'Metal-enclosed switchgear' refers to a type of electrical equipment that is completely enclosed in a metal casing, which contains various electrical components such as circuit breakers, switches, and fuses. According to IEC 62271-200, metal-enclosed switchgear installations must be designed to meet specific standards for insulation capacity, degree of protection, current carrying capacity, switching capacity, and mechanical function as determined by testing provisions.
Verification is achieved through a type test on a prototype panel. Furthermore, a routine test is conducted on each completed panel or transport unit. It is important to note that standard IEC 62271-200 must always be observed in conjunction with IEC 62271-1.
Type-tested switchgear installations with insulated enclosures fall under the IEC 62271-201 standard.
Table of Contents:
1. Rated Voltage
The insulation level values for a switchgear installation should be chosen according to the system's requirements at the site of installation, as specified in the selection tables of IEC 62271-1. Table 1 presents the selection values for rated voltages up to 52 kV.
The voltage values "across the isolating distance" are applicable solely to switching devices that must satisfy the safety standards for the open contacts of disconnectors.
Table 1 presents two pairs of values that can be chosen as the rated lightning impulse voltage level for nearly all specified voltages.
Table 1 – Rated voltages and insulation levels in the medium voltage range
Rated voltage Ur/kV (r.m.s. value) | Rated short-duration power-frequency withstand voltage Ud / kV (r.m.s. value) | Rated lightning impulse withstand voltage Up / kV (peak value) | ||
Line-earth, line-line, across contact gap | Across isolating distance | Line-earth, line-line, across contact gap | Across isolating distance | |
(1) | (2) | (3) | (4) | (5) |
3.6 | 10 | 12 | 20, 40 | 23, 46 |
7.2 | 20 | 23 | 40, 60 | 46, 70 |
12 | 28 | 32 | 60, 75 | 70, 85 |
17.5 | 38 | 45 | 75, 95 | 85, 110 |
24 | 50 | 60 | 95, 125 | 110, 145 |
36 | 70 | 80 | 145, 170 | 160, 195 |
52 | 95 | 110 | 250 | 290 |
In selecting equipment, it is crucial to consider the risk of lightning and switching overvoltages, the neutral treatment method, and, if relevant, the type of overvoltage protection. For installations and equipment susceptible to atmospheric overvoltages, such as those directly connected to overhead lines, the higher value pairs should be chosen.
Lower value pairs are suitable for installations that are either not subject to atmospheric overvoltages or are safeguarded against such overvoltages by the use of arresters.
2. Insulating Media
IEC 62271-200 encompasses switchgear where atmospheric air serves as the gaseous insulator within the enclosures, as well as switchgear where a fluid-insulating medium other than atmospheric air (for example, SF6) is employed. Consequently, this results in two types of switchgear: air-insulated switchgear (AIS) and gas-insulated switchgear (GIS).
3. Degree of Protection
The metallic and grounded enclosure safeguards personnel from exposure to live components and moving parts. It also secures the installation from the intrusion of foreign objects. For switchgear according to IEC 62271-200, one can choose from three distinct degrees of protection.
The distinction lies in whether the enclosure is designed to repel fingers or similar objects (IP 2X as per IEC 62271-1, minimum requirements for metal-enclosed switchgear), rigid wires larger than 2.5 mm in diameter (IP 3X), or rigid wires larger than 1 mm in diameter (IP 42).
4. Compartments, Accessibility and Service Continuity
The term "metal-enclosed" previously encompassed three distinct categories: "metal-clad," "compartmented," and "cubicle" switchgear, differentiated by the design of their internal compartments. However, this structural approach to compartmentalization has been superseded in IEC 62271-200 with a classification system based on the accessibility of compartments containing high-voltage components.
Three out of four classes are defined by the control of access and whether it is necessary to open the compartment during regular operation or solely for maintenance. Compartments that require tools for access are not intended to be opened as part of normal operations. The fourth class pertains to compartments that are not accessible, such as those in gas-insulated switchgear.
Regarding the accessibility of compartments, distinctions are categorized as follows:
Interlock-controlled accessible compartment: Integral interlocks facilitate the opening of compartments for regular operations and maintenance tasks.
Procedure-based accessible compartment: Access to the compartment for normal operation and maintenance is governed by an appropriate procedure that includes locking mechanisms.
Tool-based accessible compartment: The compartment is designed to be opened using tools, but this is not required for regular operation or maintenance.
Non-accessible compartment
Categorization is based on the loss of service continuity, centring on the primary switching device. The LSC categories stem from the range of switchgear components that must be removed from service upon opening a compartment:
4.1 LSC1 Category Functional Unit
This category represents the lowest level of service continuity. It pertains to an accessible compartment within a panel that necessitates the removal of at least one additional panel from service upon opening. Opening a busbar compartment requires de-energizing all panels in the corresponding section.
4.2 LSC2 Category Functional Unit
This form is designed to ensure the network's maximum service continuity when accessing high-voltage compartments within switchgear and controlgear. It implies that one can open the accessible high-voltage compartments of a functional unit while keeping the other functional units in the same section energized.
This indicates that at least one busbar should remain energized. The use of a removable partition can achieve this category.
Category LSC2 stipulates that the connection compartment can be opened while maintaining the busbar(s) in an energized state.
4.3 LSC2A Category Functional Unit
Category 2A refers to a type of panel that must be entirely removed from service if one of its compartments is opened. This panel is designed with partition walls that isolate it from neighbouring panels and contains at least two compartments, along with a specified isolating distance.
4.4 LSC2B Category Functional Unit
Category 2B offers minimal restrictions on service continuity, ensuring that all other panels in the installation, as well as all cable termination compartments (including those in the concerned panel), stay operational when a compartment is opened. It necessitates partition walls between adjacent panels and at least three compartments and two isolation distances for each panel.
When a compartment is opened, the partitions and shutters adjoining the compartment or panel offer a level of protection from live high-voltage components. The "partition class" denotes if the partition is entirely metallic or includes sections made of insulating material.
4.5 Examples of Applied Categories
The subsequent diagrams depict three functional units within ABB's UniSec medium voltage switchgear, each classified under high categories (LSC2, LSC2A, and LSC2B). These classifications guarantee the highest level of service continuity along with the necessary accessibility for maintenance and servicing tasks.
SBC (circuit-breaker with switch-disconnector),
WBC (unit with withdrawable circuit breaker) and
HBC (unit with vacuum circuit breaker and gas-insulated disconnector)
4.6 Class PM
"Partition of Metal" refers to metallic shutters and partitions that separate live parts from an open compartment. Class PM is the optimal selection for personnel safety within the compartment as it guarantees a uniformly earthed structure. Figure 7 depicts the metallic shutters, which are closed when the circuit breaker is retracted, ensuring connection to the structure of the circuit-breaker compartment.
4.7 Class PI
"Partition of insulating material" refers to a break in the metallic barrier or shutter between live components and an open compartment, which is shielded by insulating material. While both types of partitions offer equivalent protection against accidental contact for workers, a metallic partition additionally blocks the electric field.
The choice of installation category for any particular case rests with the user, prioritizing the safety of personnel during maintenance and cable work within the metal-enclosed switchgear and controlgear. Limiting fault effects is crucial only after the compartment walls' resistance to arcing is confirmed and the compartmentalization provides a genuine potential separation (class PM).
5. IAC (Internal Arc Classified) Switchgear
It is a consensus among experts that both manufacturers and users should exert all possible efforts to avoid faults in switchgear installations where internal arcing might occur. Nevertheless, it is recognized that it is not possible to entirely prevent such faults in every instance. Consequently, it is anticipated that modern switchgear designs will be tested for their response to internal arcing.
Internal short-circuit arcs during operation can be caused by overvoltage, faulty insulation, or improper control. The test involves inducing the arc with an ignition wire connected across all three phases. The arc reaches temperatures of approximately 4,000 K at its footing points and around 10,000 K or more in the arc column area.
Immediately upon ignition of the arc, the gas in the vicinity rapidly heats up, leading to a sharp increase in pressure within the compartment. This pressure would escalate to the enclosure's load limit if it were not for the built-in pressure relief vents.
The sealing membranes of these vents react within approximately 5 to 15 milliseconds, opening the pathway for the heated gases to escape (Figure 9). This distinct process is influenced not only by the response time of the pressure relief valves but also by the mechanical inertia of the mass of heated gas.
The peak pressure achieved depends on the compartment's volume where the fault occurs and the magnitude of the short-circuit current. The largest amount of heated gases is released into the area surrounding the switchgear during the expansion phase. The pressure stress on the panel surpasses its peak at approximately 15 ms, while the building's maximum stress is attained after about 40 ms.
A forceful expulsion of still-warm, low-density gases and luminous particles takes place during the subsequent emission and thermal phases.
Where,
Compression Phase (pressure build-up),
Expansion Phase (pressure relief),
Emission Phase (hot gases released),
Thermal Phase (ejection of glowing particles) a) isochronous pressure rise, b) opening of pressure relief valves.
Guidelines on testing metal-enclosed switchgear for internal arcing response are available in IEC 62271-200.
The test conditions stipulate that internal arcing should be initiated using a thin ignition wire in each compartment of the panel under examination. The ignition point and energy flow direction are designated to ensure the arc burns for as long as possible at the location furthest from the feeder. The short-circuit test facility powering the test object, comprising at least two panels, must possess adequate power to sustain a short-circuit current equal to the rated short-time withstand current in a three-phase manner across the internal arcing for the test's agreed duration (suggested times are 1.0, 0.5, or 0.1 seconds).
This encompasses the standard protection grading durations at full short-circuit current. The test outcome is limited to determining if the compartment can endure the stress from internal overpressure during this short-circuit interval.
In the course of the test, fabric indicators (black, cretonne, or cotton-wool batiste) are mounted vertically at specified intervals on metal frames adjacent to the panels' accessible walls and horizontally at a height of 2 meters above the area where personnel would operate the installation.
5.1 IAC Classification
Classifications should be indicated as follows:
Classification: IAC (Internal Arc Classified)
Type of accessibility: A, B, C
Classified sides of the enclosures: F, L, R
Rated values of three-phase arc fault: current [kA] and arcing time [s]
Rated values of single-phase arc fault (when applicable): current [kA] and arcing time [s]
In metal-enclosed switchgear, there are three levels of accessibility at the installation site: Accessibility type A is for authorized personnel only; Accessibility type B allows for unrestricted public access; and Accessibility type C specifies access under certain restrictions.
Accessibility type A: For authorized personnel only
Accessibility type B: Unlimited access for the general public
Accessibility type C: accessibility restricted by installation out of reach and beyond the zone accessible to the public
Test conditions are defined to align with specific accessibility types (refer to figure 8-19). Various sides of a switchgear installation may offer different levels of accessibility. These are denoted by the identification code using the letters F (front), L (lateral), or R (rear).
The manufacturer must clearly mark the front side. The classification of sides is not applicable to switchgear with accessibility type C.
Upon completing the short-circuit test, the performance of the tested panels is documented according to five criteria:
Criterion 1: Doors and covers must stay closed. Acceptable deformations are those where no part touches the indicators or walls.
Criterion 2: The enclosure must not fragment during the test. However, the ejection of small parts, each weighing no more than 60 g, is permissible.
Criterion 3: Arcing should not create holes in the outer sides of the enclosure that are accessible and below a height of 2 m.
Criterion 4: Hot gases must not ignite horizontal and vertical indicators. Exceptions are allowed for ignition caused by burning paint, decals, or incandescent particles.
Criterion 5: The effectiveness of earth connections must be confirmed through visual inspection.
5.2 Few IAC Examples:
Example #1: IAC AFL 12.5 kA 1s switchgear
The switchgear is shielded on three sides (front and side) for an arc fault current of 12.5 kA and an arc fault duration of 1 second. It comes in two versions:
Version 1 – The switchgear is mounted against the wall, creating a duct between the switchgear's rear and the wall for gas to flow through.
Version 2 – The switchgear is positioned away from the wall, allowing for filters in the back of the unit to cool the gas and reduce its pressure prior to environmental release.
Access to the rear part of the switchgear is strictly prohibited during operation as it lacks protection.
Example #2: IAC AFLR 25 kA 1s switchgear
The switchgear is safeguarded on all four sides—front, lateral, and rear—to withstand an arc fault current of 25 kA and an arc fault duration of 1 second. Filters in each unit are designed to cool the gas and reduce its pressure before release into the environment.
Additional insights into internal arc phenomena can be acquired through high-speed camera imagery or video recordings captured during testing, making them highly advisable.
Passing the arc fault test, particularly in a type test context, is recorded with the IAC ("Internal Arc Classified") designation on the type plate. This certification is further detailed with additional information such as the type of accessibility, the accessible sides, the test current, and its duration.
In addition to criterion 5 of the assessment, it should be noted that the switchgear and controlgear are not the primary factors concerning the effects in the event of a hot gas ejection.
Reflections off ceilings and walls during the emission and thermal phases (Figure 8-18) can redirect hot gases from pressure relief vents into areas occupied by personnel, creating dangerous conditions. The most significant damage typically occurs within the switchgear and controlgear during this phase. The discharge of extremely hot gases becomes particularly hazardous when, due to the supply direction (from below), electromagnetic forces cause the arc to remain near the pressure relief vent.
A panel type can only be deemed fully tested after taking this particular case into account.
Countermeasures to protect operating personnel from these effects can include the installation of screens or discharge plates. For high short-circuit currents, the ideal solution is hot gas conduits with blow-out facilities and absorbers that discharge into the switchgear room.
Enhanced outcomes can be attained without extra installations if the arc duration can be confined to roughly 100 ms through suitable trip times. Since the system protection's grading times typically prohibit short-term tripping of the feeder circuit-breaker, the inclusion of additional sensors, like the Ith-limiter, becomes necessary.
When a pressure relief valve opens during a persistent short-circuit current, it triggers an immediate trip command to the feeder circuit breaker, extinguishing the internal arc in less than 100 ms. The pressure exerted on the walls, ceilings, doors, and windows of the switchgear installation room results from the gas ejection during the expansion phase (Figure 9). Generally, the withstand capability cannot be verified through testing.
Major manufacturers offer calculation programs to determine the pressure development within the switchgear installation compartments, which helps ascertain the necessity for pressure relief vents in the installation room.
Source: #ABB
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