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Several commenters requested that OSHA totally remove the
structural metal frame of a building as an acceptable grounding method
(Exs. 3-2, 4-13, 4-15, 4-18, 4-21). For example, NECA believed that
this grounding technique is obsolete and unsafe (Ex. 3-2). NECA noted
that 2002 NEC Section 250.136(A) states: "The structural metal frame
of a building shall not be used as the required equipment grounding
conductor for ac equipment." Other commenters argued that this
allowance is incongruent with the 2004 and prior editions of NFPA 70E
(Exs. 4-13, 4-15, 4-18, 4-21). For example, Mr. Michael Kovacic stated
that this has been prohibited for ac circuits since the 1978 edition of
the NEC. He presented the reason for this as follows:
This requirement [in proposed paragraph (g)(7)(i) for equipment
grounded by an equipment grounding conductor that is contained
within the same raceway, cable, or cord, or runs with or encloses
the circuit conductors] is to keep conductors grouped close together
so magnetic fields generated by the flow of ac electricity, which
reacts with the circuit conductors, will cancel each other out,
thereby minimizing the total circuit impedance for safety reasons
(preventing electrocution in the event of a breakdown or fault in
the equipment by rapid operation of the overcurrent device). In the
case of dc circuits, there are no pulsating magnetic fields and
consequently no circuit reactance, which increases the circuit
impedance to negatively affect the grounding path of equipment. [Ex.
4-18]
OSHA agrees with these comments. In fact, the Agency provided
similar rationale in prohibiting the use of the metal structure of a
building for grounding electric equipment when it adopted the existing
standard in 1981 (46 FR 4034, 4046, January 16, 1981). However, at that
time, the Agency also decided not to apply this prohibition
retroactively, reasoning as follows:
[F]rom the standpoint of employee safety, installations where
electric equipment is secured to, and in metallic contact with, the
grounded structural frame of a building are essentially free of
electrical shock hazards. This condition occurs because the electric
equipment enclosures and the metal building frame will be
approximately at the same potential if a ground fault occurs and
will provide a measure of employee safety. [46 FR 4046]
In that rulemaking, OSHA agreed with comments that it would be
impractical to require changes to installations that had been permitted
by the NEC for many years before 1978.
OSHA believes that this rationale continues to apply today. Nothing
in the record has convinced the Agency that the conclusion drawn in the
existing standard in 1981 is incorrect. Also, the Agency does not
believe that the substantial cost to employers of changing these
grounding connections is worth the slight possible reduction in risk
associated with moving from the use of the structural metal frame of a
building to a separate equipment grounding conductor. In addition, in
actual practice, such a change might not lead to an overall reduction
in risk at all. To reconfigure a branch circuit and run new conductors
back to a point where an acceptable connection to the ground is
available,\25\ an employee would need to deenergize the existing
circuits connected. An employee could inadvertently contact an
energized part during the recircuiting process.
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\25\ Section 250.130(C) of the 2002 NEC lists acceptable
grounding methods.
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Consequently, the final rule in Sec. 1910.304(g)(8)(iii) continues
to allow the use of the grounded structural metal frame of a building
as the equipment grounding conductor for equipment secured to, and in
metallic contact with, the metal frame only for installations made
before April 16, 1981. However, unlike the existing standard, the final
rule requires such grounds to be replaced any time work is performed on
the branch circuit. In such cases, the circuit needs to be deenergized
anyway, and there would be no increased risk during the installation of
a new equipment grounding conductor. Additionally, the costs of
installing an acceptable equipment grounding conductor in such cases
would be minimized.
