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Power and Grounding Requirements
5-7
DMS-MTX DualMode Metrocell Cell Site Description
scheme if the system input power is less than 50V thus not requiring any
ground (see CEC par 10-102).
CEC par 10-102
Two wire direct-current systems supplying interior wiring and operating at
not more than 300 V or less than 50 V between conductors shall be grounded,
unless such system is used for supplying industrial equipment in limited areas
and the circuit is equipped with a ground detector.
The interpretation of "objectionable current" is to be aligned with the leakage
current limits as defined in CSA 950 (maximum 5% current rating) or CSA
225 (maximum 10% current rating). The NEC definition of "objectionable
current" is any current not suitable for a particular installation; which would
include leakage current limits, grounding conductor size, electrochemical
potential between dissimilar metals, etc.
Grounding requirements for the Metrocell is to keep the total return current on
the grounding network below 5% of the total system DC current draw. This is
done by:
1. Making the desired return path a much lower resistance than the
undesired return path (that is, current divider principle). Eliminating the
grounding conductor at the power plant will help discourage return
current flow through the supplementary grounding conductor.
2. Minimize equalization currents between frames via the grounding
conductors and antenna coax, etc. This is achieved by adhering to an
isolated mesh grounding concept. The mesh concept means that all the
metal surfaces (frames, shelves, PCP ground planes and module chassis)
within the system are bonded together with ideally as little contact
resistance as practically possible.
Isolation means that the system grounding mesh only makes contact with
other grounded systems at the local ground reference or BPG. This helps
to reduce the chance of ground currents from other systems from flowing
through the Metrocell grounding conductors. Isolation from building steel
should be facilitated by providing an isolation pad underneath each frame.