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Call points BG/pB Ex E VErsions
TM244 May 2019 www.eaton.com
Call points BG/PB Ex e Versions
English
8.0 Functional safety – BG
Introduction
The BG Call Point has been designed for use in potentially
explosive atmospheres and harsh environmental
conditions . The glass reinforced polyester enclosures are
suitable for use offshore or onshore, where light weight
combined with corrosion resistance is required .
The function of the BG is to raise an alarm manually once
verification of a fire or emergency condition exists, by
breaking the Glass .
The BG Break glass Unit is configured with either a single
series Resistor (R1) or an R1 and end of line resistor
known as R2 . No Current passes through R1 in either
configuration until the glass has been broken and the
switch contacts have closed . Upon closure of the switch
the current is then sent through R1 and hence changes the
resistive value in the circuit which triggers the alarm . The
R2 resistor always has a current flowing through which
provides the reference resistive value for the circuit . The
circuit can diagnose an open circuit failure in R1 without
having to trigger the system via a continuity check which is
a form of diagnostics for determining if an R1 resistor has
failed Open Circuit . This form of diagnostics does however
require a proof test to be conducted in order to identify
the failure and depending on the set up of the system the
defective Resistor may not be easily traceable if there are
several BG break glass Units in the system . The end of line
resistor R2 is used for the purpose of detecting an open or
short circuit in the supply conductors .
The safety function of the Call Point is to raise the alarm
when the Glass is Broken .
Under No fault (Normal) Operating conditions the BG
Break Glass Unit will raise the alarm upon operating the
switch via breaking the glass .
Under fault conditions the failure mode of the Break
Glass is a failure to raise the alarm . For the failure rate
associated with this failure mode please refer to the
following table .
Assessment of functional safety
The BG Call Point is intended for use in a safety system
conforming to the requirements of IEC61508 . Sira Test &
certification Ltd has conducted a Failure Modes Effect and
Diagnostic Analysis (FMEDA) of the BG Break Glass unit
against the requirements of IEC61508-2 using a proof test
interval of 8760hrs . The Call Point is classed as a
Type A device .
BG breakglass unit
Safety Function of BG Break Glass Unit:
‘To raise the alarm upon breaking the glass’
Architectural
constraints:
Type A
HFT=0
SFF= 97%
Proof Test Interval
=8760Hrs
MTTR = 8 Hrs
SIL3
Random hardware
failures:
λ
DD
= 0
λ
DU
= 5.72E-07
λ
SD
= 0
λ
SU
= 2.04E-05
Probability of failure
on demand:
PFD
AVG
=2.51E-03
(Low Demand Mode)
SIL2
Probability of
Dangerous failure on
safety function:
PFH = 5.72E-07
(High Demand Mode)
SIL2
Hardware safety
integrity compliance
[1]
Route 1
H
Systematic safety
integrity compliance
Route 1
S
Systematic Capability
SC2
Overall SIL-capability
achieved
SIL 2 (Low Demand)
SIL 2 (High Demand)