Retro perspective 30 years ago: Metal-Clad or Metal-Enclosed MV Switchgear? What to choose?

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Eleven bays of metal-enclosed switchgear
Eleven bays of metal-enclosed switchgear assure a high level of power continuity for critical loads at a large commercial installation. The split bus primary selective switchgear incorporate latest solid state controls and features two-way source transfer with a bus-tie switch, providing for automatic transfer between two utility lines and a standby generator.

There are two types of switchgear commonly applied today for switching and protection of in-plant medium voltage (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 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 also contains insulated bus which, when tested periodically, requires a shotdown of the gear.

Metalenclosed switchgear is available with interrupter switches and  fuses that require no scheduled maintenance, and the air-insulated bus does not require periodic dielectric testing. Annual maintenance normally consists of little more than a visual inspection through the windows of the gear. This switchgear should be seriously considered if only infrequent interruptions can be tolerated by plant operations.

How much load will be interrupted for fault protection or for maintenance?

Consider maintenance as well as cost when selecting switchgear with fuses and interrupters (metalenclosed) or with breakers and relays (metal-clad).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 provided to allow selective switching to minimize the number of loads interrupted. Circuit breakers are used in applications requiring a very high (above 720 A) continuous current carrying and load-interrupting capability. While this capability may be an advantage in some cases. a higher degree of service continuity can often be achieved with less expensive power fuses by subdividing the system into a larger number of discrete segments, with the result that load switching or fault interruption on one segment of the system will affect fewer loads (see Fig. 1).

Figure 1 - Segmenting a plant's load into small blocks
Figure 1 - Segmenting a plant's load into small blocks supplied by multiple radial circuits assures a high level of power continuity and operating flexibility. In-plant distribution diagrammed at left subejcts all loads to interruptions for fault clearing or maintenance on any one load. Segmented scheme at right uses some amount of cable to serve loads and has approximately same switchgear cost - but provides for better service reliability.

A high degree of segmentation also allows the use of smaller transformers located strategically 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?

Figure 2 - Distribution system with both overhead and insulated cable
Figure 2 - Distribution system with both overhead and insulated cable construction. Protective device A clears transient faults on overhead system and recloses automatically, with only momentary interruption of power to loads. Faults in cable portion of system are interrupted by device B and in transformer by device C. Permanent nature of cable and transformer faultsmakes automatic reclosing undesirable.

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 transformers are rare. and those that do occur 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) in as much 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 metal-enclosed switchgear with fuses and interrupter switches or metal-clad switchgear with circuit-breakers and relays may be used.

More complex protection devices 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 usually  accomplished with ac control power from one or more voltage transformers which may also function as voltage sensing devices. Metal-clad switchgear with circuit-breakers and relays usually needs DC control power, and therefore the addition of a stationclass battery. Not only does this take considerable space, it often requires more maintenance than the switchgear itself.

There are many recorded cases of damage to switchgear and plant which could have been avoided if the batteries had been 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. 3). the switches are automatically opened if a single-phasing condition occurs. interrupting and isolating all three phases of the load feeder.

Figure-3 - Metal enclosed switchgear feeder bay
Figure-3 - Metal enclosed switchgear feeder bay has provisions for protection from open-phase conditions. Open-phase detector (located in low voltage compartment in switchgear bay at lower right) in conjuction with the power-operated interruper-switch protects from all open-phase conditions, including single-phasingcaused by blown fuses.

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 apparatus 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. Metal-enclosed 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.

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. 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.

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.

RESOURCE: Electrical Energy Management; June/July 81

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About Author


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on