Ashcroft A Series - N4 Pressure Switch - Manual (Page 4)

A-S ERIES S WITCH S AFETY M ANUAL
I&M009-10210
REV A - 4/19/2013
1.5 Reference Standards
Functional Safety
• IEC 61508: 2010 Functional safety of electrical/electronic/ programmable
electronic safety-related systems
• ANSI/ISA 84.00.01-2004 (IEC 61511 Mod.) Functional Safety – Safety
Instrumented Systems for the Process Industry Sector
2 DEVICE DESCRIPTION
The A-Series Pressure switch is an electrical switch which is actuated via inlet
pressure. The switch will change state from normally closed (NC) to normally open
(NO) as pressure increases. It will change state again from NO to NC as pressure
decreases.
The switch is available in several configurations. The unit can be purchased as a
factory set model (APS) where the switch point is set at the time of manufacture
and is not adjustable, or a field adjustable model (APA) which can be adjusted by
the end user. Each of these styles is available in a watertight housing or an
explosion-proof housing. There are a variety of female or male pressure inlets
available. There are also several electrical connectors available (18 awg wire leads,
½” conduit fitting, cable, spade terminals, Micro Din Form C connector). The
switch can be purchased as a single-pole double throw switch (SPDT) or a double
pole double throw switch (DPDT). Electrical ratings range from .1amps at 125Vdc
to 5amps at 250Vdc depending on the type of switch ordered. Pressure ranges are
available from -15psi through 7500psi with setpoints no greater than the
maximum range of the product.
3 DESIGNING A SIF USING A MANUFACTURER PRODUCT
3.1 Safety Function
The A-Series switch will change states with changes in inlet pressure. Once an
actuation pressure is achieved the switch will change state as described in Section
2 of this document. The designer of the SIF must consider if the alarm condition is
on fall pressure or increasing pressure and adjust the switch accordingly. If a
factory set switch is desired the product must be specified to change state at a
predefined pressure and in the desired direction. When using the A-Series product
with dual switches only one setpoint pressure can be used. That pressure will
actuate both switches.
The A-Series switch is intended to be part of final element subsystem as defined
per IEC 61508 and the achieved SIL level of the designed function must be verified
by the designer.