42 ELECTRICALCONTRACTOR | FEB. 17 | WWW.ECMAG.COM
POWERQUALITY BY RICHARD P. BINGHAM
The utility crew that replaced the
wires had explained that there was a
break in one of the conductors caused
by the tree contact. It resulted in the
damage to the equipment in the house.
The question: “But why didn’t the main
breakers open to protect my equipment?”
Trying to draw air schematics with
my fingers didn’t really convey the
explanation very well. The figure above
shows more clearly how the 13-kilovolt
(kV) distribution feeders are converted
down to the 240-volt (V) split-phase circuit that comes into the house through
the grounded conductor and two-phase
conductors. Whereas the wye circuit has
each phase 120 degrees apart from each
other, the voltage on each phase conductor in a split-phase circuit is 180 degrees
out of phase with the other.
Most loads in a residence are fed from
one of the two 120V circuits that are
derived from the 240V by the midpoint
voltage reference resulting from the
grounded (neutral) conductor. Current
from each 120V leg sums in the neutral,
which, in the ideal world, cancels each
other out because of the 180-degree
phase shift. Some larger loads are powered across the two-phase conductors,
240V. A grounding electrode is driven at
the service entrance, which also comes
into the breaker panel and is bonded to
all the equipment grounding wires from
each circuit in the house, along with
each neutral or grounded conductor. Zng
is ideally a low impedance, less than the
25-ohm value. Only very small amounts
of leakage current should be flowing in
the grounding conductor under normal
circumstances.
Back to the damaged equipment and
why the main breakers didn’t protect it.
The residence was located in a sandy
area, so the impedance (Zng) of the ground
rod was probably relatively high. National
Electrical Code Article 250.56, Resistance
of Ground Rod Electrode, states, “When
the resistance of a single ground rod is
over 25 ohms, an additional electrode
is required to augment the ground rod
electrode, and it must be installed not less
than 6 feet away.”
This house had one ground rod, but it is
unlikely the test was done at installation—
and certainly not since—to check the
impedance. When the neutral conductor
broke, the return path to the utility trans-
former was no longer low impedance.
Without a good ground reference, the
neutral-to-line voltage at the dwelling’s
various receptacles was no longer 120V
relative to ground on both sides. Based on
current flows from the loads and imped-
ances on each phase, the “mid-point”
reference of the neutral is no longer in
the middle of the 240V. One phase could
be 150V and the other 90V, but still 240V
phase-to-phase. Equipment powered by
such voltage is likely to be damaged.
Also, most electronic equipment has
a three-prong plug, with some compo-
nents within the equipment referenced
to the safety ground. Without adequate
external or built-in surge suppression,
semiconductor and capacitors can
go “pop-pop-pop” in the presence of
excessive voltage on the phase conduc-
tor. Unless the protection is properly
designed to open the circuit when the
protection devices get abused for too
long, things will still go pop.
The homeowner still wanted to know
why the main breakers didn’t operate
to protect the equipment. Breakers (or
fuses in the service panel) are designed
to protect the wiring from being dam-
aged from excessive current causing
overheating, insulation melting, conduc-
tor failures and subsequent fire. The I2*t
curve requires an overcurrent condition
of a particular level to be exceeded for
a specified time to trip. The higher the
current, the faster the trip. Even so, the
equipment could be damaged in millisec-
onds before the breaker operates.
Fuses within equipment are often an
indicator that the equipment was damaged rather than protected, as failure of
components inside the equipment often
result in the overcurrent condition that
blows the fuse. The main 100 ampere
(A) breakers never saw enough current
to trip. It was undetermined if individual
circuit breakers tripped, but if a 15A or
20A breaker experienced enough current
to trip, you can bet the TV or dishwasher
or sensor spotlight electronics had
already been damaged. The homeowner
did switch off the main breakers after
determining that something was wrong,
but the damage had been done.
This wasn’t a one-off occurrence.
Another homeowner had a similar neutral conductor failure that resulted in
loss of the furnace and other equipment
during a snowstorm. In October 2006, I
covered the story of a phase conductor
open-circuiting and the strange voltage
phenomena that occurred for hours to
houses fed from that transformer.
Loss of a phase in industrial/com-mercial facilities can wreak havoc on
three-phase loads, such as electric motors.
All conductors’ integrity is critical to safe
operation of a facility, even the innocently
named “neutral” conductor.
But Why?
Understanding when things go wrong
WHEN I ARRIVED RECENTLY at a residential construction site to build a covered
porch, the homeowner was busy cutting a tree away from the power lines to the
house with a chain saw. It could have led to disaster, but his chain saw skills and
carefully attached ropes dropped the tree away from the wires. He walked over
with a section of the wires that had previously run to the house service from the
utility transformer secondary. The evil in his hand was the source of destruction
of numerous electronic and other electrical loads in the house earlier in the week.
BINGHAM, a contributing editor for power quality, can be reached at 732.248.4393. IM
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