Generator service glossary
The trade's working vocabulary, defined the way it gets used on a truck and in a contract. Where a term has a full guide behind it, the definition links there.
The system
- Standby generator
- An engine-generator set installed to carry a building or process when utility power fails. Distinct from prime and continuous duty machines, which are built and rated to run as a routine power source rather than in reserve. The rating a unit carries is on its nameplate and governs how it should be applied.
- ATS (automatic transfer switch)
- The switch that stands between utility power, generator power, and the load. It senses the loss of the normal source, signals the engine to start, transfers the load to the generator, and transfers back when utility power returns. Exercising the generator does not test it; only a transfer test does. Generator ATS testing
- EPSS (emergency power supply system)
- The whole chain that delivers emergency power: the energy source and engine-generator set together with the transfer equipment, controls, and accessories between them and the load terminals. The useful mental model is that the EPSS is a system, and a test of the engine alone is not a test of the system.
- Switchgear
- The assembly of disconnects, breakers, and control equipment that distributes and protects power on larger installations. On multi-generator sites the switchgear is where paralleling happens, and it carries its own inspection and maintenance scope beyond the generator itself.
- Paralleling
- Operating two or more generators electrically locked together so they share load as one source, matched in voltage, frequency, and phase. Common on large facilities for capacity and redundancy, and a discipline of its own to test and maintain.
- Day tank
- A smaller fuel tank mounted at or near the engine, fed from the main storage tank, holding the working supply of fuel the engine draws from. A day tank has its own level controls, alarms, and inspection points.
- Annunciator
- The alarm panel, local or remote, that reports the generator system’s status and trouble conditions: run, fault, low fuel, low battery, and the rest. Standards for emergency systems expect alarms to be seen where people can act on them.
- Block heater
- An engine coolant heater that keeps a standby engine warm so it can start and accept load promptly in cold conditions. A failed block heater is a quiet defect: everything works until a cold-morning outage, and then the engine starts hard or not at all.
- Bi-fuel
- An engine configured to run on a mixture of diesel and natural gas, typically to extend runtime on stored diesel. The fuel program and the maintenance scope both change when a unit is bi-fuel, and the OEM’s documentation governs.
Testing and maintenance
- PM (preventive maintenance)
- The scheduled inspection and service program that keeps a standby unit ready: fluids, filters, batteries, belts, connections, and the readings that prove condition. In a service business, PM is the contracted recurring scope everything else hangs off. PM scheduling
- Load bank test
- Running the generator against a portable or installed artificial load so it works at a real output level regardless of how light the building load is. The test proves the machine can carry sustained load and produces the readings the record needs. Required load and duration come from the governing standard, the adopted edition, and the AHJ. Load bank testing
- Transfer test
- A test that simulates loss of normal power so the ATS runs its full sequence: sense, start, transfer, run under load, retransfer, cool down. The point is to test the switch and the system, which a no-load engine exercise never touches. ATS testing explained
- Transfer time
- The elapsed time from loss of the normal source to the load being carried by the emergency source. Captured on transfer tests, and worth trending: drift across successive tests is an early warning of a tiring controller, relay, or battery.
- Wet stacking
- The buildup of unburned fuel and carbon in a diesel engine’s exhaust path from running too long at light load. The classic cause is years of no-load or light-load exercise. Running the engine at proper load, as a load bank test does, is both the prevention and part of the cure.
- Fuel polishing
- Circulating stored diesel through staged filtration and water separation to remove water, sediment, and microbial contamination, then returning it to the tank. Part of a fuel program, not a substitute for one: polishing cleans the tank’s contents; it does not fix the water source or reverse chemical aging. Diesel fuel maintenance
- Battery load test
- Testing the starting batteries under load rather than trusting a resting voltage reading. A battery can show healthy voltage and still collapse under cranking demand, and a weak starting battery is a common cause of a no-start.
- Insulation resistance (megger) test
- Applying test voltage to windings and cabling to measure insulation condition. A trending record across years is the value: declining readings flag moisture or deteriorating insulation before a failure does.
- Exercise timer
- The programmed schedule on which the system starts and runs itself between service visits. Useful, and dangerous to trust blindly: settings changed during troubleshooting and never restored are a known failure mode, so verifying the exerciser is part of testing.
- Hour meter
- The engine’s odometer. Hour readings drive maintenance intervals, support warranty positions, and, for emissions purposes, prove how much a unit ran and for what category of reason. A reading captured on every visit is cheap; a missing year of readings is not reconstructable. Emissions recordkeeping
Standards and compliance
- NFPA 110
- The standard for emergency and standby power systems: how they are installed, maintained, operated, and tested, and the records that work should produce. It has legal force where adopted, in the edition the jurisdiction adopted, which is why edition-specific numbers belong to the site rather than to a blog post. NFPA 110 testing requirements
- Level, Class, and Type (NFPA 110)
- The three-axis way the standard describes an installation: Level reflects how critical the load is to life safety, Class describes how long the system must be able to run, and Type describes how quickly power must be restored. The values for a given site come from the adopted codes and the AHJ. NFPA 110 Level 1, explained
- NFPA 99
- The health-care facilities standard. Where NFPA 110 covers the power system itself, NFPA 99 addresses how the essential electrical system serves a healthcare facility, including how load is prioritized across branches so the most critical loads come back first. Healthcare generator compliance
- AHJ (authority having jurisdiction)
- The person or office that enforces the adopted codes for a site: a fire marshal, a building official, an inspector, sometimes several at once. The AHJ determines which editions apply and how, which makes the AHJ the answer to most "exactly how often" questions.
- The Joint Commission
- A widely recognized U.S. healthcare accrediting organization. Its surveyors evaluate facilities against its published requirements, and for standby power that includes looking for evidence that testing and maintenance happened, on the record.
- CMS
- The federal agency behind Medicare and Medicaid. For participating hospitals, its conditions of participation sit on top of the fire and life-safety framework, which is one of the reasons healthcare standby power carries more oversight than any other occupancy.
- EPA tiers
- Successive generations of federal emission standards for diesel engines, phased in by engine power and model year, each stricter than the last. The tier attaches to the engine as built. For standby fleets, the operating rules turn less on the tier and more on the emergency classification and the permit. EPA tiers and recordkeeping
- ULSD (ultra-low-sulfur diesel)
- The low-sulfur diesel grade modern engines and rules commonly require. Relevant to service work twice: permits can expect fuel documentation, and modern fuel chemistry is commonly described as more prone to water and microbial trouble in long storage, which is what a fuel program manages.
- DEF and aftertreatment
- Exhaust-side emissions systems on many newer engines: diesel particulate filters that trap soot, and selective catalytic reduction systems fed with diesel exhaust fluid. They add fluid handling, regeneration behavior, and components to the maintenance scope, on the OEM’s terms.
The business
- WO (work order)
- The record of one job: the customer, site, equipment, scope, time, parts, photos, readings, and sign-offs. In a systemized shop the work order is where field reality becomes billable, documented history. Work orders
- Service agreement
- The standing contract for recurring PM: covered equipment, scope, cadence, response terms, and the line items (load bank, fuel, travel, after-hours) that decide whether the deal holds margin. For compliance customers the agreement is also a records commitment. What belongs in the agreement
- T&M (time and materials)
- Work billed by hours and parts rather than under a contracted price. The reactive repair revenue that a PM book tends to pull behind it, which is why shops track the ratio between contracted and reactive work. Pricing the PM contract
- Gate (PowerOps)
- A workflow transition that only a named, authenticated person with the right permission can perform, by an explicit click, with a confirmation of what is being attested. Quote to won, scheduled to dispatched, completed to invoiced: each is a gate, and none can be fired by a timer, a webhook, or an AI.
- Attestation
- What a gate click means: the person is putting their name on a specific state of the record at a specific moment. The confirmation panel shows what they are attesting to; the audit row preserves it.
- Audit row
- The append-only record a gate click writes: who, when by server time, from and to state, a snapshot of the record, and the permission that allowed it. Written once, never edited, enforced at the database layer. The reason an auditor’s question gets a name and a timestamp instead of a shrug. Security & trust
The software speaks this language too.
Equipment records with ATS and kW fields, test results with pass/fail sign-offs, and an audit row behind every one of them.