Siemens SIMOCODE pro System Manual page 685

Programmable logic controller (PLC)
Programming device
Protective extra low voltage (PELV)
SELV
SFB
SFC
SIMOCODE pro
GWA 4NEB 631 6050-22 DS 03
A controller whose functions are stored as a program on the
control device. The PLC consists of a CPU, memory, input/
output modules and an internal bus system. The I/O devices
and the programming language are based on the needs of the
control engineering.
A compact and transportable PC, suitable for industrial
purposes. It is characterized by a special hardware and
software configuration for SIMATIC programmable logic
controllers.
Protective measure against electric shock (previously
"protective extra low voltage with safe isolation").
Active components and exposed conductive parts may be
grounded and connected to the protective conductor in
contrast to SELV. Safe isolation means that the primary circuit
of the transformer must be isolated from the secondary
circuit by means of double or reinforced insulation. PELV is
used where active low voltage conductors or exposed
conductive parts must be grounded. This is the case, for
example, if equipotential bonding is required to prevent
sparks in containers and hazardous areas. However
dangerous leakage currents can flow over the exposed
conductive parts through the enclosure ground (independent
of the low voltage), if faults occur in the higher-level network.
Safety extra low voltage
Low electric voltage (up to 50 V AC or 120 V DC), that offers
significant protection against electric shock due to its low
value and insulation.
Devices that are operated with SELV and do not generate
high voltages are categorized as Class III according to DIN EN
61140 (VDE 0140-1).
System function block
A block integrated in the S7 CPU operating system that can
be called like a function block (FB) in the user program if
required.
System function:
A function integrated in the S7 CPU operating system that
can be called like a function (FC) in the user program if
required.
Glossary
Glossary-13