Avoid electric shock by carefully handling low-voltage wiring

June 1, 1996
Workers new to the field frequently assume that low-voltage cabling does not present a shock hazard. A 50- to 60-volt direct current (DC) is normally present on an idle tip-and-ring pair. Sometimes a ringing current of 90V alternating current (AC) is present and, under certain circumstances, can deliver an electrical shock. Many accidents have occurred because of improper or careless handling of such cabling and failure to observe proper safety procedures.

Don McConaghy, Leviton Telcom

Installer Tips

Problem

Workers new to the field frequently assume that low-voltage cabling does not present a shock hazard. A 50- to 60-volt direct current (DC) is normally present on an idle tip-and-ring pair. Sometimes a ringing current of 90V alternating current (AC) is present and, under certain circumstances, can deliver an electrical shock. Many accidents have occurred because of improper or careless handling of such cabling and failure to observe proper safety procedures.

Solution

Always assume that hazardous voltages exist in any wiring system. A safety check, using a known and reliable voltage measuring or detection device, should be made immediately before work begins and whenever work resumes. There are other important safety rules for avoiding electrical shock, some of which are included here.

Procedure

1) Always use insulated tools and avoid all contact with bare terminals and grounded surfaces.

2) Disconnect the dial-tone service from the premises wiring while working on it. If you cannot disconnect the service, take the telephone handset off-hook. The DC level will drop and normally no AC ringing current will be delivered.

3) When cutting or drilling, be careful not to cut through or drill into concealed wiring or pipes. Make a small inspection opening before you start cutting.

4) When running telephone wire on or near metallic siding, check for stray voltages. On mobile homes or trailers with metallic surfaces, always test for stray voltages and bond to ground before beginning work.

5) Keep telephone wiring away from bare power wires or lightning rods, antennas, transformers, steam or hot-water pipes, and heating ducts. Do not place telephone wire in a conduit, channel, box, duct or other enclosure containing power or lighting circuits of any type, and provide adequate separation between telephone and electrical wiring.

6) Do not work on telephone wiring if you wear a pacemaker. Telephone-circuit voltages can disrupt them.

7) Most electrical injuries involving telephone wiring result from sudden surges of high voltage on normally low-voltage wiring. Because a fatal lightning surge can be carried over telephone wire for many miles, never install or connect telephone wiring during electrical storms.

8) Follow all guidelines issued by the telephone company, local code requirements and the provisions in Article 800 of the National Electrical Code.

Don McConaghy is an applications engineer at Leviton Telcom, Bothell, WA. This installer tip is excerpted from the company`s recently published book, Installation Strategies for Long Term Cabling System Success.

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