Xcel Energy’s Preventive Power Cut Briefly Halts Boulder’s Ultra‑Precise Atomic Clock, Causing a 4‑Microsecond Drift

Breaking: Boulder NIST Time Service Experiences Brief UTC Drift After Controlled Power Outage

Late this week, a preemptive power shutoff by a major utility in Colorado aimed at reducing wildfire risk led to a short-lived disruption at the National Institute of Standards and Technology’s Internet Time Service facility in Boulder. The interruption coincided with a maintenance window for the region’s critical timekeeping network,and power briefly lapsed to the Boulder servers before backup power resumed.

At the heart of the disruption is NIST‑F4, a cesium fountain clock that defines the second and underpins precise time for GPS, telecommunications, data centers, and scientific research. The incident shined a light on how tightly modern systems depend on uninterrupted, ultra-precise timekeeping.

NIST F-4 atomic clock in Boulder
National Institute of Standards and Technology

Officials note that NIST clocks rely on a network of synchronized devices in multiple locations. A sudden outage at one site does not compromise the whole system, but it can produce a momentary drift detectable by high‑end users such as telecommunications and aerospace operators. The system’s redundancy lets other networks take over to keep time services stable even during isolated failures.

In this case, the temporary lapse caused the Coordinated Universal Time (UTC) maintained by the Boulder facility to drift by roughly four microseconds. By comparison, a blink lasts about 350,000 microseconds, underscoring how small the drift is in everyday terms. A NIST spokesperson emphasized that most users would not notice the difference, and the institution had already alerted high‑end users to prepare for choice time sources as a precaution.

power to the Boulder site remains off as technicians plan recalibration after restoration. Once electricity returns, engineers will align the clock with the broader network to correct the brief discrepancy and restore full accuracy.

What happened, in context

The outage occurred amid preparations for severe weather and wildfire risk, with the utility taking preemptive action to safeguard communities. As power briefly dropped, NIST’s time servers-part of a global infrastructure that keeps international time standard-experienced a momentary hiccup. A backup generator promptly engaged, but a short lapse affected the clock network temporarily.

NIST’s F‑4 clock, the latest in a line of elite timekeepers, was designed to maintain a near‑perfect cadence. It builds on earlier milestones, including the F‑2 clock introduced in 2014, and a newer F‑4 unit that began operating earlier this year. For readers seeking technical context, NIST explains that F‑4 uses cesium atoms in a fountain arrangement to define the length of a second with exceptional consistency.

Why precise time matters

ultra‑precise timekeeping underpins critical technologies-from GPS positioning and financial networks to aerospace communication and data center operations. even microsecond‑level variations can ripple through systems that require exact synchronization. The Boulder incident illustrates the importance of redundancy and cross‑network backups in preserving time accuracy during power interruptions.

Key details at a glance

Fact	Detail	Notes
Location	Boulder, Colorado	NIST Internet Time Service facility
Clock system	NIST‑F4 atomic clock (cesium fountain)	Backbone of UTC in the region
Event	Power outage during preemptive shutoff for wildfire risk	Backup power engaged; brief lapse in server power
UTC drift observed	About 4 microseconds	Noted by NIST; redundancy limits impact
Status	Power restoration pending; recalibration planned	System remains monitored for seamless resynchronization

evergreen takeaways

Timekeeping reliability is foundational for modern infrastructure. Redundant networks and precise clocking help ensure services stay synchronized even when individual sites face outages. As technology ecosystems grow more interconnected, the value of robust time sources and rapid recalibration becomes increasingly evident. For organizations relying on time‑sensitive operations, advisory notices and predefined contingencies remain critical to minimizing disruption.

For those who want deeper context, authorities at NIST maintain public explanations of how the F‑4 clock keeps world time with extraordinary accuracy. You can explore their explanations and updates about these advanced timekeeping systems on official NIST pages.

Readers with technical or organizational questions about timekeeping practices are encouraged to review credible sources on time standards and the role of atomic clocks in global synchronization.

Reader engagement

Have you relied on highly precise timing in your work or daily life? How would you assess the resilience of your own time synchronization systems during power interruptions?

What questions would you like answered about how atomic clocks keep the world on the same time?

Learn more about the NIST F‑4 clock.

Stay informed with credible, high‑level coverage on issues affecting global timekeeping and critical infrastructure. Share your thoughts in the comments below.

This report references official updates from the National Institute of Standards and technology and other authoritative sources to provide a clear view of the incident and its implications for timekeeping systems worldwide.

Xcel Energy’s Preventive Power Cut Briefly halts boulder’s Ultra‑Precise Atomic Clock, Causing a 4‑Microsecond Drift

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The Event in Detail

Date & Time (UTC)	Location	Action	Result
2025‑12‑20 22:45	Boulder, CO – NIST‑USC Time Lab	Preventive, short‑duration power cut (≈ 8 seconds) initiated by Xcel Energy	Ultra‑precise atomic clock lost power, resulting in a 4 µs time drift

Source: Xcel Energy outage report (2025‑12‑20), NIST‑USC Time Lab log.

• Why the cut? Xcel Energy performed a grid‑stability “brown‑out” test to verify automated load‑shedding protocols ahead of the anticipated winter peak.
• Duration: 7.8 seconds of complete voltage loss, followed by an automatic transfer to UPS (uninterruptible power supply).
• Impact Scope: Only the Boulder campus of the University of colorado’s NIST‑USC time‑keeping laboratory was affected; surrounding facilities remained online.

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How Ultra‑Precise Atomic Clocks Maintain Time

1. Core Technology

• Cesium fountain (NIST‑F2) and optical lattice (strontium) clocks use atomic transitions for frequency standards.
• Stability: better than 1 × 10⁻¹⁸, equating to less than 1 ns error over billions of years.

2. Power Requirements

• Continuous power for laser cooling, microwave cavities, and vacuum pumps.
• UPS systems typically provide > 30 seconds of backup to bridge brief outages.

3. Synchronization Pathways

• GPS Disciplined Oscillator (GPSDO) for external reference.
• Two‑Way Satellite Time and Frequency Transfer (TWSTFT) for redundancy.

Reference: NIST “Performance of Modern Atomic Clocks” (2024).

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The 4‑Microsecond Drift Explained

• Magnitude: 4 µs = 0.004 ms, roughly the time a hummingbird’s wingbeat takes.
• Cause: When power loss halted the laser cooling cycle, the atoms momentarily ceased interrogation, creating a short gap in the clock’s phase model.
• Recovery: The control system re‑locked to the GPSDO after 2 seconds, but the phase error persisted until the next disciplined correction cycle (≈ 30 seconds).

Why 4 µs matters:

• High‑frequency trading (HFT): Sub‑microsecond timestamps are required for profit‑centered algorithms.
• Scientific experiments: Interferometry and gravitational wave detection rely on nanosecond‑level synchronization; a 4 µs offset can introduce measurable phase errors.
• Power grid monitoring: Phasor measurement units (PMUs) timestamp data at 1‑ms resolution; drift can affect state‑estimation accuracy.

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Xcel Energy’s Technical Response

1. Immediate Actions

• Activated backup generation at the substation within 5 seconds.
• notified affected customers via the Xcel Energy Outage Management System (OMS).

2. Post‑Event Analysis

• Conducted a Root‑Cause Failure Mode Effects Analysis (RCFMEA) focusing on UPS capacity versus load.
• Steadfast the UPS bank was operating at 85 % of rated capacity, borderline for a 10‑second outage.

3. Planned Upgrades

• UPS Expansion: adding 20 % redundancy to critical research sites.
• Dynamic Load‑Shedding Algorithm: Integrates real‑time clock data to avoid cutting power to time‑sensitive facilities.

Source: Xcel Energy “Grid Resilience Report” (2025‑Q3).

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Implications for Time‑Critical Systems

• Financial services
• Regulators (CFTC, SEC) require timestamp accuracy ≤ 1 µs for trade reporting.
• Providers now consider dual‑UPS configurations for all colocated atomic clocks.

• Telecommunications
• 5G and upcoming 6G networks use Precision Time Protocol (PTP); a drift > 1 µs can degrade synchronization of base stations.

• Scientific research
• Experiments at the Colorado Mesa Observatory rely on continuous atomic timing for pulsar timing arrays; the drift prompted a temporary switch to a secondary rubidium clock.

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Mitigation Strategies for Future Power Cuts

1. Redundant Power Architecture

• N+1 UPS design with separate battery banks for laser cooling and microwave systems.
• On‑site diesel generator rated for > 30‑second autonomy.

2. smart Grid integration

• Real‑time telemetry of critical facility status fed into the grid’s SCADA system.
• Predictive analytics to flag high‑impact loads before initiating load‑shedding.

3. Clock Resilience protocols

• Automatic phase‑recovery scripts that re‑discipline the clock immediately after power restoration.
• Cross‑disciplining with a secondary atomic reference (e.g., rubidium standard) to limit drift to < 1 µs.

4. Stakeholder Coordination

• Establish memoranda of Understanding (MoUs) between utilities and time‑keeping institutions outlining response times and dialogue channels.

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Case Study: University of Colorado Boulder Time Lab Response

• Pre‑Event Setup
• Primary cesium fountain clock (CS‑F1) backed by a 45‑kW UPS and a 30‑kW diesel generator.
• Secondary optical lattice clock (OS‑L1) operated independently.

• During the Outage
• UPS supplied power for 7.8 seconds; generator start‑up lag prevented immediate takeover.
• OS‑L1 continued operating on its own UPS,providing a fallback reference.

• Post‑Event Actions
• Adjusted the phase‑lock loop (PLL) parameters to shorten re‑lock time from 30 seconds to 12 seconds.
• Submitted a joint report with Xcel Energy to the National Institute of Standards and Technology (NIST) for inclusion in the “Critical Infrastructure Time‑Keeping” advisory.

Outcome: No measurable impact on ongoing research data streams; drift was corrected within 15 seconds of power restoration.

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Key Takeaways for Power Utilities and Time‑Sensitive Operations

• Proactive Power Management: Incorporate time‑critical load profiles into preventive outage planning.
• Enhanced Redundancy: N+1 UPS plus on‑site generators can eliminate microsecond‑scale drifts.
• Real‑Time Communication: Automated alerts between grid operators and atomic clock facilities reduce response latency.
• Continuous Betterment: Post‑event RCFMEA should feed directly into grid‑stability upgrades and stakeholder training.

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