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 ELECTRICAL
Grounds and Grounding Systems-Continued
From equipment served by rigid conduit to the nearest grounding
cable attachment on conduit runs of less than 25 feet, allowable
resistance is 10 ohms
n Maximum permissive resistance for grounds and grounding systems
between equipment or structure being grounded and solid (earth) ground:
For generating stations, maximum permissive resistance is 1 ohm
For main substations, distribution substations, and
switching-stations on primary distribution systems, maximum
permissive resistance is 3 ohms
For secondary distribution system (neutral) grounding,
noncurrent-carrying parts of the distribution system itself, and
enclosures of electrical equipment not normally within reach of
other than authorized and qualified electrical operating and
maintenance personnel, maximum permissive resistance is 10 ohms
For individual transformer and lighting-arrester grounds on
distribution system, maximum permissive resistance is 10 ohms
n When total resistance in either of the above tests exceeds allowable,
measure resistances of individual portions of the circuits to determine the
points of excessive resistance and report:
n Substandard resistance values resulting from poor contact between
metallic portions of grounding system and earth:
n Structural steel, piping, or conduit run exceeding 25 ft used as a
current-carrying part of grounding circuit for protection of equipment:
n Absence of ground-cable connections:
7.6.13 Instruments: Comply with all current safety precautions. Determine if
preventive maintenance inspections are adequate. Check for:
n Electrical Instruments:
improper shielding, mounting, or enclosures when located near
strong magnetic fields, subject to vibrations, extremes in
temperature, moisture, metallic and other dust, and acid or
corrosive vapors
inadequately, improperly or not neatly stored in cases or cabinets
that are free from dust, corrosive fumes, excessive heat, moisture,
vibration, strong magnetic fields
not clean
7-28
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