Grounding Separately Derived
Alternating-Current Systems
Introduction: and 3 vs 4 Pole Switching
John J. Stark
Marketing Services
Coordinator, Russelectric
Inc.
Ground fault protection
requirements
in distribution
systems have
been in place in the NEC since the very early 70’s. Since then,
closer scrutiny has been given as to when to switch the neutral, I.E.,
the grounded conductor, in Wye systems, when switching between
two independent sources of power.
Consider the GFP problem when applying transfer
switches :
There are many applications where switching the neutral is the most
practical of all methods.
The most widely accepted method is to utilize a 4 pole transfer
switch, to separate the grounds between two or more than two
sources.
This will satisfy code requirements and will insure that faults to
ground will not have multiple paths to follow. This will allow the
GFP to monitor the actual faults.
Ground Fault Protection Requirements:
National Electric Code, Section 230.95:
230.95 Ground-Fault Protection of Equipment
Ground-fault protection of equipment shall be provided for
solidly grounded wye electric services of more than 150
volts to ground but not exceeding 600 volts phase-to-phase
for each service disconnect rated 1000 amperes or more.
The grounded conductor for the solidly grounded wye system
shall be connected directly to ground through a grounding
electrode system, as specified in 250.50, without inserting
any resistor or impedance device.
The rating of the service disconnect shall be considered
to be the rating of the largest fuse that can be installed or the
highest continuous current trip setting for' which the actual
overcurrent device installed in a circuit breaker is rated or
can be adjusted.
The provisions of 230.95 covering the ground-fault protection
of equipment were first required in the 1971Code
(pertinent
verbiage)
National Electric Code, Section 230.95:
In addition to the requirements of the N.E.C., Ground Fault Protection
for systems which are less than 1000 amperes, using the same code
section as a guide, is accepted by most engineers.
2011 National Electric Code, Section 250.30 - Grounding and Bonding Systems
(We’ll use two examples at 120/208, 3 ph, 4 w):
Let’s take a look at the one-lines of the neutral (grounded) conductor…
Keeping in mind the distinction between the Grounding conductor (equipment grounding conductor),
and the Grounded conductor (neutral conductor)…
N.E.C. Exhibit 250.12
N.E.C. Exhibit 250.11
Gen
Notice that the Neutral
conductor is solidly
connected, thus, not
separately derived.
Gen
Notice that the Neutral
conductor is switched,
(break-before-make)
thus, separately derived.
N.E.C. Informational Note No.1:
An alternate ac power source, such as an on-site generator, is not a separately derived system
if the grounded conductor is solidly interconnected to a service-supplied system grounded conductor.
An example of such a situation is where alternate source transfer equipment does not include a
switching action in the grounded conductor and allows it to remain solidly connected to the
service-supplied grounded conductor when the alternate source is operational and supplying the
load served. N.E.C.
N.E.C. Exhibit 250.12
N.E.C. Exhibit 250.11
Gen
Gen
Exhibits 250.11 and 250.12 depict a 208Y/120-volt, 3-phase, 4-wire electrical service
supplying a service disconnecting means to a building. A feeder is installed from the
service equipment to the normal power terminals of a transfer switch. The emergency
or alternate power terminals of the transfer switch are supplied by a feeder that is
supplied by a generator with a 208Y/120-volt, 3-phase output. Emergency, legally
required standby, and/or optional standby loads are supplied from the load terminals
of the transfer switch in accordance with the applicable requirements of Articles 700,
701 (Legally Required Standby Systems), and 702 (Optional Standby Systems). N.E.C.
N.E.C. Exhibit 250.12
N.E.C. Exhibit 250.11
Gen
Gen
Exhibit 250.11 depicts a 3 pole ATS with solid neutral. The neutral conductor from the generator to the load is
not disconnected by the transfer switch. The system has a direct electrical connection between the normal
grounded system conductor (neutral) and the generator neutral through the neutral bus in the transfer switch,
thereby grounding the generator neutral. Because the generator is grounded by connection to the normal system
ground, it is not a separately derived system and there are no requirements for grounding the neutral at the
generator. The conductor installed between the equipment grounding terminal of the transfer switch and the
generator frame/equipment grounding terminal is either an equipment grounding conductor or a supply side
bonding jumper, depending on where the first overcurrent device in the generator feeder circuit is located. See
250.35(B) for the requirement covering generators supplying systems that are not separately derived. N.E.C.
N.E.C. Exhibit 250.11
Gen
In Exhibit 250.12, the grounded conductor (neutral) is connected to the switching contacts
of a 4-pole transfer switch. Therefore, the generator system does not have a direct
electrical connection to the other supply system grounded conductor (neutral), and the
system supplied by the generator is considered separately derived. This separately
derived system (3-phase, 4-wire, wye-connected system that supplies line-to-neutral loads)
is required to be grounded in accordance with 250.20(B). The methods for grounding the
system are specified in 250.30(A). N.E.C.
Note: The above is only true when employing open transition (break-before-make),
fully rated neutral switching contacts.
N.E.C. Exhibit 250.12
Gen
Section 250.30(A)(l) requires separately derived systems to have a system bonding jumper
connected between the generator frame and the grounded circuit conductor (neutral).
The grounding electrode conductor from the generator is required to be connected to
a grounding electrode.
This conductor and the grounding electrode are to be located as close to the generator as
practicable, according to 250.30(A)(4). However, because the generator feeder is supplying
a building that is also supplied by an ac service, 250.58 requires both supply systems to be
connected to the same grounding electrode system. N.E.C.
N.E.C. Exhibit 250.12
Gen
If it is determined by the engineer that a separately derived
system is not required, then 3 pole transfer switches are
acceptable.
N.E.C. Exhibit 250.11
Gen
More on 4 Pole Switching:
Construction Considerations and Precautions for Specifying and
Applying 4 pole Switches:
Switching the Neutral Conductor:
1. The neutral switching contacts and related arc quenching means must
be identical to and mounted on the same shaft and mounting base as the
associated main contacts. This will insure proper mechanical
synchronism and alignment and avoid any possibility that the neutral
contacts will fail to open or close when they should. This would not
possible with an adjunct type neutral switching accessory, rather than a
true four pole switch.
2. Being completely identical, the neutral switching contact would
have the same current rating as the main contacts. It would have
identical withstand ratings should a fault occur on one line, thus
unbalancing the system. The neutral contacts can be stressed as much
as the main contacts.
Suggested Specification Insert:
“On 3 phase, 4 wire systems utilizing ground fault protection, a true 4
pole switch shall be supplied with all four poles mounted on a common
shaft. The continuous current rating and the closing and withstand
rating of the fourth pole shall be identical to the rating of the main
poles. Ancillary overlapping neutrals shall not be permitted."
Example: a 4 Pole Transfer Switchbase with Fully Rated Integral Switched Neutral
3 vs 4 Pole Physical Comparison
3P
4P
CAD Rendering (side view) of a true 4 pole switch with
insulating barriers omitted for clarity.
Notice the neutral pole and contacts are indistinguishable from the power poles and
contacts because –in fact- they are the same and they operate the same.
Conclusion:
To satisfy code requirements for GFP, and separately derived
power systems, true 4 pole switching will insure that faults to
ground will not have multiple paths to follow and will allow the
GFP system to accomplish its purpose.
To meet the stringent requirement of 4-pole switching
applications brought about by the widespread use of GFP
(Ground Fault Protection) equipment, truly synchronous contact
operation and fully rated construction as an integral design in
4-pole transfer switches is completely necessary.