By Peter Wicks
Specific types of metal-enclosed switchgear used in industrial plants are defined as (1) metal-clad switchgear, (2) low-voltage power circuit breaker switchgear, and (3) interrupter switchgear.
Metal-clad switchgear, by definition, is a specific type of metal-enclosed switchgear. There are several applicable standards in North America that cover the construction features, the design tests required, the production tests required and the ratings applicable.
Standards
In Canada, the applicable standard is CSA-C22.2 No 31-M89 which covers Switchgear Assemblies and includes Metal-clad switchgear. This Standard in general covers the construction and design features required for metal-clad switchgear. Components comply with the applicable requirements for the Canadian Electrical Code that governs such components. Equipment designed, manufactured and tested to this standard is certified by the CSA and a CSA approved label, which is accepted by all local electrical inspection authorities, is applied to the equipment at the manufacturing plant. The CSA Standard references EEMAC Standard G8-2 which is the other Canadian standard used and accepted in the Canadian market place.
EEMAC Standard G8-3.2 is more of an engineering standard than the CSA Standard listed. It covers metal-enclosed power switchgear which includes metal-clad switchgear as well as station-type cubicle switchgear. The definition of metal-clad switchgear is this standard is similar to that detailed in the CSA Standard. The EEMAC Standard references many of the applicable ANSI Standards for ratings, definitions and testing procedures.
Two other important Canadian standards are applicable and are listed in the CSA Standard for reference and compliance. One of these covers the grid for measurement of corona in switchgear assemblies, EEMAC Standard G11-1, 1972. By the CSA Standard, corona extinction test voltages are applied to a completed switchgear assembly but are not applicable to nominal voltage ratings of 13.8kV and below and such corona test voltage is not required to be applied to the circuit breaker, voltage transformers or control power transformers, at this rating. Above 13.8kV the circuit breaker has to be tested. This is considered a production test by the CSA Standard list.
The other Canadian standard now more widely accepted in North America is the EEMAC Standard G14-1, 1987 which covers the procedure for testing the resistance of metal-clad switchgear under conditions of arcing due to an internal fault. Such testing establishes the switchgear as what is commonly called Arc resistant Switchgear. More on this topic later in this article.
Of interest is the fact that the equivalent ANSI Arc Resistant Standard has yet to be adopted. The EEMAC Standard has wide acceptance in the United States when it comes to the acceptance of metal-clad switchgear having successfully passed such tests.
ANSI Standards comprise a number of standards that are applicable to metal-clad switchgear. These include C37.20.2 Metal-clad Switchgear and C37-04, C37-06 and C37-10 which cover definition, ratings, features and test procedures.
In general, each of the standards referenced above has a similar definition, ratings, construction details and test procedures. Reference to any one of these main standards determines the type of metal-clad switchgear required and its ratings. IEC Standards for switchgear are often referred to in technical specifications. Caution is advised in that some European Standards relative to metal-clad switch-gear, are quite different from what is expected of switchgear designed and tested to the North American Standards listed here. Ratings and test results are also different.
Approval for use in Canada
Metal-clad switchgear approval for use in Canada can be handled in a number of ways. Equipment that includes a CSA approval label is the most universal. It involves a manufacturing plant obtaining certification by CSA allowing labeling of the product. This is usually undertaken early in the design/production stage and approval is ongoing subject to regular reviews by CSA. Such labeling of the switchgear is acceptable to all electrical inspection authorities.
Another way is UL-C labeling of the switchgear. This signifies approval by UL, but to Canadian standards. Such switchgear approval product-labeling is also accepted by all electrical inspection authorities. Two other equipment approval methods are also used. These involve special inspection at the plant or on-site by CSA, on a job-by-job basis, resulting in approval. In lieu of such approval, inspection by local authorities, is acceptable in some juristrictions. This is on a job-by-job basis and not the best approach to use unless one is sure of full compliance once the equipment has been manufactured.
The definition of metal-clad switchgear in the standards listed is switchgear that is characterized by the following required features and is a subset of metal-enclosed switchgear:
- The main switching device, a breaker, is a drawout type that includes a mechanism to move it physically between connected and disconnected positions with disconnectable primary and secondary contacts.
- All major primary components within the cell are separated by grounded metal barriers and are completely enclosed by grounded metal. Control devices and control wiring are enclosed and in separated grounded metal compartments or conduit/wire ways.
- All live parts are enclosed within grounded metal compartments.
- Automatic shutters are provided that cover primary disconnecting contacts when the removable breaker is in the disconnected or test position or completely removed from the cell.
- Primary conductors and bus are fully insulated throughout.
- Mechanical interlocks are provided for proper operating sequence.
- All meter relays, instruments and control devices and their wiring are isolated from primary components by grounded metal barriers. It's acceptable to have short lengths of wire to instrument transformers physically unprotected.
- A front door is provided for access to the primary disconnect device (breaker) which can be used as a relay/metering panel as when this door is open no primary circuit components are exposed.
- Voltage transformers are mounted in drawout or tiltout compartments.
Metal-clad Switchgear Ratings
Voltage Ratings
Table 1 illustrates voltage ratings for metal-clad switchgear.
Some metal-clad switchgear manufacturers can offer 38kV class switchgear with a 170kV BIL. Even higher BIL levels are possible with SF-6 insulated switchgear.
Continuous Current ratings
Current ratings of bus within the metal-clad switchgear are 1200A, 2000A and 3000A to the standards. Most manufacturers can supply 4000A continuously rated bus.
Breaker continuous current ratings are normally 1200A, 2000A and 3000A to 15kV class. Some manufacturers can increase these breaker ratings to 3500A, 3750A and 4000A by forced air cooling of the breaker. This is accomplished by adding cooling fans to the breaker itself or by adding the cooling fans to the cell, allowing a standard 3000A breaker to be applied to a forced air cooled cell, providing a total breaker cell rating of 4000A continuous.
At 27kV and 38kV class, breaker ratings are 1200A and 2000A. Some manufacturers offers higher ratings, 3000A, and one offers a 600A rating.
Maximum symmetrical interrupting capability of breakers at 4.76kV (max) breaker IC ratings is 36 and 49kA, at 8.2kV is 41kA, at 15kV is 23, 36 and 48kA, at 27kV is 25 and 40kA, and at 38kV is 25, 31.5 and 40kA.
Drawout Breaker Interchangeability
Normally the drawout breaker associated with metal-clad switchgear is designed to ensure that only a breaker rating suitable for the cell it was designed for can be inserted into that cell and that all breakers of similar ratings are completely interchangeable with one another. A breaker of a higher continuous rating and/or interrupting capacity can be inserted into any cell, but a lower rated breakers can only be inserted into a cell of matching rating. This allows the user to purchase one spare breaker only, of the highest rating and utilize it in any cell, which reduces the cost of spare breakers.
Tests
Two types of test procedures are normally applicable to metal-clad switch-gear and these procedures are covered by the various standards listed above. The initial tests are associated with the design and development of the product. In general these include such tests as:
- Dielectric tests which include normal frequency withstand tests and impulse tests and corona tests.
- Tests for bus bar insulation
- West tests on entrance bushings: when applicable to the application and type of metal-clad switchgear
- Rated continuous current test-heat run
- Short time current withstand tests
- Momentary endurance tests
- Insulation material tests, flame resistance and tracking resistance tests
- Full breaker short circuit tests to establish all ratings claimed in accordance with the applicable standards
- Paint qualification tests
- Rain tests for outdoor switchgear application
Such design tests are conducted on the basic equipment and do not normally expect to have to be repeated as long as the metal-clad switchgear and its major components are not modified in any way that could change the ratings. This would require re-testing. Other design testing such as seismic testing is also carried out to determine suitability of the switchgear for use in seismic areas.
Production tests are carried out prior to shipment on each and every Metal-clad Switchgear assembly. These generally include:
- Normal frequency withstand tests
- Mechanical operation tests which generally include interlocks, drawout element operation, shutter operation, interchangeability of breakers etc.
- Effectiveness of grounding of instrument transformer case or frame grounding
- Electrical operation and control wiring tests including correctness of wiring and insulation tests, polarity, verification and sequence tests
It is not uncommon to see impulse tests and heat run tests specified as a production tests. While these are generally considered a design test these can be undertaken as a production test for an added cost to the customer.
Arc Resistant Metal-clad Switchgear
This class of product has gained popularity in North America over these past few years and in particular in Canada where it originated.
The applicable North American Standard used for testing EEMAC G14-1, as previously mentioned. Arc resistant switchgear has been promoted due to the safety concerns of operating and maintenance personnel and the inherent increased reliability of the equipment. Unlike arc resistanct switchgear, conventional metal-clad switchgear subjected to an internal fault, while the equipment is being operated or maintained, could result in personal injuries due to the emission of flame, gases and particles outside the enclosure.
Arc resistant switchgear is now certifiable by UL and UL-C (for Canada) and can now be labeled.
Some metal-clad switchgear manufacturers have tested their arc resistant switchgear to IEC Standards. Such tests are different from those required by EEMAC and are in general less arduous.
EEMAC tested arc resistant switchgear includes 3 basic types of accessibility:
- Accessibility Type A which is arc resistant construction at the front of the equipment only.
- Accessibility Type B which is arc resistant construction at the front, back and sides of the equipment.
- Accessibility Type C which is arc resistant construction at the front, back and sides and between compartments within the same cell or adjacent cells.
There is an exception to Type C allowed by the standard in which a fault in the bus bar compartment of a feeder cell is allowed to break into the bus bar compartment of an adjacent feeder.
Some manufacturers have undertaken tests on metal-clad switchgear to preclude this from taking place and, instead, contain the arc within the bus compartment of any feeder cell.
Arc resistant tests based on one high and two high breaker design have been successfully undertaken.
Arc resistant test criteria to EEMAC Standard include:
- Properly secured doors, covers etc, do not open
- No hazard due to flying parts
- Arcing does not cause holes in sides of switchgear covers
- Test indicators do not light
- Grounding connection remains effective
The effects of an internal arc fault are:
- Mechanical stress from pressure increase
- Thermal stress from radiated heat
- Release of toxic gases
- Direct injury from pressure wave, contact with energized parts or radiation from intense light
- Indirect injury from airborne parts
There are basically 4 stages of internal arc faults:
- Compression
- Expansion
- Emission
- Thermal
The compression stage starts at time zero and ends when pressure can no longer increase. This typically lasts 5 to 10 milliseconds.
The expansion stage starts when the pressure relief doors, located at the top of the switchgear, begin to open. Gas flow is characterized by wave motion. Expansion stage typically lasts 5 to 10 milliseconds.
Emission stage starts when the pressure relief doors have opened and ends when the gas in the cell reaches arc temperature.
This typically lasts 50 to 100 milliseconds.
Thermal stage lasts until the arc is extinguished and combusting materials are consumed. This lasts until the arc is extinguished and every thing has cooled down to room temperature. Such thermal energy heat melts and vaporizes parts of the equipment and causes the greatest damage to the switchgear.
In accommodating arc-resistant types of switchgear in an electrical room, consideration should be given to adequate room ventilation and to clearance above the switchgear which should be at least 36 inches.
Metal-clad switchgear is often accommodated in an outdoor house or power control room and if arc resistant switchgear is used, it's important to ensure arc resistant tests have been conducted on not only the switchgear itself but the combination switchgear and weatherproof enclosure to ensure complete safety and compliance with the applicable standards.
Service conditions in applying Metal-clad Switchgear
Its important to know the specific service conditions under which the switchgear will operate.
This includes ambient temperatures if above 40 degrees C or below-30 degrees C, snow loading if outdoor switchgear is being used, the seismic zone, chemicals in the atmosphere, damaging fumes, vapors, steam etc, sea coast location, salt spray concerns, altitude if above 3300Ft, solar radiation if it's significant, exposure to significant dust, exposure to hot and humid climates, requirement for vermin proof, anti-fungus, and explosive mixtures.
All such concerns can be addressed by the manufacturer and designs and modifications are available to cater to any unusual service conditions, as long as these conditions are known ahead of the design start.
Peter Wicks is with Switchgear Technical Services Inc. He can be reached at wicks@on.aibn.com