The incoming and outgoing lines of a low-voltage circuit breaker cannot be reversed because the dielectric properties of the operating mechanisms on the moving and stationary contact sides are different, resulting in different arc root movement patterns on the moving and stationary contacts. If a bottom-entry line is used, the circuit breaker may need to be derated, meaning the actual operating current will be less than the rated current. Unless the circuit breaker has the same protection function for the neutral (N) line as the phase line, and the settings are identical, it is not recommended to swap the phase and N lines. Let's first look at the schematic diagrams of top and bottom-entry lines for low-voltage circuit breakers:
The area within the dashed boxes in Figures 1 and 2 is the circuit breaker body. Notice that the circuit breaker is four-pole. Within the dashed box of QF, three phase lines have semi-circular short-circuit protection symbols and semi-rectangular overload protection symbols, indicating that this circuit breaker has overload and short-circuit protection functions. In Figures 1 and 2, the pole containing the N line has no protection; the circuit breaker here merely functions as a disconnector. Therefore, the wiring of this circuit breaker is a 3P+N type.
Please note two things:
① It is recommended that the incoming line be from top to bottom, that is, to adopt the top incoming line method. If the bottom incoming line is used, the circuit breaker may need to be derated, that is, the actual operating current will be less than the rated current.
② Unless the circuit breaker has the same protection function for the neutral line as for the phase line, and the setting values are the same, it is not recommended to swap the phase line with the neutral line.
Let's look at why the incoming line direction for a low-voltage circuit breaker is best from top to bottom, rather than from bottom to top .
This issue relates to two reasons: ① The dielectric properties of the operating mechanisms on the moving contact side and the stationary contact side of the circuit breaker are different. Let's look at Figure 3. Figure 3 is a physical diagram of a miniature circuit breaker. The upper side is the incoming terminal, and the lower side is the outgoing terminal. We can see from the figure that the mechanical complexity of the outgoing terminal exceeds that of the incoming terminal.
Figure 3. Physical image of ABB's S200 low-voltage miniature circuit breaker MCB
The incoming side consists of the stationary contacts of the circuit breaker, while the outgoing side consists of the moving contacts. The stationary contacts have a relatively simple structure and better heat dissipation, therefore the dielectric properties, i.e., the insulation properties, of the stationary contacts are better than those of the moving contacts.
The dielectric properties of circuit breaker insulation materials are related to temperature rise. The higher the current, the higher the temperature rise of the appliance, and the relatively lower the dielectric properties. Therefore, when a circuit breaker receives a reverse-current connection, it needs to be appropriately derated, typically to 75% of the rated current . For example, if the rated current of the circuit breaker is 100A, its operating current after reverse-current connection is 75A. For high-quality circuit breakers, derating is not necessary. This will be explained in the circuit breaker's instruction manual or sample materials.
② The movement of the arc root on the moving and stationary contacts of a circuit breaker differs. For the contacts of low-voltage electrical appliances , once an arc is generated, the part of the arc close to the electrode is called the arc root. The arc contracts sharply at the arc root, and a bright, circular spot is formed on the electrode surface; this bright spot is called the arc root spot.
For the cathode arc root, the cathode spot is the electron emission point, with a very high current density, reaching 10⁴ A/cm². Therefore, the electrode material rapidly vaporizes, forming metal vapor that enters the arc gap, thus creating a pit on the cathode surface. For the anode arc root, it is the entry point for electrons into the anode, with a larger area than the cathode spot, a lower current density, and a higher temperature; the electrode material also reaches its boiling point. At the moment the circuit breaker trips, the circuit is polarized, potentially acting as either the anode or cathode; the specific polarity is determined at the instant of tripping. Let's look at Figures 4 and 5.
In the contacts of the switching devices shown in Figures 4 and 5, the upper part is the stationary contact and the lower part is the moving contact. In Figure 4, the stationary contact is the negative or cathode, and in Figure 5, the stationary contact is the positive or anode. Let's look at the arc movement. Figures 4-1 and 5-1: The moving and stationary contacts have just broken, and an arc appears between the contacts; Figures 4-2 and 5-2: As the contact gap increases, the arc is also lengthened, and its middle part bends towards the arc-extinguishing chamber under the action of the electric field force; Figures 4-3 and 5-3: In Figure 4-3, the arc root has left the contact and is moving towards the arc-extinguishing chamber; In Figure 5-3, we see that the anode region of the arc has left the contact, while the cathode region remains on the moving contact; In Figure 5-4, after a period of time, the cathode region of the arc remaining on the moving contact (cathode) leaves the contact and moves towards the arc-extinguishing chamber.
What is the reason? As mentioned earlier, when the circuit breaker contacts open, an electric arc appears, and the resistance between the electrodes continuously increases. The arc forms arc spots on the two electrodes. Due to the burning of the arc, some of the contact material melts and vaporizes, forming a metallic arc. A metallic arc is supported by ions of metal vapor; we call it a metallic phase arc. The characteristics of a metallic phase arc are: the diameter is relatively large, and the arc is essentially stationary; as the arc lengthens with the increase in contact distance, under the influence of an external magnetic field, surrounding gas enters the arc, causing the temperature at the arc center to rise, the current to concentrate towards the center, the arc to become thinner, and the arc changes from a metallic phase arc to a gaseous phase arc. Only then does the arc have the possibility of movement. We know that low-voltage electrical appliances have arc-extinguishing chambers. For the arc to enter the arc-extinguishing chamber, it will inevitably encounter various corners and steps. What effect do these corners and steps have on the movement of the arc ?
A renowned scholar named R. Michal studied the mechanism of arc root movement and concluded that: ① The anodic arc root has the ability to jump over obstacles. That is, the anodic arc can leap over steps and gaps. ② The movement of the cathode arc root must be continuous; it can only move along the surface of the obstacle. When the arc encounters a step, the cathode arc root must climb from below the step along the vertical surface to above the step before continuing, while the anodic arc root directly jumps over it, thus causing the arc to tilt and stop. For specific circuit breakers, if a bottom-entry line is adopted, the lag in arc root movement becomes even more pronounced. In other words, if the circuit breaker's entry direction is top-entry, and the corresponding contacts are stationary, then when the contacts open, regardless of whether the cathode is in the stationary or moving contact, the arc can enter the arc-extinguishing chamber relatively smoothly. At the same time, the dielectric properties are also better; conversely, if the incoming line direction of the circuit breaker is bottom-in and the corresponding contact is a moving contact, the time for the arc to enter the arc-extinguishing chamber will be slower than the former, and the dielectric properties will be slightly worse.
Therefore, the quality of top and bottom incoming lines in a circuit breaker is largely related to the circuit breaker's manufacturing technology. For domestically produced circuit breakers, bottom incoming lines are best avoided. If bottom incoming lines are unavoidable, derating is necessary. For imported or joint-venture circuit breakers, such as those from Schneider Electric, Siemens, and ABB, the capacities of top and bottom incoming lines are the same, requiring no derating.
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