Owner's manual

ELECTRICAL CONNECTIONS 4
23
TT 51 SERIES
www.krohne.com09/2010 - 4000754201 - MA TT 51 R01 en
4.7 Connection diagram of rail-mount transmitter (intrinsically safe)
4.8 Cable length
In order to ensure reliable HART
®
communication, the maximum cable length of the output
circuit must be observed.
To calculate the maximum cable length for the output circuit, determine the total resistance of
the output loop (load resistance + approximate cable resistance). Find out the capacitance of the
cable being used. In the following tables you can find the maximum cable length based on the
typical values for 1 mm
2
cables. CN is the abbreviation for "Capacitance Number" which is
multiple of 5000 pF present in the device.
Figure 4-4: Connection diagram
1 Input (intrinsically safe)
2 Classified hazardous area (potentially explosive area e.g. zone 0, 1 or 2)
3 Classified hazardous area (potentially explosive area e.g. zone 1 or 2)
4 Safe area
5 Modem / Ex (intrinsically safe)
6 R
Load
(intrinsically safe)
7 Voltage supply 10…30 VDC (intrinsically safe - terminals 21, 22)
DANGER!
In the Ex version, please note that the maximum cable length is determined by a resistance, an
inductance and a capacitance of the cable. The total capacitance and inductance of the cable
must be within the limits for the transmitter described in the Ex certificate.
Field device Cable insulation
PVC Polyethylene Polyethylene foam
[m] [ft] [m] [ft] [m] [ft]
1 (CN = 1) 600 1969 1100 3609 2000 6562
10 Multidrop (CN = 1) 500 1640 900 2953 1600 5249
10 Multidrop (CN = 4.4) 85 279 150 492 250 820
Table 4-1: Maximum length for typical 1 mm
2
cables