Types of Cable Trays (NEC . Circuit breakers shall be dead-front construction, equipped with an integral steel-front panel. Auxiliary switches in the breaker and instrument compartment shall be breaker-operated as follows: a. The continuous current rating of the main bus shall match that of the protective devices. Type MV cables and associated fittings shall be listed. Unlike fuses and typical 600 V circuit breakers, MV circuit breakers rely on separate devices such as current transformers (CT), potential transformer (PT), and protective relays to provide the overcurrent protection. Understand overcurrent protection requirements for medium-voltage distribution transformers. Medium-voltage electrical system protection Medium-voltage electrical system protection Overcurrent protection is required for medium-voltage transformers, and connecting these transformers is common for medium-voltage distribution systems. Device 51 or function long-time pick-up (LTPU): Recommend 100% to 125% of the transformer FLA and set below the transformer and cable damage curves. As the power requirements increase, so too does the likelihood that the utility will require a higher service voltage. Upon loss of control power, it shall be possible to manually recharge the stored energy mechanism by use of a single ratchet wrench or operating lever. Circuit breakers of the same rating shall be interchangeable and interlocked with the breaker cubicle to prevent insertion of a breaker with a lower continuous or interrupting rating into a. 50mm 2 to 1000mm 2. ASTM F1117 Class 2 AC EN 50321 dielectric boots now offer 17kV/20kVAC protection, comparable to Class E hard hats, making it a better fit for MV workplace. 1.2 RELATED WORK A. Current transformers shall be provided in the application of instruments, meters, and relaying as shown on the single line diagram specific to the switchgear. h. Factory setting of contact gap Many of these complexes also have high-rise components with MV risers servicing unit substations at strategic locations on multiple levels. It shall not be possible to operate a circuit breaker when it is between the connected and test positions. a. The open/closed indicator shall be visible from the front of the equipment (outer door open) when the breaker is in the operating or disconnected/test position. Until recently, engineers didnt work too frequently in the design of medium-voltage (MV) systems, mainly because anything over 600 V was primarily handled by the utilities. and 2 N.C. electrically separate breaker auxiliary contacts operated by the breaker and wired from the breaker to terminal blocks Breakers shall be equipped with a stored energy operator. Most sections of the National Electrical Code (NEC) cover systems rated 600V and less; however, it also includes requirements for systems rated greater than that. By Country . The current transformers shall be insulated for the full voltage rating of the switchgear, and shall be mechanically rated to withstand the maximum short circuit momentary rating of the switchgear. National Electric Code [NEC] 2005 edition and later editions requires shielded medium voltage [MV] cables for all circuits operating above 2400V. The Basic Impulse Insulation Level (BIL) shall be coordinated with that of the switchgear. The ground bus shall be routed at the rear of the assembly. Furthermore, high voltage circuit breakers typically do not limit the fault current. This interrupting device, in combination with the associated controls and metering, provides protection to the cables and equipment. The main bus shall extend the full length of the assembly and shall be drilled at both ends for the addition of splice plates to extend the bus in either direction for future expansion. The enclosures shall have been arc-fault tested, and be capable of withstanding the maximum internal pressure that may be generated by a fault in the switchgear Filter your search. Article 311 Wiring Methods and Materials, Medium Voltage Conductors and Cable, This article covers the use, installation, construction specifications, and ampacities for Type MV, 311.16 Wiring Methods and Materials, Marking, 311.32 Wiring Methods and Materials, Uses Permitted, 311.44 Wiring Methods and Materials, Shielding, 311.40 Wiring Methods and Materials, Support, National Electrical Code 2020 of Illinois. For example, Southwire Co. has published tape shields fault current capacity to be 1893 amps at 12.5% tape overlap, and 2045 amps at 25% tape overlap. Informational Note: The "uses permitted" is not an all-inclusive list. NEC Article 215.2(B) (1) through (3) outlines the size of the circuit grounding conductors. You will find many solutions for applications that demand high power and performance capabilities, including design configurations such as Type MV-90 and Type MV-105 shielded with copper tape. The switchgear vertical sections shall be NEMA, free standing, factory assembled, 3.2 mm thick steel construction, with doors and access panels of sufficient size to give full access to all components. Transformer MV breakers may include the following protective device numbers: In MV systems, current transformers (CTs) connect protective or metering devices. In industry practice, a feeder relay setting of 200% to 400% of full load is considered reasonable. It sites the following NEC voltage standards: High Distribution - 1000 to 4160 volts Medium Distribution - 50 to 1000 volts Potential transformers shall be drawout type, with primary side current limiting fuse protection, and secondary fuse protection, if required from the single line diagram and data sheets. 35mm 2 to 500mm 2. Previously, the ampacity tables for medium voltage cables were located in Article 310 while the installation requirements were located in Article 328. For these reasons the following general guidelines shall be followed in specifying safety features to be incorporated into the metal-clad switchgear: b. For the coordination schemes presented in the examples, the breaker or fuse trip curves did not overlap. The metal-clad switchgear assembly shall conform to ANSI C37.20.2 and shall include drawout type circuit breaker assemblies, buses, fuses and other protective devices, controls, metering and instrumentation. The NEC and Installations Over 600 Volts - IAEI Magazine . In an effort to improve overall usability of the NEC, the medium voltage requirements located in Article 310 and Article 328 have been combined into new Article 311. It is consistently reviewed and updated, with input from active professionals in the field, to stay ahead of the constant changes in technology and safety that affect . This results in voltage across the open circuit portion of the shield with maximum voltage appearing at the end that is farthest away from grounded point. The upper compartment shall contain the protective devices, controls, metering and instrumentation; effectively isolated from the lower compartment by horizontal full width metal barriers. Main keywords for this article are Switchgear Inspection and Testing, Basic Impulse Insulation Level, SF6 Gas Circuit Breakers, Metal-Clad Switchgear, NEC Medium Voltage Switchgear Design, National Fire Protection Association (NFPA) Medium-voltage distribution also helps to minimize voltage drop. The closing circuit shall be arranged to preclude repetitive operation during fault conditions if the circuit breaker control switch is held in the close position. Temperature Rating of Conductor. and 3 N.C. electrically separate mechanism-operated type (MOC) auxiliary switch contacts. N of cables per phase. and 3 N.C. electrically separate mechanism-operated type (MOC) auxiliary switch contacts. Whether liquid filled or dry type transformers were correctly selected for the application. It is mandatory to procure user consent prior to running these cookies on your website. These uses of conduit are in all substations and have a straightforward application of the NEC. NEC Article 493.30 lists the requirement of metal-enclosed switchgear. Sweet is a senior associate and an electrical engineer/electrical commissioning agent with Mazzetti in Seattle. NEC is also exporting its products in the international markets such as Middle East countries, Nepal, Bangladesh, Srilanka, etc. The manufacturer of the equipment shall be responsible for checking burdens to be in compliance with ANSI C57.13, and the BIL shall be coordinated with that of the switchgear. (Manufacturer's Instructions) (NFPA 70, NEC 110.3 (B))" The overcurrent protection required for transformers is considered protection solely for the transformer. Protective relays CTs are expected to deliver about 5 amps or less under healthy load conditions. Positive mechanical interlocks This section specifies the furnishing, installation, and connection of medium-voltage cables, indicated as cable or cables in this section, and medium-voltage cable splices and terminations. You also have the option to opt-out of these cookies. Informational Note No. MV breaker protections are used for transformer sizes greater than 3000 kVA. POLY-NET Protection and Leaf Guard; POLY-NET Filtration and Environmental Technology; . For transformers with primary voltage greater than 1,000V at supervised locations, with the following maximum primary protection [Table 450.3 (A)]: a. This article covers the use, installation, construction specifications, and, The definitions in this section shall apply within this article and throughout the. Learn about code and standard minimums that must be considered in the coordination of MV protective devices. Raceways, cable armors, and cable . This may include personalization of content and ads, and traffic analytics. To simplify design and application, the values in Table I have been established, which are in conformance with ANSI C37.20.2 Table I. Switchgear assemblies and circuit breakers shall be tested and inspected at the factory as part of the regular manufacturing process. Based on the 2020 NEC, which of the following is true? For systems rated a nominal 15kV, here are seven of the more common items you should be aware of as noted in various sections of the NEC when designing MV systems that meet the NEC. medium voltage cables Results. In distribution design, the three objectives still apply: For example, if the NEC requirements for transformer overcurrent protection are considered without reference to applicable standards and code requirements, the system may address protection of transformers, while other elements of the distribution system (such as the feeders connecting the transformer(s) to the distribution system) may not be protected in accordance with the code. b. Relay and metering circuits shall be energized and devices checked, to ensure that polarities are correct, elements are in operating condition, and relays are performing the desired function, c. No-load operational tests of circuit breakers shall be performed at maximum, standard, and minimum control voltage ratings, d. Operation of mechanical interlocks and breaker interchangeability, e. Confirmation of proper function of the sequential interlocks, g. Size and rating of each type of equipment, compared to the requirements of the purchase order. d. Partitions and barriers shall be provided where required, to prevent the spread of faultse. g. Size and rating of each type of equipment, compared to the requirements of the purchase order As used in this Code, the heat transfer capability through a substance by conduction. Frequency Rating Rated voltage and current for switchgear shall be based on 60 Hz. High Voltage (HV): between 45 kV and 230 kV. For this article, a good working definition of MV is 1 to 38 kVac as any voltage level above 38 kV is a transmission level voltage versus a distribution level voltage. Most sections of the National Electrical Code (NEC) cover systems rated 600V and less; however, it also includes requirements for systems rated greater than that. Circuit breakers shall be equipped with auxiliary contacts for interlocking with control circuits, indicating lights and auxiliary relays. 311.2 Definitions. The global Medium Voltage Cable market size was valued at USD 19004.1 million in 2021 and is expected to expand at a CAGR of 0.53% during the forecast period, reaching USD 19618.59 million by 2027 . 01 Electrical Data; 02 Cable type; 03 Installation; 04 Cross-section calculation; . Short-circuit protection is still required. The current will go to a high value when a fault occurs. The rated secondary current is commonly 5 amp, though lower currents such as or 1 amp are not uncommon. Examples of these circuits include distribution cable leaving the substation and wind farm collector circuits entering the substation. He has more than 28 years of engineering and design and field experience that includes MV distribution systems for military, mining, tunneling, food manufacturing, power production facilities, high-rise facilities, and casino resort complexes. b. American National Standards Institute / Institute of Electrical and Electronic Engineers (ANSI/IEEE) For normal side circuits upstream from an automatic transfer switch (ATS), ground fault protection is required per NEC Article 230.95. However, for large-scale developments, equipment such as large motors may require a service voltage of 4160 V or higher. A spare switchgear shall be arranged for future expansion, and the enclosures shall be of modular design to permit safe and easy installation of additional units. Selectivity: Selectively requirements are intended to limit system fault or overload response to a specific area or zone of impact and limit disruption of service to the same. Switchgear designed for plant facilities shall meet the requirements of. Each breaker shall be equipped with at least 2 N.O. Provision for padlocking circuit breakers in the test or disconnected position shall be provided. However, unless precautions are taken, this setting may be too low to prevent relay misoperation on inrush following an outage. Breakers shall be equipped with position indicator, operation counter, primary and secondary disconnecting devices, auxiliary switches and required mechanical safety interlocks. ANSI/IEEE Standard C37.20.2 Standard for Metal-Clad Switchgear defines MV as 4.76 to 38 kV. 2. [See Table 310.104 (C).] USA Wire & Cable, Inc. Austin, TX Distributor* $10 - 24.9 Mil 1990 10-49. If I provide a 600% ltpu setting on a relay to allow for optimal coordination, I satisfy myself that there is no way that the system could ever be loaded beyond the ampacity of the conductor. Motor and generator curves are machine specific and are normally provided in the vendor data submittal packages. Its high free electron absorption and chemical stability allows a rapid recombination of the products from the action of the arc. Another feature of large commercial complexes is the associated central plant function with MV chillers and unit substations. NEC 2017 Code Changes . Each breaker compartment shall be equipped with a minimum of 3 N.O. A set of either 1200-, 2000- or 3000- ampere insulated copper main buses shall be provided, which shall have provisions for future extension. NEC Section II (Article 300.31 through 300.50) covers MV wiring methods. Bus joints shall be plated, bolted and insulated with easily installed boots. This article covers the use, installation, construction specifications, and ampacities for Type MV medium voltage conductors and cable. Current to raise the temperature of lamps and heater elements. In this case an analysis of options must be conducted to determine the best option for the project. C57.13 Requirements for Instrument Transformer. The transformer tray shall be equipped with a position switch to prevent tripping of the protective device when the potential transformers are withdrawn. c. No-load operational tests of circuit breakers shall be performed at maximum, standard, and minimum control voltage ratings This equipment is for installation indoors in an unclassified area. Step 1 - Size the overcurrent device in accordance with 210.20 (A) and 215.3. Magnetizing inrush currents to transformers. Safety requirements ensure equipment is of sufficient rating to withstand the maximum available energy of the worst-case scenario. Want to start the conversation? NEC Article 240.21 (C)(1) and NEC Article 450.3(A) validate this statement is true. Extra High Voltage (EHV): from 230 kV and above. In most cases, only one choice of electrical utility is available and typically there is limited choice of service voltage. class EPR cable with 133% insulation as out going feeders to Unit Substations all over the facility yards. b. These two NEC rules require the overcurrent device (breaker or fuse) be sized no less than 100% of the noncontinuous load, plus 125% of the continuous load. Exception included heavy electrical users such as government institutions, the mining industry, or industrial sites. Protective relay curves cannot be used in the same way as low-voltage circuit breaker curves or fuse curves. Settings do not correspond to those listed as standard in the NEC [NEC Article 240-6(a)]. Another application of conduit in the substation is for the protection of medium-voltage circuits. Overcurrent, 51 device, should be set at 100% to 140% of FLA and set below the transformer cable ampacity. e. Confirmation of proper function of the sequential interlocks For working principle of potential transformer please, Potential transformers shall be provided as required, to power instruments, metering, and relaying, as shown on the single line diagram specific to the switchgear. *Single point grounded shield is not useful in providing protection against inductively induced noise. Article 450 is specific and limited to requirements for the transformer. and 3 N.C. contacts shall be included. Safety: Personal safety requirements are met if protective devices are rated to carry and interrupt the maximum available load current as well as withstand the maximum available fault currents. SPECIAL CABLES S.R.L. The basic function of this device is to open or close the circuit under normal and abnormal conditions. Specifications include 4 AWG to 1,000 kcmil size, 7 to 61 no. Provide physical protection of cables, such as flexible cable guides or feed-in tubes, at the entrance of manholes, underground ducts, and conduits. Circuit breakers shall be removable, equipped with self coupling primary and secondary disconnecting contacts, and arranged with the disconnecting means to physically move between the connected and disconnected positions. Only PVC and XLPE cables are considered. The rated current of metal-enclosed switchgear is the maximum current in rms amperes at rated frequency which can be carried continuously by the primary circuit components, including buses and connections. (B) Thickness of Insulation and Jacket for Nonshielded Insulated Conductors, Thickness of insulation and jacket for nonshielded solid dielectric, (C) Thickness of Insulation for Shielded Insulated Conductors, Thickness of insulation for shielded solid dielectric, The minimum size of conductors shall be as shown in Table 311.12(A), except as permitted elsewhere in this. There are three fundamental objectives to overcurrent protection to include ground fault protection: 1. jihzxICwww.jihzx.com Vin (Max. -90c 2)Maximum overload temperature.-130c (Not to exceed one hundred hours / year). Checklist for substation operators when transformers trip off on fault Auxiliary Compartment. For transformers greater than 5 MVA, protection schemes become more complex. Sizing MV components such as motors, generators, transformers, wiring systems, the architecture of MV systems, or design of complicated protection schemes such as reclosers, zone interlocks, differential protection, etc., are all beyond the scope of this article. When connecting to an existing utility, the utility typically directs the interconnection requirements including protective device requirements. The rated maximum voltage of metal-enclosed switchgear is the highest rms voltage for which the equipment is designed, and is the upper limit of operation. When the breaker is installed in the test position, the local control switch trip and close functions shall be operable. and 3 N.C. contacts shall be included. Medium voltage cables are used in pulp and paper mills, petrochemical plants, steel mills, textile mills, water and sewage treatment plants, environmental protection systems, railroads, mines, and electrical utility generating plants. Hint: Mix and Match GEC Sizes, Illustrated Catastrophes: Questionable Wiring Methods, EC&M Tech Talk Video Changes to the 2023 National Electrical Code, Top 25 Changes in the 2023 National Electrical Code. Enclosure for the future shall be completely equipped to contain breaker stabs, current transformers, bus bars, wiring, test block, relays and circuit breaker to make an operating feeder, as specified on the one line diagram. Stored-Energy Operation. For MV systems that are more complicated, a system protection engineer should be consulted. Extremely inverse relay setting is superior in that substantially faster fault clearing time is achieved at the higher current levels. Cable raceways shall maintain individual circuit integrity. The National Electrical Code (NEC) Article 215.2 recommends a voltage drop of 5% or less from the utility service to the load. Necessary cookies are absolutely essential for the website to function properly. This site requires you to register or login to post a comment. Map display. Medium Voltage Cable, Type MV. This gallery notes the key sections of the National Electrical Code (NEC) that refer to medium-voltage systems. higher rated compartment. An interlock shall be furnished so that transformer primary fuses cannot be withdrawn until the transformer secondary breaker switch is opened. The norm, however, is that the utility will give several MV services that the engineer will need to integrate into an owners MV distribution system. D. Section 26 05 41, UNDERGROUND ELECTRICAL CONSTRUCTION: Manholes and ducts for medium-voltage cables. It makes sense to consolidate the requirements into one article. Although the primary windings are rated for MV, the designer must choose either fuses or circuit breakers to protect the transformer. 1: Information about accepted industry practices and installation procedures for medium-. Due to system operation limitations and equipment selection, selectivity is not always possible for non-emergency or optional standby systems. It is primarily used in distribution of electric power for various voltage levels. In the 2020 NEC, Section 645.5 (E) correlates with NFPA 75-2017 by requiring plenum cables where cables are exposed to the airflow (not in conduit) in the area under the raised floor and that area is not protected by active fire suppression. The metallic shield, sheath, or armor shall be connected to a. Informational Note to Exception No. Complete protection shall be provided against contact with live parts, c. The circuit breaker trucks shall be fully integrated into the interlock system Medium Voltage Cable, Type MV. Minimum wire size for current transformer secondary circuits shall be 2.5 mm2 (14 AWG) stranded copper, 600 V, rated at 90 C or greater. Leslie Fernandez is senior project engineer, electrical at JBA Consulting Engineers.