ERICO Practical Guide to Electrical Grounding - Manual (Page 26)

18 Practical Guide to Electrical Grounding
The damage to the concrete can be explained due to its
non-homogeneous character and moisture content. During
a fault, one path from the rebar to the outside soil through
the concrete will have a lower resistance than any other. The
fault current following this path will cause heating and
vaporization of the water (moisture). The expansion, as
the water turns to steam, can cause the concrete to crack
or spill.
The Ufer grounding system is an excellent method for low
fault currents (housing, light commercial, etc.), especially
in arid regions where driven rods are less effective. But
when high current faults are possible, including lightning,
care must be exercised in designing the system, especially
since it is impossible to isolate the foundations from the rest
of the grounding system.
We recommend that the current path into the foundation
must be connected (wire ties between rebars as a minimum)
and a metallic path should be provided from the rebar to
the earth. This metallic path should be connected to
an external ground electrode. See Figure 1-42, “Ufer”
ground detail.
Bare Copper:
Size Per N E C
CADWELD
To Other
Available
Electrodes
Foundation Near
Electrical
Service Entrance
Finished Grade
Rebar Meeting
Requirements
of N E C 250-81
CADWELD To Rebar
“UFER “ Ground Detail
Fig. 1-42
Grounding Book 4/14/99 10/5/99 6:01 PM Page 18 (Black plate)