4.4.4. Power Systems

💡 First Principle: Everything runs on power. Without reliable power, every other resilience measure is irrelevant. Power systems bridge the gap between the moment power fails and the moment it's restored — whether that's seconds (UPS) or hours (generator).

Uninterruptible Power Supply (UPS) — battery-backed devices that provide immediate power during outages. UPS systems bridge the gap between a power failure and generator startup (typically 10-30 seconds). They also condition power, protecting against surges and sags.

Generators — provide long-term power during extended outages. Diesel or natural gas generators take 10-30 seconds to start and can run for hours or days depending on fuel supply.

Dual power supplies — servers with two power supplies connected to separate circuits ensure that a single circuit failure doesn't take the server offline.

Power Distribution Units (PDUs) — distribute power to equipment in data center racks with monitoring and management capabilities. Managed PDUs allow remote reboot of individual outlets and real-time power consumption monitoring — essential for capacity planning and troubleshooting.

Managed Power Distribution — centralized systems that monitor power consumption, control outlets remotely, and alert on anomalies.

Surge protectors prevent voltage spikes from damaging equipment. Power conditioners go further by smoothing out sags, surges, and electrical noise. In critical environments, both are used alongside UPS systems for layered protection.

Testing and maintenance — generators require regular testing under load (monthly is standard practice) to ensure they'll start when needed. Fuel must be stored properly and rotated. UPS batteries degrade over time and require periodic replacement — a UPS with dead batteries provides zero protection. Document and test the full failover chain: utility power fails → UPS activates → generator starts → UPS recharges.

⚠️ Exam Trap: A UPS provides short-term power (minutes to hours) while generators start up. Generators provide long-term power. They work together: UPS bridges the gap, generator sustains. Neither alone is a complete power resilience solution.