Choice of a circuit-breaker

Choice of a circuit-breaker

 

The choice of a CB is made in terms of:

 

·         Electrical characteristics of the installation for which the CB is intended

·         Its eventual environment: ambient temperature, in a kiosk or switchboard enclosure, climatic conditions, etc.

·         Short-circuit current breaking and making requirements

·         Operational specifications: discriminative tripping, requirements (or not) for remote control and indication and related auxiliary contacts, auxiliary tripping coils, connection

·         Installation regulations; in particular: protection of persons

 

 

 

Low Voltage Circuit Breakers

Based on IEC 60947-2 (LV switchgear and controlgear - Part 2: Circuit breakers):

 

In (Rated current)

 

The rated continuous / uninterrupted current that the circuit breaker can carry.

 

Icm (Rated short-circuit making current)

 

The short-circuit current that the circuit breaker can withstand as it is closing where the act of closing initiates the fault.

 

Icu (Rated ultimate short-circuit current)

 

The maximum symmetrical short-circuit current the circuit breaker can interrupt.

 

Ics (Rated service short-circuit current)

 

The breaking capacity such that the circuit breaker is tested to carry its current continuously. The test sequence verifies that the breaker can be returned to service. Ics is the maximum current the breaker can interrupt multiple times and be returned to service without being damaged and is expressed as a % of Icu.

 

Icw (Rated short time withstand current)

 

This is the steady state symmetrical fault current the breaker has to be able to carry for a duration of 0.05s to 3s without exceeding its thermal integrity.

Power factor calculation

The power factor correction capacitor should be connected in parallel to each phase load.

 

Single phase circuit calculation

 

Power factor calculation:

PF = |cos φ| = 1000 × P(kW) / (V(V) × I(A))

 

Apparent power calculation:

|S(kVA)| = V(V) × I(A) / 1000

 

Reactive power calculation:

Q(kVAR) = √(|S(kVA)|2 - P(kW)2)

 

Power factor correction capacitor's capacitance calculation:

C(F) = 1000 × Q(kVAR) / (2πf(Hz)×V(V)2)

 

Three phase circuit calculation

 

For three phase with balanced loads:

 

Calculation with line to line voltage

 

Power factor calculation:

PF = |cos φ| = 1000 × P(kW) / (√3 × VL-L(V) × I(A))

 

Apparent power calculation:

|S(kVA)| = √3 × VL-L(V) × I(A) / 1000

 

Reactive power calculation:

Q(kVAR) = √(|S(kVA)|2 - P(kW)2)

 

Power factor correction capacitor's capacitance calculation:

C(F) = 1000 × Q(kVAR) / (2πf(Hz)×VL-L(V)2)

 

Calculation with line to neutral voltage

 

Power factor calculation:

PF = |cos φ| = 1000 × P(kW) / (3 × VL-N(V) × I(A))

 

Apparent power calculation:

|S(kVA)| = 3 × VL-N(V) × I(A) / 1000

 

Reactive power calculation:

Q(kVAR) = √(|S(kVA)|2 - P(kW)2)

 

Power factor correction capacitor's capacitance calculation:

C(F) = 1000 × Q(kVAR) / (3×2πf(Hz)×VL-N(V)2)

 

Electronics & Electrical Engineering

BY

C. JAMES ERICKSON Retired Principal Consultant, E. I. du Pont de Nemours & Co., Inc.

CHARLES D. POTTS Retired Project Engineer, E. I. du Pont de Nemours & Co., Inc.

BYRON M. JONES Consulting Engineer, Assistant Professor of Electrical Engineering,

University of Wisconsin—Platteville.

DOWNLOAD :

http://www2.hcmuaf.edu.vn/data/phamducdung/thamkhao/Mark's%20Standard-Handbook/Electronics.pdf

Medium Voltage Design Guide (Schneider Electric)

 

DOWNLOAD :

http://www.schneider-electric.ie/documents/panelbuilders/en/shared/technical-ressources/mv_design_guide.pdf

Low voltage switches

DOWNLOAD :

 

http://www.engineering.schneider-electric.dk/Attachments/ed/guide/low_voltage_swiches_3_guide.pdf

 

LV Switchgear Functions & Selection

DOWNLOAD :

http://www.schneider-electric.com.au/documents/electrical-distribution/en/local/electrical-installation-guide/EIG-H-LV-switchgear.pdf