Nace CP 1, Course Manual

Page: 144 / 535

Safety 4:7
CP 1–Cathodic Protection Technician Course Manual
©
NACE International, 2000
12/01/04
•
Avoid measurements during thunderstorms since hazardous voltages
may occur on structures as a result of even remote lightning strikes.
•
Use caution when working near power transmission lines or, when
measuring across isolating devices and polarization cells, use one-hand
connection method with insulated test leads and clips.
•
Consider the possible shock hazards from operating CP systems in
water.
Induced AC Voltages
In cases where a structure or tracer wires parallel a high voltage AC
(HVAC) power transmission circuit, significant AC potentials may be
encountered. Hazardous AC potentials can occur on a structure as a result
of induction, ground return currents, or faulted power circuits.
The AC voltage-to-ground of a structure should always be measured first if
there is a possibility of hazardous potentials. Be careful not to make direct
physical contact with the measurement circuit. Use a multimeter to measure
AC potentials between the structure and a copper-copper sulfate reference
electrode in contact with the earth or other low resistance ground. If an AC
voltage in excess of 15 V is measured, the structure is considered
hazardous; and steps must be undertaken to reduce the hazardous voltage
level. If the potentials are determined to be less than 15 V
ac
, no specific
action is necessary; however, caution should be maintained as this voltage
can change at any time with a change in power line load.
NACE Standard RP0177,
Mitigation of Alternating Current and Lightning
Effects on Metallic Structures and Corrosion Control Systems,
contains
valuable AC safety information. You should become thoroughly familiar
with this document especially Section 5, “Personnel Protection.” NACE
Standard RP0177 is included in the Appendices section of the course
manual.
A pipeline paralleling an HVAC transmission line can reach dangerous AC
voltages when it is welded together on skids, but not backfilled. Special
safety precautions must be taken when working around such lines. The
safety precautions may include temporary electrical grounding of the pipe,
avoiding physical contact with the pipe and other grounded structures and
avoiding physical contact across electrically isolating joints. Safety