UFC 3-560-01
6 December 2006
Including change 1, 7 December 2006
grounding equipment. For the Air Force, grounding sets and jumpers should be
workbench assembled and an annual visual inspection performed for the equipment.
7-1.3.1 Clamps. Use the alloy (copper or aluminum) matching the conductor or
device to which it is attached and meeting or exceeding the current-carrying capacity of
the associated cable. Use smooth jaw clamps on buses to avoid surface marring. Use
serrated clamp jaws to bite through corrosion products for attachment to conductors or
metal products. Self-cleaning jaws are recommended for use on aluminum. Never use
hot-line clamps for grounding.
7.1.3.2 Cable. Cables will be preferably ASTM F 855, Type I, of a minimum 2/0
AWG copper and be able to withstand the available fault currents for 15 cycles for
substation use and for 30 cycles for line use. Sharp bends and continuous flexing of
cable can break conductor strands. Excessive cable lengths must be avoided as this
increases resistance, and twists and coils also reduce their current-carrying capacity.
As a general rule, limit the length of grounding cables to 30 ft (9 m) for line use and 40 ft
(12 m) for substation use. Derate the Table 7-1 fault current capability by 10 percent
when using multiple ground cables (which must all be of the same size and length).
Cables prepared by facility personnel for grounding applications should be highly
flexible and rugged; specialty wires intended for electric welders or for railroad
locomotives are typically used.
Table 7-1. Maximum Fault Current Capability for Grounding Cables1
Cable Size (AWG)
Fault Time (Cycles)
RMS Amperes (Copper)
15
27,000
2/0
30
20,000
15
36,000
3/0
30
25,000
15
43,000
4/0
30
30,000
1
These current values are the "withstand rating" currents for grounding cables and cables as per
conductors. They represent a current that a cable should be capable of conducting without being
damaged sufficiently to prevent reuse.
7.1.3.3
Ferrules. Use ASTM F 855, Type IV (threaded stud copper base
compression type) when installed on grounding cables by facility personnel. Ferrules
should have the filler compound vent hole at the bottom of the cable so that employees
can visually check that the cable is fully inserted into the ferrule. Heat shrink or springs
should be installed over a portion of the ferrule to minimize strand breakage caused by
bending. In all cases, the manufacturer's recommendations should be followed. Do not
use aluminum alloy ferrules as they will not provide a lasting snug fit. Check for
tightness periodically.
7-2
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