back up protection

7/30/2019 Back Up Protection

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Page

Discrimination and Back-up Protection

Introduction 12/2

Discrimination 12/3Current discrimination, time discrimination 12/4Back-up protection 12/5

Discrimination limits

Series circuit-breakers miniature circuit-breakers 12/6Series circuit-breakers circuit-breakers 12/10

Back-up protection values

Series circuit-breakers miniature circuit-breakers 12/24

Discrimination miniature circuit-breakers 12/26

td

= 80 ms

NSK-8274b

A

Q1

E

K2

A

Q2

E

K1

A

E

Q3

Q4 Q5

A A

E E

tzss

= 50 ms

td

= 150 ms

tzss

= 50 ms

td

= 0 ms

tzss

= td

td

= 0 ms

tzss

= td


2/17


Introduction

It is not the performance of an individual

component that determines the reliability of

a low-voltage system, but rather the team-

work of all such components.Optimal cooperation of switching devices

depends on discriminative line and deviceprotection. Each feeder and each device

must be protected as well as possible

against overload and short-circuit, withoutbeing adversely affected by any switching

operations intended to protect the rest of

the system.

When a system is planned, it is customary

to select protective devices with breaking

capacities to suit the maximum possible

short-circuit currents at the points of installa-tion.

VDE and IEC standards allow back-up pro-tection of a switching device by one of the

upstream protective devices with an appro-

priate switching capacity, if thereby boththe feeder and the downstream protective

device are protected.

This means in practical terms that at a

point in the distribution system with a

possible short-circuit current of e.g. 50 kA,

a cost-effective protective device with a

breaking capacity of only 20 kA can also

be used, entailing however other advan-

tages (e.g. less space required and better

protection), if it is given back-up protectionby an upstream device with a breaking ca-

pacity of at least 50 kA.

Discriminative switching with SIEMENS de-

vices means: maximum possible availa-

bility of supply, with at the same time a

high degree of system and device pro-tection by way of specific shutdown of

the faulty circuit without affecting the

other loads.

12/2 Siemens NS PS 1999


3/17


In series-connection of circuit-breakers, pro-

tection against overload and short-circuit is

referred to as discriminative if, in the direc-

tion of power flow, only that device up-stream of the fault is tripped.

Current discrimination

With overload, the discrimination can be

worked out from a comparison of the

time/current characteristics. With short-

circuit, this comparison produces values

that are too low. The reason for this is the

behaviour of the releases in the event ofshort-circuit currents, which deviates from

the behaviour in the long-time range, i.e. inthe event of overload (see diagram below).

With adequately different short-circuitcurrents at the installation points of two cir-

cuit-breakers, the instantaneous short-cir-cuit releases can generally be set so that in

the event of short-circuit after the down-stream switch, only the latter operates.

With almost identically high short-circuitcurrents at the installation points of the cir-

cuit-breakers, grading of the operating cur-rents of the short-circuit releases allows dis-

crimination only up to a certain short-circuit

current. This current is referred to as thediscrimination limit.

The limit values for discrimination are

shown in the tables on Pages 12/8 to 12/25.

If the value worked out from the short-

circuit calculation (e.g. to VDE 0102) at theinstallation point of the downstream switch

lies below the discrimination limit read offfrom the table for the chosen combination,

discrimination is assured for all possibleshort-circuit occurrences at the installation

point.

If the short-circuit current worked out at the

installation point lies above the discrimina-

tion limit, discriminative tripping by thedownstream switch is assured only up to

the value in the table. The planner must

assess whether the value can be con-sidered adequate, as the probability of, for

example, the maximum short-circuit currentoccurring is low. Otherwise a switch combi-

nation must be selected, with a discrimina-

tion limit higher than the maximum short-

circuit current.

Discrimination

In = 10 A

I"k = 20 kA

Ue = 400 V

= 1600= 120

= 120 A

= 1600 AI>

I>

3RV

3VF

NSK-8220a

In = 160 A

I"k = 6 kA

I

I>

3RV

I>

3VF

Discrimina-tion limitworked outby tests5 kA

Opening time

Current

A

Discrimination limit fromcomparison of characteristics1600 - 20% = 1280 A

Fig. 12/1 Example of the effect of discrimination on the selection of circuit-breakers

Tripping characteristics forcircuit breakers

In = rated currentIk = short-circuit currentI> = response current

1

Siemens NS PS 1999 12/3


4/17


Current discrimination, time discrimination

Fig. 12/2 Fig. 12/3

Q4 Q2 Q1

150 ms

80 ms50 ms

NSK-8221a

Opening time

Current

A

td = 80 ms

NSK-8222a

A

Q1

E

K2

A

Q2

E

K1

A

E

Q3

Q4 Q5

A A

E E

tzss = 50 ms

td = 150 ms

tzss = 50 ms

td = 0 ms

tzss = td

td = 0 ms

tzss = td

3VF current-limiting circuit-breakers

for distribution system protection toIEC 60 947

The 3VF circuit-breakers are available invarious short-circuit breaking capacities. Toeliminate the need to create a table for

every breaking capacity where the discrimi-

nation limit is dependant on the breaking

capacity, the discrimination limit is given inthe sequence e.g. 40/70/100 kA. The corre-

sponding value is dependant upon thebreaking capacities of the upstream and

downstream breakers. Decisive is always

the smallest value.

Time discrimination

Another method of assuring discrimination

in the event of almost identically high short-

circuit currents at the installation points istime discrimination. The upstream breakerrequires time-delay short-circuit releases,

so that in the event of a fault only the down-

stream breaker isolates the part of the sys-

tem affected by the fault.

Both the release delays and the operating

currents of the short-circuit releases aregraded.

In order to avoid undesirably long tripping

times with series-connection of several cir-cuit-breakers, SIEMENS has developed

microprocessor-driven short-time gradingcontrol (ZSS) for the 3WN/3WS circuit-

breakers.

This control permits reduction of the trip-

ping delay time to a maximum of 50 ms for

the breaker upstream of the short-circuit.

Upstream

circuit-breaker

kA 100 70 40 70

Downstreamcircuit-breaker

kA 40 100 100 70

Discriminationlimit

kA 40 70 40 70

td = Maximum operative timeof the blocking signal ordelay time in the event ofnon-activated short timegrading control.

tZSS = Delay time of all switcheswhich sense the short-circuit and receive noblocking signal.

A: Output,transmits blocking signal

E: Input,receives blocking signal

Communication lines

Function:A short-circuit at point K1 issensed by Q4, Q2 and Q1.When the ZSS is activated,Q1 and Q2 are temporarilyblocked by Q4 and Q2 respec-tively.As Q4 receives no blockingsignal, it trips after 10 ms.

A short-circuit at point K2 issensed only by Q1; as Q1receives no blocking signal,it trips after 50 ms. WithoutZSS, tripping would not takeplace until after 150 ms.

12/4 Siemens NS PS 1999


5/17


Back-up protection

When a circuit-breaker or miniature circuit-

breaker is selected, it is mandatory that thebreaker be able to handle the short-circuit

current I"k at the installation point, either onits own or with the assistance of an up-

stream protective device (back-up protec-

tion). This means that by virtue of the back-

up protection, the breaking capacity of thedownstream breaker is allowed to be lowerthan the short-circuit current at the installa-

tion point. This represents a degree of plan-

ning freedom, which makes above all for

economical system design and also op-timizes component protection.

The tables on pages 12/26 and 12/27 show

the increased permissible short-circuit cur-

rents for all breaker combinations, madepossible by back-up protection.

Appraisal of the back-up protection for thecircuit-breakers to IEC 60 947 and for theminiature circuit-breakers according toIEC 60 898 was based on the breaking

capacity test with test sequence Icu, in

according to IEC 60 947-2.

Fig. 12/4 Example of the effect of back-up protection on the selection of circuit-breakers

3VF3

5SX

NSK-8223a

Icn = 40 kA

In = 160 A

I"k = 28 kA

Icn = 10 kA

In = 16 A

5SX

Icn = 10 kA

In = 16 A

3VF3

Icn = 40 kA

In = 160 A

I"k = 28 kA

I"k = 9 kA I"k = 14 kA

Overload and short-circuit protectivewithout advantage of back-up protectionIn Rated currentI"k Short-circuit currentIcn Rated short-circuit

breaking capacity

Overload and short-circuit protectivetaking into account back-up protectionin accordance with the tableon page 12/26 and 12/27.

Back-up-protectedup to 15 kA

1

Siemens NS PS 1999 12/5


6/17


7/17


8/17

S. 10-11


Series-connection circuit-breakers miniature circuit-breakers AC 400 V, 50 Hz1)

Downstream circuit-breakers Upstream circuit-breakers

3RV1.1 3RV1.2

In (A) 1 1.25 1.6 2 2.5 3.2 4 5 6.3 8 10 12 1 1.25 1.6 2 2.5 3.2

I > (A) 12 15 19 24 30 38 48 60 76 96 120 144 12 15 19 24 30 38

Icn (kA) 100 100 100 100 100 100 100 100 100 50 50 50 100 100 100 100 100 100

Discrimination limits (kA)

3RV1.1 1 12 100 0.1 0.1 0.2 0.3 0.6 50 50 0.1

1.25 15 100 0.1 0.1 0.1 0.2 0.3 0.4 0.8 0.1

1.6 19 100 0.1 0.1 0.1 0.1 0.2 0.3 0.3 0.1

2 24 100 0.1 0.1 0.1 0.1 0.2 0.2 0.3

2.5 30 100 0.1 0.1 0.1 0.1 0.2 0.2

3.2 38 100 0.1 0.1 0.1 0.1 0.2 0.2

4 48 100

5 60 100 6.3 76 100

8 96 50

10 120 50

12 144 50

3RV1.2 1 12 100 0.1 0.1 0.1 0.6 50 50 50 0.1

1.25 15 100 0.1 0.1 0.2 0.2 0.6 30 50 0.1

1.6 19 100 0.1 0.1 0.1 0.2 0.3 0.4

2 24 100 0.1 0.1 0.1 0.2 0.3 0.5

2.5 30 100 0.1 0.1 0.1 0.2 0.2 0.3

3.2 38 100 0.1 0.1 0.1 0.1 0.2 0.2

4 48 100

5 60 100

6.3 76 100

8 96 100

10 120 100

12.5 150 100

16 192 50

20 240 50

22 264 50

25 300 50

3RV1.3 16 192 50

20 240 50

25 300 50

32 384 50

40 480 50

45 540 50

50 600 50

3RV1.4 16 192 100

20 240 100

25 300 100

32 384 100

40 480 100

50 600 100

63 756 100

75 900 100

90 1080 100

100 1140 100

In = Rated current

I> = Response current for instantaneous short-circuit releases

Icn = Rated short-circuit breaking capacity acc. to IEC 60 947-2 for mccbs


9/17

S. 12-13


Series-connection circuit-breakers miniature circuit-breakers AC 400 V, 50 Hz1)


3RV1.3 3RV1.4

In (A) 16 20 25 32 40 45 50 16 20 25

I > (A) 192 240 300 384 480 540 600 192 240 300

Icn (kA) 50 50 50 50 50 50 50 100 100 100


3RV1.1 1 12 100 50 50 50 50 50 50 50 100 100 100

1.25 15 100 50 50 50 50 50 50 50 100 100 100

1.6 19 100 0.6 1.5 50 50 50 50 50 0.6 1.5 100

2 24 100 0.4 0.6 1.2 3 50 50 50 0.4 0.6 1.2

2.5 30 100 0.3 0.4 0.6 1.2 2 3 5 0.3 0.4 0.6

3.2 38 100 0.3 0.3 0.5 0.8 1.2 1.5 2 0.2 0.3 0.5

4 48 100 0.4 0.6 0.8 1 1.2 0.4

5 60 100 0.4 0.6 0.6 0.8 1 0.46.3 76 100 0.4 0.5 0.6 0.8 0.8 0.4

8 96 50 0.4 0.5 0.6 0.6 0.8 0.4

10 120 50 0.4 0.5 0.6 0.6 0.8 0.4

12 144 50 0.5 0.6 0.6 0.8

3RV1.2 1 12 100 50 50 50 50 50 50 50 100 100 100

1.25 15 100 50 50 50 50 50 50 50 100 100 100

1.6 19 100 6 50 50 50 50 5 0 50 100 100 100

2 24 100 0.8 2.5 50 50 50 50 50 1.2 8 100

2.5 30 100 0.5 1 1.5 8 50 50 50 0.6 1.2 4

3.2 38 100 0.4 0.6 0.6 2 3 6 12 0.4 0 .6 0 .6

4 48 100 0.4 0.6 1 1.5 2.5 3 0.3 0.5 0.6

5 60 100 0.4 0.6 0.8 1.5 1.5 0.3 0.4

6.3 76 100 0.5 0.6 0.6 0.6 0.6 0.3 0.5

8 96 100 0.4 0.6 0.8 1 1.2 0.4

10 120 100 0.4 0.6 0.8 1 1 0.4

12.5 150 100 0.4 0.5 0.6 0.8 1 0.4

16 192 50 0.5 0.6 0.8 0.8

20 240 50 0.6 0.8

22 264 50 0.6

25 300 50 0.6

3RV1.3 16 192 50 0.5 0.6 0.6 0.8

20 240 50 0.5 0.6 0.6 0.8

25 300 50 0.4 0.5 0.6 0.6

32 384 50 0.6

40 480 50 0.6

45 540 50

50 600 50

3RV1.4 16 192 100 0.5 0.6 0.6 0.8

20 240 100 0.5 0.6 0.6 0.8

25 300 100 0.4 0.5 0.6 0.6

32 384 100 0.6

40 480 100 0.6

50 600 100

63 756 100

75 900 100

90 1080 100

100 1140 100

In = Rated current

I> = Response current for instantaneous short-circuit releases


1) In 240/415 V, 50 Hz networks the discrimination limits must be reduced by 10%.


10/17


11/17


12/17


13/17

S. 20-21


Series-connection circuit-breakers circuit-breakers AC 400 V, 50 Hz1)


3VF5 3VF6 3VF7 3VF

In (A) 200 250 315 400 315 400 500 630 800 800-1250 16

I >> (A) 2000 2500 3150 4000 3200 4000 5000 6300 6400 15000

Icn (kA) 45/70/100 45/70/100 45/70/100 45/70/100 45/70/100 45/70/100 45/70/100 45/70/100 50 50/70/100


3VF4 125 1250 40/70/100 2 2 2.5 3.4 3.4 4 4 5.1 5.1 33 4

for line protection 160 1600 40/70/100 2 2 2.5 3.4 3.4 4 4 4.9 4.9 33 4

200 2000 40/70/100 2.5 3.4 3.4 4 4 4.9 4.9 33 4

250 2500 40/70/100 2.5 3.4 3.4 4 4 4.9 4.9 33 4

3VF4 125 2000 70 3 4 5 5 6 6 27

for starter 160 2500 70 3 4 5 5 6 6 27

combinations 200 3000 70 3 5 5 6 6 27

3VF4 250 3000 40/70 3 5 5 6 6 27

Non-automatic

circuit-breakers

3VF5 200 2000 45/70/100 2.4 3.4 3.4 4 4 4 4 30 4

for line protection 250 2500 45/70/100 2.4 3.4 3.4 4 4 4 4 28 4

315 3150 45/70/100 3.4 4 4 4 4 28 4

400 4000 45/70/100 4 4 28 4

3VF5

for motor proteciton 315 4800 40/65 5 5 27

3VF5 200 3000 65 3.4 5 5 5 5 27

for starter 250 3800 65 5 5 5 27

combinations 315 4800 65 5 5 27

3VF5

Non-automatic 315 4800 40/65 5 5 27

circuit-breakers

3VF6 315 3200 45/70/100 4.4 4.4 21

for line protection 400 4000 45/70/100 4.4 4.4 21

500 5000 45/70/100 21 630 6300 45/70/100 21

800 6400 50

3VF6

for motor protection 500 7500 40/65 17

3VF6 315 4800 65 17

for starter 400 6000 65 17

combinations 500 7500 65 17

3VF6

Non-automatic 500 7500 40/65

circuit-breakers

3VF7 800 15000 50/70/100

fo r l ine p ro tect ion 1250 15000 50/70/100

3VF7 800 15000 50/70

Non-automatic 1250 15000 50/70

circuit-breakers

3VF8 1600 20000 70/100

fo r l ine p ro tect ion 2000 20000 70/100

3VF8 1600 20000 65

Non-automatic 2000 20000 65

circuit-breakers

In = Rated current

I>= Response current for instantaneous short-circuit releases


1) In 240/415 V, 50 Hz networks the discrimination limits must be reduced by 10%.

The discrimination values apply for adjustable releases at maximum setting.


14/17


15/17

S. 24-25

Back-up protection values

Series-connection circuit-breakers-miniature circuit-breakers AC 400 V, 50 Hz

AC 230/400 V, 50 Hz

D ow ns tre am c ir cui t- br ea ke rs U ps tre am c ir cui t- br eak er s

3VF3 3VF3 3VF4 3VF5

adjustable fixed setting

In (A) 50 63 80 100 125 160 50 63 80 100 125 160 125 160 200 250 200 250

I >> (A) 500 630 800 1000 1250 1600 400 500 630 800 1000 1280 1250 1600 2000 2500 2000 2500

Icn ( kA)

40/70

100

40/70

100

40/70

100

40/70

100

40/70

100

40/70

100

40/70

100

40/70

100

40/70

100

40/70

100

40/70

100

40/70

100 40/70/100 40/70/100 40/70/100 40/70/100 45/70/100 45/70/1

Back-up protection up to (kA)

5SX2 0.5-2 6kA/ 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50

5SX4 3 10kA 35 35 35 35 35 35 35 35 35 35 35 35 35 35 35 35 35 35

A-Characteristics 4 35 35 35 35 35 35 35 35 35 35 35 35 35 35 35 35 35 35

B-Characteristics 6 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15

C-Characteristics 8 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15

D-Characteristics 13 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15

16 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15

20 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15

25 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15

32 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15

40 10 10 10 10 10 10 10 10 10 10 10 10 8/10 8/10 8/10 8/10 8/10 8/10

50 10 10 10 10 10 10 10 10 10 10 8/10 8/10 8/10 8/10 8/10 8/10

In = Rated current

I >= Response current for instantaneous short-circuit releases



16/17

S. 26

Discrimination miniature circuit-breakers

In distribution networks without any fuses, mcbs provide selectivity between themselves within close limits. This is dependentpeak current of the downstream mcb and the tripping current of the upstream mcb.

The following table specifies up to which short-circuit current in kA, discrimination is provided between mcbs connectedin series at 230 V AC.

Downstream miniature circuit-breakers Upstream miniature circuit-breakers

5SX4-7 5SX7-7 5SX7-8

In (A) 20 25 32 40 50 63 80 100 63 80

I >> (A) 200 250 320 400 500 630 800 1000 945 1200

Icn ( kA) 10 10 10 10 10 10 10 10 10 10


5SX2 6 30 6/10 0.2 0.2 0.3 0.5 0.5 0.5 0.8 1.5 1.5 3

5SX4 10 50 6/10 0.2 0.2 0.3 0.5 0.5 0.5 0.8 1.2 1.5 3

B-Characteristics 13 65 6/10 0.2 0.2 0.3 0.4 0.5 0.5 0.8 1.2 1.5 2

16 80 6/10 0.2 0.2 0.3 0.4 0.5 0.5 0.8 1.2 1.5 2

20 100 6/10 0.2 0.3 0.4 0.5 0.5 0.8 1.2 1.5 2

25 125 6/10 0.4 0.4 0.4 0.6 1.2 1.2 1.5

32 160 6/10 0.4 0.4 0.4 0.6 1.2 1.2 1.5

40 200 6/10 0.4 0.4 0.6 1.2 1.2 1.5

50 250 6/10 0.4 0.6 1 1.2 1.5

5SX2 0.5 5 6/10 0.2 0.3 0.5 0.8 0.8 0.8 1.2 4 5 6/10

5SX4 1 10 6/10 0.2 0.3 0.5 0.8 0.8 0.8 1.2 4 5 6/10

C-Characteristics 1.5 15 6/10 0.2 0.3 0.5 0.8 0.8 0.8 1.2 4 5 6/10

2 20 6/10 0.2 0.3 0.5 0.8 0.8 0.8 1.2 4 5 6/10

3 30 6/10 0.2 0.2 0.3 0.5 0.5 0.5 0.8 1.5 1.5 3

4 40 6/10 0.2 0.2 0.3 0.5 0.5 0.5 0.8 1.5 1.5 3

6 60 6/10 0.2 0.2 0.3 0.5 0.5 0.5 0.8 1.5 1.5 3

8 80 6/10 0.2 0.2 0.3 0.4 0.4 0.4 0.6 1.2 1.5 2.5

10 100 6/10 0.2 0.2 0.3 0.4 0.4 0.4 0.6 1.2 1.5 2.5

13 130 6/10 0.2 0.2 0.3 0.4 0.4 0.4 0.6 1.2 1.2 2

16 160 6/10 0.2 0.2 0.3 0.4 0.4 0.4 0.6 1.2 1.2 2

20 200 6/10 0.2 0.3 0.4 0.4 0.4 0.6 1.2 1.2 2

25 250 6/10 0.3 0.4 0.4 0.6 1 1.2 1.5

32 320 6/10 0.3 0.4 0.4 0.6 1 1.2 1.5

40 400 6/10 0.8 1 1.5

50 500 6/10 0.8 1 1.5

63 630 6/10 0.8 1.2

In = Rated current

I> = Response current for instantaneous short circuit releases



17/17

S. 26

on the

100

1500

10

5

4

3

3

3

3

3

2.5

2.5

6/10

6/10

6/10

6/10

4

4

4

3

3

3

3

3

2.5

2.5

2

2

1.5

back up protection

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