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Page 26
MERCAP – INSTRUCTION MANUAL
7ML19981CM01.1
Incorrect power connection will not damage the MST9500. However, it can lead to a larger
current (~40 mA) through the loop, and it will not operate with incorrect polarity.
The MST9500 is isolated from the power supply that provides for the opportunity of grounding
either line (positive or negative) if requirements for Ex safety are followed and the power
supply voltage is less than 33 Vdc.
Caution
: During connection, do not leave moisture or metal scrap (of the cable shielding
etc.) in the housing. This can interfere with transmitter operation.
Interconnection
Supply
The supply voltage requirements for the MST9500 are shown in the installation figures on
page 27. Because the MST9500 uses a switched power supply circuit, the required terminal
terminal
terminal
terminal
voltage
voltage
voltage
voltage depends on the total measuring current. In case of a higher current value, a lower
terminal voltage is allowed.
For example, when using a 250 Ohm measuring resistance without barrier and cable resistan-
ce, the supply voltage should be at least 14.5V. A 250 Ohms measuring resistance, a barrier
of 280 Ohm, and 20 Ohm cable resistance (500 m) results in a total of 550 Ohm, therefore a
minimum supply voltage of 20.5 Volts (approx.). In case of a multi-drop application, where the
measuring current is fixed to 4 mA, the supply voltage on the terminals of the MST9500 should
be at least 12 Volts.
Cable
The selection of the cable is mainly determined by two criteria:
1.
The resistance of the copper conductor (Ohm)
2.
The cable capacity (pF)
The copper resistance influences the voltage drop over the cable. The cable capacitance
influences the HART
TM
signals and is important for intrinsically safe applications. If, for
example, it has a diameter of 1 mm
2
, the result is a copper resistance of 36.8 Ohm/km and a
capacitance of 100 pF/m. To maintain reliable transfer of the HART
TM
modem signals, it is
indicated that the RC time of the connection parts should never be more than 65 µSec. For
output signals (from the MST9500), only the cable and barrier resistance counts. For input
signals it is less favourable since the measuring resistance also counts.
(RB + RM) x CC should be max. 65 µSec. (R in Ohm, C in Farad, T in Sec). For a standard 28 V
280 Ohm barrier and a 250 Ohm measuring resistance, a field capacitance of 0.123 µF is
allowed. This is higher for IIC (I/S) applications than allowed; therefore, attenuation of HART
signal will not occur.
In IIB applications, where the maximum allowed capacity value is 0.33 µF, the cable length
allowed will be longer than actually allowed for HART™. Depending on cable specifications,
the maximum length lies between 1 and 3 km.
When making Ex calculations, only the cable capacitance at the transmitter side of the barrier
counts. For damping calculations, the cable capacity at the other side of the barrier should
also be considered.