GAI-Tronics 495-001 Mine Dial / Page Phone Interface Cabinet - Manual (Page 7)

Pub. 42004-198D
Model 495-001 Mine Dial/Page Phone Interface Cabinet Page: 7 of 22
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Single-pair cables can be used to connect each 491 Series Mine Dial/Page Phone to the nearest
distribution point. For distances up to one mile, unshielded twisted wire can be used. Shielded cable
should be used for longer distances and in situations where the cable is routed near ac power distribution
facilities, ac-to-dc rectifiers, or cables.
All underground cables should be installed in accordance with Federal requirements 30 CFR 75.516-2
and any applicable state requirements. The location of stations at main portals and working sections
must be in accordance with Federal requirements 30 CFR 75.1600 and any applicable state
requirements.
If the mine cable (not in metal conduit) is exposed aboveground out-of-doors; adequate line-to-ground
transient protection, such as nominal 450 V carbon air-gap lightning arrestors, must be installed between
each conductor and ground.
The U.S. Code of Federal Regulations (30 CFR 75.521) requires arrestors within 100 feet of the
point a circuit enters a coal mine and that they be grounded separately from power system grounds.
Similarly, 30 CFR 77.508 requires arrestors on exposed wires entering buildings at the point of
entry.
Except where splices or connections are necessary outside the mine to meet this arrestor requirement, the
mine cable must run one continuous length from the point of connection to the interface cabinet. This
continuous cable run is a requirement for the system permissibility rating. The continuous cable is
necessary because transient line-to-line protection for the mine cable is provided within the interface
cabinet PCBA Rack Assembly between the pair of lines to each 491 Series Mine Dial/Page Phone. This
arrestor installation limits the coupling of transients that might appear across the pair of wires to each
phone.
PABX Telephone Switchboard Requirements
The GAI-Tronics Mine Dial/Page Phone system uses a loop start DTMF (Dual Tone Multi Frequency)
line for each independent 491 Series Mine Dial/Page Phone station. In most situations, a Private
Automatic Branch Exchange (PABX) at the coal mine site best provides this function.
There are no special requirements for this PABX, and conventional telephones at aboveground locations
can also be used with the PABX. This PABX can be provided by a public telephone company, leased or
purchased from an “interconnect” telephone equipment company.
The PABX should provide a standard two-wire loop-start 48 V dc analog line connection with
bridged ringing (17 to 67 Hz) for each Mine Dial/Page Phone extension. Three additional lines will
be required for Monitor and All-Call functions. Refer to Installation section under “Wiring
Telephone Interface Cabinet to Telephone System” on page 9 for detailed information.
We recommend that the PABX switchboard have standby batteries in the event of an ac power outage;
however, this is not a requirement. Lightning protection should be provided for the PABX per the
manufacturer’s recommendation.