CODECOMMENTS BY MICHAEL JOHNSTON
Disadvantages
One disadvantage of an ungrounded system is that a first phase-to-ground fault
condition can be difficult to find. The
voltage-to-ground in an ungrounded
system is 0V (in theory) because there is
no ground connection from any system
conductor. However, there is distributed
leakage capacitance present throughout
such systems. Phase-to-ground voltage
levels can appear during voltage testing
and are usually the result of capacitance-coupling effects from the system circuits.
Voltage to ground
The definition of “voltage-to-ground”
covers another important point about
voltage-to-ground levels in ungrounded
systems. It clarifies that the voltage-to-ground of a grounded system is the
voltage between the given conductor and
the grounded circuit point or conductor.
For example, in a 120/240V, single-phase system, the voltage is 120V from
any ungrounded phase conductor to
ground. For ungrounded systems, the
greatest voltage between the given conductor and any other circuit conductor is
also the phase-to-ground voltage.
On a 480V, three-phase, 3-wire,
ungrounded delta system, the phase-to-phase voltage is 480V. Based on the
definition, this is also the phase-to-ground
voltage for this system. This vital information must be applied when selecting
circuit breakers for ungrounded systems.
The circuit breakers must typically have
a straight voltage rating (e.g., 480V) as
opposed to slash-rated breakers (e.g.,
480/277) (see 240.85 and the associated
informational note). The phase-to-ground
voltage for ungrounded systems is the
phase-to-phase voltage of the system.
Ground-detection equipment
For the systems addressed in Section
250.21(A), grounding is not required.
Where a system is not grounded and operates at not less than 120V and at 1,000V
or less, ground detectors are required.
The ground-detection requirement provides the ability to monitor ungrounded
systems to detect a first phase-to-ground
fault on the system. The first phase-to-ground fault will not cause overcurrent
device operation, so continued service is
achieved. However, personnel responsible
for monitoring the system must react to
the annunciation, investigate the first
phase-to-ground condition and remove it.
If the first phase-to-ground condition is
not cleared and a second phase-to-ground
fault develops on a different phase, the
result is a simultaneous phase-to-phase
short circuit and phase-to-ground fault
event, which can lead to significant equipment destruction and downtime.
There are some benefits of operating
a system ungrounded where the National
Electrical Code (NEC) permits, but it is
important to monitor it and react appro-
priately if a phase-to-ground condition
develops. Ungrounded systems are often
installed and used in industrial facili-
ties where power continuity is desired
for assembly lines and other continu-
ous processes that would be damaged or
could cause personal injury if a phase-
to-ground fault event were to result in
power interruption.
The choice to install and operate this
type of system is determined by the process’ nature, the process’ operational
characteristics and the desired operation
method. Where ground detectors are
installed on an ungrounded system, the
sensors must be located as close to the
supply source as possible [250.21(B)( 2)].
Listed ground-detection equipment is
available for use on ungrounded systems.
Marking required
According to Section 250.21(C), all
enclosures containing equipment and conductors for an ungrounded system must be
field-marked, “Caution: Ungrounded System Operating – ____ Volts,” at the source
or at the first system-disconnecting means.
This marking must meet the requirements
in 110.21(B) and be durable enough for
the environment in which it is installed.
This marking requirement also applies to
switchboards and panelboards that contain ungrounded systems, as covered in
Section 408.3(F)( 2).
This marking gives qualified people
an additional notification of the type
of system contained within the enclosure. The phase-to-ground-voltage
readings are not as familiar to workers
as grounded system voltage readings.
The marking requirement provides an
additional level of safety for personnel
who must work on ungrounded systems either to troubleshoot or to add
to the systems. Sections 110.2(D)( 1) and
110.4(A) of NFPA 70E, Standard for Electrical Safety in the Workplace, provide
requirements for qualifications, training
and use of test instruments.
Disadvantages and Regulations
Code rules for ungrounded systems
THE DECISION TO INSTALL AND OPERATE an ungrounded system is typically a combined effort that includes a design or engineering team, the owner, the
operators and sometimes the authority having jurisdiction. Ungrounded systems
can provide continuity of electrical operation and limit system outage downtime.
Common ungrounded, three-phase electrical systems are as follows: 240-volt (V),
three-phase, 3-wire, delta-connected; 480V, three-phase, 3-wire, delta-connected;
2,300V, three-phase, 3-wire, delta-connected; 4,600V, three-phase, 3-wire, delta-connected; and 13,800V, three-phase, 3-wire, delta-connected.
JO HNS TON is NECA’s executive director of standards and safety. He is chair of the NEC
Technical Correlating Committee; a member of the IBEW; and an active member of the NFPA
Electrical Section, Education Section and the UL Electrical Council. Reach him at
mj@necanet.org. I S T
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