Installation with IS earth – connection via shunt-diode safety
barrier
• A suitable shunt-diode safety barrier is selected on the basis
of gas group compliance (IIB) and loop calculations. The
entity parameters for safety barrier and load (voltage, current,
capacitance and inductance) must match up correctly in order for
the loop to be approved intrinsically safe.
• MTL7787+ is an example of a suitable shunt-diode safety barrier,
with U
max
= 28 V, I
max
= 93 mA, R
min
= 300 ohm. Refer to Figure
6-5.
• The earth terminal of the safety barrier shall be connected to IS
earth.
• The safety barrier is normally mounted on a DIN rail connected to
the Intrinsically Safe (IS) earth.
• The cable screen shall be connected to IS earth in the safe area
but always left floating on the detector side.
Installation without IS earth-connection via galvanic isolated
safety barrier/current repeater
• If no IS earth is used, the detector may be connected via a
galvanic isolating safety barrier. A suitable barrier is selected on
the basis of gas group compliance (IIB) and loop calculations. The
entity parameters for safety barrier and load (voltage, current,
capacitance and inductance) must match up correctly in order for
the loop to be approved intrinsically safe.
• MTL5541 is an example of a suitable galvanic isolated barrier, with
U
max
= 28 V, I
max
= 93 mA, R
min
= 300 ohm. Refer to Figure 6-5.
3.8.1 Checking the setting of RS485 termination resistor with dip
switches
Check that the dip switches are set correctly. For normal operation,
make sure that the switches are in the ‘open’ (fully down) position
as shown in Figure 3-6. If switch number two is set to ‘closed’ (up)
position, the device connects the 120 Ω termination resistor to the
RS485 loop.
May2024 Quick Start Guide
Quick Start Guide 29
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