24157075-Basics-of-CBs

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Folienthema/Folie1/Jahreszahl

A&D / BD-CD June 04

X MER

ACB B C ACB

MCCBACBACB

TOCCP

TOMCC1

TOMCC2

TOETP

TOEM.LIGHT

TOUTILITYMCC

MCCB SDF SDF

Basics of Low Voltage System & Circuit Breakers

G

ACB

ACB

B C ACB

X MER

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Typical Transformer specifications

Rating 1000 kVAPrimary Voltage 11 kVSecondary Voltage 440 Volts

%ge Impedance 5 %Vector Group DYn11

G

Typical Generator specifications

Rating 1000 kVAVoltage 440 Volts%ge Impedance 16 %

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How to calculate system fault level

T/F rating 1000 kVA

IFL of the transformer 1391 amps kVA x 1000

1.732 x 415

Isc of the transformer 27,825 amps IFL0.05

So the fault level at T/F terminals works out to be 27.8 kA

Similarly the fault level at the generator terminal works out to be 8.7 kA

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Effects of addition of T/F in parallel

T/F rating 1000 kVA

IFL of the transformer 1391 amps kVA x 1000

1.732 x 415

Isc of the transformer 55,650 amps IFL2 x 0.05

So the fault level at T/F terminals works out to be 55.7 kA

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Considerations for the fault level calculations

1. What is the size of the transformer

2. How many transformers are in parallel

3. Size of the Captive Generator, if used

4. Are all these units used in parallel ?

5. Any large size motor fed from the main bus ?

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Folienthema/Folie6/Jahr

eszahl

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Criterion for Breaker selection

1. Full load current of the transformer / Generator

2. Fault level at the point of installation

3. Other issues such as

1. 3 pole / 4 pole

2. Manual / motorized

3. Type of the release

4. Any other points .

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eszahl

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Terminologies associated with breakers

1. ICMRated making capacity (expressed as peak value)

2. ICURated ultimate breaking capacity (expressed as rms value)

3. ICS Rated service breaking capacity (expressed as rms value)

4. ICW Rated short time withstand capacity (expressed as rms value)

5. ICCRated short time withstand capacity for non automatic breakers

(expressed as rms value)

Test sequence ICU = O t CO (t = 3 min)

ICS = O t CO t - CO (t = 3 min)

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eszahl

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Types of breakers

1. Medium of arc quenching - Oil

- Air,

- Vacuum

2. Principle of operation - Current zero type of breakers

- Current limiting breakers

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What is current limiting ?

A principle where short circuit currentis interrupted before it reaches thefirst peak.

Current limitationin to Sentron VL

10 ms4 ms t

iProspectivecurrent

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Folienthema/Folie10/Jah

reszahl

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Advantages of current limiting

1. Faster fault clearing

2. Lower let through value

3. Devices subjected to lesser stresses

4. Enhanced protection levels with

Cascading

5. ProperDiscrimination possible

Current limitationin to Sentron VL

10 ms4 ms t

iProspectivecurrent

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reszahl

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What is Cascading ?

Cascading refers to the technique of series connection of

protective devices,which results in enhanced breaking capacity.

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reszahl

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What is Cascading ?

Isc = 35 kA

3VL, 630 A ,45 kAIsc = 40 kA

VL160X, 100A, 40 kA

VF100, 100A, 18 kA

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reszahl

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CascadingChart

Upstream

MCCB frame

Br.Cap.

(Icu)

VF100 VL160x VL160x VL160 VL250 VL250 VL400

18 kA 40 kA 40 kA 40 kA 40 kA 40 kA 45 kA

Release TM TM TM TM / ETU TM / ETU TM / ETU TM / ETU

Ratings 100A 100A 160A 160A 200A 250A 315A

VF100 TM 18 kA 100A

40 kA 100A

70 kA 100A

40 kA 160A

70 kA 160A

40 kA 160A

70 kA 160A

40 kA 200A 40 kA

70 kA 200A 50 kA

40 kA 250A 40 kA 40 kA

70 kA 250A 50 kA 70 kA

45 kA 315A 45 kA 45 kA

70 kA 315A 50 kA 70 kA

45 kA 400A 45 kA 45 kA 45 kA 45 kA

70 kA 400A 50 kA 70 kA 70 kA 70 kA

45 kA 630A 45 kA 45 kA 45 kA 45 kA 50 kA 50 kA

70 kA 630A 50 kA 70 kA 70 kA 70 kA 70 kA 70 kA

50 kA 800A 50 kA 50 kA 50 kA 50 kA 50 kA 50 kA 50 kA

70 kA 800A 50 kA 70 kA 70 kA 70 kA 70 kA 70 kA 70 kA

TM / ETU

Downstream MCCB frame

VL160x TM

VL250 TM / ETU

TM

TM / ETU

TM / ETU

VL630

VL800

TM / ETU

TM / ETU

ETU

VL160x

VL160

VL250

VL400

VL400

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reszahl

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What is Discrimination ?

Discrimination implies that the protective devices

closest to the point of fault will operate in the quickest possible

manner to avoid any wide spread shut down.

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What is Discrimination ?

Isc = 35 kA

3VL, 630 A ,45 kAIsc = 40 kA

VL160X, 100A, 40 kA

VF100, 100A, 18 kA

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Types of Discrimination

1. Current based

2. Time based

3. Energy based

4. Logic based (ZSI)

1. Partial Discrimination

2. Total Discrimination

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