3. Low-Voltage Power System Basics · Page 5 of 6
Short-Circuit Levels and Backup Protection
SPDs fail safely only when backup protection is correct.
Prospective fault current
If an SPD reaches end of life as a short, the upstream fuse or breaker must clear the fault. Check the manufacturer's backup protection table against your board's fault level.
Dedicated vs shared protection
Some designs use a dedicated SPD backup breaker; others allow connection under a main incomer with specified fuse ratings. Follow the datasheet—guessing is a fire risk.
Selectivity
SPD backup devices should clear SPD faults without blacking out the whole facility when practical.