Substantiation: There are many, many issues that need to be addressed in this
Code allowance. First, the allowance itself is very restrictive, when you look at
the parameters that must be followed in order to use this allowance. Consider
item (2), which requires that no continuous metal paths are installed between
the structures. This is too difficult to enforce, when this permission has been
used, only to have another tradesperson install such a metal path after the
original installation. While I understand that Panel 5 cannot predict future
violations, I think some proactive thinking is in order here. Panel 5 exercised
good judgment last code cycle when it set forth the sizing requirements for a
common grounding electrode conductor for multiple separately derived
systems, based on the possibility of a future change to premises wiring system
[250.30(A)(4)(a)]. Such logic should be used once again for this code rule,
which is not only difficult to enforce, but also could create very significant
hazards.
In addition to the enforcement issues, let us examine the safety issues that
might result from this rule:
Multiple neutral-to-ground connections, that would occur if parallel
continuous metallic paths are installed at a later date, create a condition where
neutral current and ground-fault current will flow through conductive metal
parts of a building or electrical system. This current flow can cause death from
electric shock and property damage from fires. Preventing these two issues is
the very purpose of the Code, as set forth in 90.1(A).
Electric Shock - Electric shock can occur if the feeder grounded conductor to
a separate structure is open because the allowed neutral-to-ground connection
permits neutral current to flow onto the metal parts of the electrical system.
Electric Shock from No Safety Ground. If the feeder grounded conductor is
open, the low-impedance path used to clear ground-fault current is lost. Under
this condition, a ground-fault will not be cleared and all metal parts of the
electrical system will be energized to line-voltage.
Fire. A fire is created when heat is sufficient to cause ignition. In electrical
systems, heat is generated whenever current flows. The temperature rise is
dependent on the square of the current flow (I) and the resistance of the
material (R), as well as the duration of the current flow (12R). A neutral-toground
connection (even if it meets the NEC requirements) can cause a fire,
and sometimes an explosion, due to an electric arc if the grounded conductor is
open.
When the grounded conductor is open, neutral current flows onto the metal
parts of the electrical system because a neutral-to-ground connection is allowed
within the structure disconnect enclosure. When the grounded conductor is
opened in wood frame construction, neutral current seeking a return path to the
power supply travels into the moist wood members. After many years, the
wood is converted into charcoal (wood with no moisture) because of the heat
generated from the current flow. The ignition temperature of the wood is
decreased and the temperature of the wood is increased because of neutral
current.
For the purposes of correlation, companion proposals have been submitted to
the following sections:
250.134, 250.24, 250.30, 250.142, 338.10(B), 450.5, 501.30, 502.30, 503.30,
505.25, 506.25, 547.9, 550.33, and 551.76
