SIEMIENS MCCB Series Short Circuit Rating Guide

SIEMIENS MCCB Series Short Circuit Rating Guide

Basic and Core Concepts

Positioning and Scope

Applicable products: Siemens molded case circuit breakers (MCCBs) and matching fuses, covering three voltage levels of 240V, 480V, and 600V, mainly used in industrial and commercial distribution systems.

Core Value: Addressing the issue of insufficient breaking ratings for branch circuit breakers - By combining CSA certification for main circuit breakers/fuses and branch circuit breakers, it allows branch equipment to break ratings below the system's available fault current, reducing costs while ensuring safety.

Basic rules and definitions

Series short-circuit rating: The main equipment (line side) and branch equipment (load side) are connected in series, and the combined breaking rating has been tested and certified to meet the requirement of "combined rating ≤ main equipment rating".

General rating standard: The default breaking rating for individual circuit breakers is 5000A rms symmetrical current, and for individual fuses it is 10000A rms symmetrical current (excluding equipment labeling values).

Load side equipment types: including branch circuit breakers, branch line circuit breakers, built-in main circuit breakers, and remote main circuit breakers, all of which must meet the requirements of series combination.

240V level series connection scheme

1. Circuit breaker series combination (240V Breaker)

Covering 4 types of series rated values, the core combination parameters are shown in the following table (excerpted key data):

Series rated value (A) Main circuit breaker model Main circuit breaker maximum current (A) Branch circuit breaker model Branch circuit breaker pole number Branch circuit breaker current range (A) Branch circuit breaker voltage (V)

22,000 QPH,BQH,BLH 70(1P)、125(2P)、100(3P) QP,BQ,BL 1P 15-70 120/240

22,000 QNH,QNRH (2P) 200 QAF2,BAF2 1P 15-20 120

65,000 HQP,HBQ,HBL 70(1P)、125(2P)、100(3P) QPH,BQH,BLH 1P 15-70 120/240

65,000 ED4,ED6 (2,3P) 125 ED2 3P 15-100 240

100,000 CED6 (2,3P) 125 QP,BQ,BL 2P 15-125 120/240

200,000 LPG (3P) 1600 PD6,PXD6 3P 1200-1600 480

2. Series combination of fuses (240V Fuse)

Using fuses as the main equipment, paired with branch circuit breakers, the core combination is as follows:

Series rated value (A) Main fuse type Main fuse maximum current (A) Branch circuit breaker model Branch circuit breaker pole number Branch circuit breaker current range (A)

6,000 L (1,2,3P) 600 QPH,BQH,BLH 3P 15-100

65,000 T (300V) 200 QAF,BQAF,BAF 1P 15-20

100,000 J,R (2,3P) 600 NJGA,NJJA 3P 200-400

480V series connection scheme

1. Circuit breaker series combination (480V Breaker)

Covering 9 series rated values, core parameters (excerpt):

Series rated value (A) Main equipment type Main equipment model Main equipment maximum current (A) Branch circuit breaker model Branch circuit breaker voltage (V)

25000 circuit breakers NGB, HGB, LGB (2,3P) 125 BQD, CQD 277/480

35000 Circuit Breaker JD6-A, JXD6-A (3P) 400 NGB 277/480

50000 Circuit Breaker MD6, MXD6 (3P) 800 NJGA 480

65000 circuit breaker HMG (3P) 800 MD6, MXD6 480

100000 circuit breakers LMG (3P) 800 HFD6, HFXD6 480

150000 Circuit Breaker CJD6 (- A) (3P) 400 JXD6, JD6 480

200000 Circuit Breaker CED6 (3P) 125 ED4, ED6 480

2. Series combination of fuses (480V Fuse)

Combination of main fuse and branch circuit breaker, core example:

Series rated value (A) Main fuse type Main fuse maximum current (A) Branch circuit breaker model Branch circuit breaker pole number Branch circuit breaker current range (A)

50,000 J (2,3P) 400 ED4 3P 15-100

100,000 T,J,R (3P) 1200 HLXD6-A 3P 450-600

600V series connection scheme

1. Circuit breaker series combination (600V Breaker)

Covering 8 types of series rated values, core parameters (excerpt):

Series rated value (A) Main circuit breaker model Main circuit breaker maximum current (A) Branch circuit breaker model Branch circuit breaker pole number Branch circuit breaker voltage (V) Branch circuit breaker current range (A)

14,000 NGB2 (2,3P) 125 BQD6 1P 347 15-70

22,000 HGB2 (2,3P) 125 QP,BQ,BL 2P 347/600 15-70

35,000 HJD6(-A) (2,3P) 600 HFD6 2P 600 70-250

50,000 HMD6,HMXD6 (2,3P) 800 JD6(-A),JXD6(-A) 2P 600 200-400

100,000 CED6 (2,3P) 125 NGB,NGB2 2P 347/600 15-125

2. Series combination of fuses (600V Fuse)

The main fuse is J/R/T type, with a series rated value of 100000A. The branch circuit breakers are JD6 (- A), HJD6 (- A) and other series, with a voltage of 600V and a current range of 200-600A.

Key issue

Question 1: In a 240V distribution system, if a circuit with a series short-circuit rating of 65000A needs to be designed, and the main circuit breaker is selected as HQP series (1P, 70A), what models of branch circuit breakers can be matched? What are the key parameter requirements that need to be met?

Answer: According to the 240V circuit breaker series connection scheme in the document (pages 5-20), when the main circuit breaker is HQP (1P, 70A), the available branch circuit breakers and key parameter requirements are as follows:

Can be matched with branch circuit breaker models: QP, BQ, BL, QPH, BQH, BLH, QT, QPF, BQF, BLF, QE, BE, BLE, etc.

Key parameter requirements:

Number of poles: 1P (corresponding to 1P of the main circuit breaker);

Current range: 15-70 A（QP/BQ/BL/QPH/BQH/BLH）、15-50A（QT）、15-30A（QPF/BQF/BLF/QE/BE/BLE）；

Voltage: 120/240V (QP/BQ/BL/QPH/BQH/BLH/QT), 120V (QPF/BQF/BLF/QE/BE/BLE);

Core limitation: The individual breaking rating of branch circuit breakers can be lower than 65000A, but the breaking rating of series combinations must not exceed the rated value of the main circuit breaker HQP, and all combinations must comply with the table entry corresponding to the "65000A series rating" in the document (such as the combination of HQP 1P 70A and QP 1P 15-70A).

Question 2: What is the core design logic of the "series short-circuit rating" in the document? Why is it allowed for the individual breaking rating of branch circuit breakers to be lower than the available fault current? What prerequisites does this design need to meet?

answer:

Core design logic: Utilizing the "pre current limiting effect" of the main circuit breaker/fuse - when a short circuit occurs in the system, the main equipment (line side) triggers the current limiting or breaking action first, reducing the fault current that the branch equipment (load side) needs to withstand, so that the branch equipment can safely break under working conditions lower than its individual breaking rated value, thereby balancing "system safety" and "cost optimization" (without the need to configure high rated equipment for the branch).

The reason for allowing the rated value of the branch to be reduced: The series combination has been tested by CSA certification to verify the suppression effect of the main equipment on fault current, ensuring that the actual fault current borne by the branch equipment is ≤ its breaking capacity. Therefore, there is no need for the branch equipment to separately meet the system's available fault current requirements.

prerequisite:

Combination compliance: The main branch equipment combination clearly listed in the document must be used, and it is prohibited to replace unverified models;

Rated value limit: The breaking rated value of the series combination is ≤ the breaking rated value of the line side equipment (main circuit breaker/fuse);

Clear identification: The equipment must be labeled with the correct breaking rating, voltage level, and other information, and installed in accordance with the specifications of the power distribution system.

Question 3: What are the differences in the rated range, core equipment model, and application scenarios of the series connection schemes for three voltage levels of 240V, 480V, and 600V?

Answer: The differences in the series connection schemes of the three voltage levels are mainly reflected in the rated value range, equipment selection, and application scenarios. The specific comparison is shown in the following table:

Comparison dimension 240V level 480V level 600V level

Series rated range 22000A~200000A 25000A~200000A 14000A~100000A

Core main equipment model  QP/BQ/BL、QPH/BQH/BLH、ED4/ED6、HQP/HBQ NGB/HGB/LGB、FD6-A/FXD6-A、MD6/MXD6、HMG NGB2/HGB2/LGB2、HJD6(-A)、HMD6/HMXD6、CED6

Model of core branch equipment  QP/BQ/BL、QAF/BAF、ED2、QJ2/QJH2 BQD/CQD、NGB、NJGA、LD6/LXD6 BQD6、HFD6、JD6(-A)/JXD6(-A)

Voltage adaptation: 120V, 240V, 120/240V, 277V, 480V, 277/480V, 347V, 600V, 347/600V

Typical application scenarios include civil building power distribution (such as residential and commercial buildings), small industrial equipment industrial plants (such as motor control and production line power distribution), and large commercial facilities high-voltage industrial scenarios (such as heavy machinery, high-voltage motors, and main power distribution in large factories)

The key difference point supports 1P equipment (single-phase distribution), with the most diverse combination types mainly consisting of 3P equipment (three-phase distribution). The rated value covers the highest voltage range in the medium to high range, and the equipment needs to withstand higher insulation levels. The upper limit of the rated value is lower