Functional Safety Terminology

Fault Tolerance

IEC 61511-1

Ability of a functional item to continue to perform a required function in the presence of faults or errors

Functional Safety

IEC 61508-4

Part of the overall safety relating to the Equipment Under Control (EUC) and the EUC control system that depends on the correct functioning of the Electrical/Electronic/Programmable Electronic (E/E/PE) safety-related systems and other risk reduction measures

Safe Failure

IEC 61508–4

1. results in the spurious operation of the safety function to put the EUCEUC: Equipment under control* (or part thereof) into a safe state or maintain a safe state; or

2. increases the probability of the spurious operation of the safety function to put the EUCEUC: Equipment under control* (or part thereof) into a safe state or maintain a safe state.

Safe Failure Fraction

IEC 61508–4

The ratio of the rate of safe failures to the total failure rate of the system.

Safe State

IEC 61511-1

State of the process when safety is achieved

IEC 61800-5-2

State of the PDS(SR) Safety related power drive systems* when safety is achieved

Safe Stop

IEC 61800-5-2

The Safe Stop functions are defined as:

• Safe Torque Off (STO)

  • This function prevents force-producing power from being provided to the motor.

  • This safety sub-function corresponds to an uncontrolled stop in accordance with stop category 0 of IEC 60204-1.

• Safe Stop 1 (SS1)

  • Safe Stop 1 deceleration controlled: SS1-d initiates and controls the motor deceleration rate within selected limits to stop the motor and performs the STO function (see 4.2.3.2) when the motor speed is below a specified limit; or

  • Safe Stop 1 ramp monitored: SS1-r initiates and monitors the motor deceleration rate within selected limits to stop the motor and performs the STO function when the motor speed is below a specified limit; or

  • Safe Stop 1 time controlled SS1-t initiates the motor deceleration and performs the STO function after an application specific time delay.

Safety Function

IEC 61800-5-2

Function to be implemented by a safety-related system or other risk reduction measures, that is intended to achieve or maintain a safe state for the equipment or machinery driven by the PDS(SR) Safety related power drive systems*, in respect of a specific hazardous event

Safety Integrity Level (SIL)

IEC 61508

The standard IEC 61508 defines four Safety Integrity Levels (SILs) for safety functions: SIL 1 is the lowest integrity level and SIL 4 is the highest.

A hazard analysis and risk assessment serves as a basis for determining the required safety integrity level.

Safety Related System

IEC 61800-5-2

Designated system that both

• implements the required safety functions necessary to achieve or maintain a safe state for the equipment or machinery driven by the PDS(SR)Safety related power drive systems*; and

• is intended to achieve, on its own or with other risk reduction measures, the necessary safety integrity for the required safety functions

Subsystem

IEC 61800-5-2

Part of the top-level architectural design of a safety-related system, failure of which results in failure of a safety-related function