ERICO Practical Guide to Electrical Grounding - Manual (Page 101)

93 Chapter 6: Special Grounding Situations
and/or are damaged when the pipe is installed. The breaks
in the coating (called holidays) are protected by the
cathodic protection system. Since the amount of steel
exposed at the holidays is very small compared to a bare
pipeline, the amount of current required to protect the
pipeline is reduced in a direct ratio.
To protect the pipeline in the case of stray current, the pipe
must be bonded to the negative side of the DC power supply
station with a low resistance conductor. This provides a
direct metallic path for the return current to follow as it
leaves the pipe (Fig. 6-6).
A few basic rules in designing a cathodic protection system
include:
1. Bonding together of all structures (tanks, pipes,
both across joints and between different pipes, etc.)
is of absolute necessity for proper protection. This
will provide a metallic return current path for any
cathodic current.
2. A study is needed to determine any effect of the
cathodic protection system on any “foreign” (owned
by others) nearby structures. Any cathodic
protection current picked up by a foreign structure
must also leave that structure - which may cause
corrosion.
CADWELD Connections. Let us look at the electrical
connections required in a cathodic protection system and
why they are different than those required for a grounding
system.
Cathodic connections are low current connections rather
than grounding connections. Grounding connections are
required to withstand damage while conducting huge
surges of ground fault current. Cathodic protection
connections are required to carry only a small but
continuous current. Therefore cathodic protection
connections do not have to be as massive as grounding
connections.
A very low resistance system is required for a cathodic
protection system, and it must remain low in resistance over
the life of the system. The higher the resistance, the less
efficient is the cathodic protection system. CADWELD
Cathodic Protection Connections meet this low resistance
requirement, both when installed and over the life of the
system.
The pipe used in transmission pipeline systems is usually a
highly stressed thin wall steel pipe. Any connection to this
pipe by the cathodic protection wires or the test leads must
not damage the pipe. CADWELD Cathodic Protection
Connections use a special alloy weld metal (designated as
F-33) developed to minimize the effect the weld has on the
pipe. These connections have been proven by independent
tests not to be detrimental to the pipe, and more than 45
years of usage without any detrimental effects have
provided field proof to the tests.
CADWELD Weld Metal for cathodic protection has a
green cap on the weld metal tube to properly identify it as
F-33 alloy. The CADWELD Weld Metal used for
grounding connections should not be used to make cathodic
connections to high stressed pipe. (CADWELD cathodic
connections should never be used to make high current
grounding connections.)
Making Connections. Cadweld cathodic protection
connections can be made to live pipelines and to fuel tanks
with certain restrictions. ANSI/ASME Codes (B31.4 and
B31.8) allow cathodic connections to be made to liquid
petroleum transmission lines and to gas transmission and
distribution lines with a limit of a 15 gram (CADWELD
CA15) weld metal. The lines must be full of product with
no air pockets and when welding to tanks, the weld must be
+
-
dc
Power
Supply
Pipe Line
Stray DC Current
Fig. 6-6
Grounding Book 4/14/99 10/5/99 6:02 PM Page 93 (Black plate)