Instruction Manual Supplement
D104484X012
785C Actuator
April 2019
Safety Manual for Fisher™ 785C Actuators
Purpose
This safety manual provides information necessary to design, install, verify, and maintain a Safety Instrumented
Function (SIF) utilizing the Fisher 785C actuator.
WARNING
This instruction manual supplement is not intended to be used as a stand-alone document. It must be used in conjunction
with the following manual:
Fisher 785C Actuator Instruction Manual (D104483X012)
Failure to use this instruction manual supplement in conjunction with the above referenced manual could result in personal
injury or property damage. If you have any questions regarding these instructions or need assistance in obtaining any of
these documents, contact your Emerson sales office
Introduction
.
This manual provides necessary requirements for meeting the IEC 61508 or IEC 61511 functional safety standards.
Figure 1. Fisher 785C Actuator
785C Double-Acting Springless Actuator
www.Fisher.com
785C Actuator
April 2019
Instruction Manual Supplement
D103398X012
Terms and Abbreviations
Safety: Freedom from unacceptable risk of harm.
Functional Safety: The ability of a system to carry out the actions necessary to achieve or to maintain a defined safe
state for the equipment / machinery / plant / apparatus under control of the system.
Basic Safety: The equipment must be designed and manufactured such that it protects against risk of injury to persons
by electrical shock and other hazards and against resulting fire and explosion. The protection must be effective under
all conditions of the nominal operation and under single fault condition.
Safety Assessment: The investigation to arrive at a judgment based on the facts of the safety achieved by
safetyrelated systems.
FailSafe State:
D State where valve actuator is deenergized and spring is extended. (785C single-acting spring return constructions).
D State where valve actuator is driven to or held in a predefined position by the control medium. (785C double-acting
springless constructions).
Fail Safe: Failure that causes the valve to go to the defined failsafe state without a demand from the process.
Fail Dangerous: Failure that does not respond to a demand from the process (i.e. being unable to go to the defined
failsafe state).
Fail Dangerous Undetected: Failure that is dangerous and that is not being diagnosed by automatic stroke testing.
Fail Dangerous Detected: Failure that is dangerous but is detected by automatic stroke testing.
Fail Annunciation Undetected: Failure that does not cause a false trip or prevent the safety function but does cause
loss of an automatic diagnostic and is not detected by another diagnostic.
Fail Annunciation Detected: Failure that does not cause a false trip or prevent the safety function but does cause loss of
an automatic diagnostic or false diagnostic indication.
Fail No Effect: Failure of a component that is part of the safety function but that has no effect on the safety function.
Low demand mode: Mode, where the frequency of demands for operation made on a safetyrelated system is no
greater than twice the proof test frequency.
Acronyms
FMEDA: Failure Modes, Effects and Diagnostic Analysis
HFT: Hardware Fault Tolerance
MOC: Management of Change. These are specific procedures often done when performing any work activities in
compliance with government regulatory authorities.
PFD
SFF: Safe Failure Fraction, the fraction of the overall failure rate of a device that results in either a safe fault or a
diagnosed unsafe fault.
SIF: Safety Instrumented Function, a set of equipment intended to reduce the risk due to a specific hazard (a safety
loop).
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: Average Probability of Failure on Demand
AVG
Instruction Manual Supplement
D103398X012
SIL: Safety Integrity Level, discrete level (one out of a possible four) for specifying the safety integrity requirements of
the safety functions to be allocated to the E/E/PE safetyrelated systems where Safety Integrity Level 4 has the highest
level of safety integrity and Safety Integrity Level 1 has the lowest.
SIS: Safety Instrumented System – Implementation of one or more Safety Instrumented Functions. A SIS is composed
of any combination of sensor(s), logic solver(s), and final element(s).
785C Actuator
April 2019
Related Literature
Hardware Documents:
Bulletin:
61.2:785C, Fisher 785C Piston Actuator: D104412X012
Instruction Manual:
Fisher 785C Piston Actuator: D104483X012
Guidelines/References:
D Safety Integrity Level Selection – Systematic Methods Including Layer of Protection Analysis, ISBN 1556177771,
ISA
D Control System Safety Evaluation and Reliability, 2nd Edition, ISBN 1556176388, ISA
D Safety Instrumented Systems Verification, Practical Probabilistic Calculations, ISBN 1556179099, ISA
Reference Standards
Functional Safety
D IEC 61508: 2010 Functional safety of electrical/electronic/ programmable electronic safetyrelated systems
D ANSI/ISA 84.00.012004 (IEC 61511 Mod.) Functional Safety – Safety Instrumented Systems for the Process Industry
Sector
Product Description
The 785C Series pneumatic actuators are medium to large spring-return single acting or double-acting springless
piston actuators that provide accurate, high thrust output for short to long travel applications. The 785C actuators can
be installed on sliding-stem valves for throttling control or on-off applications.
The single-acting actuator constructions feature an internal bias spring that forces the actuator piston rod to extend or
retract upon a loss of supply pressure, thereby ensuring a fail-closed or fail-open mode of operation. This effectively
eliminates the need for a trip valve and volume tank in most constructions.
An optional manual override (top mounted handwheel or side mounted handpump) is capable of extending or
retracting the actuator manually and can be engaged at any position from full open to full close. The top-mounted
handwheel utilizes an engagement lever that couples the handwheel and piston rod. The handpump has a hydraulic
cylinder attached to the piston rod. This enables the handpump to operate the actuator manually, unless the
handpump is set to bypass position.
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