USBM Contract No. SO144082 

TECHNICAL BULLETIN 

PAGER PHONES 

INSTALLATION 

April 1975 

Bureau of Mines 

Department of the Interior 

Washington, D.C., 20240 

Arthur D Little, fnc. 
The views and conclusions contained in this document are those of the authors 

and should not be interpreted as necessarily representing the official policies of 

the Interior Department's Bureau of Mines or the U.S. Government. 

Arthur D Little. Inc 

J 
TECHNICAL BULLETIN 

PAGER PHONES 

INSTALLATION 

INTRODUCTION 

The pager phones used in many underground coal mines are batteryoperated, party-line telephones with provisions for loudspeaker paging. The 

system is two-wire, non-polarized, and is operated by self-contained batteries. 

Many of the individual units are certified as permissible. 

Requirements for installation are specified in state and federal regulations, 

such as in the Code of Federal Regulations, Title 30, Mineral Resources, Part 75. 

The major federal regulations that affect communication in coal mines are noted 

below, with a brief description of the subject matter in each of the relevant 

paragraphs. Refer to the cited paragraphs of Title 30 for explicit details of these 

regulations. (This brief description is for reference purposes only.) 

Minimum Requirements (paragraphs 75.1600, 75.17 13-2, and 

75.1003-1) 

Approved telephone or two-way communication will be provided 

between the surface and each main shaft landing and each working 

section ; to the nearest medical assistance; and shall be protected 

from contact with trolley wires or trolley feeder wires. 

Installation Requirements (paragraphs 75.5 16-2, 75.52 1, and 

77.508-1 ) 

Communication wires will be supported on insulated hangers, 

attached to messenger cables, or buried, to protect against 

mechanical damage ; installed opposite trolley wires; and insulated 

near power conductors . . . each cable will have lightning arresters 

within 100 feet of going below ground . . . and arresters will be 

provided wherever telephone wires enter a building. 

Wire and Cable Requirements (paragraphs 75.5 16-2, 75.5 17, and 

75.52 1) 

Communication cable means two or more insulated conductors, 

covered by an additional abrasion-resistant covering; power wires 

and cables shall be adequately insulated and fully protected; and 

lightning arresters shall be connected to low-resistance grounds, 

more than 25 feet from neutral grounds. 

Arthur I3 Little Inc 
This technical bulletin is based on: the requirements of Title 30; installation 

practice recommended by equipment manufacturers; the experience of repair 

personnel; industrial telephone communication experience; and Rural Electrification Authority (REA) practices. 

A description of the many telephones available is included in the bulletin 

Introduction; the types of wire and cable to be used in these installations are 

described in the bulletin Cable Selection; and proper care and procedures for connecting and splicing the interconnecting cable are described in the bulletin Splices 

and Connections. The following text includes excerpts from those bulletins, to 

supplement the installation recommendations. Detailed information on each subject should be obtained from the individual technical bulletins. 

MOUNTING 

Pager phones are designed to be mounted on an upright support at the 

desired location. For convenience, the phone should be mounted five feet above 

the floor where there is no obstruction to using the handset or removing the 

cabinet front cover for servicing or battery replacement. In low coal situations, a 

suitable height for installation should be selected convenient to the normal 

operator's position at the site selected. About 12 inches of free space on each side 

of the phone should be provided for cabinet access. Four 114-inch or 5116-inch 

holes are normally provided in a 6-inch x 10-314-inch pattern on a rear bracket, 

for mounting with lag screws, bolts, or spikes. The phone should be protected 

against direct exposure to dripping water and should not be allowed to rest in a 

puddle of water. The mounting location should be convenient to a work location 

and have a safe, unobstructed area for a worker to stand and use the phone. The 

phone must be in a location where the worker will not be in the path of moving 

vehicles or falling debris. Each telephone is normally well insulated, low voltages 

are used, and the current is limited to a safe value, but it is still good practice to 

provide an insulating mat or dry planking for the user to stand on. 

CABLE 

The cable wire size depends on a series of factors that include: the total 

number of telephones in an installation, the total length of cable run (distance 

between the farthest phones), the configuration of branch lines, available battery 

voltage, and the type of paging relay used. The interaction of these factors and 

their impact on the choice of wire size is discussed in the technical bulletin Cable 

Selection. The preferred cable, regardless of wire size, is a twisted pair of solid 

conductor wires, with individual insulation around each wire in the pair and an 

outer abrasion-resistant covering of waterproof, flame-retardant material. 

Arthur D Little Inc 
Cable strung in an entry should be supported by insulated "J" hooks, or 

other suitable insulating hooks, spaced close enough to prevent excessive sag in 

the cable. Additional support can be provided at branch points or near each 

telephone location. This additional support and strain relief, as shown below, will 

reduce the hazards of broken cables and prevent undue strain on the connections 

to the telephone. The cable must be supported free of any standing water, 

protected from leaks or dripping water, and hung so it will not snag passing 

vehicles or personnel. 

The branch cable, where it connects to the telephone, should be slack and 

formed into a "drip loop" near the phone installation, as shown below. 

Cable installed in track entries should be hung on the side of the entry that is 

free of trolley wires and trolley feed wires. Telephone cable should not run close 

to power conductors, but if that is necessary it should cross the power line at a 

sharp angle to reduce the length of cable that is close to the power conductor. 

Additional insulation and support, such as running the cable in a slit length of 

plastic pipe, should be provided at points where the telephone cable must pass 

over a power conductor or trolley wire. The additional protective insulation 

should extend at least four feet to either side of the crossing point. 

If telephone cables are installed in bore holes that contain non-armored 

power cable, either the telephone cable or the power cable must be armored or 

protected at both the top and bottom of the bore hole with insulating transformers. However, because the pager phone is a dc paging system, the use of 

insulating transformers is impractical, so the only choice is to provide armor 

protection to either the power cable or the telephone cable in this situation. 

Lighting protection must be provided for telephone circuits where the wire 

and cable go underground. Details of lightning protection are given in the 

technical bulletin on Circuit Protection. 

Many telephones used underground, particularly those used at the working 

face, are subject to periodic relocation. To allow for this, and to reduce the 

problems associated with repeated cable splicing, some convenient length of wire 

(say, 500 feet) should be included as part of the branch line. The extra wire can 

be kept reeled, or neatly coiled in a bundle and secured with a few wraps of 

plastic electrical tape. The extra wire should be hung near the telephone in a place 

free of dripping water or water accumulation, and should be supported by an 

insulated hanger that is isolated from power or trolley wires. 

Arthur D Little Inc 
SUPPORT MAIN LlNE 

AND TAP LlNE 

STRAIN RELIEF 

Arthur D Little Inc 
DRIP LOOP 

Arthur D Little Inc 
CONNECTIONS 

For handling convenience, the branch, tap, drop line, or connecting cable to 

each individual telephone, can be a lighter wire gauge than that of the main cable. 

Each connection to the main line should be a good electrical and mechanical 

joint, protected by a careful double wrap of plastic electrical tape. Detailed 

illustrations for a variety of recommended connecting schemes are included in the 

technical bulletin Splices and Connections. 

Connections at the phone cabinet depend on each manufacturer's design and 

on individual state or local requirements. A majority of the phones provide two 

exposed spring-loaded terminals for the attachment or connection to the wires. 

For proper connection, first strip the insulation away from each conductor in the 

pair, then seal off the exposed area of the cable with plastic electrical tape to keep 

out moisture, as shown below. Then insert one of the exposed conductors in each 

of the cabinet terminals, as shown in the figure. Some states, such as Pennsylvania, do not allow the use of exposed terminals at the face area of gassy 

mines. For such applications, some phones are equipped with twist-lock connectors at the end of a short cable. Each connector is mated with a similar 

connector on the drop or branch line to complete the installation, as shown 

below. In either type of installation, there should be a drip loop below the cabinet 

to prevent condensate from running down the cable and into the cabinet. 

BATTE R I ES 

Replacement 

Pager phones are usually operated by one (or two) 12-volt, dry-cell batteries, 

NEDA No. 923 or No. 926 (National Electronic Distributors Association). To 

install batteries, open the pager phone cabinet and inspect the batter compartment. Remove the old battery by loosening the retaining clamp, and either 

unscrew the battery terminals to release the battery wires or remove the battery 

plug, depending on the battery type. Remove the battery, and carefully wipe out 

the battery compartment to remove dirt and residual moisture. Place a fresh 

battery in the compartment, and secure the retaining clamps tight enough to 

restrain the battery without crushing or bending the battery case. Reconnect the 

battery wires, being careful to observe the polarity markings noted on the case. If 

the plugconnector type is used, do not force the connector. Correct polarity is 

maintained when the larger connector pin fits in the larger hole. The difference in 

pin sizes is not great, so a mismatch can be forced. If the connector does not mate 

easily, reverse it and try again without forcing. Afterreplacing the battery, close 

the cabinet and mark the date of battery replacement either on the outside of the 

cabinet or in a continuing log. 

Arthur D Little Inc 
+8 

I 

MINIMUM l INCH 

CABLE- END SE-ALING 

--! %4 INCHk 

STRIP 

Arthur D Little Inc 
ARE IN LINE - THEN 

INSERT WIRE 

PUSH TERMINALS 

TWIST-TO- LOCK CONNECTOR 

8 

3h INCH 

ALLOW ENOUGH 

SLACK FOR DRIP LOOP 

Arthur I3 Little Inc 
CAUTION: Pager phone circuits are normally designed to provide sufficient current limiting with the specified battery. If other battery types 

are used - such as the nickel-cadmium rechargeable type or one of the 

alkaline, long-life, highcurrent varieties, the circuit may not be able to 

limit the available current to a safe value. 

Testing 

Battery life is not easy to predict, because of the many operating variables 

that affect the average current drain. Voltmeter measurement of a battery can be 

made as described in the Technical bulletin Testing -- Part I. In general, the 

batteries in a telephone system that are used many times a day may have to be 

replaced every four to six weeks, while a telephone system that is seldom used 

may keep its batteries at usable strength for four to six months. 

Each battery change should be recorded, either on the telephone cabinet or 

in a central log. Experience gained over a period of time will help predict when a 

battery in a particular phone is reaching the end of its useful life. Periodic 

verification of battery status at each phone should be made with a voltmeter and 

recorded in the log. When the battery voltage drops to a value that is 75 to 80% of 

the installed level, then it should be replaced. For example: for a 12-volt battery, 

the replacement level is about 9 volts. 

FUSES 

Fuses are provided in pager telephones as an added precaution against 

excessive current in the external circuit. Current-limiting circuitry is normally 

provided in the telephone, but the fuse is an additional safeguard. No provision is 

made in most phones to store a spare fuse. It is good practice to tape two 

additional spare fuses to the inside of the cabinet when the phone is first installed. 

Then, the correct fuse will be available at the phone if it is needed. 

AMPLl FlER LOUDNESS 

Each pager phone has a loudspeaker, powered by its own internal amplifier, 

that is switched on by the dc paging signal. The available audio power is about 5 

watts, which is adequate to be heard above most mine noises. Many telephones 

have a volume control for the speaker. During installation, the speaker should be 

oriented, and the volume set, to ensure adequate coverage in the area. 

During set-up, have another person page from a distant location to the phone 

being installed. Adjust the volume control to the desired level during the paging. 

The telephone cabinet should be positioned to direct maximum sound to the 

work area. 

Arthur D Ltttle. Inc 
SWITCHING DISTRIBUTION 

In some mines, it has been found that a total party-line system is not 

desirable. In these installations, some sort of switchboard set-up is provided at a 

central site, such as the dispatcher's location. Then, certain sections, or specific 

phone locations, are isolated and wired separately to the switching center. To call 

within one of these branches, the normal procedure is used. To call a station 

outside such a branch, or to call a special phone set in a single circuit, the call 

must go through a switchboard. 

The equipment used for this switching function is quite often modified 

public-service telephone-type equipment. Because the pager phone is a d-c system, 

care must be taken to ensure that the switching system provides a continuous 

circuit for both wires in each cable. If the switching unit contains "loading coils," 

they should be removed. Each of the loading coils will load the phone line as if it 

was another phone. 

GENERAL 

The design of cable runs, branching and interconnections depends on individual mine topography and communication needs. Choice of wire or cable size 

depends on the type of phones, number of phones, total distance, and combination of branch schemes for a particular mine, as described in the technical bulletin 

Cable Selection. Most phone locations are fixed, except those in the face area 

that must move as work progresses. A number of portable telephones are becoming available that can be used with existing installations, if provision is made 

for an easy tap into the line. At major junction points, terminal strips or open 

splice boxes, such as those shown below, are the ideal method of providing for 

temporary connection of a portable telephone. 

Arthur D Little, Inc 
2 PAlR 

r BINDING m3Tb 

MOUNTING 

3 PAlR 

TERMINAL BLOCKS 

11 

Arthur D little Inc 
MOUNTING HOLES-, 

I --BINOIUG POSTS 

1 

-WIRE ENTRY 

TWO PAIR TERMINAL BLOCK 

MESSENGERCLAMPSBINDING POSTS -/ / y (TO PHONE) 

DROP WIRE TERMINAL BLOCK 

Arthur I3 httle Inc 
/ BINDING POSTS 

26 PAIR TERMINAL 

(COVER REMOVED) 

These pole mounted Wire Terminals are used as distribution points for either the jacketed Figure 8 or the 

non-jacketed multiple-pair distribution wires. 

MULTI-PAIR WIRE TERMINALS 

CAPACITY 

(PAIRS) 

6 

11 

12 ' 

16 

18 

26 

Arthur I3 Little Im 

DIMENSION, INCHES 

A 

4% 

4% 

4% 

4% 

8% 

8% 

I 

13% 

18% 

18% 

18% 

17% 

17% 

DEPTH 

2% 

2 Y2 

2% 

2% 

2% 

2%