Quantcast Metal Frameworks of Buildings

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MIL-HDBK-419A
2.5.6 Metal Frameworks of Buildings. The metal frameworks of buildings may exhibit a resistance to earth of
less than 10 ohms, depending upon the size of the building, the type of footing, and the type of subsoil at a
particular location.  Buildings that rest on steel pilings in particular may exhibit a very low resistance
connection to earth. For this low resistance to be used advantageously, it is necessary that all elements of the
framework be bonded together.
2.5.7 Water Pipes. Metal underground pipes have traditionally been relied upon for grounding electrodes. The
resistance to earth provided by piping systems is usually quite low because of the extensive contact made with
soil. Municipal water systems in particular establish contact with the soil over wide areas. For water pipes to
be effective, any possible discontinuities must be bridged with bonding jumpers. The NEC requires that any
water metering equipment and service unions be bypassed with a jumper not less than that required for the
grounding connector.
However, stray or fault currents flowing through the piping network into the earth can present a hazard to
workmen making repairs or modifications to the water system. For example, if the pipes supplying a building
are disconnected from the utility system for any reason, that portion connected to the building can rise to a
hazardous voltage level relative to the rest of the piping system and possibly with respect to the earth. In
particular, if the resistance that is in contact with the soil near the building happens to be high, a break in the
pipe at even some distance from the building may pose a hazardous condition to unsuspecting workmen. Some
water utilities are inserting non-conductive couplings in the water mains at the point of entrance to buildings to
prevent such possibilities.  For these reasons, the water system should not be relied upon as a safe and
dependable earth electrode for a facility and should be supplemented with at least one other ground system.
2.5.8 Incidental Metals. There may be a number of incidental, buried, metallic objects in the vicinity of the
earth electrode subsystem. These objects should be connected to the system to reduce the danger of potential
differences during lightning or power fault conditions: their connection will also reduce the resistance to earth
of the earth electrode subsystem. Such additions to the earth electrode subsystem should include the rebar in
concrete footings, buried tanks, and piping.
2.5.9 Well Casings. Well casing can offer a low resistance contact with the earth. In some areas, steel pipe
used for casing in wells can be used as a ground electrode. Where wells are located on or near a site, the
resistance to earth of the casing should be measured and, if below 10 ohms, the well casing can be considered
for use as a ground electrode.
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