Abstract: Different types of circuit breakers with varying short-circuit breaking capacities are selected to meet the different expected short-circuit current requirements of different lines (when I is the same). The selection principle for circuit breakers is: the short-circuit breaking capacity of the circuit breaker ≥ the expected short-circuit current of the line. Keywords: Circuit breaker, key points, power distribution lines. 1. Different types of circuit breakers should be selected for different loads. The most common loads are power distribution lines, motors, and household and similar household appliances (lighting, household appliances, etc.). Correspondingly, there are distribution protection type, motor protection type, and household and similar household protection type circuit breakers. The protection properties and characteristics of these three types of circuit breakers are different. For distribution type circuit breakers, there are Class A and Class B: Class A is non-selective, and Class B is selective. Selective means that the circuit breaker has three-stage protection characteristics: overload long-time delay, short-circuit short-time delay, and short-circuit instantaneous. Most of the DW15, DW17(ME), AH, and DW40/DW45 series universal (also known as frame-type) circuit breakers are type B. However, the DZ5, DZ15, DZ20, TO, TG, CM1, TM30, and HSM1 series, as well as certain specifications of the universal DW15 and DW17 series, only offer two-stage protection: long-delay overload protection and instantaneous short-circuit protection. These are non-selective Class A circuit breakers. Selective protection: When a short circuit occurs at point F, only the circuit breaker QF2 closest to point F operates, while the circuit breaker QF1 above does not operate. This is selective protection (because QF1 does not operate, the unfaulted branches QF3 and QF4 continue to be powered). If both QF2 and QF1 are Class A circuit breakers, then when a short circuit occurs at point F and the short-circuit current reaches a certain value, QF1 and QF2 will operate simultaneously, causing a complete power outage in the QF1 circuit and its downstream branches, thus failing to provide selective protection. The reason selective protection is possible is that QF1 is a Class B circuit breaker, possessing short-circuit short-delay capability. When a short circuit occurs at point F, the short-circuit current flows through both the QF2 branch and the QF1 circuit. The instantaneous trip unit of QF2 will trip (its total breaking time is typically no more than 0.02s). Due to QF1's short-delay capability (its short-delay is ≥0.1s or 0.2, 0.3, or 0.4s), QF1 will not trip within 0.02s. When QF2 trips and disconnects the faulty line, the entire system returns to normal. Therefore, to achieve selective protection, the upstream circuit breaker should be a Class B circuit breaker with three-stage protection. For motor protection circuit breakers that directly protect motors, two-stage protection performance—overload long-time delay and short-circuit instantaneous—is sufficient. This means that Class A circuit breakers (including molded case and universal types) can be selected. Series such as DZ5, DZ15, TO, TG, GM1, TM30, HSM1, and DW15, in addition to their distribution protection capabilities, also offer motor protection functions in their 630A and below specifications. Protection for household and similar applications (formerly known as conductor protection or lighting protection) also utilizes small Class A circuit breakers, with typical products including C45N, PX200C, and HSM8. Overcurrent protection circuit breakers for distribution (lines), motors, and household appliances vary in their overload current withstand capabilities (including motor starting current and starting time) due to differences in the protected objects (such as transformers, wires and cables, motors, and household appliances). Therefore, the protection characteristics of the selected circuit breakers also differ. (1) Inverse-time breaking characteristic of distribution protection circuit breaker Note: Returnable characteristic: Considering that there are groups of motors in the distribution line, since the motors are only part of the load and a group of motors will not start at the same time, it is determined to be 3In (In is the rated current of the circuit breaker, In≥IL, IL is the rated current of the line). The circuit breaker is tested. When the test current is 3In, it is held for 5s (In≤40A), 8s (40A＜In＜250A), and 12s (In＞250A). Then the current is returned to In, and the circuit breaker should not operate. This is the return characteristic. (2) Inverse-time breaking characteristic of motor protection circuit breaker Note: According to the nature of the motor load, it can be selected to operate within 2, 4, 8, or 12 minutes. Generally, 2 to 4 minutes is selected. 7.2In is also a returnable characteristic. It must avoid the starting current of the motor (5 to 7 times In). Tp is the delay time. The operating time Tp can be selected according to the load characteristics of the motor. It is 2s < Tp ≤ 10s, 4s < Tp ≤ 10s, 6s < Tp ≤ 20s and 9s < Tp ≤ 30s. Generally, 2s < Tp ≤ 10s or 4s < Tp ≤ 10s is selected. (3) The instantaneous setting current of the distribution protection type is 10In (with an error of ±20%). In is 400A and above. It can be selected from 5In and 10In (as proposed by the user and set by the manufacturer). The instantaneous setting current of the motor protection type is 12In. In is generally equal to the rated current of the motor in the design. (4) Overload tripping characteristics of circuit breakers for household and similar applications Note: Types B, C, and D are instantaneous tripping types: Type B tripping current > 3-5In, Type C tripping current > 5-10In, Type D tripping current > 10-50In. Users can choose one of them according to the needs of the protected object. (5) Short-circuit short-delay characteristics of Class B circuit breakers DW15 type circuit breaker: 3-10In (Inm is the frame current when Inm is 1600A), 3-6In (Inm is 2500A or 4000A), short delay time is 0.2 or 0.5s. ME type circuit breaker: 3-12In, short delay time is adjustable from 0 to 0.3s. DW45 type circuit breaker: 0.4-15In, short delay time is adjustable from 0.1, 0.2, 0.3 and 0.4s. When designing engineering projects, circuit breakers with different protection characteristics (as mentioned above) should be selected based on different load objects to avoid serious consequences due to improper selection. In practice, the most common confusion is misselecting motor load protection as distribution protection or household protection. Miniature circuit breakers (MCBs) also have motor protection types, such as the Tianjin Merlin Gerin C45AD. 2. Selecting circuit breakers with different short-circuit breaking capacities to adapt to different expected short-circuit current requirements (when I is the same). The principle for selecting circuit breakers is: the short-circuit breaking capacity of the circuit breaker ≥ the expected short-circuit current of the line. Assume a power source (SL710/0.4kV transformer) has a capacity of 1600kVA, a secondary current of 2312A, and a short-circuit current of 42.96kA at 5m from its output terminal. A branch circuit has a rated current of 125A. Since this branch circuit is very close to the transformer, say at a distance of 10m, the circuit breaker for this branch circuit should be an HSM1_125H type molded case circuit breaker (its ultimate short-circuit breaking capacity is 400V, 50kA). However, at a distance of 50m from the transformer, due to the resistance and reactance of the busbar, the short-circuit current drops to 34.5kA, and at 100m, it drops to 28.8kA. Therefore, an HSM1_125M type molded case circuit breaker (its ultimate short-circuit breaking capacity is 400V, 35kA) can be selected. Currently, many circuit breaker manufacturers in China classify the short-circuit breaking capacity of the same frame size into levels such as E, S, M, H, L (HSM1 series from Hangzhou Zhijiang Switchgear Factory), C, L, M, H (CM1 series from Changshu Switchgear Factory), or S, H, R, U (TM30 series from Tianjin Low Voltage Electrical Appliances Company). Among these, E represents the economic type, S the standard type, M the medium short-circuit breaking type, H the high breaking capacity, L the current-limiting type, C the economic type, L the low breaking capacity; M the high breaking capacity, H the ultra-high breaking capacity; S the standard type, H the high breaking capacity, R the current-limiting type, and U the ultra-high breaking capacity. Taking the HSM1_125 molded case circuit breaker as an example, the ultimate short-circuit breaking capacity of the E type is 400V, 15kA; the S type is 400V, 25kA; the M type is 400V, 35kA; and the H type is 400V, 50kA. Their prices also differ significantly. For example, if type E is valued at 1, then type S is 1.2, type M at 1.4, and type H at 2. That is, the cost of one type H circuit breaker can buy two type E circuit breakers. Users should not artificially add a so-called "safety factor" when designing and selecting circuit breakers, to avoid waste. 3. Regarding the ultimate short-circuit breaking capacity, operational short-circuit breaking capacity, and short-time withstand current ultimate short-circuit breaking capacity (Icu) of a circuit breaker, these refer to the short-circuit current that can be connected and disconnected under certain test parameters (voltage, short-circuit current, power factor) and a certain test procedure, after which the circuit breaker can no longer carry its rated current. Its test procedure is 0—t (online)C0 ("0" means disconnection, t is the interval time, generally 3 minutes, and "C0" means immediate disconnection after connection). After the test, the tripping characteristics and power frequency withstand voltage must be verified. Operating short-circuit breaking capacity (Ics) refers to the ability to connect and disconnect a short-circuit current under certain test parameters (voltage, short-circuit current, and power factor) and after a certain test procedure. After this connection and disconnection, it must continue to carry the rated current. Its test procedure is 0—t(online)C0—t(online)C0. Short-time withstand current (Icw) refers to the circuit breaker's ability to withstand short-time currents of 0.05, 0.1, 0.25, 0.5, or 1 second without tripping under certain voltage, short-circuit current, and power factor conditions. Icw is an indicator of the circuit breaker's electrical and thermal stability during short-time tripping, and it is specific to Class B circuit breakers. The minimum Icw value is typically 12In or 5kA when In ≤ 2500A, and 30kA when In > 2500A (Icw for DW45_2000 is 400V, 50kA; Icw for DW45_3200 is 400V, 65kA). The test conditions for operational short-circuit breaking capacity are extremely stringent (single breaking, secondary breaking). Since it must continue to carry rated current after the test (the number of times is 5% of its lifespan), it verifies not only tripping characteristics and power frequency withstand voltage but also temperature rise. IEC 947_2 (and the 1997 revised version IEC 60947_2) and the Chinese national standard GB 140482 stipulate that Ics can be 25%, 50%, 75%, and 100% of the ultimate short-circuit breaking capacity Icu (50%, 75%, and 100% for Class B circuit breakers; the 25% limit is omitted for Class B circuit breakers as they are mostly used for mainline protection). An important principle for selecting circuit breakers, as mentioned above, is that the circuit breaker's short-circuit breaking capacity ≥ the expected short-circuit current of the line. This short-circuit breaking capacity usually refers to its ultimate short-circuit breaking capacity. Regardless of whether it's a Class A or Class B circuit breaker, its operational short-circuit breaking capacity is almost always less than its ultimate short-circuit breaking capacity Icu. Class A: DZ20 series Ics = 50%～77%Icu, CM1 series Ics = 58%～72%Icu, TM30 series Ics = 50%～75%Icu (some products have Ics = Icu). Class B: DW15 series Ics = approximately 60% of Icu (some, such as 630A, have Ics = Icu, but their short-circuit breaking capacity is only 30kA at 400V); DW45 series Ics = 62.5%~80% Icu. Regardless of whether it's a Class A or Class B circuit breaker, as long as its Ics meets the Icu percentage value specified in IEC947_2 (or GB14048.2) standard, it is a qualified product. When designing and selecting, users only need to ensure that the circuit breaker's ultimate short-circuit breaking capacity is greater than or equal to the expected short-circuit current of the line. For the line itself, for example, in the example above with a transformer capacity of 1600kVA, the possible short-circuit current is approximately 43kA, calculated only at a distance of 5m from the transformer. The probability of such a short circuit is extremely small. When selecting a circuit breaker, as long as its ultimate short-circuit breaking capacity is greater than 43kA, for example, 50kA is sufficient. After one "0" and one "C0" cycle, its mission is complete, and a new circuit breaker must be replaced. Its short-circuit breaking capacity, for example, 50% of Icu, reaches 25kA. It can perform primary breaking, secondary switching (at a 25kA short-circuit current), fault current, and also carry its rated current—a very demanding task. Some users believe that the circuit breaker should be designed with an operating short-circuit breaking capacity (Ics) ≥ the expected short-circuit current of the line, which is a misunderstanding and unnecessary. Some manufacturers advertise in their catalogs that their products have Ics = Icu. If this is true, it means their Icu specification has a margin; if not, it indicates inaccuracies and should not be fully trusted. Moreover, circuit breakers with Ics = Icu are much more expensive and not cost-effective. For decades, a type of cascade protection (also known as backup protection) has been popular abroad. As shown in Figure 2, the ultimate short-circuit breaking capacity of the selected QF2 circuit breaker is less than the expected short-circuit breaking capacity of its line (for example, the line rated current is 250A, while the expected short-circuit current is 50kA). In this case, QF2 is selected as HSM1_250S circuit breaker (Icu is 400V, 35kA). When a short circuit occurs at point F (short-circuit current reaches 50kA), QF1 (assuming the rated current at QF1 is 400A, QF1 is selected as HSM1_400H, whose Icu is 400V, 65kA) and QF2 will break the circuit together. QF2 will only bear a portion of the short-circuit current breaking, and the rest will be borne by QF1. For most of the fault current less than 35kA at point QF2, QF2 will bear the fault. This type of cascaded protection also has certain conditions. For example, the adjacent branch is not a critical load (because once QF1 trips, the QF3 circuit will also lose power), and the instantaneous setting values ​​of QF1 and QF2 must be coordinated. The main purpose of this cascaded protection is to save investment. It should be mentioned that the short-circuit breaking capacity of all circuit breakers (whether Icu or Ics) is the effective value of the periodic component. In the short-circuit test, the current of "C0" (close) is the peak current Ich. When the short-circuit breaking test is performed at the test station, the voltage, short-circuit current (effective value), and power factor (cos) have been adjusted, and its closing current is determined. The closing current test ("C" test) uses the peak current to assess the ability of the contacts and other conductors to withstand the electrodynamic repulsion and thermal stability. Whatever the effective value current (breaking current) is, the corresponding peak current will be. Users do not need to consider the peak current parameter. Peak current (impulse current) ich = kch (√2Ic), where Ic is the effective value of the periodic component, kch is the impulse coefficient (1 < kch < 2), and kch × 2 is the peak coefficient. 4. Selection of Four-Pole Circuit Breakers: Four-pole circuit breakers are necessary in the following situations: 1) Systems requiring dual power supply switching must use four-pole circuit breakers to meet the isolation needs during maintenance, testing, and repair of the entire system; 2) The main single-phase switch for each household in a residential building should be a two-pole switch with an N pole (a four-pole circuit breaker can be used); 3) Residual current operated protective devices (RCDs) must ensure that all live conductors in the protected circuit are disconnected. Therefore, for circuits requiring residual current operation protection, RCDs with an N pole (such as four poles) should be selected. Currently, there are six types of four-pole molded case circuit breakers supplied in the domestic market: 1) The N pole of the circuit breaker does not have an overcurrent trip unit, and the N pole opens and closes the circuit together with the other three phase poles; 2) The N pole of the circuit breaker does not have an overcurrent trip unit, and the N pole is always connected and does not disconnect together with the other three phase poles; 3) The N pole of the circuit breaker has an overcurrent trip unit, and the N pole opens and closes the circuit together with the other three phase poles; 4) The N pole of the circuit breaker has an overcurrent trip unit, and the N pole is always connected and does not disconnect together with the other three phase poles; 5) The N pole of the circuit breaker is equipped with a neutral line breakage protector, and the N pole opens and closes the circuit together with the other three phase poles; 6) The N pole of the circuit breaker is equipped with a neutral line breakage protector, and the N pole is always connected and does not disconnect together with the other three phase poles. Types 1) and 2) are suitable for normal conditions where the neutral current does not exceed 25% of the phase current (transformer connection group designation is Yyno). Type 2) is suitable for TN-C systems (PEN line must not be disconnected). Types 3) and 4) are suitable for three-phase load imbalances and loads containing a large number of electronic devices (with high harmonic content), causing the N line current to be equal to or greater than the phase current, resulting in N line overload and the inability to cut off the overload fault by means of the overcurrent tripping of the three phase lines. Type 4) is suitable for TN-C systems. Types 5) and 6) are suitable for disconnecting the three phases and neutral line when the neutral line is broken to protect single-phase equipment from damage and indirect electric shock accidents. Type 6) is suitable for TN-C systems.