BEKA BA384E, Manual

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11.4 4/20mA output
The BA384G Rate Totaliser can be supplied with a
factory fitted galvanically isolated 4/20mA current
sink output which may be conditioned to represent
the composite rate or total display.
11.4.1 Intrinsic safety
The 4/20mA output has been certified as a separate
galvanically isolated intrinsically safe circuit
complying with the requirements for simple
apparatus. This allows terminals C1 and C2 to be
connected to any intrinsically safe circuit protected
by a certified Zener barrier or galvanic isolator
providing the output parameters do not exceed:
Uo ≤ 28V dc
Io ≤ 200mA dc
Po ≤ 0.84W
The maximum equivalent capacitance and
inductance of the 4/20mA output is:
Ci = 2.2nF
Li = 4µH
To determine the maximum permissible cable
parameters, these figures should be subtracted from
the maximum cable capacitance and inductance
specified by the certificate for the Zener barrier or
galvanic isolator powering the 4/20mA output circuit.
11.4.2 System design
The Rate Totalisers 4/20mA output is a passive
current sink i.e. not powered, but it is totally isolated
from all other Rate Totaliser circuits. It is effectively
a 2-wire 4/20mA transmitter requiring a minimum
supply of 5V with the output current controlled by the
Rate Totaliser’s composite rate or total. Subject to
complying with intrinsic safety interconnection
requirements, the terminals C1 and C2 may be
connected to another instrument which supplies
power and will accept a 4/20mA transmitter input.
The 4/20mA current output may also be transferred
to the safe area via a galvanic isolator or Zener
barriers. Terminals C2 and C4 are internally linked
and may be used for joining a return 4/20mA wire.
Fig 19 shows how a 2-channel Zener barrier may be
used to transfer the 4/20mA current output into the
safe area, alternatively a galvanic isolator may be
used.
Fig 19 Application of 4/20mA output