new chapter 7 nec requirements and coordination basics

7/25/2019 NEW CHAPTER 7 NEC Requirements and Coordination Basics

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CHAPTER 7

NEC REQUIREMENTS ANDCOORDINATION BASICS

Chapter 7 Page 1 NEC Requirements and Coordination Basics


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NEC REQUIREMENTS AND COORDINATION BASICS

CONDUCTOR PROTECTION

Types of Conductor Circuits

Types of Protection

NEC Requirements

Factors Affecting Protection

Ampacity Tables

Cable Damage

Protection and Coordination Criteria

TRANSFORMER PROTECTION

Nameplate Data

Basic Transformer Protection

Thermal Protection

Fault Protection

Factor Affecting Transformer Protection

NEC Requirements

TIME/CURRENT COORDINATION INTERVALS

Introduction

Electromechanical Relay to Electromechanical Relay

Electromechanical Relay to Electromechanical Relay with an Instantaneous Trip

Numerical Relay to Numerical Relay

Numerical Relay to Numerical Relay with an Instantaneous Trip

Numerical Relay-to-Current Limiting Power Fuse

Numerical Relay-to-Current Limiting Power Fuse

Numerical Relay-to-Expulsion Power Fuse

Numerical Relay with Instantaneous Trip-to-LVPCB

Instantaneous Trip Numerical Relay and Power Fuse Coordination



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Instantaneous Trip Numerical Relay and LVPCB Coordination

EXERCISES

Exercise 1 Instantaneous Relay Trip Settings

Exercise 2: Relay Coordination



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CONDUCTOR PROTECTION

TYPES OF CONDUCTOR CIRCUITS

Power cables (conductors) are divided into two voltage classes: low voltage (600 V

and below) and medium voltage (above 600 V).

Low Voltage: In general, NEC Article 240-3 requires that conductors that are rated600 V or less be protected in accordance with their ampacities; however, there areexceptions to the basic rule, such as motor and motor control circuits and tappedconductors. The NEC should be reviewed for these special cases (exceptions).

Medium Voltage: Protection of medium voltage conductors are further separatedinto two sub-categories: aerial lines and cables in conduit.

Aerial Lines: Although aerial lines usually sustain more faults over their life than

do cables in conduit, aerial lines faults are also usually self-clearing, which meansthat the faults were caused by high winds, lightning strikes, or animals (e.g., birdsand squirrels). Distribution fuses (e.g., K or T links), NEMA Type E power fuses,and relays (medium voltage power circuit breakers) are used to protect aeriallines.

Cables in Conduit: NEC Article 240-100 only requires that medium voltagefeeders have short circuit protection in each ungrounded conductor. The samearticle sets limits on the protective devices maximum ratings that are in excess ofthe conductors full load (continuous) current ratings. In general, the NEC isspecifying relay (breaker) and fuse ratings to protect the medium voltage cablesfrom phase and ground faults and they (the NEC) leave overload protection of thecables to the designers preference (choice).



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TYPES OF PROTECTION

Conductors must be protected against overloads, phase, and ground fault conditionsthat may range from 100 to 250% of the conductors continuous current ratings foroverloads, and anywhere from 500 to 2000% of the conductors continuous current

ratings for faults.

Overloads

The bimetallic thermal elements of an MCCB, the long time functions (LTPU andLTDB) of an LVPCB, the overcurrent time-delay relay (ANSI Device No. 51), and upto approximately 250% of a fuses continuous current rating (Icont) provide overloadprotection for conductors (Figure 7-1).

Phase Faults

The magnetic (instantaneous) element of an MCCB, the short time (STPU andSTDB) and instantaneous (IT) functions of an LVPCB, an instantaneous relay (ANSIDevice No. 50), and the fuse (>250% of Icont) provide phase fault (short circuit)protection for conductors (Figure 7-1).

Ground Faults

Ground fault protection of conductors requires separate devices. To trip a MCCB onground faults requires a special shunt-trip attachment or a separate ground sensing(ANSI Device No. 50G) relay. LVPCBs have separate built-in ground fault functions(GFPU and GFT) as shown in Figure 7-1b. Ground fault relays are available in both

time-delay (ANSI Device No. 51G) and instantaneous (ANSI Device No. 50G)models (Figure 7-1c), which are basically the same models as their phase faultcounterparts, except that the overcurrent relay tap settings are much more sensitive(e.g., 0.5 A versus 5 A). Fuses provide very poor to no protection for ground faults.



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TYPES OF PROTECTION

Figure 7-1. Conductor Protection



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NEC REQUIREMENTS

NEC Articles 110, 240, and 310 have specific requirements that relate to theprotection of conductors.

Under 600 Volt Conductors: Per NEC Article 240-3, low voltage conductors shouldbe protected against overcurrent (overloads and short circuit) in accordance withtheir ampacities as specified in NEC Article 310-15 and Tables 310-16 through 310-19.

NEC Article 240-3(b) permits the use of the next higher-rated standard overcurrentdevice that is rated above the ampacity of the conductors that are being protected,as long as the next higher-rated standard device rating does not exceed 800amperes. NEC Article 240-3(b) is sometimes called the round-up rule forconductor protection. NEC Article 240-6 specifies the standard ampere ratings offuses and inverse time circuit breakers as listed in Table 7-1.

1* 25 60 125 300 601* 20003* 30 70 150 350 700 25006* 35 80 175 400 800 3000

10* 40 90 200 450 1000 400015 45 100 225 500 1200 500020 50 110 250 600 1600 6000

*Fuses only

Table 7-1. NEC Article 240-6 Fuse and Breaker Standard Ampere Ratings

NEC Article 240-3(c) specifies that the conductors ampacity must be greater thanthe protective devices rating if the next standard higher-rated device rating exceeds800 amperes. NEC Article 240-3(c) is sometimes called the round-down rule forconductor protection.



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FACTORS AFFECTING PROTECTION

Over 600 Volt Conductors: NEC Article 240-100 only specifies short circuit

protection for conductors that are rated over 600 volts. If a standard fuse is beingused to protect the conductor, its rating cannot exceed 300% of the conductorscontinuous current ampacity rating. If a breaker with relays or an electronically-actuated fuse is used for protection, their trip settings cannot exceed 600% of theconductors continuous current ampacity rating.

AMPACITY TABLES

NEC Article 310-15 governs the ampacity ratings for both low and medium voltageconductors and cables. In particular, NEC Tables 310-16 (Table 7-2) through 310-19, and their accompanying notes, specify the NEC allowable ampacity ratings for

conductors that are rated 0 to 2000 volts. NEC Tables 310-67 through 310-86, andtheir accompanying notes, specify the NEC allowable ampacity ratings forconductors that are rated 2001 through 35,000 volts.



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AMPACITY TABLES

Size Ampacities of Insulated Copper Conductors, 0-2000 V*

AWG

kcmil

60oC

(140oF)

Types

TW,

UF

75oC

(167oF)

Types

THHW,

THW,

THWN,

XHHW

90oC

(194oF)

Types

THHN,

THHW,

XHHW

14

12

10

8

6

4

2

1

1/0

2/0

3/0

4/0

250

500

750

1000

20

25

30

40

55

70

95

110

125

145

165

195

215

320

400

455

20

25

35

50

65

85

115

130

150

175

200

230

255

380

475

545

25

30

40

55

75

95

130

150

170

195

225

260

290

430

535

615

TempoC Ampacity Correction Factors

21-25

26-30

31-35

36-40

41-4546-50

1.08

1.00

.91

.82

.71.58

1.05

1.00

.94

.88

.82

.75

1.04

1.00

.96

.91

.87

.82

*No more than 3 conductors in a raceway or cable or earth (direct burial), based on an ambient

temperature of 30oC (86

oF).

Source: NEC Table 310-16

Table 7-2. Low Voltage Copper Conductor Ampacities



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CABLE DAMAGE

A cable must be protected from overheating due to excessive fault (short circuit)currents. The fault point may be on a section of the protected cable or on any otherpart of the electric system. During a phase fault, the I2R losses in the phase

conductor elevate first the temperature of the conductor, followed by the insulationmaterials, protective jacket, raceway, and surroundings. Because the fault currentshould be interrupted either instantaneously or in a very short time by the protectivedevice, the amount of heat that is transferred from the metallic conductors outward tothe insulation and to other materials is very small, and, therefore, the heat from I

2R

losses is almost entirely in the conductors. For practical purposes, it is assumed that100 percent of the I

2R losses are consumed to elevate the conductor temperature.

During the period that the fault current is flowing, the conductor temperature shouldnot be permitted to rise to the point where it may damage the insulation.

Operating Temperatures Versus Short Circuit Temperatures: The operating

temperature (T1oC) is the initial temperature of the conductor prior to the occurrenceof the fault. The operating temperature of a low voltage conductor is assumed to bethe temperature of the conductor in a 30

oC ambient temperature environment and at

rated ampacity, which is 60oC, 75

oC, or 90

oC (Table 9-3). The operating

temperature of medium voltage conductors also varies, with 90oC (e.g., XLP) being

the most common temperature. The short circuit temperature (T2oC) is the final

temperature of the conductor after the fault occurs. This short circuit temperature isassumed to be the maximum temperature rise that the conductor can sustain for aspecified period of time without damaging the insulation (Table 7-3).

Type of InsulationContinuous (Initial)

Temperature Rating (T1oC)

Short Circuit (Final)Temperature Rating (T2

oC)

Rubber

Cross-LinkedPolymer (XLP)

Silicone Rubber

Thermoplastic

Paper

Varnished Cloth

75

90

125

60, 75, 90

85

85

200

250

250

150

200

200

Table 7-3. Typical Conductor Temperatures



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CABLE DAMAGE

Conductor Temperature Rise: On the assumptions that all heat is absorbed by theconductor metal (copper or aluminum) and that there is no heat transmitted from theconductor metal to the insulation, the temperature rise is a function of the conductor

size (area in circular mils), the magnitude of the short circuit current (asymmetricalamperes), and the fault duration (time in seconds). These variables are related bythe following formula:

(I/A)2t = (k1) log10[(T2+ k2)/(T1+ k2)]

where: I = short circuit current (asymmetrical) in amperes (A)A = conductor cross-sectional area in circular mils (cmils)t = time of short circuit in seconds (sec)T1= initial conductor temperature in degrees Celsius (

oC), as

listed

by the manufacturerT2= final conductor temperature in degrees Celsius (

oC), after

theshort circuit, as listed by the manufacturer

k1= 0.0297 for copper = 0.0125 for aluminumk2= 234 for copper = 228 for aluminum

The initial (T1) and final (T2) conductor temperatures are predetermined on the basisof the conductors continuous current rating and the insulation material. For lowvoltage thermoplastic conductors, T1 will equal 60

oC, 75oC, or 90oC and T2 willalways equal 150

oC (Table 7-3).

The operating time of the protective device must allow the device to clear the fault forall values of asymmetrical fault current (I) in less time than the time (t) that iscalculated in the above equation.

Cable damage charts and curvesare often used to ensure that the protective devicewill clear the fault prior to damaging the conductor. Figure 7-2 shows an ICEAformat for presenting medium voltage cable damage curves, and Figures 2-3 and 2-4show cable damage curves that are used by many coordination engineers.

Example A: How quickly must a protective device clear a 30 kA (asymmetrical

current) fault to protect a medium voltage AWG No. 4/0 copperconductor?

Answer: Per Figure 7-2, an AWG No. 4/0 copper conductor can withstand a 30

kA fault for approximately 16 cycles(0.2667 sec).



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CABLE DAMAGE

Figure 7-2. ICEA Medium Voltage Cable Damage Curves



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CABLE DAMAGE

Figure 7-3. Cable Damage Curves for Thermoplastic Insulation(75

oC-150

oC)



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CABLE DAMAGE

Figure 7-4. Cable Damage Curves for XLPE Insulation (90oC-250oC)



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PROTECTION AND COORDINATION CRITERIA

The rating or setting of the protective device must protect the low or medium voltageconductors, and it must also coordinate with other protective devices in the system.

Low Voltage Conductors: Because the NEC requires that low voltage conductorsbe protected in accordance with their listed NEC ampacities, protection andcoordination in the cable overload ranges (< 250%) is not a problem. In the shortcircuit (fault) ranges (> 250%), the protective device ratings or settings must beselected or adjusted such that the TCC curves of the protective device fall to the leftand below the conductors damage curve.

Medium Voltage Conductors: Because the NEC permits fuse protection not toexceed 300% and breaker (with relays) protection not to exceed 600%, protectionand coordination of the cables in the overload ranges is sometimes a problem. Thedesign engineer must very often compromise between protection and coordination.

In the short circuit ranges, similar to low voltage conductor protection andcoordination, the protective devices ratings must be selected or adjusted such thatthe TCC curves of the device fall to the left and below the conductors damagecurve.



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TRANSFORMER PROTECTION

NAMEPLATE DATA

As a minimum, the following transformer data, most of which comes directly from the

transformer nameplate, are required to perform a coordination study:

kVA rating

Primary and secondary voltages

Full-load amperes (primary and secondary)

Cooling classes (e.g., OA, FA, AFA, etc.)

Connections (e.g., wye-delta, delta-wye, etc.)

Percent impedance (Z%)

Cooling medium (e.g., liquid-filled or dry-type)

Overload capabilities (capacity)

ANSI/IEEE damage curves

In accordance with ANSI/IEEE standards, a nameplate must be provided with eachtransformer. For both liquid-filled and dry-type transformers, the nameplate isrequired to be made of a corrosion-resistant, durable metal, and it must provide (list)the ratings and other essential operating data for the transformer.

ANSI/IEEE C57.12.00-1987 specifies the data that must be included on thenameplate for liquid-filled transformers, and ANSI/IEEE C57.12.01-1989 specifiesthe information that must be included on the nameplate for dry-type transformers.With only a very few exceptions, the information that is required is the same for bothtypes of transformers.

As an example of the information provided on a typical nameplate, Table 7-4 lists anitemized description of the nameplate that is shown in Figure 7-5.



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NAMEPLATE DATA

Figure 7-5. Typical Transformer Nameplate



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BASIC TRANSFORMER PROTECTION

Less than 5 mva (solidly grounded):

Figure 7-6 is a one-line diagram of the minimum electrical industry-recommended

protection scheme for solidly-grounded power transformers that are rated less than 5MVA.

Figure 7-6. Transformer Protection (Primary Fuse)



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BASIC TRANSFORMER PROTECTION

5 MVA and larger (low resistance grounded):

Figure 7-7 is a one-line diagram of the minimum electrical industry-recommended

protection scheme for medium-sized power transformers (e.g., 5 MVA and larger)that are being protected by power circuit breakers.

Figure 7-7. Transformer Protection (Primary Breaker)



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THERMAL PROTECTION

Power Fuses: NEMA Type E-Rated power fusesare also used to provide thermaloverload protection for the transformer, as shown in Figure 7-8. If the power fuse isa current limiting-type fuse (CLE in Figure 7-8), it is sized at approximately 150% of

FLAT(Ipri). If the power fuse is an expulsion-type fuse (RBA in Figure 7-8), it is sizedat approximately 140% of FLAT (Ipri). Regardless of the type of power fuse that isused (CLE or RBA), it must also be coordinated with the secondary-side mainLVPCB.

Figure 7-8. Transformer Fuse Protection



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THERMAL PROTECTION

Time-Delay Overcurrent Relays (ANSI Device No. 51): Thermal protection forheavy overload conditions normally is obtained by the transformer load-side(secondary) time-delay overcurrent relay (ANSI Device No. 51 in Figure 7-7). This

device also protects the load-side bus (No. 100) and it is set to coordinate with thelargest feeder breaker protective device (ANSI Device No. 51). The source-side(primary) time-delay overcurrent relay (ANSI Device No. 51) is set to pick up at twoto four times the full load amperes (FLAT) of the transformer at maximum cooling(e.g., FA rating) and it is also set to coordinate with the load-side main breakerprotective device (ANSI Device No. 51).

FAULT PROTECTION

Instantaneous Relays (ANSI Device 50): Primary fault protection for powertransformers is typically provided by instantaneous relays (ANSI Device 50 in Figure7-7). It is typically set high enough so that it does not operate for a low side fault,which means that it must be set higher than the maximum asymmetrical fault currentthat flows on the secondary-side of the power transformer.

Power fuses (NEMA Type E-rated):Also provide short circuit or fault protection forthe transformer, as shown in Figures 2-6 and 2-8. Current limiting power fuses(CLE) provide better short circuit (fault) protection than do expulsion (non-currentlimiting) power fuses (RBA).



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FACTORS AFFECTING TRANSFORMER PROTECTION

Note: This section will cover only those devices that must be included in anovercurrent device time-current coordination study, for example, fuses, circuitbreakers, and relays. Other transformer protective devices such as differential

relays and sudden-pressure relays are sometimes present, but they do not usuallyenter into a coordination study.

The rating, selection, and/or setting of a transformer primary-side protective device isaffected by the following two factors: (1) NEC requirements and (2) ANSI/IEEEthrough-fault protection requirements.

NEC REQUIREMENTS

The NEC has specific requirements for transformers depending on whether they areprotected by only a primary-side device or by both primary and secondary-side

devices.

Transformers 600 Volts Nominal or Less (Primary): NEC Article 450-3(b)(1)specifies that transformers that are rated 600 volts nominal, or less, must beprotected by an individual overcurrent device on the primary side that is rated or setat no more than 125% of the rated primary current (FLApri) of the transformer, asshown in Figure 7-9.

Figure 7-9. Primary-Side Protection for Transformers (< 600 V)



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NEC REQUIREMENTS

Transformers 600 Volts Nominal or Less (Primary and Secondary): NEC Article450-3(b)(2) specifies that transformers that are rated 600 volts nominal, or less, andthat have an overcurrent device on the secondary side that is rated or set at not

more than 125% of the rated secondary current (FLAsec) of the transformer, are notrequired to have an individual overcurrent device on the primary side, if the primaryfeeder overcurrent device is rated or set at a current value not more than 250% ofthe rated primary current (FLApri) of the transformer, as shown in Figure 7-10. Eventhough NEC Article 450-3 permits higher ratings for the transformer primary andsecondary-side protective devices, it does not permit violation of NEC Article 240-3for conductor (or bus) protection.

Figure 7-10. Primary and Secondary-Side Protectionfor Transformers (< 600 V)

Transformers Over 600 Volts (Unsupervised Installations): NEC Article 450-3(a)(1) specifies primary and secondary-side protection for transformers that arerated over 600 volts in unsupervised locations, as shown in Figure 7-11. NEC Table

450-3(a)(1) [Table 7-5] specifies the maximum settings for the primary andsecondary-side protective devices.

Transformers Over 600 Volts (Supervised Installations): NEC Article 450-3(a)(2)specifies primary and secondary-side protection for transformers that are rated over600 volts in supervised locations, as shown in Figure 7-12. NEC Table 450-3(a)(2)[Table 7-6] specifies the maximum settings for the primary and secondary-sideprotective devices.



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NEC REQUIREMENTS

Figure 7-11. Transformers Over 600 Volts (Unsupervised Installations)

PRIMARY PROTECTION SECONDARY PROTECTION

Over 600 Volts Over 600 Volts 600 Volts or Below

Transformer

Rated

Impedance

Circuit

Breaker

Setting

Fuse

Rating

Circuit

Breaker

Setting

Fuse

Rating

Circuit Breaker

Setting or

Fuse Rating

Not more

than 6%

600% 300% 300% 250% 125%

More than

6% and not

more than10%

400% 300% 250% 225% 125%

Source: NEC

Table 7-5. Transformer Protective Device Maximum Ratings or Settings(Unsupervised Installations)



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NEC REQUIREMENTS

Figure 7-12. Transformers Over 600 Volts (Supervised Installations)

PRIMARY PROTECTION SECONDARY PROTECTION

Over 600 Volts Over 600 Volts 600 Volts or Below

Transformer

Rated

Impedance

Circuit

Breaker

Setting

Fuse

Rating

Circuit

Breaker

Setting

Fuse

Rating

Circuit Breaker

Setting or

Fuse Rating

Not more

than 6%

600% 300% 300% 250% 250%

More than

6% and not

more than

10%

400% 300% 250% 225% 250%

Source: NEC

Table 7-6. Transformer Protective Device Maximum Ratings or Settings(Supervised Installations)



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TIME/CURRENT COORDINATION INTERVALS

INTRODUCTION

An overcurrent relay TCC curve is shown as a single line, as opposed to the TCC

band that is typically shown for fuses and circuit breakers. Therefore, a time marginshould be added to the characteristic of the overcurrent relay to account formanufacturing tolerances, induction disc overtravel, circuit breaker opening(operating) time, and safety. This section summarizes the time margins that areused for induction disc (electromechanical)-type overcurrent relays. The time marginmay be decreased if field tests indicate that the system still coordinates with thedecreased margin.

If the time margins are not accounted for, there is a much higher probability ofmiscoordination; the upstream protective relay may trip before, or at the same timeas other downstream protective devices (e.g., other relays, breakers, fuses, etc.)

The following general rules apply to time/current coordination intervals:

The time margins that are recommended are minimum time marginsand they should be maintained between the TCC curves at all values ofcurrent.

The upstream overcurrent relay typically should have the same inverseTCC curve as the downstream relay or be less inverse.

The standard medium voltage power circuit breaker opening time is 5

cycles (0.083 sec). Adjustments should be made in the time marginsfor 3-cycle (0.05 sec) and 8-cycle (0.133 sec) circuit breaker openingtimes.



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ELECTROMECHANICAL RELAY TO ELECTROMECHANICAL RELAY

Figure 7-14b shows a vertical time margin of a 0.400 seconds for relay-to-relaycoordination, which is measured on the horizontal scale at the maximum availablesymmetrical fault current (F1). The time margin includes the following factors:

0.083 sec circuit breaker opening time (5 cycles)0.100 sec induction disc overtravel0.200 sec safety margin

0.383 sec total time 0.400 sec

Figure 7-14. Elecromechanical Relay-to-Electromechanical RelayCoordination Time Interval



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ELECTROMECHANICAL RELAY TO

ELECTROMECHANICAL RELAY WITH AN INSTANTANEOUS TRIP

Figure 7-15b shows a vertical time margin of 0.300 seconds for relay-to-relay with an

instantaneous trip (IIT or IOC) coordination, which is measured on the horizontalscale at the maximum available symmetrical fault current (F1). Note that the relay-to-relay margin, which remains at 0.400 seconds, is measured at the IITs pickup(P.U.). The time margin includes the following factors:

0.083 sec circuit breaker opening time (5 cycles)0.100 sec induction disc overtravel0.100 sec safety margin


Figure 7-15. Electromechanical Relay-to-Electromechanical Relay with IIT Coordination Time Interval



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NUMERICAL RELAY TO NUMERICAL RELAY

Figure 7-16b shows a vertical time margin of a 0.3 seconds for numerical relay tonumerical relay coordination, which is measured on the horizontal scale at themaximum available symmetrical fault current (F1). The time margin includes the

following factors:

0.083 sec circuit breaker opening time (5 cycles)

0.200 sec safety margin


Figure 7-16. Numerical Relay to Numerical RelayCoordination Time Interval



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NUMERICAL RELAY TO

NUMERICAL RELAY WITH AN INSTANTANEOUS TRIP

Figure 7-17b shows a vertical time margin of 0.200 seconds for relay-to-relay with an

instantaneous trip (IT) coordination, which is measured on the horizontal scale at themaximum available symmetrical fault current (F1). Note that the relay-to-relaymargin, which remains at 0.300 seconds, is measured at the IITs pickup (P.U.). Thetime margin includes the following factors:

0.083 sec circuit breaker opening time (5 cycles)

0.100 sec safety margin


Figure 7-17. Numerical Relay toNumerical Relay with IT Coordination Time Interval



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NUMERICAL RELAY-TO-CURRENT LIMITING POWER FUSE

Figure 7-18b shows a vertical time margin of 0.15 seconds for relay-to-currentlimiting power fuse (e.g., Cutler-Hammer CLE) coordination, which is measured onthe horizontal scale at the point (PCL) where the CLE fuse goes current limiting. The

CLE fuse protects the transformer and the relay provides both overload (ANSIDevice No. 51) and short circuit (ANSI Device No. 50) protection for the cable. Thetime margin includes the following factors: Note: Setting of ANSI Device No. 50 forthis application will be covered in the next section.

0.15 sec safety margin0.15 sec total time

Figure 7-18. Numerical Relay to Medium Voltage Current Limiting Power FuseCoordination Time Interval



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NUMERICAL RELAY TO EXPULSION POWER FUSE

Figure 7-19b shows a vertical time margin of 0.15 seconds for the relay-to-expulsionpower fuse (e.g., Cutler-Hammer RBA) coordination, which is measured on thehorizontal scale at the maximum available symmetrical fault current (F1) at the bus

(Bus 100) where the fuse is installed. The RBA fuse protects the transformer andthe relay (ANSI Device No. 51) provides good overload protection but poor shortcircuit protection for the cable. The time margin includes the following factors:


Figure 7-19. Numerical Relay to Medium Voltage Expulsion Power FuseCoordination Time Interval



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NUMERICAL RELAY WITH INSTANTANEOUS TRIP-TO-LVPCB

Figure 7-20b shows a vertical time margin of 0.15 seconds for the relay with aninstantaneous trip (IT)-to-low voltage power circuit breaker (LVPCB) coordination,which is measured on the horizontal scale at the maximum available symmetrical

fault current (F1) at the bus (Bus 150) where the LVPCB is installed. The timemargin includes the following factors: Note: Setting of ANSI Device No. 50 for thisapplication will be covered in the next section.


Figure 7-20. Relay with IIT-to-LVPCB Coordination Time Interval



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INSTANTANEOUS TRIP NUMERICAL RELAY AND POWER FUSECOORDINATION

When coordinating instantaneous trip relays (ANSI Device No. 50) with downstreammedium voltage current limiting power fuses, the instantaneous trip setting should be

set approximately 10% greater than the maximum fault current that the downstreamfuse permits to flow through the circuit. Figure 7-21b shows that the instantaneoustrip setting (IIT) should be set approximately 10% greater than the point (PCL), insymmetrical rms amperes, where the fuse is considered to go current limiting, whichis approximately the point where the fuses total clearing time curve crosses thehorizontal (current) axis. In this application, relay B (ANSI Device Nos. 51/50) isproviding both overload and short circuit protection for the cable.

The IIT (or IOC) setting is represented by the following formula:

IIT = [(1.1 X PCL)]/CT ratio

It is general industry practice to round the calculated setting up to thenext multiple of 5. For example, if the calculated setting equals 37.2 A,set the IT at 40 A.

Figure 7-21. IT Relay Settings with CLFs



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INSTANTANEOUS TRIP NUMERICAL RELAY AND POWER FUSECOORDINATION

Example B: Referring to Figure 7-22, assume that PCL= 3200 A and the CT ratio is200/5. What should be the setting of the IIT relay?

Answer: IIT = [(1.1 X PCL)]/CT ratio

= [(1.1 X 3200)]/(200/5) = 88; set at 90 A (3600 A)

Expulsion (Non-Current Limiting) Fuses: Figure 7-22 shows that there is noupstream instantaneous relay (ANSI Device No. 50) when the downstream powerfuse is an expulsion fuse. Before an instantaneous trip relay could coordinate withthe downstream fuse, its tap setting would have to be approximately 10% greaterthan the maximum asymmetrical fault current (Iasy) that flows through the power fuse.There would rarely be enough cable impedance between the two buses to permit

application of an instantaneous trip relay. Relay B is providing good overloadprotection but poor short circuit protection for the cable.

Figure 7-22. Relay Settings with Expulsion (Non-CLF) Fuses



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INSTANTANEOUS TRIP NUMERICAL RELAY AND LVPCB COORDINATION

Figure 7-23b shows that instantaneous trip setting (IT) should be set approximately10% greater than the asymmetrical amperes (F1(asy)) that is available for a three-phase bolted fault at the LVPCB bus (Bus 150).

The IIT (or IOC) setting is represented by the following formula:

IIT = [(1.1)(IF1(sym))(Mm)]/[(CT ratio)(Transformer ratio)]

where Mmequals the A.F.

It is general industry practice to round the setting up to the next multiple of 5.

Figure 7-23. IT Relay Settings with LVPCBs



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INSTANTANEOUS TRIP NUMERICAL RELAY AND LVPCB COORDINATION

Example C: Referring to the one-line diagram that is shown in Figure 7-24, whatshould be the setting of the IT relay?

Figure 7-24. Example E One-Line Diagram

Answer: IIT = [(1.1)(IF1(sym))(Mm)]/[(CT ratio)(Transformer ratio)]

= [(1.1)(18500)(1.32)]/[(300/5)(4.16/.48)]

= 51.5; set at 52 A (3120 A)or round up to55 A(3300 A).



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EXERCISES

EXERCISE 1: INSTANTANEOUS RELAY TRIP SETTINGS

Given the diagrams that are shown in Figures 2-25a, 2-25b, and 2-25c, what is the

correct setting for the instantaneous trip relay Figure 7-25b or Figure 7-25c?

Figure 7-25. Exercise 1 One-Line Diagram and TCC Curves



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EXERCISE 1: INSTANTANEOUS RELAY TRIP SETTINGS

Answer: Figure 7-25b requires that the IIT setting be set 10% greater than thepoint (PCL) where the fuse goes current limiting (Formula 1). Figure 7-25c requires that the IIT setting be 10% greater than the maximum

asymmetrical fault current at Bus 100 (Formula 2). Whichever of thetwo calculations that yields the highest setting is the correct answer?

Formula 1: IIT = [(1.1 X PCL)]/CT ratio

Formula 2: IIT = [(1.1)(IF1(sym))(Mm)]/[(CT ratio)(Transformer ratio)]



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EXERCISE 2: RELAY COORDINATION

Which of the two relaying schemes that are shown in Figure 7-26 is the industry-preferred relaying coordination scheme -- 2-26a or 2-26b?

Figure 7-26. Exercise 6 One-Line Diagram



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EXERCISE 2: RELAY COORDINATION

Answer: As a general rule, the upstream protective relay should have the sameinverse TCC curves as the downstream relay, or, if the relay types aremixed, as they are shown in Figure 7-27, the upstream relay TCC

curves should be less inverse. Therefore, Scheme No. 1 is theindustry-preferred relaying coordination scheme. See Figure 7-27.

Figure 7-27. Exercise 6 Solution

new chapter 7 nec requirements and coordination basics

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