Choosing Medium-Voltage Switchgear - S&C Electric Company

Electrical Energy
Management
FOR INDUSTRIAL & COMMERCIAL ENGINEERS & MANAGEMENT
Choosing
Medium-Voltage
Switchgear:
Metal-Clad or Metal-Enclosed?
Reprinted for
S&C Electric Co.
Copyright 1981
Cleworth Publishing Co., Inc.
620-R75
June/July 81

Choosing
Medium-Voltage
Switchgear:
Metal-Clad or Metal-Enclosed?
Consider maintenance as
well as cost when selecting
switchgear with fuses and
interrupters (metalenclosed)
or with breakers
and relays (metal-clad)
There are two types of switchgear
commonly applied today for switching
and protection of in-plant mediumvoltage
(4.16 kV through 34.5 kV)
power distribution systems. One is
metal-clad switchgear using draw-out
air-magnetic or vacuum circuit
breakers and relays for both load
switching and fault protection: the
other is metal-enclosed switchgear
using interrupter switches for load
By John Cooper. Director,
Application Services, Metal-Enclosed Gear,
S&C Electric Co.
switching and power fuses for fault
protection. An understanding of the
application and operating philosophies
of the two types of gear is
necessary to choose objectively the
gear that will permit the Optimum
design for an in-plant system.
The three basic functions of switchgear
in an industrial, commercial, or
institutional medium-voltage distribution
system are to:
Distribute and carry load, including
permissible overloads, with a minimum
of interruptions for scheduled
routine maintenance or for service of
the switchgear.
Identify and clear faults quickly
enough to minimize damage, while Interrupting
the least possible amount of
plant load.
Provide sufficient segmentation of
the medium-voltage system so that the
extent of circuit outages can he
limited during work on cables and
loads.
To ensure that each function has
been properly considered in relation
to both system design and plant operation,
a number of pertinent questions
should be explored - including ease
of maintenance, number of power interruptions,
reclosing, availability of
skilled personnel and cost-benefit
analysis. The following discussion provides
some basic information,
gathered from the field, for consideration.
How many outages can be
permitted for maintenance
Metal-clad switchgear contains drawout
circuit breakers which are removed
for required scheduled maintenance;
removal of a breaker interrupts
its load. Metal-clad switchgear
Electrical Energy Management June/July 81

also contains insulated bus which, minimize the number of loads interwhen
tested periodically, requires a rupted.
shutdown of the gear. Circuit breakers are used in ap-
Metalenclosed switchgear is avail- plications requiring a very high (above
able with interrupter switches and 720 A) continuous currentcarrying
fuses that require no scheduled main- and load-interrupting capability.
tenance, and the air-insulated bus While this capability may be an addoes
not require periodic dielectric vantage in some cases. a higher
testing. Annual maintenance normally degree of service continuity can often
consists of little more than a visual in- be achieved with less expensive
spection through the windows of the power fuses by subdividing the system
gear. This switchgear should be into a larger number of discrete
seriously considered if only infrequent segments, with the result that load
Interruptions can be tolerated by switching or fault interruption on one
plant operations. segment of the system will affect
How much load will be interrupted
for fault protection
or for maintenance
fewer loads (see Fig. 1). A high degree
of segmentation also allows the use of
smaller transformers located stra-
It is axiomatic to plan system protection
so that fault isolation will result in
de-energization of only the faulted segment
of the system, thus permitting
continuous service to other loads. Additionally.
there are many other
reasons why portions of the distribution
system will be taken out of service
- for example, to add transformers,
test cables or even modify circuits to
accommodate plant expansions. For
these occasions. a sufficient number
of load switching points should be pro-
tegically throughout the system,
eliminating the need for unnecessarily
long, high-ampacity secondary conductors
required where fewer, larger,
widely separated transformers are
used.
Is automatic reclosing
necessary
Automatic reclosing is neither useful
nor desirable on in-plant power systems
consisting of Insulated cables (in
conduit or bus duct) feeding transformers.
Faults on cables and trans-
vided to allow selective switching to farmers are rare. and those that do oc-
Electrical Energy Management June/July 81

cur are not transient: they are permanent.
they result in significant
damage. and they are only exacerbated
- not cleared - by automatic
reclosing operations. Metal-enclosed
switchgear has achieved widespread
use on cable systems because of the
simplicity. economy and positive action
of power fuses in providing protection
from permanent faults.
On the other hand, automatic reclosing
can be an advantage on outdoor,
overhead distribution circuits
subject to a high incidence of transient
or temporary faults caused by
falling tree branches, animal and bird
contacts. wind-borne debris, lightning
or ice. Overhead circuits are commonly
protected by metal-clad switchgear
(with circuit breakers and associated
relaying) inasmuch as a short-
time interruption of system voltage by
opening of the switchgear breaker
may result in arc extinction, permitting
an automatic reclosing operation
to restore service (see Fig. 2). In
deciding whether to utilize automatic
reclosing, consideration must be given
to the effect on synchronous motors
and large induction motors. High inrush
current resulting from automatic
reclosing may cause severe mechanical
damage to the motors. Or, it may
result in minor insulation damage
which is not apparent at the time, but
which will lead to premature failure.
This insulation damage will be accelerated
with repeated fast reclosures,
as the effects are cumulative.
Even manual reclosing may be
undesirable on a cable system: It is
often a temptation to hope that the
protective device has operated unnecessarily.
Rather than take the time
to search for the fault. even a trained
person may, under pressure from production
people, reclose in the hope
that the protective device will “hold.”
Since the condition which caused the
protective device to operate will not
have been eliminated. reclosing will
only reinitiate the fault. This will
cause further equipment damage, as
well as provide a hazard to personnel
in the vicinity. The practice of reclosing
before locating and correcting the
fault is highly questionable.
Is sophisticated relaying
required
Most in-plant system protection needs
can be satisfied by an overcurrent
protective device - a fuse or a relay
and circuit breaker. With the variety
of fuse ratings and time-current
curves available either metalenclosed
switchgear with fuses and
interrupter switches or metal-clad
switchgear with circuit-breakers and
relays may be used.
More complex protection schemes
respond to conditions other than just
overcurrent: e.g., reverse-current or
reverse-power relays, differential
relays and overcurrent relays with
harmonic restraint. These devices are
a necessity for utility high-voltage or
EHV networks, but their desirability
can be questioned for simpler in-plant
systems. Complex relaying introduces
the requirement for a much higher
level of sophistication in system
design and coordination, as well as
relay testing and calibration. Complexity
can also invite defeat by
operating personnel who are in a
hurry to restore power, and have no
time or inclination to review a complex
system designed years before.
Is dc control power available
Metal-enclosed switchgear with fuses
and interrupter switches is normally
selfcontained with no requirement
for an auxiliary power supply. Fault
protection is provided by the fuses
which use the energy of the fault current
to achieve interruption. Even
complex remote controlled or automatic
power operation of switches is
Electrical Energy Management June/July 81

usually accomplished with ac control tion of a stationclass battery. Not only
power from one or more voltage does this take considerable space, it
transformers which may also function often requires more maintenance than
as voltage sensing devices. the switchgear itself. There are many
Metal-clad switchgear with circuit- recorded cases of damage to switchbreakers
and relays usually needs dc gear and plant which could have been
control power. and therefore the addi- avoided if the batteries had been
Electrical Energy Management June July 81
maintained, if the battery charger
hadn’t been turned off-or if fuses
had been used initially.
Is single-phasing a problem
The possibility of single-phasing a load
by operation of a fuse need not be an
issue in choosing modern metalenclosed
switchgear or metal-clad
switchgear. Detectors and relays are
available for sensing single-phasing,
which could be caused by source-line
burndown. broken conductors, singlephase
switching, or by blown fuses on
the utility source or on the in-plant
feeder. When the detectors or relays
are applied in conjunction with
power-operated interrupter switches
in metal-enclosed switchgear (see Fig.
4). the switches are automatically
opened if a single-phasing condition
occurs. interrupting and isolating all
three phases of the load feeder.
Sensing and power operation will
raise the cost of metal-enclosed
switchgear significantly over that for
manual gear. But the cost will still be
on the order of 35 to 40% of that for
equivalent metal-clad switchgear with
circuit breakers and relays. And since
the addition of this feature allows
metal-enclosed gear to detect even
source single-phasing. the level of protection
is higher than that normally afforded
by metal-clad gear.
Are skilled technicians
available
Any electrical equipment should be
operated and maintained only by
qualified persons “having adequate
knowledge of the installation, construction.
. .(and) operation of the ap
paratus and the hazards involved” according
to the National Electrical
Safety Code ANSI C2. The Code, parts
of which have been adopted by local
and state jurisdictional authorities,
specifies that “the employer shall inform
each employee working on or
about communications equipment or
electric-supply equipment and the
associated lines, of the safety rules
governing the employee’s conduct
while so engaged.” In addition, such
persons shall be “regularly instructed
in methods of first aid and emergency
procedures” and have “an adequate
supply of protective devices and
equipment.” Users who cannot justify
the expense of training and equipping

employees to specialize in work on
electrical equipment should have
maintenance performed by an electrical
contractor skilled in medium
voltage. Calibration of relays and
dielectric testing of insulated bus
should be performed by a qualified
testing organization.
The choice between metal-enclosed
switchgear and metal-clad switchgear
is often made on the basis of the availability
of qualified persons and the
willingness of management to provide
funds for maintenance. Metalenclosed
switchgear is available with
non-damageable, non-aging, permanently
accurate fuses which require
no maintenance and with
switches which require no scheduled
maintenance or adjustments. A simple
visual inspection and occasional exercising
are all that is required. Conversely,
the maintenance requirements
for circuit breakers, relays and
batteries are well established.
Electrical Energy Management June July 81
Will cable size be based
on ampacity
If cable size is selected solely on the
basis of ampacity. the source protective
device should be selected to
operate fast enough to interrupt maximum
available fault current before
the insulation suffers thermal
damage. In other words, the selection
of cable sizes should be based not only
on ampacity. but also on the ability to
withstand fault current while the
source protective device detects and
clears a fault.
Fuses clear heavy fault currents In
less than .014 sec, contrasted with circuit
breakers which clear faults in
.083 to .133 sec, plus relay time. It is
desirable to protect a cable from
damage due to passage of fault current.
The choice may well become one
of whether to use fuses for protection.
or to use metal-clad switchgear with
circuit breakers and relays and
specify cables several sizes larger
than required by ampacity. An excellent
reference on cable protection
is “IEEE Recommended Practice for
Protection and Coordination of Industrial
and Commercial Power
Systems, IEEE Std. 242-1975.”
What are the economics
In light of today’s high cost of money,
it is essential to keep capital outlays
and operating expenditures to a practical
minimum. Consequently, the
economics of switchgear application
have become increasingly important
in plant design. Metal-enclosed
switchgear provides protection for an
in-plant cable system at a cost of 25 to
40% of metal-clad switchgear.
The high cost of building floor-space
may make it desirable to locate the
gear out of doors. Both metal-clad and
metal-enclosed gear may be installed
outdoors. Metal-clad gear normally
requires an additional housing or
walk-in shelter so that routine
maintenance may be performed during
inclement weather. This extra protection
is not required for outdoorstyle
metal-enclosed gear.
Metal-enclosed switchgear weighs
less than metalclad switchgear. thus,
it is easier to handle with a minimum
of rigging. Foundation or support
channels are not required, permitting
it to be located anywhere, even on
balconies or rooftops. Only a level
floor or pad is required. and room
need not be provided to accommodate
drawout of circuit breakers.
The time required for the design of
an in-plant medium-voltage distribution
system is short compared to the
many years it will be in service. Over
its life, the switchgear will be called
upon to facilitate routine scheduled
work on the power system as well as
to limit damage and lost production
due to faults.
The foregoing fundamental application
questions were modeled to assure
that the choice of switchgear will take
into account how the plant Is operated
and to help develop a switching and
protection philosophy. The user may
wish to consider other aspects. As
with any engineering decision, it is important
that the choice of type of
switchgear be made only after consideration
of all relevant factors. EEM