Page 129 - Electrical Safety of Low Voltage Systems

112 Chapter Six

FIGURE 6.19 Dielectric joint across a natural gas metal pipe.

6.10.2 Should We Bond Incoming Pipes Electrically
Separated by a Dielectric Joint?
Natural gas utilities may interpose a dielectric joint, which is highly
resistant to currents to electrically separate their metal pipes from the
user’s in a permanent fashion in order to protect their metal work
(Fig. 6.19).
The electrical connection of unlike conductive materials (i.e., steel
pipe and copper ground rod) embedded in electrolytes such as the
earth, by forming a galvanic cell (i.e., a battery) might trigger corrosion
to the detriment of the pipe. Corrosion occurs when direct currents, by
leaving metal parts, “drag” out their constituent materials. As copper
has a negative potential (i.e., −0.34 V), while the steel is positive (i.e.,
0.04 V), the latter will suffer corrosion (Fig. 6.20).
The dielectric joint prevents the above-described corrosion of the
pipe by interrupting the circuit of the galvanic cell.
Because of the dielectric joint, the metal pipe is insulated from
ground; however, for the same reasons earlier explained, it needs to
be connected to the main equipotential system for safety reasons. The

FIGURE 6.20 Galvanic corrosion.