900 Series V5.1: Installation Guide, February 2000 31
Figure 26: Schematic wiring diagram of Intrinsically Safe zone
4.1.2. Maximum loading
Due to the finite resistance of the safety barrier, there is a limit to the current drain that
can be tolerated before the voltages on the circuit fall outside the specified limits for the
Series 970 Intrinsically Safe devices. Two components of the current drain must be
considered, namely the standing current of the devices themselves and the maximum
drain caused by alarm LEDs being illuminated. The standing current of the devices can
be calculated simply by taking the sum of the individual device currents on the circuit, as
given in chapter 6 "Loop Load Calculations” for each product.
The panel software should limit the maximum number of LEDs that can be illuminated
simultaneously and, in practice, it is this parameter that causes the real limit on circuit
loading. The design of the LED drive circuits in the monitors is such that the addition of a
remote LED will not increase the current drain on the circuit and remote LEDs can
therefore be ignored in loading calculations.
Further restrictions apply when the GBX70 galvanic barrier is used with circuits having
high cable capacitance. Under these conditions communication errors might occur,
resulting in corrupt data or in devices failing to respond. In order to ensure reliable
communication when capacitive loads are connected to these barriers, observe the
following conditions:
Series 950 loop
Protocol translator
(View from top)
GBX70 barrier
(View from top)
To Next Page
Loading...
Need help?
General
