Safety Guide

XNX Universal Transmitter
Overview 5
XNX Gas Detector Transmitter
2 Overview
IEC 61508 is a generic functional safety standard. Functional safety
is defined in this standard as “part of the overall safety relating to the
Equipment Under Control (EUC) and the EUC control system which
depends on the correct functioning of the E/E/PES
1
safety related
systems, other technology safety-related systems, and external risk
reduction facilities.”
A system is considered to be functionally safe if the random and
systematic faults do not kill or injure humans, pollute the environment,
and do not result in the loss of equipment or production.
A systematic fault is defined as a failure with a definite cause. A random
fault can happen at any time and the cause is unclear. The terms fault
and failure can be used interchangeably.
A Safety Integrity Level-certified system can detect the majority of
safe and unsafe failures. XNX is SIL 2 capable per IEC 61508. XNX
is SIL 3 capable in a redundant system per IEC 61508. Table 1 and
Table 2 below outline a system’s safety integrity level in relation to its
average probability of failure to perform its design function on demand
and probability of dangerous failure per hour.
Table 1. Average Probability of Failure to Perform Its Design Function on Demand (Low Demand
System)
Safety Integrity
Level
Low demand mode of operation (Average probability of
failure to perform its design function on demand (PFD))
4 ≥ 10
-5
to < 10
-4
3 ≥ 10
-4
to < 10
-3
2 ≥ 10
-3
to < 10
-2
1 ≥ 10
-2
to < 10
-1
1 Electrical/electronic/programmable electronic systems
Table 2. Probability of a Dangerous Failure Per Hour (High Demand System)
Safety Integrity
Level
High demand or continuous mode of operation
(Probability of a dangerous failure per hour (PFH))
4 ≥ 10
-9
to < 10
-8
3 ≥ 10
-8
to < 10
-7
2 ≥ 10
-7
to < 10
-6
1 ≥ 10
-6
to < 10
-5
NOTE:
The XNX system is Type B. A Type B system uses controllers
or programmable logic per IEC 61508.
The XNX product consists of a main board, a personality board, and
a sensor.
XNX
main
board
IR Personality
board
EC Personality
board
IR sensor
mV sensor
EC sensor
XNX Transmitter Sensor
Note: Only one personality board per XNX main board
mV Personality
board
This manual outlines the proof test procedure, a required operation to
maintain the XNX’s functional safety under low demand applications.