Electric arcing in HV batteries can be a significant safety issue. See Arc-Management Methods for the design methods used to detect, contain and clear an arc once one develops.
Electrical breakdown is a transient physical process in which an insulating medium, such as a gas or a solid dielectric, loses its insulating properties and rapidly transitions into an electrical conductor. This phenomenon occurs when an applied voltage or electric field across a gap becomes sufficiently intense to cause a massive multiplication of charge carriers.
Above 24V DC it is difficult for a contactor or a relay to switch, making or breaking, because the DC arc generated in air is only extinguished by increasing the distance between the contacts. Generalising, the higher the voltage, the bigger the gap required. (AC is different as it self-extinguishes as it goes through the zero-crossing point — see DC vs AC Arcs for why.)
Creepage and Clearance
High voltage circuits can discharge across a gap, the gap can be through the air or across a surface.
- Clearance: the distance between two conductors through the air.
- Creepage: distance between two conductors on the surface.
DC Arc Flash Hazard Calculations
At present, no consensus standard provides a validated methodology for calculating direct current (dc) arc-flash incident energy. IEEE 1584-2018, IEEE Guide for Performing Arc-Flash Hazard Calculations, is limited to alternating current (ac) systems and does not address dc arc-flash. NFPA 70E includes personal protective equipment (PPE) category tables applicable to certain dc battery systems; however, it does not provide analytical or empirical calculation methods for determining dc arc-flash incident energy. Ongoing research continues to advance the understanding of arc-flash behavior in dc and battery systems, but standardized calculation models have not yet been established.
DC vs AC Arcs
Battery packs run on direct current, and that single fact makes their contactors, fuses and connectors behave very differently in a fault than the AC switchgear most electricians train on.
Thermal Runaway in High-Voltage Systems
When a cell goes into thermal runaway, it acts like a pressure vessel blowing its top. Data highlights the staggering volume of material released during these events.