Electrical connections ________________________________________________________________
Vaisala _________________________________________________________________________ 24
Installation in hazardous locations
US and Canadian requirements
USA (FM): Wiring for intrinsically safe operation is shown in APPENDIX 2.
Canada (CSA): Wiring for intrinsically safe operation is shown in APPENDIX 3.
European requirements
CATEGORY 1 (zone 0)
The HMT360 has to be connected to Exia-certified associated apparatus with galvanic
isolation, gas group IIB or IIC. NOTE! If both analog outputs are in use, the channel 1 (-) and
channel 2 (-) must be short circuited (see figure 4-3).
CATEGORY 2 or 3 (zone 1 or 2)
The HMT360 has to be connected either to a zener barrier or galvanic isolator NOTE! If both
analog outputs are in use with a galvanic isolator, the channel 1 (-) and channel 2 (-) must be
short circuited (see figure 4-3).
Figures 4-1 and 4-2 present examples of galvanic isolators and zener barrier connections (only
CH1 connected).
Maximum load calculation for the barrier
Always check the technical data of galvanic isolator or zener barrier to define maximum load.
If the maximum load is not included in specifications it can be determined as follows.
Example: Stahl 9002/13-280-093-00 (zener barrier)
General specifications of the HMT360
Supply voltage
U
in
= 24 V (12...35 V)
Maximum current
I
max
= 20 mA (100%RH)
Minimum operating voltage for HMT360
U
min
= 12 V (15 V with serial port )
Cable resistance (as an example)
R
cable
= 0.085 ohm/m/core (2*0.085 ohm
/m/pair)
Cable lenght =250 m
R
Ctot
=
2*0.085 ohm/m*250m=
45 ohm
Stahl 9002/13-280-093-00 (values taken from the specifications):
Typical end to end resistance
R
barrier
= 340 ohm
Max. additional voltage drop (20mA)
U
add
= 1 VDC
Voltage drop
U
drop
= I
max
*(R
Ctot
+ R
barrier
) + U
add
= 20 mA*(45+340)ohm + 1 V = 8.7 V
Voltage for R
l
U
Rl
= U
in
- U
min
- U
drop
= 24 V - 12 V - 8.7 V = 3.3 V
Maximum load
R
lmax
= U
Rl
/I
max
= 3.3 V / 20 mA = 165ohm ,
R
lmax
< 165ohm
