USBM Contract No. SO144082 

TECHNICAL BULLETIN 

PAGER PHONES 

CIRCUIT PROTECTION 

January 1975 

C-77212 

Bureau of Mines 

Department of the Interior 

Washington, D.C. 20240 

Arthur D Little, Inc. 
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 Lttlo Inc 
TECHNICAL BULLETIN 

PAGER PHONES 

CIRCUIT PROTECTION 

Wires leading to or from underground facilities can develop hazardous 

voltages because of lightning exposure. The locations of many mine portals are 

particularly susceptible to lightning effects. The application of protective devices 

to limit this potential danger is necessary. 

REQUIREMENT 

The Code of Federal Regulations, Title 30, Mineral Resources, Part 75, 

paragraph 75.52 1 identifies the requirement for lightning arrestors to be used on 

ungrounded, exposed power conductors and telephone wires. "Each ungrounded, 

exposed power conductor and telephone wire that leads underground shall be 

equipped with suitable lightning arrestors of approved type within 100 feet of the 

point where the circuit enters the mine." 

For telephone circuits, a wide range of protective devices are available. The 

public service telephone companies and Rural Electrification Authority have 

considerable experience with the problem. The current recommendation by these 

organizations for such installations is to use gas-tube protectors, operating with a 

well-designed and constructed grounding circuit. Although pager phones are d-c 

systems, with signals in the audio range, many installations have coupled RF 

(Radio Frequency) signals on the same cable that is used for the pager system. The 

telephones themselves are constructed with isolating circuits, so as not to disturb 

RF transmission. Lightning arrestors and protective circuits must also provide this 

same isolation and circuit integrity for the RF signals. Capacitive protective 

elements cannot be used for lightning protection of such circuits, as they would 

short out the RF signals. 

Lightning strokes, either cloud-to-cloud or cloud-to-ground, release tremendous energy. The voltage associated with cloud-to-earth lightning stokes is about 5 

to 20 million Volts, but the danger is in the large amounts of energy developed by 

the fields induced in exterior cabling. 

ARRESTOR CONNECTION 

In early telephone installations, carbon blocks of 3 mil (0.003") carbon were 

used as a protective breakdown device for phone circuits, but in the past ten years 

gas-tube devices have become more acceptable. When sufficient energy is developed between two circuits, such as a telephone line and ground reference, the 

protective device ionizes and provides a low-impedance path to ground, thus 

Arthur I3 Little, Inc 
keeping the hazardous voltage from reaching the remainder of the system. In a 

gas-tube this action takes place in a few microseconds (0.000001 second) by the 

ionization of gas at 1 50 to 500 Volts in a chamber, as shown below. 

Most installations provide a discharge path from each conductor to ground. 

This is the two-element system shown below. 

A limitation of this system is that it does not protect against energy levels 

that can build up between the line pair. Either of the two protective arrestors can 

fire on a random statistical basis with a high-energy signal applied to the line 

conductors. In the short time before the second arrestor operates, a large energy 

potential can be present between the line pair conductors. To offset this condition, the three-element system was developed. The first approach was to provide 

three separate arrestors as shown below. 

This scheme is somewhat successful, but retains the problem of significant 

time delays before all potential paths are closed to the buildup of energy to a 

danger level. A scheme to improve on this deficiency is to include all gaps within 

one enclosure so that when any gap is ionized the other two will benefit from a 

preionization and reduce the time to complete all circuits as the energy differential increases. 

GROUND WIRE 

Title 30, Part 75, paragraph 75.701-4 identifies that grounding wires for the 

type of cable used in telephone systems (less than #6 AWG) shall be at least the 

same cross-sectional area as that of the conductor cable. Good installation 

practices dictate that the choice of ground cable for this application be at least 

#14 AWG solid copper. Steel or other conductor may be used if the equivalent 

conductivity is at least equal of # 14 AWG copper (2.5 ohms/ 1,000 feet). 

The protective circuit should be connected close to the point of entry into a 

building or below ground, and in any case not more than 100 feet from where the 

circuit enters the mine. 

Ground reference should be at a short distance from the protective circuit. 

Ground can be a good electromechanical bond by direct connction to a metallic 

water pipe leading underground, or to a buried metal stake. A 314-inch-diameter, 

8-foot-long copper rod, buried vertically so that the top is 18 to 24 inches below 

the top surface, is normal practice to ensure a solid electrical ground reference. 

Arthur D Lttle Inc 
- BREAKDOWN \ POTENTIAL 

TWO-ELEM ENT SYSTEM THREE-ELEMENT STSTEM 

(separate Arrestors) 

THREE-ELEMENT SYSTEM 

(One Combined Arrestor) 

Arthur D Little, Inc 
EQUIPMENT SOURCES 

The following partial list of manufacturers of lightning protective devices is 

intended to be a guide, and not a comprehensive listing of all manufacturers. 

Several typical surge arrestors are shown below. 

Cardion Electronics Reliable Electric Company 

Long Island Expressway 1 1333 Addison 

Woodbury, L.I., N.Y. 1 1797 Franklin Park, IL 60 1 3 1 

Coil Sales & Mfg. Co. Signali t e 

P.O. Box 251 1933 Heck Avenue 

Arlington Heights, IL 60006 Neptune, N.J. 07753 

Cook Electric Telecommunication Industries, Inc. 

Morton Grove, IL 60053 1375 Akron Street 

Copiague, L.I., New York 11 726 

Joslyn Electronic Systems 

P.O. Box 8 17 

Goleta, CA 93017 

Arthur D Little, Inc 
THREE-ELECTROOE. GAS-TUBE SURGE ARRESTOR 

5 

Arthur I3 Little Inc 
1 HREE-ELECTROD€ GAS-TUW SURGE ARRESTOR 

Arthur I3 Little, Inc 
THREE-ELECTRODE GAS-f UBE. SURGE ARRESTOR 

WIT H CARBOhl- BLOCK BACK- UP 

Arthur D Little, Inc