## 5 Discussion  
Low-current diffuse-type intensive arcs with highly evaporative cathode roots are mainly formed at the edges of the contacts and slots. At the position of a particularly intensive cathode spot, this can result from the continued heating of the localized hot area of the anode [30]. This also influences the density of the metal vapor at current zero. The thermal hysteresis will maintain a hot cathode surface for some time. Even though the current decline rate is very fast, the hot cathode continues to emit plasma for some time [31]. As a result, when the transient recovery voltage is increased, breakdown occurs and interruption fails. Meanwhile, local overheating would not only cause major contact erosion, but also reduce the breaking capacity because of the increased metal vapor generation and the pronounced afterglow effect [32]. Once the intensive arc has developed, the arc root moves slowly and more plasma is emitted to the gap. At the position of the intensive arc, the amount of residual metal vapor is much more when the arc current passes through zero. Hence, the probability of re-ignition increases and the breaking capacity reduces.  
On comparing the arc appearances of the two types of contacts, no intensive arc mode is observed in the CuCr50 contact. As we know, the melting and boiling points of CuW80 are much higher than that of CuCr50, for which the arc and cathode surface temperatures are higher in the type A contact. Thus, the extinguishment and movement of cathode spots are impeded, which causes the arc to burn continuously at the local region. Therefore, it can be inferred that the occurrence of an intensive arc mode is related to the contact material, and that the possibility is greater for a high-melting-point contact material. Besides, in [33], it is noted that severe erosion occurs at both the anode and the cathode under the intensive arc mode, and that the intensive arc mode tends to occur at shorter gap lengths.  
At the arc root, the charge carriers collide with the contact surface and transfer energy to the contact [21]. The total arcing energy can be calculated from the measured arc voltage, arc current, and arcing time, and can be expressed as follows:  
\[W_{arc}=P_{arc}\cdot t_{arc}=l^{2}_{arc}\times R_{arc}\times t_{arc} \tag{9}\]  
Figure 12: Breaking experiment results of the type A vacuum interrupter with 1 to 6 ms arcing times.  
where \(P_{arc}\) is the mean power input (in Watts), \(W_{arc}\) is the arc energy (in Joules), \(t_{arc}\) is the arc time, \(R_{arc}\) is the arc resistance, and \(I_{arc}\) is the arc direct current (in Amperes). In the DC-arcing stage, the arc current and arc voltage change by small amounts. According to Equation (9), the arcing energy is affected by the arcing duration \(t_{arc}\) proportionally. With a long arcing time, the DC-arcing stage is prolonged, and more energy is input to the contact. The energy transferred to the contact is mainly expended as heat conduction into the contact and evaporative cooling, which leads to higher temperatures on the contact surface. This may be a necessary condition for the occurrence of the intensive burning arc. The intensive arc mode only develops if the arcing time is long enough. Besides, the amount of residual metal vapor in the gap distance is more with a longer arcing time than with a shorter arcing time, leading to a high metal vapor density, weak dielectric strength, low breakdown voltage, and long recovery time. Therefore, a long arcing time is detrimental for the recovery of dielectric strength after current zero [9].