Medium-Voltage Distribution System Design

A. Single Bus, Figure 1.1-40

The sources (utility and/or generator(s)) are connected to a single bus. All feeders are connected to the same bus. 

This configuration is the simplest system; however, outage of the utility results in total outage. 

Normally the generator does not have adequate capacity for the entire load. A properly relayed system equipped with load shedding, automatic voltage/ frequency control may be able to maintain partial system operation. 

Any future addition of breaker sections to the bus will require a shutdown of the bus, because there is no tie breaker.

Figure 1.1-40. Single Bus

B. Single Bus with Two Sources from the Utility, Figure 1.1-41

Same as the single bus, except that two utility sources are available. This system is operated normally with the main breaker to one source open. Upon loss of the normal service, the transfer to the standby normally open (NO) breaker can be automatic or manual. Automatic transfer is preferred for rapid service restoration especially in unattended stations. 

Retransfer to the “Normal” can be closed transition subject to the approval of the utility. Closed transition momentarily (5–10 cycles) parallels both utility sources. Caution: when both sources are paralleled, the fault current available on the load side of the main device is the sum of the available fault current from each source plus the motor fault contribution. It is recommended that the short-circuit ratings of the bus, feeder breakers and all load side equipment are rated for the increased available fault current. 

If the utility requires open transfer, the disconnection of motors from the bus must be ensured by means of suitable time delay on reclosing as well as supervision of the bus voltage and its phase with respect to the incoming source voltage. 

This busing scheme does not preclude the use of cogeneration, but requires the use of sophisticated automatic synchronizing and synchronism checking controls, in addition to the previously mentioned load shedding, automatic frequency and voltage controls. 

This configuration is more expensive than the scheme shown in Figure 1.1-40, but service restoration is quicker. Again, a utility outage results in total outage to the load until transfer occurs. Extension of the bus or adding breakers requires a shutdown of the bus.

If paralleling sources, reverse current, reverse power and other appropriate relaying protection should be added as requested by the utility.

Figure 1.1-41. Single Bus with Two-Sources

C. Multiple Sources with Tie Breaker, Figure 1.1-42 and Figure 1.1-43

This configuration is similar to the configuration shown in Figure 1.1-41. It differs significantly in that both utility sources normally carry the loads and also by the incorporation of a normally open tie breaker. The outage to the system load for a utility outage is limited to half of the system. Again, the closing of the tie breaker can be manual or automatic. The statements made for the retransfer of the configuration shown in Figure 1.1-41 apply to this scheme also.

Figure 1.1-42. Two-Source Utility with Tie Breaker

If looped or primary selective distribution system for the loads is used, the buses can be extended without a shutdown by closing the tie breaker and transferring the loads to the other bus. 

This configuration is more expensive than the configuration shown in Figure 1.1-41. The system is not limited to two buses only. Another advantage is that the design may incorporate momentary paralleling of buses on retransfer after the failed line has been restored to prevent another outage. See the Caution for Figure 1.1-41, Figure 1.1-42 and Figure 1.1-43.

In Figure 1.1-43, closing of the tie breaker following the opening of a main breaker can be manual or automatic. However, because a bus can be fed through two tie breakers, the control scheme should be designed to make the selection. 

The third tie breaker allows any bus to be fed from any utility source.

Caution for Figure 1.1-41, Figure 1.1-42 and Figure 1.1-43:

If continuous paralleling of sources is planned, reverse current, reverse power and other appropriate relaying protection should be added. When both sources are paralleled for any amount of time, the fault current available on the load side of the main device is the sum of the available fault current from each source plus the motor fault contribution. It is required that bus bracing, feeder breakers and all load side equipment is rated for the increased available fault current.

Figure 1.1-43. Triple-Ended Arrangement

Summary

The medium-voltage system configurations shown are based on using metal-clad drawout switchgear. The service continuity required from electrical systems makes the use of single-source systems impractical. In the design of a modern mediumvoltage system, the engineer should:

1. Design a system as simple as possible.

2. Limit an outage to as small a portion of the system as possible.

3. Provide means for expanding the system without major shutdowns.

4. Design a protective relaying system so that only the faulted part is removed from service, and damage to it is minimized consistent with selectivity.

5. Specify and apply all equipment within its published ratings and national standards pertaining to the equipment and its installation.

Simple Spot Network Systems

The ac secondary network system is the system that has been used for many years to distribute electric power in the high density, downtown areas of cities, usually in the form of utility grids. Modifications of this type of system make it applicable to serve loads within buildings. 

The major advantage of the secondary network system is continuity of service. No single fault anywhere on the primary system will interrupt service to any of the system’s loads. Most faults will be cleared without interrupting service to any load. Another outstanding advantage that the network system offers is its flexibility to meet changing and growing load conditions at minimum cost and minimum interruption in service to other loads on the network. In addition to flexibility and service reliability, the secondary network system provides exceptionally uniform and good voltage regulation, and its high efficiency materially reduces the costs of system losses. 

Three major differences between the network system and the simple radial system account for the outstanding advantages of the network. First, a network protector is connected in the secondary leads of each network transformer in place of, or in addition to, the secondary main breaker, as shown in Figure 1.1-39. Also, the secondaries of each transformer in a given location (spot) are connected together by a switchgear or ring bus from which the loads are served over short radial feeder circuits. Finally, the primary supply has sufficient capacity to carry the entire building load without overloading when any one primary feeder is out of service. 

A network protector is a specially designed heavy-duty air power breaker, spring close with electrical motor-charged mechanism, with a network relay to control the status of the protector (tripped or closed). 

The network relay is usually a solid-state microprocessor-based component integrated into the protector enclosure that functions to automatically close the protector only when the voltage conditions are such that its associated transformer will supply power to the secondary network loads. It also serves to automatically open the protector when power flows from the secondary to the network transformer. 

The purpose of the network protector is to protect the integrity of the network bus voltage and the loads served from it against transformer and primary feeder faults by quickly disconnecting the defective feeder-transformer pair from the network when backfeed occurs. 

The simple spot network system resembles the secondary-selective radial system in that each load area is supplied over two or more primary feeders through two or more transformers. In network systems, the transformers are connected through network protectors to a common bus, as shown in Figure 1.1-39, from which loads are served. Because the transformers are connected in parallel, a primary feeder or transformer fault does not cause any service interruption to the loads. 

The paralleled transformers supplying each load bus will normally carry equal load currents, whereas equal loading of the two separate transformers supplying a substation in the secondary-selective radial system is difficult to obtain. The interrupting duty imposed on the outgoing feeder breakers in the network will be greater with the spot network system. 

The optimum size and number of primary feeders can be used in the spot network system because the loss of any primary feeder and its associated transformers does not result in the loss of any load even for an instant. In spite of the spare capacity usually supplied in network systems, savings in primary switchgear and secondary switchgear costs often result when compared to a radial system design with similar spare capacity. 

This occurs in many radial systems because more and smaller feeders are often used in order to minimize the extent of any outage when a primary fault event occurs. 

In spot networks, when a fault occurs on a primary feeder or in a transformer, the fault is isolated from the system through the automatic tripping of the primary feeder circuit breaker and all of the network protectors associated with that feeder circuit. This operation does not interrupt service to any loads. After the necessary repairs have been made, the system can be restored to normal operating conditions by closing the primary feeder breaker. All network protectors associated with that feeder will close automatically. 

The chief purpose of the network bus normally closed ties is to provide for the sharing of loads and a balancing of load currents for each primary service and transformer regardless of the condition of the primary services. 

Also, the ties provide a means for isolating and sectionalizing ground fault events within the switchgear network bus, thereby saving a portion of the loads from service interruptions, yet isolating the faulted portion for corrective action. 

The use of spot network systems provides users with several important advantages. First, they save transformer capacity. Spot networks permit equal loading of transformers under all conditions. Also, networks yield lower system losses and greatly improve voltage conditions.

Figure 1.1-39. Three-Source Spot Network

The voltage regulation on a network system is such that both lights and power can be fed from the same load bus. Much larger motors can be started across-the-line than on a simple radial system. This can result in simplified motor control and permits the use of relatively large low voltage motors with their less expensive control. 

Finally, network systems provide a greater degree of flexibility in adding future loads; they can be connected to the closest spot network bus. 

Spot network systems are economical for buildings that have heavy concentrations of loads covering small areas, with considerable distance between areas, and light loads within the distances separating the concentrated loads. They are commonly used in hospitals, high rise office buildings, institutional buildings or laboratories where a high degree of service reliability is required from the utility sources. Spot network systems are especially economical where three or more primary feeders are available. 

Principally, this is due to supplying each load bus through three or more transformers and the reduction in spare cable and transformer capacity required. 

They are also economical when compared to two transformer doubleended substations with normally opened tie breakers. 

Emergency power should be connected to network loads downstream from the network, or upstream at primary voltage, not at the network bus itself.

Sparing Transformer System

The sparing transformer system concept came into use as an alternative to the capital cost intensive double ended secondary unit substation distribution system (see Two-Source Primary— Secondary Selective System). It essentially replaces double-ended substations with single-ended substations and one or more “sparing” transformer substations all interconnected on a common secondary bus—see Figure 1.1-38. 

Generally no more than three to five single-ended substations are on a sparing loop. 

The essence of this design philosophy is that conservatively designed and loaded transformers are highly reliable electrical devices and rarely fail. Therefore, this design provides a single common backup transformer for a group of transformers in lieu of a backup transformer for each and every transformer. This system design still maintains a high degree of continuity of service. 

Referring to Figure 1.1-38, it is apparent that the sparing concept backs up primary switch and primary cable failure as well. Restoration of lost or failed utility power is accomplished similarly to primary selective scheme previously discussed. It is therefore important to use an automatic throw-over system in a two source lineup of primary switchgear to restore utility power as discussed in the Two-Source “Primary” scheme—see Figure 1.1-37. 

A major advantage of the sparing transformer system is the typically lower total base kVA of transformation. In a double-ended substation design, each transformer must be rated to carry the sum of the loads of two buses and usually requires the addition of cooling fans to accomplish this rating. In the “sparing” concept, each transformer carries only its own load, which is typically not a fan-cooled rating. In addition to first cost savings, there is a side benefit of reduced equipment space.

The sparing transformer system operates as follows:

■■ All main breakers, including the sparing main breaker, are normally closed; the tie breakers are normally open

■■ Once a transformer (or primary cable or primary switch/fuse) fails, the associated secondary main breaker is opened. The associated tie breaker is then closed, which restores power to the single-ended substation bus

■■ Schemes that require the main to be opened before the tie is closed (“open transition”), and that allow any tie to be closed before the substation main is opened, (“closed transition”) are possible .

With a closed transition scheme, it is common to add a timer function that opens the tie breaker unless either main breaker is opened within a time interval. 

This closed transition allows power to be transferred to the sparing transformer without interruption, such as for routine maintenance, and then back to the substation. This closed transition transfer has an advantage in some facilities; however, appropriate interrupting capacities and bus bracing must be specified suitable for the momentary parallel operation. 

In facilities without qualified electrical power operators, an open transition with key interlocking is often a prudent design.

Note: Each pair of “main breaker/tie breaker” key cylinders should be uniquely keyed to prevent any paralleled source operations. 

Careful sizing of these transformers as well as careful specification of the transformers is required for reliability. Low temperature rise specified with continuous overload capacity or upgraded types of transformers should be considered. 

One disadvantage to this system is the external secondary tie system, see Figure 1.1-38. As shown, all single-ended substations are tied together on the secondary with a tie busway or cable system. Location of substations is therefore limited because of voltage drop and cost considerations. 

Routing of busway, if used, must be carefully layed out. It should also be noted, that a tie busway or cable fault will essentially prevent the use of the sparing transformer until it repaired. Commonly, the single-ended substations and the sparing transformer must be clustered. This can also be an advantage, as more kVA can be supported from a more compact space layout.

Figure 1.1-38. Sparing Transformer System 

Two-Source Primary— Secondary Selective System

This system uses the same principle of duplicate sources from the power supply point using two primary main breakers and a primary tie breaker. The two primary main breakers and primary tie breaker being either manually or electrically interlocked to prevent closing all three at the same time and paralleling the sources. Upon loss of voltage on one source, a manual or automatic transfer to the alternate source line may be used to restore power to all primary loads. 

Each transformer secondary is arranged in a typical double ended unit substation arrangement as shown in Figure 1.1-37. The two secondary main breakers and secondary tie breaker of each unit substation are again either mechanically or electrically interlocked to prevent parallel operation. Upon loss of secondary source voltage on one side, manual or automatic transfer may be used to transfer the loads to the other side, thus restoring power to all secondary loads. 

This arrangement permits quick restoration of service to all loads when a primary feeder or transformer fault occurs by opening the associated secondary main and closing the secondary tie breaker. If the loss of secondary voltage has occurred because of a primary feeder fault with the associated primary feeder breaker opening, then all secondary loads normally served by the faulted feeder would have to be transferred to the opposite primary feeder. 

This means each primary feeder conductor must be sized to carry the load on both sides of all the secondary buses it is serving under secondary emergency transfer If the loss of voltage was due to a failure of one of the transformers in the double-ended unit substation, then the associated primary fuses would open taking only the failed transformer out of service, and then only the secondary loads normally served by the faulted transformer would have to be transferred to the opposite transformer. 

In either of the above emergency conditions, the in-service transformer of a double-ended unit substation would have to have the capability of serving the loads on both sides of the tie breaker. For this reason, transformers used in this application must have equal kVA ratings on each side of the double-ended unit substation. The transformers are sized so the normal operating maximum load on each transformer is typically about 2/3 base nameplate kVA rating. 

Typically these transformers are furnished with fan-cooling and/or lower than normal temperature rise such that under emergency conditions they can continuously carry the maximum load on both sides of the secondary tie breaker. Because of this spare transformer capacity, the voltage regulation provided by the double-ended unit substation system under normal conditions is better than that of the systems previously discussed. 

The double-ended unit substation arrangement can be used in conjunction with any of the previous systems discussed, which involve two primary sources. Although not recommended, if allowed by the utility, momentary re-transfer of loads to the restored source may be made closed transition (anti-parallel interlock schemes would have to be defeated) for either the primary or secondary systems. 

Under this condition, all equipment interrupting and momentary ratings should be suitable for the fault current available from both sources. 

For double-ended unit substations equipped with ground fault systems special consideration to transformer neutral grounding and equipment operation should be made—see Grounding/Ground Fault Protection section of this Design Guide. Where two single-ended unit substations are connected together by busway or external tie conductors, it is recommended that a tie breaker be furnished at each end of the tie conductors. The second tie breaker provides a means to isolate the interconnection between the two single-ended substations for maintenance or servicing purposes.

Figure 1.1-37. Two-Source Primary—Secondary Selective System

Voltage & Frequency Variation

Variation from Rated Voltage:

In accordance with NEMA MG 1, 12.44, motors shall operate successfully under running conditions at rated load with variation in the voltage up to the following percentages of rated voltage:

1. Universal motors except fan motors - plus or minus 6 percent (with rated frequency).
2. Induction motors - plus or minus 10 percent (with rated frequency).

Performance within these voltage variations will not necessarily be in accordance with the standards established for operation at rated voltage.

Variation from Rated Frequency:

Alternating - current motors shall operate successfully under running conditions at rated load and at rated voltage with a variation in the frequency up to 5 percent above or below the rated frequency. Performance within this frequency variation will not necessarily be in accordance with the standards established for operation at rated frequency.

Combined Variation of Voltage and Frequency:

Alternating - current motors shall operate successfully under running conditions at rated load with a combined variation in the voltage and frequency up to 10 percent above or below the rated voltage and the rated frequency, provided that the frequency variation does not exceed 5 percent. Performance within this combined variation will not necessarily be in accordance with the standards established for operation at rated voltage and rated frequency.

Effects of Variation of Voltage and Frequency Upon the Performance of Induction Motors:

1. Induction motors are at times operated on circuits of voltage or frequency other than those for which the motors are rated. Under such conditions, the performance of the motor will vary from the rating. The following is a brief statement of some operating results caused by small variations of voltage and frequency and is indicative of the general changes produced by such variations in operating conditions.
2. With a 10 percent increase or decrease in voltage from that given on the nameplate, the heating at rated horsepower load may increase. Such operation for extended periods of time may accelerate the deterioration of the insulation system.
3. In a motor of normal characteristics at full rated horsepower load, a 10 percent increase of voltage above that given on the nameplate would usually result in a decided lowering in power factor. A 10 percent decrease of voltage below that given on the nameplate would usually give an increase in power factor.
4. The locked-rotor and breakdown torque will be proportional to the square of the voltage applied.
5. An increase of 10 percent in voltage will result in a decrease of slip of about 17 percent, while a reduction of 10 percent will increase the slip about 21 percent. Thus, if the slip at rated voltage were 5 percent, it would be increased to 6.05 percent if the voltage were reduced 10 percent.
6. A frequency higher than the rated frequency usually improves the power factor but decreases locked-rotor torque and increases the speed and friction and windage loss. At a frequency lower than the rated frequency, the speed is decreased, locked-rotor torque is increased, and power factor is decreased. For certain kinds of motor load, such as in textile mills, close frequency regulation is essential.
7. If variation in both voltage and frequency occur simultaneously, the effect will be superimposed. Thus, if the voltage is high and the frequency low, the locked-rotor torque will be greatly increased, but the power factor will be decreased and the temperature rise increased with normal load.
8. The foregoing facts apply particularly to general-purpose motors. They may not always be true in connection with special-purpose motors, built for a particular purpose, or as applied to very small motors.

Operation of General-Purpose Alternating-Current Polyphase 2, 4, and 8 Pole, 60 Hertz Integral-Horsepower Induction Motors Operated on 50 Hertz:

While general-purpose alternating-current polyphase 2, 4, 6 and 8 pole, 60 Hertz integral-horsepower induction motors are not designed to operate at their 60 Hertz ratings on 50 Hertz circuits, they are capable of being operated satisfactorily on 50 Hertz circuits if their voltage and horsepower ratings are appropriately reduced. When such 60 Hertz motors are operated on 50 Hertz circuits, the applied voltage at 50 Hertz should be reduced to 5/6 of the 60 Hertz horsepower rating of the motor.

When a 60 Hertz motor is operated on 50 Hertz at 5/6 of the 60 Hertz voltage and horsepower ratings, the other performance characteristics for 50 Hertz operation are as follows:

1. Speed
   The synchronous speed will be 5/6 of the 60 Hertz synchronous speed and the slip will be 6/5 of the 60-Hertz slip.
2. Torque
   The rated load torque in pound-feet will be approximately the same as the 60 Hertz rated load torque in pound-feet. The locked-rotor and breakdown torques in pound-feet of 50 Hertz motors will be approximately the same as the 60 Hertz locked-rotor and breakdown torques in pound-feet.
3. Locked-Rotor Current
   The locked-rotor current (ampere) will be approximately 5 percent less than the 60 Hertz locked-rotor current (amperes). The code letter appearing on the motor nameplate to indicate locked-rotor KVA per horsepower applies only to the 60 Hertz rating of the motor.
4. Service Factor
   The service factor will be 1.0.
5. Temperature Rise
   The temperature rise should not exceed 90 °C.

Effects of Voltages over 600 Volts on the Performance of Low-Voltage Motors:

Polyphase motors are regularly built for voltage ratings of 575 volts or less and are expected to operate satisfactorily with a voltage variation of plus or minus 10 percent. This means that motors of this insulation level may be successfully applied up to an operating voltage of 635 volts.

Based on motor manufacturers high-potential tests and performance in the field, it has been found that where service voltages exceed 635 volts, the safety factor of the insulation has been reduced to a level inconsistent with good engineering procedures.

In view of the foregoing, motors of this insulation level should not be applied to power systems either with or without grounded neutral where the voltage exceeds 630 volts, regardless of the motor connection employed.

Unbalanced Voltage On Polyphase Induction Motors

Three phase induction motors are designed and manufactured such that all three phases of the winding are carefully balanced with respect to the number of turns, placement of the winding, and winding resistance. When line voltages applied to a polyphase induction motor are not exactly the same, unbalanced currents will flow in the stator winding, the magnitude depending upon the amount of unbalance. A small amount of voltage unbalance may increase the current an excessive amount. The effect on the motor can be severe and the motor may overheat to the point of burnout.

The voltages should be evenly balanced as closely as can be read on the usually available commercial voltmeter.

Effect on performance - General

The effect of unbalanced voltages on polyphase induction motors is equivalent to the introduction of a "negative sequence voltage" having a rotation opposite to that occurring with balanced voltages. This negative sequence voltage produces in the air gap a flux rotating against the rotation of the rotor, tending to produce high currents. A small negative sequence voltage may produce in the windings currents considerably in excess of those present under balanced voltage conditions.

Unbalance Defined

The voltage unbalance (or negative sequence voltage) in percent may be defined as follows:

Percent		Maximum Voltage Deviation
Voltage	= 100 *	From Average Voltage
Unbalance		Average Voltage

Example:

With voltages of 220, 215 and 210, the average is 215, the maximum deviation from the average is 5, and the percent unbalance = 100 X 5/215 = 2.3 percent.

Temperature rise and load carrying capacity

A relatively small unbalance in voltage will cause a considerable increase in temperature rise. In the phase with the highest current, the percentage increase in temperature rise will be approximately two times the square of the percentage voltage unbalance. The increase in losses and consequently, the increase in average heating of the whole winding will be slightly lower than the winding with the highest current.

To illustrate the severity of this condition, an approximate 3.5 percent voltage unbalance will cause an approximate 25 percent increase in temperature rise.

Torques

The locked-rotor torque and breakdown torque are decreased when the voltage is unbalanced. If the voltage unbalance should be extremely severe, the torque might not be adequate for the application.

Full-load speed

The full-load speed is reduced slightly when the motor operates at unbalanced voltages.

Currents

The locked-rotor current will be unbalanced to the same degree that the voltages are unbalanced but the locked-rotor KVA will increase only slightly.

The currents at normal operating speed with unbalanced voltages will be greatly unbalanced in the order of approximately 6 to 10 times the voltage unbalance. This introduces a complex problem in selecting the proper overload protective devices, particularly since devices selected for one set of unbalanced conditions may be inadequate for a different set of unbalanced voltages. Increasing the size of the overload protective device is not the solution in as much as protection against heating from overload and from single phase operation is lost.

This information is based on NEMA standard MG1-14.35.

Unbalanced Line Conditions And Motor Protection

Unbalanced line voltage can be caused by at least four significant factors:

1. Shunted single-phase load.
2. Unbalanced primary voltage.
3. Use of Delta-Wye or Wye-Delta transformers.
4. Defective transformers.

Effect on the motor performance:

1. Can cause serious reduction in starting torque due to low voltage in one or more phases.
2. Can cause excessive and unbalanced full load current.
3. Can cause nuisance overload tripping.
4. Can cause premature failure of motor windings if proper overload protection is not used and known to be functional.

How to determine source of unbalanced motor current:

In many cases only slight variance in supply voltage may be measured, but considerable variance in motor current may be observed. To determine cause of unbalance rotate line connection to motor.

For example, T2 shows high current when connected normally. If after line-to-motor rotation T2 no longer shows high current, unbalance is in supply line.

Primary Selective System—Secondary Radial System

The primary selective—secondary radial system, as shown in Figure1.1-33, differs from those previously described in that it employs at least two primary feeder circuits in each load area. It is designed so that when one primary circuit is out of service, the remaining feeder or feeders have sufficient capacity to carry the total load. Half of the transformers are normally connected to each of the two feeders. When a fault occurs on one of the primary feeders, only half of the load in the building is dropped. 

Duplex fused switches as shown in Figure 1.1-33 and detailed in Figure 1.1-35 may be utilized for this type of system. Each duplex fused switch consists of two load break three-pole switches each in their own separate structure, connected together by bus bars on the load side. Typically, the load break switch closest to the transformer includes a fuse assembly with fuses.

Mechanical and/or key interlocking is furnished such that both switches cannot be closed at the same time (to prevent parallel operation) and interlocking such that access to either switch or fuse assembly cannot be obtained unless both switches are opened.

Figure 1.1-35. Duplex Fused Switch in Two Structures

One alternate to the duplex switch arrangement, a non-load break selector switch mechanically interlocked with a load break fused switch can be used as shown in Figure 1.1-36. The non-load break selector switch is physically located in the rear of the load break fused switch, thus only requiring one structure and a lower cost and floor space savings over the duplex arrangement. The non-load break switch is mechanically interlocked to prevent its operation unless the load break switch is opened. The main disadvantage of the selector switch is that conductors from both circuits are terminated in the same structure.

Figure 1.1-36. Fused Selector Switch in One Structure

This means limited cable space especially if double lugs are furnished for each line as shown in Figure 1.1-33. The downside is that should a faulted primary conductor have to be changed, both lines would have to be de-energized for safe changing of the faulted conductors. 

A second alternative is utilizing a threeposition selector switch internal to the transformer, allowing only one primary feeder to be connected to the transformer at a time without the need for any interlocking. The selector switch is rated for load-breaking. If overcurrent protection is also required, a vacuum fault interrupter (VFI), also internal to the transformer, may be utilized, reducing floor space. 

In Figure 1.1-33 when a primary feeder fault occurs, the associated feeder breaker opens and the transformers normally supplied from the faulted feeder are out of service. Then manually, each primary switch connected to the faulted line must be opened and then the alternate line primary switch can be closed connecting the transformer to the live feeder, thus restoring service to all loads. Note that each of the primary circuit conductors for Feeder A1 and B1 must be sized to handle the sum of the loads normally connected to both A1 and B1. Similar sizing of Feeders A2 and B2, etc., is required. 

If a fault occurs in one transformer, the associated primary fuses blow and interrupt the service to just the load served by that transformer. Service cannot be restored to the loads normally served by the faulted transformer until the transformer is repaired or replaced. 

Cost of the primary selective—secondary radial system is greater than that of the simple primary radial system of Figure 1.1-27 because of the additional primary main breakers, tie breaker, two-sources, increased number of feeder breakers, the use of primary-duplex or selector switches, and the greater amount of primary feeder cable required. 

The benefits from the reduction in the amount of load lost when a primary feeder is faulted, plus the quick restoration of service to all or most of the loads, may more than offset the greater cost. 

Having two sources allows for either manual or automatic transfer of the two primary main breakers and tie breaker should one of the sources become unavailable.

The primary selective-secondary radial system, however, may be less costly or more costly than a primary loop—secondary radial system of Figure 1.1-29 depending on the physical location of the transformers. It also offers comparable downtime and reliability. The cost of conductors for the types of systems may vary depending on the location of the transformers and loads within the facility. The cost differences of the conductors may offset cost of the primary switching equipment.