Issue 049, Jan 11, 2023
Grant Justice, Sr. Research Metallurgist
The increasing utilization of high-power DC circuits in electric vehicles, solar installations, and even grid level systems make it important to differentiate between AC and DC arc behavior and the resulting effect
on the contacts.
An electrical arc will form between opening contacts in a switch when there is sufficient voltage and current in the system. These arcs will persist until the voltage required to sustain the arc is greater than the voltage supplied by the circuit. The primary way to extinguish these arcs is to continue to open the contacts increasing the gap forcing the arc sustaining voltage higher until it exceeds the circuit voltage, an example of the relationship between gap, circuit voltage, and circuit current can be found in Figure 1. This primarily applies to circuits with voltages greater than 12 V and currents greater than 0.5 A, circuits under those values can still form unstable or transient arcs during opening that will self-extinguish.¹
Figure 1: General diagram from Pitney demonstrating the gap required to extinguish an arc from silver electrodes in air.¹
There is a major advantage to arc interruption with AC power; the voltage will periodically drop to zero forcing the arc to extinguish, and at 60hz this limits the maximum arc time to less than 8 ms. The voltage across the contact will increase again after it passes though zero, however since the arc was extinguished it will need to restrike. It would initially appear that a restrike is likely since a residual pocket of plasma exists between the electrodes. However, as voltage starts to rise again in an AC circuit the polarity will be reversed, swapping the cathode and the anode. This causes free electrons in the residual plasma to be repelled by the new cathode’s negative charge creating a small (10-⁷ m thick) zone of only positive ions.
These positive ions near the negative cathode act to reduce the effective electric field across the gap between the contacts making it more difficult to restrike the arc. To restrike the arc with the sheath of ions around the cathode the voltage and resulting electric field must be high enough to initiate electron emission from the cathode. If the cathode is still cold, approximately 300 V tends to be required to overcome this sheath.² For circuits with greater than 100 A the arc can sufficiently heat the cathode to
reduce the restrike voltage; similarly, if contact material is a thermionic emitter like tungsten the voltage required to restrike can be much lower.
A practical consequence of this is that most AC switches that operate under 300 V can be relied upon to extinguish the arc the as the circuit voltage passes though zero and that the charge sheath will prevent reignition. This vastly simplifies the design of switches within this operational window. Even for light industrial switches that operate at 480 V two contacts can be used in series to double the restrike voltage as demonstrated in Figure 2.
Equation 2: Switch contact arrangements to take advantage of charge sheaths to prevent restrikes.
DC circuits do not enjoy these advantages and switches for DC circuits require more careful engineering. The voltage will not pass though zero so the arc must be extinguished by driving the voltage higher than the circuit can sustain by separating the contacts. There are several design modifications that help drive the voltage of the arc higher resulting in more rapid arc extinguishment, but they are outside the scope of this note. There are two major consequences to the differences in performance that influence the choice of contact material for these contact setups: first DC arcs will generally burn longer compared to AC arcs and the second is the polarity of a DC arc will be consistent compared to the more random AC arc.
The increased duration of DC arcs will result in greater arc erosion of the contacts. This means for circuits of similar power more advanced contact materials and/or larger contacts will be required to reduce the arc erosion that occurs in DC contacts. Because the polarity of the arc in DC contacts will be consistent, one contact will wear at a higher rate compared to the other. This can be mitigated by using asymmetric contact materials or contact sizes. This direction of the asymmetric wear is dependent on the
switch design with short arc duration generally causing more anode erosion and long arc duration generally causing cathode erosion, assuming similar contact geometries and materials.
Understanding the consequences of the AC vs DC switching is critical for the successful design of an electrical switch. Deringer-Ney offers expertise in the fabrication of contacts from a wide range of arcing contact materials and geometries to support the manufacture of a wide range of switches.
Paliney® and Neyoro™ are registered trademarks of Deringer-Ney Inc.
References:
- Pitney KE. General Contact Theory. In: Ney Contact Manual: Electrical Contacts for Low Energy Uses. Bloomfield, Connecticut: The J. M. Ney Company; 1973. p. 1‒45.
- Slade, P. “The Arc and Interruption” Electrical Contacts: Principles and Applications, edited by Slade, Paul G. CRC press, 2017.pp. 654-657