BEKA BA334E, Руководство пользователя, Страница: 45 / 64

45
Fig 18 Setpoint adjustment from the totalisation
mode
To adjust an alarm setpoint select
5P1
x or
5P2
x and
press
(
which will reveal the existing value. The
flashing digit of the setpoint may be adjusted using
the
&
or
*
push button and the ( button to
move control to the next digit. When the required
setpoint has been entered, pressing
)
will return
the display to the
5P1
x or
5P2
x prompt from which
the other setpoint may be selected, or the
instrument may be returned to the totalisation mode
by pressing
)
again.
Note: Direct access to the alarm setpoints is only
available when the menu is enabled - see
section 10.3.13
10.4 4/20mA output
The BA334G Rate Totaliser can be supplied with a
factory fitted galvanically isolated 4/20mA output
which may be configured to represent the rate or
total display.
10.4.1 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 C3 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.
10.4.2 System design
The Rate Totalisers 4/20mA output is a galvanically
isolated 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
its current being controlled by the Rate Totaliser.
Subject to complying with intrinsic safety
interconnection requirements, the terminals C1
and C3 may be connected to another instrument,
which 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.