Michell Instruments Easidew PRO, User Manual, Page: 24 / 58

I.S. Field Display User’s Manual
18 97321 Issue 2.3, May 2016
OPERATION
3.3.1 Solid State Output
Each alarm has a galvanically isolated single pole solid state switch output which is
shown in Figure 9. The output is polarized and current will only fl ow in one direction.
Ron = less than 5 Ω + 0.7 V
Roff = greater than 1M Ω
S
(8, 10) (8, 10)
(9, 11) (9, 11)
1M
5Ω 0.7 V
8μH
24nF
+
Figure 9
Equivalent Circuit of Each Alarm Output
3.3.2 Intrinsic Safety
Each alarm output is a separate galvanically isolated intrinsically safe circuit. The EC-
Type Examination Certi fi cate states that for intrinsic safety considerations, under fault
conditions, the output voltage, current, and power at terminals 8 & 9 and 10 & 11 will
not exceed those speci fi ed by clause 5.7 of EN 60079-11 for simple apparatus. This
simpli fi es system documentation and allows the alarm output terminals 8 & 9 and 10 &
11 to be connected to almost any intrinsically safe circuit protected by a Zener barrier,
or galvanic isolator, providing the output parameters of the circuit do not exceed:
Uo
= 30 V DC
lo = 200 mA
Po = 0.84 W
The maximum equivalent capacitance and inductance between each set of alarm
terminals is:
Ci =
24nF
Li = 8 μ H
To determine the maximum permissible cable parameters these fi gures should be
subtracted from the maximum permitted cable capacitance and inductance speci fi ed
by the certi fi cate powering the alarm circuit, such as the solenoid driver and switch
transfer galvanic isolators shown in Figure 10.
Hazardous Area
Safe Area
ppm
V
Easidew PRO I.S. Field Display
P E
3 4 8 9 10 11 2 1
S
S S
S
+
+ +
0
9999
4-20 mA
supply
+
0
Solenoid
driver
Alarm 2
Alarm 1
4-20 mA
2-wire
TX
Intrinsically safe
solenoid valve
C tifi d
+
0
Switch
transfer
Pump
Figure 10 Typical Alarm Application
(Shown without recommended screened cables)