Essentially these circuits function by controlling the supply of pressure fluid to the machine operating devices. The control can be direct (power interlocking) or indirect (control interlocking). Actuation of the control or interposed valves is by fluid from an interlocking valve or by a solenoid energized from an electrical interlocking switch.
Pearce Ridley Type of interlocking Electrical / Electronic low- complex circuits Hydraulic circuits Pneumatic circuits 1a 8–32 Single channel All electronic with or without software elements Single channel single Single channel 1b control Hardwired with interposing devices between pneumatic interlocking pneumatic interlocking
interlocking the interlocking device and the power switching valve actuating control through interposed element (Figure 9.8). valve (Figure 10.1). valves (Figure 11.2).
1c Hardwired with no interposing devices between Single channel
the interlocking device and the power switching opposed mode
element (Figure 9.9). interlocking through
1d 8–32 Single channel Including key exchange systems (Figure 9.10). Single channel power interposed valves or
power interlocking (Figure 10.2) equalizing valve
interlocking (Figures 11.3 and 11.4).
2a 32–128 Single channel Passive monitor is an audio/visual indication of Dual channel mixed Dual channel single
monitored failure (Figure 9.11(b). media opposed mode medium opposed
2b interlocking Active automatic monitoring at predetermined interlocking (Figure 10.3). mode interlocking
intervals (Figure 9.11(c). through interposed
valves (Figure 11.5).
3 128–256 Dual channel Includes mixed media, solenoid operated locking Dual channel mixed media Dual channel mixed passive and dissipation of energy means to enhance a opposed mode interlocking media opposed mode monitored single interlocking system (Figures 9.12, 9.13 with control through an interposed valves
interlocking and 9.14) electrical safety-related (Figure 11.6)
control circuit (Figure 10.4)
4 128–256 Multi-channel Includes cross monitoring or other reliable Dual channel mixed media Dual channel mixed active monitored means of detecting safety related failures and opposed mode interlocking media opposed mode interlocking includes electronic circuits (Figure 9.15) with cross monitoring interlocking with
(Figure 10.5) cross-monitoring
(Figure 11.7)
Categories1 Summary of requirements System behaviour2 Principles to achieve safety B Safety-related parts of control systems and/or their The occurrence of a fault can lead Mainly characterized by
protective equipment, as well as their components, to the loss of the safety function selection of components.
shall be designed, constructed, selected, assembled and combined in accordance with relevant standards so that they can withstand the expected influence.
1 Requirements of B shall apply. Well tried safety The occurrence of a fault can lead Mainly characterized by components and well tried safety principles shall be to the loss of the safety function selection of components.
used. but the probability of the
occurrence is lower than for category B.
2 Requirements of B and the use of well tried safety The occurrence of a fault can Mainly characterized by
principles shall apply. lead to the loss of the safety structure.
Safety function shall be checked at suitable intervals function between the checks.
by the machine control system. The loss of the safety function is detected by the check.
3 Requirements of B and the use of well tried safety When the single fault occurs the Mainly characterized by
principles shall apply. safety function is always selection of components.
Safety-related parts shall be designed so that: performed.
a single fault in any of these parts does not lead to Some but not all the faults the loss of the safety function, and will be detected.
whenever reasonably practicable the single fault Accumulation of undetected
is detected. faults can lead to the loss of the
safety function.
(Continued)
Table 4.11 (Continued)
Categories1 Summary of requirements System behaviour2 Principles to achieve safety
4 Requirements of B and the use of well tried safety When faults occur the safety Mainly characterized by
principles shall apply. function is always performed. structure.
Safety related parts shall be designed so that: The faults will be detected in
a single fault in any of these parts does not lead to time to prevent the loss of the the loss of the safety function, and safety function.
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 a loss of the safety function.
1The categories are not intended to be used in any given order or in any given hierarchy in respect of safety requirements.
2The risk assessment will indicate whether the total partial loss of the safety function(s) arising from faults is acceptable.
Increasing degrees of protection are provided by the use of dual media and opposed mode interlocking devices plus cross-monitoring of the state of the valves. Examples of typical circuit arrangements are referred to in Table 4.10.