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The March/April issue of the IAEI News had an article on high voltage Equipment that discussed the new areas of application. The article discussed how these systems have evolved into some non-traditional areas along with some of the safety concerns for those working around this equipment. Codes and standards were discussed in general along with some issues that come about when over 600-volt equipment is installed. This article will focus on some of the National Electrical Code requirements and common problems being experienced in the field today. Due to space limitations, not every aspect of high voltage installations can be covered in an article like this. All that can be done is to give some insights into common problem areas and hopefully stimulate the reader’s interest to go farther and learn about the proper way to install these systems. Many of the problems seen and discussed in this article are due to lack of training and limited experience on the parts of the design engineers, electrical installers and inspectors. Unfortunately, there are inspectors being asked to do plan reviews and field inspections who have never actually worked on "high voltage" equipment and systems. The combination of an installer with limited qualifications and an inspector with limited experience in this area can make for dangerous situations.

Part B of NEC Article 100 adds some definitions that are specific to over 600-volt equipment and systems. These definitions generally cover two broad categories, fuses and switching devices. It is important that these terms are understood so the requirements using these terms later on in the NEC are understood. There are several definitions for fuses including electronically actuated fuses, expulsion fuse, nonvented power fuse, power fuse unit, vented power fuse and multiple fuse to identify the several types that may be encountered. While most of the above fuses operate similarly to low voltage fuses by melting, there are fuses that are actuated electronically which have different rules applied for overcurrent settings and allowances. While fuses for under 600-volt equipment are prohibited from expelling hot gases or other materials, this is not necessarily true for fuses rated over 600 volts. High voltage fuses may or may not expel or vent hot gases that are generated by the arcing as the fuse opens. The energy levels are very high and that pressure generated from the internal melting and arcing needs to be released for some designs. One thing to watch for with fuses that do vent is that there is proper clearance of other components and especially any combustible materials in the direction of the venting. The example photo 1 shows is a case where the power cable entry point and termination is below the vent for the fuse. The fuse in this photograph is vented through a muffler in the lower part to mitigate the noise and arrest any burning materials that are expelled. Other items such as space heaters or control wiring are found installed directly below a vented type power fuse. Another feature of medium voltage fuses is that they may be ganged with two or even three fuses in parallel. While this is generally not acceptable for low voltage fuses, it is fairly common in medium voltage metal enclosed switchgear applications. An item to watch with this is the ratings. It is possible to have a single barrel fuse rated 600 amps, to have a two barrel fuse rated 600 amps or have a three barrel fuse rated 600 amps. The fuse rating is of the single barrel or the combination for the multiple barrels, not the rating of each barrel as has been mistakenly applied in the past with catastrophic results.

The other main group of definitions deals with switching devices. These include circuit breakers (CBs), cutouts, disconnecting switches, interrupter switches, oil cutouts, and oil switches. Circuit breakers also come as air CBs, vacuum CBs, oil-filled CBs and SF6 CBs. Circuit breakers rated over 600 volts are very different from what is seen with breakers rated 600 volts or less. One main difference is the "circuit breaker" is really an electrically operated switch. There are no overcurrent devices built in. The overcurrent protection and control comes from using current transformers (CTs) that may or may not be mounted on the breaker body and protective relays that are mounted in the switchgear assembly. Power is derived from a control source, batteries or control power transformer external to the breaker assembly. Typical breaker ratings are 1200, 2000 and 3000 amps, while the actual application rating depends on the CT ratio and the relay settings. Similarly cutouts, disconnect switches and interrupter switches may or may not be rated for opening load current or fault currents. Familiarity with the equipment and the applicable ratings is essential to understanding the installation requirements and restrictions.

Article 110, Part C provides the general requirements for over 600-volt applications. Section 110-30 provides that the requirements of Article 110 Part A, "General Requirements," apply along with the sections that follow which supplement or modify those requirements. As discussed in the last issue, medium voltage equipment may be enclosed in steel enclosures or it may be open where the room or a fenced area constitute the "enclosure." In either case, access to over 600-volt equipment is specifically limited to qualified personnel and there must be applicable warning signs. Sections 110-32, 110-33 and 110-34 address the work-space and access requirements to over 600-volt equipment. One key characteristic of over 600-volt equipment is that it is generally physically larger than the under 600-volt counterparts. Also with the higher energy levels present that could severely injure or kill someone, more space is needed. As shown in photo 2, a worker in the blast zone needs personal protective equipment and adequate space to operate the insulated tools, such as shotgun and other hot sticks. Conditions 1, 2, and 3 for working space in Section 110-34 are the same as for under 600-volt equipment in Section 110-26. The voltage references are for the voltage to ground. Remember the definition for "voltage to ground" in Article 100 states that for ungrounded systems, the phase-to-phase voltage is taken as the "voltage to ground" for purposes of determining the working clearance. Also, note that this working clearance for over 600-volt equipment does not depend on the likelihood of accessing while energized as provided in Section 110-26.

The last two items for discussion from Article 110 are the installation of circuit conductors and terminal temperature ratings. Section 110-36 permits several wiring methods and references several sections in Article 300 and Article 490. Where open conductor or bus installations are completed in the field, Section 110-36 also requires the supports and insulators holding these conductors in place have an adequate mechanical rating to withstand the maximum magnetic forces that would be present in the event of a short circuit. As seen in photo 3, this switchyard has overhead bus supported on several types of structures and insulators. These structures and insulators have mechanical load and stress ratings that have to be considered in the design and installation. Section 110-40 on terminal temperatures finds that over 600-volt equipment actually simplifies things. Medium voltage equipment, terminal lugs and cables are all rated at 90°C so ampacities can be taken right from the applicable tables. A review of the cable ampacity tables finds ampacities listed at 90°C and 105°C. The 105°C ampacity could be used for derating calculations where necessary, but the installation cannot exceed the 90°C ampacity maximum allowed for the terminations or equipment.

Article 225, Part C deals with outdoor feeders that operate at over 600 volts. This part requires warning signs where unqualified persons can approach the feeder or equipment as well as isolating switches. Where the building disconnecting means on a feeder is an oil switch or an air, oil, vacuum or sulfur-hexafluoride circuit breaker, an additional disconnecting switch must be installed. The purpose of this switch, like the one shown in photo 4, is to provide visible open contacts for all ungrounded conductors to ensure power is disconnected when needed. The exception permits this disconnect to be eliminated only when the oil switch or the circuit breakers installed in switchgear allows the unit to be racked out of its cell and show positively there is an open circuit. Section 225-53 requires the building disconnecting means to open all ungrounded conductors simultaneously as well as being rated to close into a fault of the maximum available short circuit current available. What this does is effectively rule out using single cutouts for poly-phase systems.

Article 230, Part H addresses services of over 600 volts. As discussed in the previous article, this is becoming very common in many areas. Section 230-200 provides that all the previous requirements for services need to be followed with the section in Part H modifying or supplementing those requirements. A fine print note sends the user to the National Electrical Safety Code (NESC) ANSI/IEEE C2 to determine the required clearances for conductors over 600 volts when installed over sidewalks, roadways, etc. This would be a case where the jurisdiction may have to adopt the NESC to make those provisions enforceable. As with outside feeders in Article 225, warning signs are required where unauthorized persons could come into contact with live parts. This does not apply to metal enclosed and locked equipment. Section 230-204 mimics Section 225-53 requiring the isolating switch so visible contacts are available to verify an open circuit. The same exception for draw out type breakers is also provided. An additional requirement for services is in Section 230-204(d) where a means for grounding the load side conductors after being disconnected must be provided.

As with outside feeders, the service disconnecting means must open all ungrounded conductors simultaneously. This has been a common problem where the "service disconnect" was three individual fuse cutouts on a pole. Where a facility owner owns and controls the over 600-volt distribution system and transformers to provide the utilization voltage, he must have a method to disconnect this equipment as well as provide the proper overcurrent protection. This generally means some sort of ganged switch, pole mounted or in switchgear, or a circuit breaker needs to be installed in the primary of the system. One issue raised by many jurisdictions is having the "service equipment" marked as suitable for service equipment. Unfortunately, the present standards do not cover this aspect for over 600-volt equipment. Some accredited testing laboratories have provided field evaluation services to meet this need by applying the philosophy of UL 869A and adapting it along with the applicable ANSI/IEEE C-37 standard for the over 600- volt equipment.

Service overcurrent protection is required by Section 230-208. It should be noted that only short circuit protection is required. The maximum rating for a fuse is three times the conductor ampacity and a protective relay may be set up to six times the conductor ampacity. This is allowed due to the construction of medium voltage cables that have a long term overload capacity. While only short-circuit protection is required by Code, most designs also provide for some level of overload and ground-fault protection. This is explained more in Section 240-100(c).

Going on to Article 240, over 600-volt systems are covered in Part I. Part I has two sections that address feeders and branch circuits. Overcurrent (short-circuit, ground-fault and overload) protection is required in each ungrounded conductor for both feeders and branch circuits. Section 240-100 requires the overcurrent protection to be selective to prevent damage or dangerous temperatures in conductors or the conductor insulation under a short-circuit condition. Actual settings for limits are not specified. It should be noted that at least one large equipment manufacturer has a fused switch / vacuum interrupter combination that will provide short-circuit and ground-fault protection. Section 240-101 addresses limits applicable to feeders only, similar to services. These limits are three times the conductor ampacity for fuses and six times the conductor ampacity for setting of protective relays on circuit breakers.

Grounding is covered in Article 250 Part K. One interesting note is that this part deals with the grounding of systems over 1000 volts where all the previous discussions have dealt with systems over 600 volts. Most of this part addresses the grounding requirements for system neutrals. Section 250-182 provides that system grounding can be accomplished through a grounding transformer such as a zigzag or Scott-T type unit. The neutral derived from this transformer and connected to earth is the grounding connection for the high voltage system. Section 250-184 provides that solidly grounded high voltage systems may be grounded at one location for each system, as typically seen with low voltage systems, or may be grounded in multiple locations under specified conditions which is more like the typical utility distribution system. This multiple grounding concept is specifically prohibited or severely limited on low voltage systems.

Whenever the neutral of the system is solidly grounded, Section 250-184(a) permits the neutral conductor to have 600-volt rated insulation or it may be bare when used as service entrance, underground portions of a feeder or an overhead portion of a feeder. It should be noted that where the outdoor bare neutral conductors come within 10 feet of buildings, the neutral must be covered.

Another option for system grounding is in Section 250-186 where impedance grounding is permitted. This is slightly different than with low voltage systems as detailed in Section 250-36. The impedance may be a resistor or a reactor and may be a "low impedance grounded system" or a "high impedance grounded system." There are advantages and disadvantages to each of these methods and many times the system voltage and loads being served will dictate which to use. Low impedance systems allow for greater fault currents and many times have some sort of ground fault-tripping scheme. The high impedance systems limit the ground fault current to a low value tolerable on a continuous basis and have an alarm system instead of tripping. One key item to remember is that the neutral conductor must now be properly identified per Article 200 and must be fully insulated for the system voltage. The reason for this is that under ground-fault conditions, the potential between the neutral conductor and any of the ungrounded conductors will approach the system phase-to-phase potential. Where the impedance grounded system is applied, no other connection of the neutral to ground is permitted. The only connection is through the impedance at the source of supply or at the first disconnecting means.

Portable equipment operating at over 1000 volts is covered by Section 250-188, which has several provisions. The supply system serving this type of equipment is required to be impedance grounded or have a grounding transformer for delta systems to derive a ground reference. Equipment grounding conductors are required to bond all non-current carrying parts of portable or mobile equipment. The sizing of the equipment grounding conductors is based on not developing more than 100 volts potential rise between ground and the faulted equipment under maximum ground- fault conditions. Ground detection and relaying is required to de-energize the equipment in the event of a ground fault and additionally, the equipment grounding conductor path has to be continuously monitored for continuity. Many times there is a redundant equipment ground installed with a sensing circuit established between the two equipment grounds such that when interrupted, the supply is immediately de-energized. Lastly, the grounding electrode used for the impedance grounding must be a minimum 20 feet away from any other system grounding electrode and shall not have any direct connection to a fence, underground metal pipe or similar underground metal structures.

Equipment around high voltage systems such as non-current carrying metal enclosures, associated fences, housings and supporting structures are required to be grounded. This is seen in typical substation arrangements where all aboveground exposed metal structures are grounded directly to the grid constructed under the substation. It should be noted that the metal shield of most medium voltage cables is not suitable to be used as an equipment grounding conductor. These shields shown in photo 5 have limited ratings for current withstand and are there to provide a ground reference around the cable insulation to even out the voltage stress and to provide a return for cable faults only. One exception could be the use of a concentric neutral of a cable assembly for the equipment ground return. The concentric neutral has larger conductors, shown in photo 6 as compared to normal stress control wire shielding, and therefore has greater withstand to meet the ground-fault current-carrying requirements.

Article 300 addresses wiring methods. The first section that addresses over 600 volt systems is Section 300-3(c)(2) which prohibits conductors of circuits rated over 600 volts from occupying the same equipment wiring enclosure, cable, or raceway with conductors of circuits rated 600 volts nominal or less unless specifically permitted. So, a simple question arises. For a solidly grounded medium voltage system where the neutral is permitted to be insulated at 600 volts as discussed above, is the neutral permitted to be installed in the same enclosure, raceway or cable tray with the ungrounded conductors? Many jurisdictions have interpreted this section to say that this is prohibited. What has to be read carefully is that this is not a prohibition based on insulation rating but on the rating of the circuit. The 600-volt insulated neutral is part of that over 600 volt circuit and is not part of a circuit rated 600 volts nominal or less. It would seem that the answer to the question would be that it is allowed. Some of the provisions where over and under 600 volt circuits may occupy the same enclosure, raceway or cable include electric discharge lighting as provided in Section 300-3(c)(2)(a) and (b). Certain motor or generator high and low voltage conductors are permitted to be together. It is also permitted in switchgear, MCC’s and control gear and in manholes if there is separation and securing. Photo 7 shows where an installation of some metering was retrofitted into an existing metal enclosed 15 kV class switch. In this case the low voltage wiring is permitted because this meter is part of the system. The violation here is from Section 490-35(b) requiring the meter equipment itself to have barriers and also that there is inadequate clearance between the uninsulated live high voltage parts and the low voltage wiring, as well as infringement on the air space for heat dissipation on this one phase.

Article 300 Part B addresses over 600 volt wiring methods. Section 300-32 is about conductors of different systems and sends the reader right back to Section 300-3(c)(2), which was just discussed.

Section 300-34 addresses cable bending radius. To understand the reasons behind the bending radius requirements, one needs to understand the differences in cable construction between low and high voltage conductors. Low voltage conductors generally have a single layer of insulation extruded onto the conductor. The voltage stress on the insulation is minimal for the thickness used and the insulation provides adequate mechanical protection. Medium voltage cables are constructed as a system to provide the insulation level needed, seal for gaps to prevent damage from corona discharge, and to provide shielding for an even ground reference. Cables rated 2000 volts or less may have a metal shield layer or not. Cables rated above 8000 volts must have a metal shield. These requirements are from Section 310-6 and the exception.

The typical medium voltage cable construction starts with the conductor just like low voltage conductors, but this is where the similarity ends. On the medium voltage cables, a thin semi-conductive layer is extruded onto the conductor to fill the voids between the strands and make a voltage stress transition to the insulation layer. Most cables today use a solid dielectric insulating material such as cross linked polyethylene (XLPE) or ethlene-propolyene rubber (EPR). The thickness of this insulation layer is dependent on the voltage level and also from being rated for use on a grounded system or an ungrounded system. For example, the 15 kV cable in photo 6 would have a thickness of 175 mils for a 100 percent rating and 220 mils for a 133 percent rating. Over this insulation is another semiconductive layer that is extruded over the insulation. This layer is to provide a voltage stress transition between the outer surface of the insulation to the metal shielding. This metallic shield can be a tape or stress control wire type as shown in photo 5. The tape is overlapped to provide a complete envelope, while the wire type is spiral wound around the cable. The final layer is a jacket that is extruded on to provide mechanical protection when pulling into a conduit or from other physical damage.

The minimum bending radius for an unshielded (no metallic shield) cable is 8 times the overall diameter of the cable assembly. For a shielded cable, this minimum radius is 12 times. If the cable is bent too tight as shown in photos 8 and 9, the insulation system is damaged and premature cable failure will occur. In photo 8, the diameter of the cable is approximately 1 inch. With the full circle bend, the space needed would be 2 times the minimum bending radius or 24 times the cable diameter. This equates to 24 inches. Since the conduit enters in the center, this cabinet would have to be at least 48 inches wide. In fact, this cabinet is just under 36 inches wide, so either the conduit should have entered toward a side or the cable should not have been routed with a full circle. Photo 9 is a 21 kV transformer installation recently completed at a residence. The cable diameter is approximately 1-1/2 inches. The left cable bend going to the load break elbow is 1.1 times this diameter where the minimum is 12 times. The installer commented he knew nothing in the Code, manufacturer’s instructions or UL that specifies bending radius so there could not be a problem. The manufacturer was sent the picture for comment and the reply came back with the installation instructions that are published with a bending radius of 12 times. The manufacturer predicts that this cable will fail at that bending point within a very few years.

Section 300-35 specifies that installations shall be made to prevent inductive heating. This means that all cables of a circuit need to be routed together where going through metallic conduit or penetrating enclosures. For a circuit with parallel conductors, each conduit needs to have one conductor of each phase, a neutral if applicable, and equipment grounding conductor as applicable. Photo 10 is the result of an installation of three 1000 kCmil 15 kV conductors run in parallel that has all the ‘A’ phase conductors in one conduit, all the ‘B’ phase in the second and all the ‘C’ phase in the third. No neutral was installed and an equipment grounding conductor was installed in each conduit. The raceway was non-metallic rigid conduit except for one sweep ell in each run that was rigid steel conduit. The cable fault occurred a couple of days after being energized at light load due to inductive heating. The fault path was from one phase to the conduit ell through the concrete encasement to the next phase in another conduit.

Section 300-37 provides the acceptable above-ground wiring methods for over 600 volt installations. Wiring methods that have been in the NEC for many years include rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, cable tray, busways and cablebus. New to the 1999 NEC was the addition of electrical metallic tubing. Additional wiring methods that can be used include other identified raceways, open runs of metal-clad cable suitable for the use and purpose, and where access is limited to only qualified persons, open runs of MV cable, bare conductors and bare busbars. It should be noted that the "other raceways" not specifically listed above have to be "identified." Article 100 defines this as "recognizable as suitable for the specific purpose, function, use, environment, application, etc., where described in a particular Code requirement." The fine print note following this definition offers the guidance that the determination of suitability may be achieved by evaluation, listing and labeling by a recognized testing laboratory, inspection agency or other organization concerned with product evaluations.

Cable terminations are covered in Section 300-40. The requirements seem pretty simple where one has to cut back the metallic and semiconductive shielding layers for a sufficient distance depending on the system voltage. After stripping back these shields, a stress reduction means needs to be applied at the termination of the shielding along with the termination of the actual conductor. Lastly, the metallic shield and associated semiconductive components must be grounded. Many years ago, splicing or terminating high voltage cables was a specialty with some art and a lot of skill involved. In photo 11, the bottom 15 kV XLPE conductor has an irreversible crimp lug on the conductor and hand made stress cone made from successive layers of tape including semiconductive tape, a filler tape and finally an overall covering for mechanical protection. Note the long distance from the end of the conductor to the beginning of the stress cone. This is that cut back distance addressed by Section 300-40. Today, most splices and terminations are done with kits and completed by individuals with a wide variance of skill or training in the proper installation of the kit. Most of the termination manufacturers will provide training and some supervision for free or at a nominal cost, but not all installers will take advantage of this service. The top termination in the photo is an example of a stress cone kit on a 15 kV EPR cable. The extra boot or skirt in the top photo is to increase the surface distance between the conductor and the metallic shield where subject to contamination and weather. The metallic braid seen at the top left is the grounding connection to where the metallic shield ended. The majority of cable failures today, either found through high potential testing or as failures in service, are the result of poor installation of the splices or stress cone terminations.

The last part of Section 300-40 addresses the grounding of the metallic shield. The requirement is for this shield to be grounded wherever it is exposed, meaning at all splices and terminations. This provides the highest level of safety, whereas grounding at only one point could have an exposed shield on an energized cable at a higher than ground potential presenting a shock hazard. A problem can arise for long runs of cable where the shield is installed essentially in parallel with the equipment grounding conductor. The shield has a voltage induced on it from the magnetic field generated by the current in the main conductor. The circuit set up by multiple connections between the shield and the equipment grounding conductor will have some circulating current. The magnitude of these currents is directly related to the load current and the length of the circuit. There is controversy and misunderstanding in the industry on the shield grounding requirement. One of the issues comes from Section 310-60(c)(1) which states the ampacities given are for a metallic shield grounded at one point only and that if grounded at more than one point, the ampacity must take into account the heating effect from shield currents. Some inspectors have gone to the point of enforcing this note as a prohibition of the grounding of the shield at more than one location no matter what. Some utilities, which are not under the NEC requirements, have a practice to only ground the shield of any medium voltage circuit in one location, typically at the source. A lot of design engineers, installers and inspectors logically look to the utilities for information due to their extensive systems and years of experience. The utility can be a good reference source, but one must also understand some of the background to their practices and not blindly accept them. Utility practices are predicated on very limited access by their own personnel whom they control and qualify. In non-utility installations, this level of limited access is almost impossible so consideration has to be made for the wider access by lesser-qualified people. Also, the typical utility circuit is many miles long, which is very different from most commercial and industrial installations. One cable manufacturer has indicated that circulating currents on the shield usually do not present a problem until the circuit exceeds 2500 to 3000 feet. Therefore, the 30 foot jumper or the 200 foot feeder circuit is generally not a problem unless the load is very near the cable ampacity.

One last comment on terminations is that the proper cable termination devices must be used. As seen in photo 12, the lug used has smooth surfaces and rounded edges which is different than the 600-volt rated lug with cut edges and square corners. The smooth and rounded design is there to minimize any concentration of corona. Corona can be very damaging to insulation over time and will concentrate where there are any sharp points. These type lugs are listed and marked for use on the applicable voltage system.

The termination also has to serve another purpose and that is sealing the conductor, insulation and shields from the entry of moisture where that exposure is possible. Section 300-42 makes this requirement and sets the conditions where it is necessary. The entry of any moisture will create another situation where the insulation will deteriorate due to the increased voltage stress in that area. Again, looking at photos 11 and 12 one can see that the taping or the heat shrink around sealing compound effectively seals the cable. Also note in photo 12 that the barrel of the lug where the cable is crimped does not have any openings like one would see on a comparable 600-volt lug.

Underground installations are covered by Section 300-50. Table 300-50 provides minimum cover depths for three types of wiring methods and three voltage ranges. The wiring methods are similar to under 600 volts systems, except where rigid nonmetallic PVC conduit is used for the exit of the cable out of the ground it must be schedule 80 from the burial depth up to a minimum of 8 feet above finish grade. Taps or splices are permitted in direct buried cables without a splice box, where the materials used are suitable for the application and the taps or splices are watertight and protected from physical damage. As with under 600 volt systems, raceways entering buildings or equipment from underground need to have a raceway seal with an identified compound or approved sealing device.

The requirements for conductors rated 2001 to 35,000 volts can be found in NEC Section 310-60. This section provides some additional definitions for "ducts" and for "thermal resistivity" as well as the methods and requirements to determine ampacity. Most electrical people are familiar with Tables 310-13 and 310-16 to 310-19 that deal with low voltage, 0 to 2000 volt rated conductors. There is a companion set of tables in Article 310 for conductors 2001 to 35,000 volts. Tables 310-61, 310-62, 310-63 and 310-64 provide information on insulation characteristics and thickness. The notes to Table 310-64 provide more information on the 100 percent and 133 percent insulation rating that was discussed above under cable construction. Tables 310-67 to 310-86 provide ampacities for individual conductors and cable assemblies of more than one conductor in various wiring methods. It is important to note the specific condition of wiring method, ambient temperature, conductor arrangement and other factors that define these table ampacities. Where these conditions are not met, then correction factors must be applied that can increase or reduce the ampacity. One last note about ampacity is that where an installation transitions from one set of conditions to another, such as from underground to aboveground outside and then aboveground inside, the ampacity has to be determined for each condition. The final ampacity is the condition that produces the smallest ampacity for the system, unless other actions are taken to change the most restrictive condition. Section 318-13 specifies different ampacities when multiconductor cables such as MC or single MV cables are installed in cable trays. The ampacity is based on the type of conductor or assembly, spacing, and cable tray configuration.

Article 326 provides details on MV cables. These would typically be single or multiconductor solid dielectric type assemblies. Most of this article directs the reader to other sections of the Code, such as the ampacity tables. It should be noted the type MV cable is not suitable for exposure to sunlight unless identified for that use with a marking like "sunlight resistant" or "Sun Res." Also, type MV cables being installed in cable tray have to be marked with a "TC" to indicate suitability for installation in cable trays. These requirements are in Section 326-4.

Part B of Article 364 provides requirements for busways rated at over 600 volts. There is an identification requirement in Section 364-21 that has several ratings in addition to the typical nameplate information for equipment. These ratings need to be understood by the specifier, installer and inspector to ensure the proper busway is being applied for the system conditions. A reference to ANSI/IEEE C37.23-1987 (R1991) is made where one can research these markings and how they are established. Section 364-23 requires that adjacent metal structures are not to have an induced current with sufficient temperature rise to cause a fire or personnel hazard. Other requirements for sizing the neutral, providing seals and barriers, drainage where exposed to wet environments, installation of ventilated busways, terminations and connections and switches are also covered in this article.

Article 370, Part D addresses manholes used as enclosures for conductors and equipment. These provisions apply to both under and over 600-volt installations. Some key items include providing sufficient size for a safe workspace around any equipment installed and cabling. Sections 370-52, 370-53 and 370-54 specifically address cable installations with regard to cable location, bending space and equipment access. Many times, over 600 volt installations will have cabling spliced in the manhole or have the cable make a complete circle on the sides to provide for movement in the duct or to relieve stress on a splice point. The bending radius issues raised before apply here. Again, as a safety note, extreme caution needs to be exercised when entering a manhole with energized high voltage conductors. Contact with the energized conductor with any part of the body should be avoided.

Flexible cables for use over 600 volts are covered in Part C of Article 400. The application for these cables is to serve mobile equipment and machinery. Typical applications are for equipment used in tunnels as discussed in Article 110 Part D, but also mobile cranes and other equipment. The conductors must be a minimum No. 8 AWG stranded for all conductors with the exception of the ground check conductor that is permitted to be No. 10 AWG per Section 400-31(a) and the exception. Shielding of the conductors and grounding of the shields, equipment grounding conductor size and grounding are the same as previously discussed. The minimum bending radius for these cable assemblies is not restricted as with the conductors used for permanent installations. The minimum bending radius is permitted to go smaller up to the point that there is not damage to the cable. Connectors in these cables have to be the locking type per Section 400-35 and have to be arranged so they cannot be opened while energized. No splices are allowed outside of enclosures except where it is the molded vulcanized type allowed by Section 110-14(b). Terminations are to be accessible only to qualified personnel.

In many heavy industrial plants, the application of "high voltage" motors has been common for many years. Originally 2300 volts was a common application voltage, but today motors operating at 13.8 kV are not uncommon. Over 600 volt motors can now be found in other light industrial and commercial installations, for example large chiller units for air conditioning. These chillers very typically are operated at 4.16 kV. Part K of Article 430 addresses motors rated at over 600 volts. As with many parts before, Section 430-121 recognizes the increased hazard with the higher voltage motors and has the requirements in this part to supplement or modify the requirements that apply to under 600 volt motors. Section 430-122 requires an additional marking on the controller to indicate the control voltage. While the power is at the high voltage level, the controls are typically at 24 and 120 volts. Flexible metal raceway not exceeding 6 feet in length is permitted to connect the motor terminal box to the supply circuit raceway or enclosure per Section 430-123. Section 430-125 provides for the short-circuit, groundfault and overload protection of the motor. Be aware that conductor protection in accordance with Article 240 may be different than what is required to protect the motor. Whichever requirement is the most restrictive must be applied. Section 430-126 establishes the maximum overload trip setting at 115 percent of the motor nameplate continuous current rating. This is different for under 600 volt motors where temperature and service factor ratings may allow different multipliers. The last item is Section 430-127 that requires the motor disconnecting means to be capable of being locked in the open position.

Article 450 deals with the installation and protection of transformers. Right in the beginning, is the overcurrent Section 450-3(a) and Table 450-3(a) that addresses protection of transformers rated over 600 volts. The key difference with the over 600 volt transformers is that the transformer impedance and the level of supervision of the installation is used to determine the rating or setting for fuses or circuit breakers. The dividing line for this impedance is 6 percent. Again, like with conductor protection, the 300 percent and 600 percent values appear for ratings or settings. Under this table, there are several notes that also have to be taken into account. For example, note 4 provides that electronically actuated fuses are permitted to be set like circuit breaker protection, up to 600 percent and note 5 permits coordinated thermal protection by the manufacturer to substitute for secondary overcurrent protection. The last area in this article to be aware of is Part C for vaults. While this part is from the transformer article, previous sections for oil-filled fuse cutouts, oil circuit breakers and similar oil-filled equipment send the reader to the applicable sections in Part C of this article for requirements such as fire resistive construction, curbs and other vault construction design considerations.

Article 460 addresses the installation of capacitors. It is common to see the use of capacitors on medium voltage distribution systems or for specific inductive loads for power factor correction. Section 460-24 in Part B requires that capacitors have switches that are group operated and must be capable of carrying 135 percent of the capacitor rated current. In addition, the switch must be able to interrupt the maximum continuous load current of that part being switched, withstanding the maximum inrush current and carry the currents due to faults in the capacitor or capacitor bank. In addition, the switch must also provide an isolating means with visible contacts in the open position. If the isolating switch does not have a load interrupting rating, then another device such as a circuit breaker with the proper ratings could be used and the isolation switch is there to provide the visible open indication. In this arrangement, there has to be interlocking or warning signs posted to ensure the correct switching sequence. Section 460-25 gives the overcurrent protection requirements. A means to detect and interrupt an overcurrent condition must be provided. This can be done with single phase or multiphase units and the protection may be for a single capacitor or a bank of capacitors. A means to discharge the energy when power is removed must be provided. The discharge level of 50 volts is the same as for under 600 volt systems, but the time allowance is longer at five minutes for over 600 volt systems, where it is one minute for under 600 volts.

Resistors and reactors are covered by Part B of Article 470. Section 470-18 requires resistors and reactors to be protected from physical damage. This can be accomplished by enclosures or by elevating the device to protect personnel from contacting live parts. Combustible materials need to be kept clear, and proper clearances maintained to grounded surfaces based on the voltage level being applied. Section 470-20 requires oil-filled reactors to be installed in accordance with the applicable requirements for vaults found in Part C of Article 450.

The last article for discussion is Article 490 – "Equipment, Over 600 volts, Nominal." For those individuals using Codes before 1999, these requirements are found in Article 710 since over 600 volt systems were considered as "special systems." With the very wide spread of systems over 600 volts, they are no longer really "special" or limited in use and, therefore, the requirements were relocated to chapter 4. Article 490 has five parts which breakdown over 600-volt equipment based on usage. In Part A, the term "high voltage" is defined as meaning over 600 volts nominal. Section 490-3 addresses oil-filled equipment in general. The requirement for all oil-filled equipment containing 10 gallons of flammable oil or more, except transformers already covered by Article 450, is that the equipment must comply with Parts B and C of Article 450. This requirement is redundant with several other articles on specific equipment that refers the user to Part C of Article 450. This general requirement is still needed for something that may not be covered by a specific equipment section.

Part B of this article provides specific requirements for equipment starting with circuit breakers and power fuses and fuseholders. Many of the requirements have already been discussed. One new requirement is for specific warning signs. Medium voltage equipment is many times installed in loop systems. What this means is that circuit breakers or switches could have an energy source to either the top or the bottom of the device, or both in the case where it is used as a tie. Never assume the top is the source and the bottom is the load for any equipment operating over 600 volts. The warning sign should state "Warning – Switch May Be Energized by Backfeed." Prove it is de-energized before coming into contact. Medium voltage switches are rated as load interrupting, meaning the switch can be opened or closed up to the nameplate rated load current, or they are non-load interrupting. Non-load interrupting switches need to have another load interrupting device in the circuit and have signs, interlocks or other provisions to prevent the non-rated switch from opening under load.

Section 490-24 and Table 450-24 provide the requirements for minimum space between live parts and ground or other live parts. It should be noted this section table applies only to field wiring and not to the construction of the equipment. This section and table should not be applied to the equipment as manufactured when inspecting in the field. The equipment is evaluated and qualified through testing using both a high potential (hipot) test as some value above rated voltage and also by an impulse test to determine if adequate spacing is provided. The test is called a Basic Impulse Level (BIL) and is similar to a controlled lightning strike. Unfortunately there are no nice rules of thumb or tables to find the proper spacing. On the other hand, the field terminations and other field wiring with exposed live parts can be evaluated using Table 490-24. As can be seen when using the table, the rated nominal voltage and the BIL rating need to be known. Generally, this is on the nameplate of the equipment. There are different values for indoor and outdoor equipment primarily due to the levels of contamination and presence of water. Clearly, outdoor exposed equipment will gather dust and other contaminates and when mixed with rainwater can start tracking. Remember the stress cone in photo 11 with the extra skirt? That skirt adds to the distance along the surface and the underneath portion is not subject to the contamination and water from rain.

An example of a problem created in the field is shown in photo 13. The fuse is very close to the metal enclosure door when it is closed. In fact, the clearance is less than 1/4 inch and the voltage level is 7.2 kV to ground or a nominal voltage rating of 12.47 kV phase to phase. Since this potential transformer was field installed, using Table 490-24 the minimum clearance for indoor (within the enclosure) is 5 inches and if this was outdoors it would be 7 inches. This installation did fail the hipot test at 37.5 kV AC. The BIL test at 110 kV, to match the existing rating, was not performed due to the hipot test failure. The installer originally did not see that there was any problem, mainly because he had not considered the installation with the door closed. If he had skipped any third party evaluation, as required by the local jurisdiction, this equipment would have suffered a catastrophic failure when energized and possible personnel injury for anyone standing close by.

Sections 490-32 and 490-33 deal with guarding of the high voltage and low voltage parts within enclosures. For high voltage parts, access is limited to qualified persons only and there are requirements for additional barriers. Low voltage exposed terminals on doors require an additional barrier over them and the door must have a positive latch mechanism to hold the door open per Section 490-38. This latch is to prevent the door from being moved or blown into a worker, where he could be shocked by the exposed live parts. Section 490-37 requires positive grounding of all instruments, metering, relaying or other metallic device cases. Sometimes, mounting on the enclosure is sufficient, while in some cases a bonding conductor to a solidly grounded metal frame member is necessary.

Part D of Article 490 deals with mobile and portable equipment such as mobile substations, switch houses, mobile shovels, cranes, pumps and underground excavators. These requirements are very similar to previous sections for motors or other specific equipment. Section 490-51(b) is clear that these requirements are in addition to other prescribed sections in Articles 100 to 725. Special attention is called to the provisions in Article 250. The requirements include physical protection, disconnecting means and overcurrent protection.

Article 490 Part E is for high voltage electrode boilers. This is another area where higher voltages are found more commonly. Electrode boilers operating at 13.8 kV are known to be installed at several food processing plants as well as other facilities needing large volumes of steam or hot water. Section 490-71 indicates that electrode boilers operating above 600 volts must be supplied from a 3-phase 4-wire solidly grounded system. Controls are not to exceed 150 volts and must also be supplied from a grounded system. Branch circuit requirements include the circuit being rated at 100 percent of the total load, having an overcurrent device with a common trip, have phase-fault protection and ground-current detection. For the ground-fault system, Section 490-72(d) provides for a trip setting of 5 amps or 7-1/2 percent of the boiler full load current for 10 seconds or an instantaneous trip when the ground fault current reaches 25 percent of the boiler full load rating. In addition to the electrical protection requirements, electrode type boilers must have pressure and temperature controls that will interrupt the circuit to the boiler when the temperature or pressure rating is exceeded.

As can be seen from this article, there are a lot of requirements and provisions in the National Electrical Code for over 600 volt applications. It is very easy for one to think there are only some requirements here or there, somewhat by the way the requirements are structured. In all the Code, only Article 490 is dedicated to systems and equipment operating at over 600 volts. The article presented here does not cover all the sections that may apply to any installation, but should give the reader a feel for the depth and breadth that is included. In addition to the National Electrical Code, one has to have access to and use several other resources and standards to ensure they are designing, installing or inspecting over 600 volt systems correctly.

Chuck Mello is the Manager Conformity Assessment for Electro-Test, Inc. He is a principle member of NEC panel 5 representing the International Electrical Testing Association (NETA) and has been involved with Codes and Standards development for over 20 years.

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