Endress+Hauser PROline promag 53 II3G Mise En Service page 38


16
Step 3 – current calculation
The number of field devices that can be connected to the H1 segment depends on the
fieldbus power supply selected (supply voltage and supply current) and the current
consumption of the field devices. Thus, the number of devices that can be connected
is reduced if a field device consumes a basic current of more than 10 mA, e.g. 20 mA.
By adding up the basic currents I
the event of a fault for the FDE (Fault Disconnect Electronics) and the current for the
data modulation I (+9 mA) you can determine the minimum supply current that the
MOD
fieldbus power supply must supply.
If, on switching on the device, the startup current I
rent, then the startup current must be taken into consideration in the calculation. The
response current for the FDE (I
ence between the maximum current in the event of a fault and the basic current. The
field device with the greatest response current is taken into consideration in the current
balance sheet.
Provided that no more than one FDE responds, the following condition must be taken
into consideration in the calculation of the segment current I
≥
I I
Seff. SEG
where
Σ
I = + max I
SEG IB
In the example the calculation formula results in the following segment current con-
sumption I , where I
SEG FDE
additional current in the event of a fault and the startup current is less than the bus cur-
rent:
Σ I
I = + max. I
SEG B
= 45 mA + 0 mA + 9 mA + 0 mA
= 54 mA
The next step checks whether the current consumption I
rent limits of the fieldbus power supply. The maximum available supply current I
calculated taking into consideration the current drops caused by the source impedance
R of the fieldbus power supply and then by the effective resistance per unit of length
Q
of the fieldbus line. Also, the condition must be met that the supply voltage at the fur-
thest field device must be at least 9 V. To make the overall considerations easier it is
assumed that all the field devices FD (refer to Fig. 7) are connected at the end of the
fieldbus line.
Fig. 7: Replacement circuit diagram
PROline Promag 53 FOUNDATION Fieldbus
of the field devices plus the response current I
B
) is calculated for each field device from the differ-
FDE
+ I + I
FDE MOD startup
= 0, I = 0, since the field devices used do not consume
startup
+ I + I
FDE MOD startup
is greater than the basic cur-
startup
:
SEG
calculated is within the cur-
SEG
Endress+Hauser
in
FDE
is
Seff.