One interruption, different clocks
A fictional local electrical interruption affects a home router, a water pump and one railway traction section. We deliberately give other equipment different supplies or limited backup. These are chosen teaching assumptions, not predictions of outage duration, water pressure, safety or any real operator's response.
Define what has actually lost supply
The selected electrical interruption affects particular connections in our imaginary district. It does not mean the entire electricity system has stopped. For this example, the home router, a reservoir's inlet pump and one railway traction section lose their normal supply. A nearby mobile site and a wastewater lift station retain support from other arrangements. Those boundaries are assumptions we need before reasoning about the effects.
Mark the affected electrical connections individually instead of crossing out every city service.
The location and extent of an interruption determine which dependencies are tested.
Read the complete journey
- 1Electricity
Define what has actually lost supply
The selected electrical interruption affects particular connections in our imaginary district. It does not mean the entire electricity system has stopped. For this example, the home router, a reservoir's inlet pump and one railway traction section lose their normal supply. A nearby mobile site and a wastewater lift station retain support from other arrangements. Those boundaries are assumptions we need before reasoning about the effects.
Notice: Mark the affected electrical connections individually instead of crossing out every city service.
Explore Distribution networks
Why: The location and extent of an interruption determine which dependencies are tested. - 2Internet
Lose one connection before the phone runs out
Our home router has no backup, so its Wi-Fi connection stops when its supply disappears. The phone's battery still contains energy. In this scenario, a supported mobile site and an available onward network offer a different connection. That outcome is conditional: a phone battery cannot keep a cell site operating, and a radio signal alone does not establish that the requested internet service is reachable.
Notice: Compare the phone's remaining charge with the separate availability of its home and mobile connections.
Explore The access network
Why: A local device, an access link and the wider service have different failure boundaries. - 3Water
Use an inventory already in place
The reservoir's inlet pump stops, but our chosen layout can supply a gravity-fed zone from usable treated-water storage for a time. Its volume decreases when outflow exceeds inflow. This delays a volume shortage; it does not prove adequate pressure or drinking-water quality at every tap. The remaining service depends on the real layout, demand, usable inventory and other constraints, which this journey does not calculate.
Notice: Follow the stored inventory decreasing while the inlet remains unavailable.
Explore Treated-water storage
Why: Storage can separate the time of an upstream interruption from a downstream volume shortage. - 4Electricity
Ask what the backup actually supports
We assign a battery to selected station information equipment, rather than to the traction supply. It can support only compatible connected loads within its power and energy limits. A battery's energy inventory helps determine duration; its power limit constrains what it can supply at once. No runtime is specified here, because load, usable energy and conversion losses would all be needed for that calculation.
Notice: Draw the battery's connection to its selected equipment and stop the line there.
Explore Energy storage
Why: The presence of backup does not establish that every nearby function is backed up. - 5Urban rail
Separate a lit station from a powered railway
The backed-up information equipment can remain available while the affected railway section lacks its normal traction supply. Our example therefore does not equate a lit display with a train able to depart. Railways use different supply arrangements, and some vehicles have additional energy sources; none is assumed here. The purpose is to distinguish the electrical needs of passenger information from those of moving the train.
Notice: Compare the functioning display with the interrupted energy path to the train.
Explore Traction power
Why: Visible activity can represent a surviving supporting function rather than full service. - 6Urban rail
Keep movement permission as a separate condition
Even if traction supply becomes available, the railway still needs the applicable detection, route protection and movement permissions. The information display does not establish those conditions. For this fictional interruption, the operating status must therefore be considered separately from each electrical status. We do not model a particular signalling design or recovery procedure; the lesson is that several requirements must be satisfied together before normal service is available.
Notice: Keep separate markers for traction supply, train-control readiness and passenger information.
Explore Signals and separation
Why: Restoring one supporting service does not establish every condition required for train movement. - 7Water
Compare a second kind of water dependency
At the wastewater lift station, our selected backup arrangement continues powering the pump. This changes the immediate outcome from the drinking-water inlet pump, even though both depend on electricity. The distinction comes from their actual support arrangements, not from one service being inherently immune. If backup becomes unavailable, consequences would depend on inflow, storage, network layout and the utility's response; this journey does not predict an overflow.
Notice: Compare two pumps with different electrical support, rather than assigning one status to all water infrastructure.
Explore Sewers & lift stations
Why: Similar equipment can have different service outcomes under the same wider event. - 8Electricity
Treat alternatives as designed capabilities
A separate facility in our city has a microgrid designed to support selected loads while disconnected from the wider grid. Its available generation, storage, controls and loads must work together. Ordinary rooftop solar is not assumed to provide this capability by itself. Across the district, stored water, batteries, alternative supplies and island operation change particular dependencies, but none establishes that every service continues indefinitely.
Notice: Identify the defined boundary and supported loads of the fictional islanded facility.
Explore Microgrids & island operation
Why: Resilience comes from capabilities matched to a service, with explicit limits.
Which observations would be insufficient to conclude that the whole district's services are normal?
Reveal the explanation
A charged phone, remaining reservoir volume and a lit station display each describe only part of a service. They do not establish wider network reachability, water pressure or quality, or railway traction and movement permissions. The relevant dependencies and limits must be considered separately.
Sources & further reading
Original explanations informed by these sources. Diagrams are conceptual; local systems differ.
- Infrastructure Resilience Planning Framework (IRPF) Cybersecurity and Infrastructure Security Agency (CISA) · 2025-03 · Source & rights record
- Power Resilience Guide for Water and Wastewater Utilities U.S. Environmental Protection Agency, Office of Water · 2023-05-03 · Source & rights record
- Stay connected in severe weather Ofcom · 2026-10-22 · Source & rights record
- How electricity is delivered to consumers U.S. Energy Information Administration · Not stated · Source & rights record
- Energy storage for electricity generation U.S. Energy Information Administration · 2023-08-28 · Source & rights record
- Measuring electricity U.S. Energy Information Administration · Not stated · Source & rights record
- Solar Integration: Distributed Energy Resources and Microgrids Basics U.S. Department of Energy · Not stated · Source & rights record
- Solar Integration: Inverters and Grid Services Basics U.S. Department of Energy · Not stated · Source & rights record
- Water System Planning Guideline Sydney Water · 2014-09 · Source & rights record
- Protecting Water Quality Through Distribution System Pressure Management US Environmental Protection Agency · 2021-12 · Source & rights record
- Primer for Municipal Wastewater Treatment Systems US Environmental Protection Agency · 2004-09 · Source & rights record
- Electrification Network Rail · Not stated · Source & rights record
- Signals explained Network Rail · 2018-09-05 · Source & rights record
- RFC 1122: Requirements for Internet Hosts — Communication Layers IETF / RFC Editor; R. Braden (editor) · 1989-10 · Source & rights record
Another way through the system.
One morning, four systems
Follow an ordinary commute through the services that make it possible.
When rain crosses the network map
Follow one shared hazard across boundaries drawn for different services.
Power is back. What else must recover?
Follow service restoration through several independent conditions.