Switching arc
A switching arc is a serial arc (known colloquially as a spark ) that occurs when two electrical contacts through which current flows are separated . With small currents, only so-called tear -off sparks or switching sparks occur, which go out by themselves. In the case of larger currents, the creation of an arc is prevented by special components or a rapid breakdown of the spark (e.g. by an arc extinguishing chamber ) is achieved in order to prevent the contacts from being damaged by the high temperatures. These measures are known as spark quenching .
causes
Switching sparks and switching arcs occur because the electrical current continues to flow in the form of a spark discharge or an arc discharge after the contacts have opened, as shown in the adjacent sketch. With closed contacts, under a. shown, there is an approximately homogeneous current distribution, shown by red flow threads. When the contact is separated, the current density is initially concentrated at the last contact point, under b. shown. With further opening, the arc then forms between the contacts at that point or points, as under c. shown.
The reason for this is the low dielectric strength of the insulation material, such as air between the contacts that are not yet wide open, as a result of which these insulation materials are ionized . Such a discharge is additionally promoted if, at the moment the contacts are lifted from one another, the current flow over a small cross-section and high current densities creates hot spots at the break-off points, causing glow emission and the subsequent delivery of metal ions. By impact ionization as in a gas discharge now the firing voltage decreases and makes it difficult to interrupt.
Switching off inductive loads ( motors , contactor coils , electromagnets , transformers ) is particularly problematic . Here, the energy stored in the magnetic field of the inductance causes the current to continue flowing - the voltage across the contacts then immediately increases to very high values when they are opened. A switching arc can therefore also occur here if the operating voltage is far below the burning or ignition voltage of the arc.
With alternating current and an ohmic load (voltage & current in phase), the current flow is stopped at the zero crossing until the voltage is high enough again to re-ignite the arc. In the case of an inductive load (current lags behind the voltage), the arc also extinguishes when the current crosses zero. Due to the leading voltage, however, the re-ignition voltage is reached more quickly, so that the interruption is significantly shorter than previously described. The shortened time-out also makes re-ionization of the route even easier. The switching arc therefore extinguishes more heavily than with a purely ohmic load.
With a capacitive load (current leads) it is exactly the opposite. When the current crosses zero, the voltage drops even further in the direction of 0 and thus takes significantly longer to reach a corresponding ignition voltage level again. During this time, however, the spark gap has mostly de-ionized and the resistance has increased, which makes re-ignition even more difficult.
The contacts of contactors for this reason in highly inductive loads (as AC-3 called load type) for lower switching currents than with resistive load (load type AC-1 specified), this burden is in the Utility category specified. At much higher frequencies, switching arcs have a similar problematic behavior as with direct voltage, they are even more difficult to extinguish at high frequencies , since displacement currents also contribute to ionization.
In the case of DC voltage, there is no zero crossing, so this possibility of this (independent) deletion is not applicable. Here, the isolating distance must be sufficiently large, and this distance must also be reached quickly so that the arc breaks off safely and quickly.
consequences
Switching sparks and switching arcs lead to interference emissions and contact wear. If the arc is not suppressed or extinguished quickly enough, this leads to the destruction of the switching contacts due to contact erosion, especially with high currents and voltages. In the worst case, this can lead to contacts being welded together and no longer able to be separated. Due to the extremely high temperatures of several thousand ° C, there is a risk - depending on the immediate environment - of igniting other objects and starting a fire.
Switching sparks that are self-extinguishing also lead in the long term to contact wear and premature failure of relays and switches . When specifying the maximum number of switching cycles of relays and contactors, a distinction is made between the mechanical number of cycles and the number of cycles under nominal electrical load. The two numbers of switching cycles often differ from each other by a factor of 10.
literature
- Basics of electrical energy supply. Volume 4: Switchgear. (PDF; 4.0 MB), seminar documents, requested December 9, 2010
Individual evidence
- ↑ Detection of low-current arcing faults in low-voltage switchgear. (PDF; 4.8 MB) Consumers in the network and their influence on the electric arc. Peter Müller, February 2015, pp. 74ff , accessed on January 28, 2019 .
- ↑ European teaching material for electrical engineering. Chapter 28.3 Contact materials. Issue 22, p. 522.