Mitsubishi Electric Melsec-QS Series Safety Application Manual page 26

3
RISK ASSESSMENT AND SAFTY LEVEL
The requirements of standards for the safety category are shown in Table3.1.
*1
Category
Safety-related parts of control systems and/or
their protective equipment, as well as their
components, shall be designed, constructed,
B
selected, assembled and combined in
accordance with relevant standards so that
they can withstand the expected influence.
Requirements of B shall apply.
1 Well-tried components and well-tried safety
principles shall be used.
Requirements of B and the use of well-tried
safety principles shall apply.
2
Safety function shall be checked at suitable
intervals by the machine control system.
Requirements of B and the use of well-tried
safety principles shall apply.
-- a single fault in any of these parts does not
3
lead to loss of the safety function, and
-- whenever reasonably practicable the single
fault is detected.
Requirements of B and the use of well-tried
safety principles shall apply.
-- a single fault in any of these parts does not
lead to loss of the safety function, and
4 -- the single fault is detected at or before the
next demand upon the safety function. If
this is not possible, then an accumulation
of faults shall not lead to loss of the safety
function.
3
3.2 Safety Category
- 4
Table3.1 Summary of safety category requirements
Summary of requirements
*1:The categories are not intended to be used in any given order or in any given hierarchy in respect of
safety requirements.
*2:The risk assessment will indicate whether the total or partial loss of the safety function(s) arising from
faults is acceptable.
*2
System behaviour
The occurrence of a fault
can lead to loss of the
safety function.
The occurrence of a fault
can lead to loss of the
safety function, but the
probability of occurrence is
lower than for category B.
-- The occurrence of a
fault can lead to loss of
the safety function
between the checks.
-- The loss of safety
function is detected by
the check.
-- When a single fault
occurs, the safety
function is always
performed.
-- Some but not all faults
will be detected.
-- Accumulation of
undetected faults can
lead to loss of the safety
function.
-- When te faults occur the
safety function is always
performed.
-- The faults will be
detected in time to
prevent loss of the
safety function.
Principles to achieve
safety
Mainly characterized by
selection of components
Mainly characterized by
structure
(Referred to EN954-1.)