connection be made to any neutral downstream from the main neutral-ground bonding point (this would put some of the return load current on the ground).

250-30 Grounding separately derived Alternating-Current Systems
(3) Grounding Electrode. The grounding electrode shall be as near as practical to and preferably in the same area as the grounding conductor connection to the system. The grounding electrode shall be the nearest one of the following:
(a) An effectively grounded structural metal member of the structure.
(b) An effectively grounded metal water pipe within 5ft. (1.52m) from the point of entrance into the building.
(c) Other electrodes as specified in sections 250-50 and 250-52 (ex. grounding rods) where electrodes
specified by (a) or (b) above are not available.

 

• Metallic conduit is encouraged because it shields against Radio Frequency Interference (RFI). However, metallic receptacles that are connected to conduit will pick up noise and interference from other equipment in the building. The requirement to ground metallic boxes and conduit can be accomplished with the conduit runs back to the Main Service panel. To isolate the receptacle ground from boxes/conduit, use isolated receptacles (orange color) or non-metallic boxes. To isolate equipment from conduit use non-metallic fittings.

The isolated ground wire run between the Meridian equipment and the grounding electrode should be insulated to avoid contact with other grounds. On a DC system the isolated ground can be run separate from the hot and neutral wires. On an AC system, the grounding conductor must be run with the hot and neutral wires to cancel reactance.

 

1999 National Electrical Code (NEC) references:

250-146 Connecting Receptacle Grounding Terminal to Box.
An equipment bonding jumper shall be used to connect the grounding terminal of a grounding type receptacle unless grounded as in (a) through (d).
(d) Isolated Receptacles.
Where required for the reduction of electrical noise (electromagnetic interference) on the grounding circuit, a receptacle in which the grounding terminal is purposely insulated from the receptacle mounting means shall be permitted. The receptacle grounding terminal shall be grounded by an insulated equipment grounding conductor run with the circuit conductors. This grounding conductor shall be permitted to pass through one or more panel-boards without connection to the panel-board grounding terminal as permitted in Section 384-20 exception, so as to terminate within the same building or structure directly at an equipment grounding conductor terminal of the applicable derived system or service.

250-134, (b), Exception No. 2: For dc circuits, the equipment grounding conductor shall be permitted to be run separately from the circuit conductors.

250-96 Isolated Grounded circuits.
(b) …an equipment enclosure supplied by a branch circuit shall be permitted to be isolated from a raceway containing circuits supplying only that equipment by one or more listed nonmetallic raceway fittings located at the point of attachment of the raceway to the equipment enclosure.
FPN: Use of an isolated equipment grounding conductor does not relieve the requirement for grounding the raceway system and outlet box.

 

• Use a Common Ground reference for the entire building.

 

1999 National Electrical Code (NEC) reference:

250-30 Common Grounding Electrode
Where an AC system is connected to a grounding electrode in or at a building as specified in Sections 250-24 and 250-32, the same electrode shall be used to ground conductor enclosures and equipment in or on that building. Where separate services supply a building and are required to be connected to a grounding electrode, the same grounding electrode shall be used. Two or more grounding electrodes that are effectively bonded together shall be considered as a single grounding electrode system in this case.

 

Correct Isolated Ground Installations:

Three illustrations of correct Isolated Ground Installations follow. Although all three examples use a single hot wire, in the USA rectifiers can be wired using two hot leads for a total of 208 or 220 volts.

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