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Rockwell Automation Publication 35-UM001A-EN-P - May 2022
ESPE
Electro-sensitive Protective Equipment
An assembly of devices and/or components working together for protective tripping or presence-
sensing purposes and includes as a minimum:
• A sensing device.
• Controlling/monitoring devices.
• Output signal-switching devices (OSSD).
HFT
Hardware Fault Tolerance
The HFT equals
n
, where
n
+1 faults could cause the loss of the safety function. An HFT of one means
that two faults are required before safety is lost.
IEC
International Electrotechnical Commission
The International Electrotechnical Commission (IEC) is the organization that prepares and publishes
international standards for all electrical, electronic, and related technologies.
IGBT
Insulated Gate Bipolar Transistors
Typical power switch that is used to control main current.
ISO
International Organization for Standardization
The International Organization for Standardization is an international standard-setting body that is
composed of representatives from various national standards organizations.
MTTF
Mean Time to Failure
The length of time that a device or other product is expected to remain reliable in operation.
PELV
Protective Extra Low Voltage
An electrical system where the voltage cannot exceed ELV under normal conditions, and under single-
fault conditions, except earth faults in other circuits.
PFD
Average Probability of a Dangerous Failure on
Demand
The average probability of a system to fail to perform its design function on demand.
PFH
Average Frequency of a Dangerous Failure per
hour
The average frequency of a system to have a dangerous failure per hour.
PL
Performance Level
EN ISO 13849-1 safety rating
PM
Permanent Magnet
In permanent magnet (PM) motors, magnets mounted on or embedded in the rotor, couple with the
current-induced internal magnetic fields of the motor generated by electrical input to the stator.
SELV
Safety Extra Low Voltage Circuit
A secondary circuit that is designed and protected so that, under normal and single fault conditions,
its voltages do not exceed a safe value.
SIL
Safety Integrity Level
A measure of a products ability to lower the risk that a dangerous failure could occur.
SSN
Safety Network Number
A unique number that identifies a section of a safety network.
STR
Spurious Trip Rate
That part of the overall failure rate that does not lead to a dangerous undetected failure.
STO
Safe Torque Off
The Safe Torque Off (STO) function is used to help prevent unexpected motor rotation during an
emergency while the drive remains connected to the power supply. When STO is activated, the torque
power cannot reach the drive, which stops and helps prevent any motor shaft rotation.
Abbreviations and Definitions (Continued)
Abbreviation
Full Term
Definition