Rack Grounding Issues:
Note this topic is related to Chassis Grounding or the more general Equipment Grounding and Chassis Stud Grounding [chassis design].
Introduction
Racks may also be called Enclosures, Cabinets, or bays
All grounding or bonding points should be tight for good continuity, identified by green color, permanently labeled, and properly prepared by cleaning metal surfaces to bare metal or by the use of serrated bushings. Anodized aluminum must be cleaned to bare metal.
Metal chassis shall be effectively bonded to a main grounding point in the rack cabinet where
necessary to assure electrical continuity and shall have the capacity to conduct safely any fault
current likely to be imposed on it.
In a chassis with AC service connected to it, the grounding terminal of its receptacle shall be
internally bonded to the chassis frame.
If solder is used, the connection of the equipment grounding conductor shall not depend on
solder alone.
The leakage current of cord connected equipment should be very low.
Equipment Rack Grounding
The equipment grounding conductor of a power-supply cord or interconnecting cable shall be sized in accordance with NEC 250.122. The minimum size equipment grounding conductor is based on the total rating of the enclosed equipments in amperes. Note that the minimum size equipment grounding conductor may be smaller than the size for the current-carrying conductors; i.e., the grounded (neutral) and ungrounded conductors, which are sized per NEC Article 310.15
Rack Grounding Requirements
1. Have a common grounding or bonding bus (normally a cabinet rail).
2. When the enclosure contains more than one bay, bond all grounding or bonding busses
together.
3. All mounted chassis within rack cabinets shall have a grounding or bonding conductor
attached to the common grounding or bonding bus when the chassis is not grounded or
bonded through the power cord.
4. The grounding or bonding conductor shall be permanent and continuous.
5. Sub-assemblies mounted in other types of enclosures should be bonded by adequate
preparation of the mounting surfaces or by the use of a bonding conductor.
6. To provide protection against grounding or bonding conductor breakage, conductors
between the common grounding or bonding bus and moveable chassis should be braided
cable or stranded wire.
7. Possible ground connections include bare copper wire, insulated copper wire or grounding strap.
---- Bare wire is not normally used for chassis to rack bonding.
----- Aluminum wire if used should be one gauge larger than copper.
---- If insulated wire is used, the insulation shall be green in color.
---- Grounding straps may be braided wire or flexible insulated laminate copper.
----- Flexible double-insulated laminates of copper (inside) and polyester film (outside), with
punched and tinned ends to facilitate termination. They provide low-impedance ground paths at RF frequencies.
Equipment Rack Installation; next design topic.
AC power distribution systems shall have the neutral conductor
grounded at the distribution transformer and to the earth electrode subsystem of the facility. The size of the
ground conductor from the first service disconnect means to the earth electrode subsystem shall be as specified
in the National Electrical Code.
In each facility served by a
common distribution transformer, the neutral shall be directly connected to the nearest point of the earth
electrode subsystem. Where delta-wye system conversion is employed, the service entrance shall be a five-wire
system consisting of - phase conductors, a grounded (neutral) conductor, and a grounding (green)
conductor, In each facility, all power distribution neutrals shall be isolated from the C-E equipment case and
the structure elements so that no AC return current flows through the equipment and fault protection subsystem
or the signal reference network. The fault protection subsystem grounding (green) conductor shall be installed
in accordance with the National Electrical Code for all C-E equipment, Conduit shall not be used in lieu of the
separate grounding (green) wire.
All grounded (neutral) conductors shall be
grounded at the first service disconnect means of each source. For delta-wye conversions, a five-wire system
shall be utilized from each source. Delta systems shall employ four-wires from the source, consisting of three
phase conductors and a grounded conductor for grounding purposes... MIL-STD-188-124
Index page; How To Specify a Equipment Chassis
