Plain-English definitions of terms used in high-voltage electrical construction and data center operations. These definitions are general information. Always follow your site’s procedures and the standards adopted for your site.
MOP (method of procedure)
A written, step-by-step plan for one specific task on critical equipment, such as maintenance or a change to the power system. It lists the scope, risks, roles, steps, checks and a back-out plan, and it is reviewed and approved before work starts.
SOP (standard operating procedure)
Written instructions for a routine task that is done the same way every time, such as daily rounds or a standard equipment check.
EOP (emergency operating procedure)
Written steps for responding to a failure or abnormal event, such as a loss of utility power or a UPS fault, to make people and equipment safe and restore service.
LOTO (lockout/tagout)
Practices that control hazardous energy. Workers isolate energy sources and place locks and tags on energy-isolating devices so the equipment cannot be energized while people work on it. In the US, OSHA 29 CFR 1910.147 covers general industry, and 29 CFR 1926.417 covers lockout and tagging of circuits in construction. NFPA 70E Article 120 describes how to establish an electrically safe work condition.
Electrically safe work condition (ESWC)
A state in which a conductor or circuit part has been disconnected from energized parts, locked and tagged, tested to confirm the absence of voltage, and grounded if needed. NFPA 70E-2024 Section 120.6 lists the steps.
Energization
Applying voltage to electrical equipment or a circuit for the first time, or again after work. It is done under an approved, written procedure with hold points.
First power
The first energization of a new building’s or system’s permanent electrical equipment.
Switching order
A written list of switching operations, in the exact order they must be done, with the expected state of each device after each step.
ATS (automatic transfer switch)
A switch that automatically moves an electrical load from one power source to another, for example from the utility to a generator, when it senses that the first source has failed, and moves it back when the source returns.
RPP (remote power panel)
In a data center, a cabinet with one or more panelboards, fed from a PDU or UPS system, that distributes power to circuits for IT racks close to the racks.
PDU (power distribution unit)
Equipment that takes power from a UPS or other source and distributes it, often through a transformer and panelboards, to IT equipment. The term is also used for rack-mounted power strips.
UPS (uninterruptible power supply)
A system that supplies power to critical loads without interruption when the main source fails, using stored energy such as batteries.
Lineup
A group of electrical equipment sections, such as switchgear or switchboard sections, installed side by side and connected as one assembly. On data center projects, people also use “lineup” for a full power path that is built, tested and energized as a set, for example a utility feed, generator, ATS and the distribution below it.
NFPA 70E
Standard for Electrical Safety in the Workplace, published by the National Fire Protection Association. It describes safe work practices for electrical hazards (shock and arc flash), including the electrically safe work condition, risk assessments and PPE. The edition at the time of writing is 2024. Employers widely use it to help meet OSHA electrical safety requirements.
Arc flash
A release of heat, light and pressure caused by an electric arc between conductors or to ground. It can cause severe burns and other injuries.
Arc flash risk assessment
A review required by NFPA 70E before work where an arc flash hazard may exist. It identifies the hazard, estimates its likelihood and severity, and decides the protective measures and PPE.
Qualified person
A person with the skills and knowledge for the construction and operation of the electrical equipment, who has received safety training to identify and avoid the hazards involved (NFPA 70E definition, in plain words).
Authorized person (LOTO)
The person who applies locks and tags and is responsible for the lockout/tagout on the equipment. OSHA calls this the “authorized employee”.
Absence of voltage test
A test with a properly rated meter, checked on a known voltage source before and after, to confirm that a conductor or circuit part is de-energized at the point of work.
Hold point
A step in a procedure where work stops until a named person checks the work and signs off.
Commissioning levels (L1–L5)
A common way to describe stages of commissioning: L1 factory testing, L2 site delivery and inspection, L3 installation and start-up checks, L4 functional performance testing, L5 integrated systems testing. Definitions vary by project.
IST (integrated systems test)
The final commissioning stage, where systems are tested together under realistic conditions, often including simulated utility failures.
Pencil-whipping
Marking a check or inspection as complete without actually doing it.
Related guides: Data center MOP guide · Lockout/tagout for electrical construction · Data center energization checklist