5-6Power and Grounding Requirements
System power protection
There are three levels of protection at a Metrocell cell site. The first level is at the power plant which may consist of a hydraulic-magnetic breaker or slow- blow fuse. This stage is not provided by Nortel. The second level of protection is located in the RIP of the frames that consists of a magnetic breaker. In some cases, a third level of protection is implemented in the equipment shelf such as the TRU/DPA shelf fans and the ATC shelf and usually consists of a faster blow fuse. This arrangement isolates faults that occur lower down in the hierarchy from affecting circuits higher up.
Grounding
UL/CSA approval
The North American electrical codes require that there be no current over the grounding conductors (see C22.1 par 10-200 and ANSI/NFPA No. 70 article 250-21) and the safety standards specify that the electrical codes be adhered to. The Metrocell uses a two-wire DC power distribution scheme. In a grounded two-wire system, the return and ground are multiply connected and an unspecified amount of the return current can flow over the grounding conductors in violation of the electrical code rules.
Therefore, each cell site has to be inspected by a safety authority (UL/CSA in North America) such that the codes requirements (refer to UL-1459 par 14.2 and 34.6 and CSA C22.2 No. 225 par 4.5.3.1a) are met in order to obtain an approval from that authority.
UL-1459 par 14.2
A product intended for permanent connection to the branch-circuit supply shall have provision for the connection of one of the wiring methods in accordance with the National Electrical Code, ANSI/NFPA No. 70.
UL-1459 par 34.6
A field-wiring terminal intended solely for connection of an equipment- grounding conductor shall be capable of securing a conductor of the size rated for the application in accordance with the National Electrical Code ANSI/ NFPA No. 70.
CSA C22.2 N0. 225 par 3.5.3.1a
Permanently connected equipment shall be provided with wiring terminals or leads for the connection of conductors not less than 14 AWG and having an ampacity not less than 125% of the rated input current.
UL would not accept the grounding of the battery return when the battery/cell site configuration is not in the same room unless the battery is floating. A dedicated battery/cell site configuration residing in the same equipment room would not raise any concerns. CSA would have no objections to a grounding
411-2021-111 Standard 01.01 June 1996
