Glossary
Key terms, acronyms, and abbreviations used across industrial control systems, machine safety, and electrical standards.
A
The maximum short-circuit current available at a given point in an electrical system, expressed in amperes. Panel SCCR must meet or exceed the AFC at the installation point. AFC is determined from the utility transformer capacity and the impedance of all conductors between the transformer and the panel. Required for NEC Article 409 compliance.
The organization, office, or individual responsible for enforcing the requirements of a code or standard, or their designated representative. In electrical work, the AHJ inspects and approves installations against the adopted edition of the NEC. The applicable AHJ varies by project location and system type — it may be a local building department, fire marshal, or federal agency such as OSHA.
The maximum fault current a protective device (circuit breaker or fuse) can safely interrupt without damage or arcing. AIC must equal or exceed the AFC at the point of installation. Also referred to as Interrupting Rating (IR). Selecting protective devices with insufficient AIC is a common and serious NEC violation.
The European regulatory and certification framework for equipment and protective systems used in potentially explosive atmospheres. ATEX markings are commonly seen alongside IEC 60079 concepts such as EPL, gas group, and T-code. For multinational projects, engineers often compare ATEX certification with IECEx acceptance and local NEC hazardous-location rules.
B
The normal operating control system for a process unit. In IEC 61511 work, the BPCS is separate from the SIS and cannot be credited as an independent protection layer if it is also the initiating cause of the hazardous scenario. Keeping BPCS and SIS boundaries clear is central to defensible LOPA and SIF design.
C
A marking indicating that a product conforms to all applicable EU directives and regulations. CE marking is mandatory for placing machinery on the EU market. For industrial machinery the law (Machinery Directive 2006/42/EC through 19 January 2027; Machinery Regulation (EU) 2023/1230 from 20 January 2027) requires a documented risk assessment and compliance with the applicable essential health and safety requirements. Applying harmonized standards such as EN ISO 12100, ISO 13849-1 or IEC 62061 is voluntary, but confers presumption of conformity for the requirements they cover.
A classification (B, 1, 2, 3, 4) defined in ISO 13849-1 describing the structural and behavioral properties of a safety-related control system. Category B is the baseline; Category 4 requires dual-channel architecture with cross-monitoring and tolerance of any single fault. Category is one input to determining Performance Level — higher categories enable higher achievable PL.
A hazardous-area document defining how associated apparatus, field devices, wiring methods, and entity parameters must be connected for an intrinsically safe loop. Control drawings identify permitted cable characteristics, segregation rules, and field termination limits so installers do not defeat the Ex i design.
D
The fraction of dangerous failures detected by automatic diagnostic tests, expressed as a percentage. Defined in ISO 13849-1 and IEC 61508. DC values are classified as None (<60%), Low (60–90%), Medium (90–99%), and High (≥99%). DC, MTTFd, and Category together determine the achievable Performance Level of a safety function.
E
A safety function that brings a machine to a safe state as quickly as possible when activated. Defined in IEC 60204-1 and NFPA 79. Stop categories 0 (immediate power removal), 1 (controlled stop then power removal), and 2 (controlled stop, power maintained) determine how the drive system responds. E-stop circuits must be designed to achieve a required Performance Level per ISO 13849-1 or SIL per IEC 62061.
The IEC 60079 classification that expresses how much protection equipment provides against ignition in explosive atmospheres. EPL maps equipment to zones, for example Ga to Zone 0, Gb to Zone 1, and Gc to Zone 2. Selecting the correct EPL is one of the first checks in hazardous-area equipment review.
A process-industry shutdown function that moves equipment or an entire unit to a defined safe state during an abnormal or hazardous condition. ESD is a broader concept than a machinery E-stop and is usually implemented through one or more SIFs within a SIS under IEC 61511.
The highest intrinsic-safety protection level defined in IEC 60079-11 for explosive-gas atmospheres. Ex ia equipment is designed to remain safe with two independent faults and is suitable for Zone 0 gas locations. It is commonly used with barriers or galvanic isolators and documented through control drawings.
H
A structured team-based hazard review used in the process industries to identify deviations, causes, consequences, and safeguards. HAZOP usually precedes LOPA in IEC 61511 projects and provides the hazardous scenarios that must be evaluated for SIF allocation and SIL targets.
The number of faults a system can tolerate while maintaining its safety function. Defined in IEC 61508 and IEC 62061. HFT 0 means one fault causes loss of safety function; HFT 1 means two simultaneous faults are required to cause loss. Higher SIL levels require higher HFT, which typically means redundant (dual-channel) architecture.
I
The international standards body responsible for electrotechnical standards, including IEC 61508 (functional safety), IEC 62061 (machinery functional safety), IEC 60204-1 (electrical equipment of machinery), IEC 61511 (process safety), and IEC 62443 (industrial cybersecurity). IEC standards are adopted by many national bodies, often with local modifications.
The international certification system used to demonstrate conformity of equipment and services for explosive atmospheres. IECEx certification is widely paired with IEC 60079 equipment selection and often compared with ATEX certification for projects that cross regional markets.
A safeguard that can prevent a hazardous event from progressing to a consequence and that is independent of the initiating cause and the other credited layers. IPLs are central to LOPA because they define how much risk reduction already exists before a new SIF is assigned.
The international standards body responsible for standards such as ISO 12100 (machine risk assessment), ISO 13849-1 (machinery safety — Performance Level), and many other engineering standards. ISO and IEC collaborate extensively on dual-logo standards in the functional safety domain.
L
A semi-quantitative risk assessment method used to determine the required Safety Integrity Level (SIL) for safety instrumented functions. LOPA identifies independent protection layers (IPLs) between a hazard and its consequence, and calculates the risk reduction needed from the SIF. Commonly used in process industries in conjunction with IEC 61511.
M
The average time until a component fails in a dangerous (undetected) way, expressed in years. Defined in ISO 13849-1. MTTFd is classified as Low (<10 yr), Medium (10–30 yr), and High (30–100 yr) per channel. Together with Category and Diagnostic Coverage, MTTFd determines the achievable Performance Level of a safety function.
N
Published by NFPA as NFPA 70, the NEC is the primary electrical installation code enforced in the United States. It is adopted into law by most states and local jurisdictions and enforced by the AHJ. For industrial machinery, NEC Article 670 governs facility installation while NFPA 79 governs machine electrical design. Key industrial articles include 250, 409, 430, and 670.
A US standards organization that publishes the NEC (NFPA 70) and NFPA 79 (Electrical Standard for Industrial Machinery), among hundreds of other codes and standards. NFPA standards are voluntary but are widely adopted into law by US jurisdictions. NFPA 79 is the primary standard for the electrical design of industrial machines sold in the US market.
O
The US federal agency responsible for workplace safety regulations. OSHA 29 CFR 1910 (General Industry) and 1926 (Construction) reference electrical standards and machine guarding requirements. OSHA enforces the use of nationally recognized testing laboratory (NRTL) listed equipment in workplace installations, which drives UL listing requirements in practice.
P
The low-demand safety integrity metric used in IEC 61511 and IEC 61508 for functions that are demanded infrequently. PFDavg is the correct quantitative basis for process SIS work; it is not interchangeable with PFHd, which is used for high-demand or continuous-demand machinery safety calculations.
The average probability of a safety function failing dangerously per hour of operation. Used in IEC 62061 and IEC 61508 to quantify SIL compliance for high-demand or continuous-demand safety functions. SIL 1 requires PFH < 10⁻⁵; SIL 3 requires PFH < 10⁻⁷. Component datasheets from safety device manufacturers provide PFH values for use in system calculations.
The high-demand or continuous-demand failure-rate metric used in IEC 62061 and commonly referenced in ISO 13849-1 comparison tables. PFHd is the rate that subsystem values are summed against for machinery SIL calculations. It contrasts with PFDavg, which is used for low-demand process SIS applications.
A discrete level (PL a–e) specifying the ability of a safety-related control system to perform its safety function under foreseeable conditions, as defined in ISO 13849-1. PL e provides the highest reliability. Required PL (PLr) is determined by risk assessment; achieved PL is calculated from Category, MTTFd, and Diagnostic Coverage. PL is the ISO 13849-1 equivalent of SIL in the IEC 62061 / IEC 61508 framework.
An industrial digital computer designed for real-time control of manufacturing and automation processes. Standard PLCs are not rated for use in safety functions. Safety PLCs (also called safety controllers or FSCs) are designed and certified to IEC 61508 / IEC 62061 / ISO 13849-1 and are required for implementing safety-rated control functions at SIL 2–3 or PL d–e.
A proof-test method for shutdown valves in which the valve is moved partway toward its trip position to reveal dangerous undetected failures without taking the full process out of service. PST improves diagnostic coverage but does not replace the need for periodic full proof testing.
The planned maximum time between proof tests of a safety function or subsystem. PTI is a design parameter, not just a maintenance preference, because longer intervals increase accumulated dangerous undetected failures and therefore worsen the achieved PFDavg or PFHd.
S
The maximum short-circuit current a panel or assembly can withstand without damage, expressed in kA. Required to be marked on industrial control panels per NEC 409.110. Panel SCCR is determined by the lowest-rated component in the assembly, calculated using UL 508A Supplement SB. The marked SCCR must meet or exceed the AFC at the installation point — this is one of the most commonly failed NEC inspection points.
The dominant semiconductor-fab host communication framework for equipment status, alarms, recipes, and remote interaction. SECS defines the messaging layer and GEM defines the equipment behavior model. Control engineers usually encounter it when tool interlocks, remote commands, or factory-host integration must be coordinated with equipment safety behavior.
An industry association that publishes safety standards for semiconductor manufacturing equipment. Key standards include SEMI S2 (environmental, health, and safety guidelines), SEMI S8 (ergonomics), and SEMI S14 (fire risk assessment). SEMI S2 compliance is required by most semiconductor fabs and is distinct from NEC, NFPA 79, and IEC standards, though it references many of them.
The proportion of all failures that are either safe failures or dangerous detected failures, expressed as a percentage. Defined in IEC 61508. SFF ≥ 90% is typically required for SIL 2 subsystems with HFT 0; SFF ≥ 99% for SIL 3. SFF and HFT together determine the maximum SIL a subsystem can claim without architectural constraints.
A specific safety function implemented by a Safety Instrumented System (SIS) to bring a process to a safe state on demand. Each SIF has its own SIL target, determined by risk assessment (typically LOPA). A SIF consists of a sensor subsystem, logic solver, and final element subsystem, each contributing to the overall SIF PFD or PFH.
A discrete level (SIL 1–4) specifying the required risk reduction a safety function must achieve. Defined in IEC 61508 and applied to machinery in IEC 62061 and to process safety in IEC 61511. Higher SIL levels require greater reliability, stricter architectural constraints, and more rigorous verification. SIL 4 is rare in practice; most industrial machinery targets SIL 1–3. SIL is the IEC 62061/61508 equivalent of Performance Level (PL) in ISO 13849-1.
The maximum SIL a subsystem architecture can claim, independent of the numerical PFHd calculation. In IEC 62061, SILCL is determined from architecture, Hardware Fault Tolerance, and Safe Failure Fraction. A subsystem can have a very low PFHd and still be capped by a lower SILCL if its architecture is not adequate.
An instrumented system used to implement one or more Safety Instrumented Functions (SIFs). A SIS consists of sensors, logic solvers (safety PLCs), and final elements (valves, shutdowns). Governed by IEC 61511 in the process industries. A SIS is independent of the Basic Process Control System (BPCS) and must be designed, tested, and maintained separately.
A discrete level (SL 1–4) defined in IEC 62443 specifying the required cybersecurity capability of an Industrial Automation and Control System (IACS). SL-T (Target) is determined by risk assessment; SL-C (Capability) is what a product or system can achieve. SL is conceptually analogous to SIL in functional safety, but addresses intentional cyber threats rather than random hardware failures.
A device designed to protect electrical equipment from voltage transients caused by lightning, switching, or other disturbances. NEC 409.70 (2023) addresses SPD requirements for industrial control panels. SPDs are particularly important where safety relay modules, PLCs, or other sensitive electronics are housed in panels exposed to transient voltage events.
The formal document that defines what a safety function must do, under what conditions it must act, what safe state it must achieve, and how it will be verified and maintained. In IEC 61511 projects, the SRS is the governing design input for SIS implementation, testing, and modification control.
T
The explosive-atmosphere temperature classification that states the maximum surface temperature equipment can reach in service. The selected T-code must stay below the autoignition temperature of the hazardous substance present. T-codes are reviewed alongside EPL, gas group, and certification records.
U
A Nationally Recognized Testing Laboratory (NRTL) that develops safety standards and certifies products. UL 508A is the standard for industrial control panels. UL listing is required by OSHA for equipment in US workplaces and is typically required by AHJs. UL 508A certification involves both the panel construction standard and Supplement SB for SCCR determination.
V
A power electronics device that controls AC motor speed by varying the frequency and voltage of the motor supply. Also called an adjustable speed drive (ASD) or inverter. VFDs introduce considerations for motor branch circuit protection (NEC Article 430), drive-rated conductors, and input harmonic filtering. Safety-rated stop functions on VFDs (STO, SS1, SS2) are covered in IEC 61800-5-2 and may be used as part of a functional safety architecture.
This site is an independent educational reference for industrial automation standards. It paraphrases and organizes guidance in original language; it is not a substitute for the official published standards, professional engineering judgment, or legal review, and content has not been independently verified against current published editions.
Items marked Review pending await technical review against the identified edition. Items marked Planned have no reviewed source material behind them. Do not rely on this site for compliance determinations, safety-critical design decisions, or legal interpretation.