900 Series V5.1: Installation Guide, February 2000 33
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All series 950 Intrinsically Safe devices have zero equivalent capacitance
and inductance.
4.2. Protocol translators
In order to enable the use of standard control and indicating equipment in Intrinsically
Safe systems, you need to use a device to "translate" voltage levels from the loop driver,
operating within the S950 limits, to levels compatible with the Intrinsically Safe
requirements. The translator also "boosts" the current pulses returned by the Intrinsically
Safe detectors, thereby ensuring compatibility with standard loop driver thresholds.
The translator is a loop-powered device that draws a low quiescent current and is
therefore transparent to both the loop driver and the Intrinsically Safe detectors. As the
translator is used within the safe area, i.e., before the safety barrier, no certification is
necessary. Refer to Figure 26.
4.3. Safety barriers
The single channel 28 V 300 Ohm barrier is the most basic type of barrier and therefore
the cheapest. The problem with these devices is that one side of the barrier must be
connected to a high-integrity (safety) earth. The earth connection has no effect on the
operation of the Series 970 Intrinsically Safe devices and is not needed for their correct
operation. However, Aritech control and indicating equipment cannot operate with this
earth connection as the control equipment incorporates earth-leakage monitoring and the
earthing of the loop causes unwanted cross talk between loops. For Aritech Series
analogue addressable fire panels the GBX70 (galvanically isolating barrier) must therefor
be used.
Galvanically isolated barriers (also known as transformer isolated barriers) differ from
conventional shunt zener barriers in that they provide electrical isolation between the
input (safe area) and the output (hazardous area). This is achieved by the use of a DC-
to-DC converter on the input side. This is connected to the hazardous area through a
voltage- and power-limiting resistor-zener combination similar to a conventional barrier.
The galvanic isolation technique means that the circuit does not need a high integrity
(safety) earth and that the intrinsically safe circuit is fully floating. By using this type of
interface, you eliminate any earth problems to controling and indicating equipment.
Although galvanically isolated barriers are widely used with conventional fire detectors,
the pulse response of standard products has always been too slow to be used in
analogue addressable systems. However, the GBX70 has been developed as a special
galvanically isolated barrier that freely transmits the Series 950 protocol pulses without
introducing severe voltage drops.
This interface is available as single or dual channel versions and is recommended for all
Intrinsically Safe applications. Both versions are BASEEFA certified under certificate
numberEx95C2064 (the KFDO- types have replaced the earlier KHDO- types.)
The galvanically isolated barrier is a two-wire device that does not need an external
power supply. Current drawn from the Series 950 loop by the barrier itself is less than
500 µA. The housing is a rail-mounted type identical to that used for the protocol
translator. Refer to Figure 26 for details.
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