MEDC BG - Manual (Page 4)

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7. Functional Safety
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.
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