4-20mA Transmitter Wiring
TECHNICAL NOTE October 2002
4-20 mA Transmitter Wiring
TM
inthe Zone
4-20 mA Transmitter Wiring Options
Transmitters are available with a wide variety of signal outputs. The 4-20mA analogue signal is by far the
most commonly used in industrial applications. Several physical 4-20mA wiring options exist. This
guidance note aims to outline these options.
Industrial transmitters are available for monitoring many parameters these including pressure, temperature
and flow. Zellweger Analytics’ gas detectors / transmitters offer 4-20mA outputs, where 4 mA equates to a
zero gas reading and 20 mA equates a full scale reading of the calibrated gas range.
This signal is sent to a remotely located control panel. The control panel uses this signal and activates
executive actions via relay contacts, e.g. audible and visual alarms or initiate plant shut down procedures.
Transmitters typically require a 24Vdc supply. This supply can be derived local to the transmitter or
supplied direct from the associated control panel.
Several transmitter wiring options exist. The design of the associated control panel dictates which option
should be used.
These wiring options include:
• Current source transmitter, non isolated (3 wire)
• Current sink transmitter, non isolated (3 wire)
• Fully isolated (4 wire)
• Two wire loop powered transmitters’
Most modern transmitters can be wired in a current sink or source format, selection is often made by the
use of specific terminals or the location of a link within the transmitter. However some transmitters may be
limited to either a sink or source configuration. Check the transmitters technical manual for the available
options.
When transmitters are wired in a current source configuration it follows that the control system will be
current sink, and vice versa. Therefore it is important to qualify whether it is the transmitter or the control
system that is to be wired in a specific configuration.
For the purposes for this guidance note it is assumed that both the transmitter and the remote control
panel require a 24Vdc supply.
Fire and flame detection
Current source transmitter, non isolated (3 wire)
This is the most common configuration of modern 4-20mA transmitters.
The transmitter and control panel can use the same 24V and 0V dc supply
lines. The 4-20mA signal flows through the 24V dc line and the signal line to
the controller.
Advantages:
• Only three cable cores are required to the transmitter.
• A common power supply can be used for both the transmitter and the
control panel.
Disadvantages:
• Any electrical interference or pick up may be transmitted along the signal line,
which could generate a spurious alarm in the control panel.
Current sink transmitter, non isolated (3 wire)
The transmitter and control panel can use the same 0V and 24V dc supply
lines. The 4-20mA signal flows through the 0V dc line and the signal line to the
controller.
Advantages:
• Only three cable cores are required to the transmitter.
• A common power supply can be used for both the transmitter and the
control panel.
Disadvantages:
• Any electrical interference or pick up may be transmitted along the signal line,
which may generate a spurious alarm in the control panel.
Fully isolated (4 wire)
The transmitter and control panel use separate 24V dc supplies. The
4-20mA signal flows through two separate cable cores between the
transmitter and control panel. It is assumed that the power to drive the
4-20mA loop is derived from the control panel.
Advantages:
• Electrical interference on the voltage supply lines will not be transferred to
the 4-20mA signal line, reducing the risk of spurious signals being received
at the controller.
Disadvantages:
• An additional cable core is required for each transmitter compared to current
sink and source options.
• A separate power supply is required for both the transmitter and control
panel.
Fire and flame detection
Two wire loop powered transmitters
This configuration supplies power and 4-20mA signal over a two wire loop connection between the
transmitter and the control panel.
Not all transmitters can be wired in this format and must be specifically designed to accommodate this
configuration.
Advantages:
• Has low power consumption.
• Only two cable cores are required to the transmitter.
Disadvantages:
• Transmitter discrete fault signalling can not be set at 0mA as this configuration continues to draw
some current in a fault condition. This configuration is not suitable for control panels that require
a 0mA signal for a fault indication.
Sub 4mA status signalling is limited due the reduced range of mA available between fault and a zero
gas reading.
Not suitable for power hungry transmitters, e.g. catalytic gas detectors, or Infrared gas detectors using
optical heating elements.
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10/2002
Please Note:
While every effort has been made to ensure accuracy
in this publication, no responsibility can be accepted
for errors or omissions. Data may change, as well as
legislation, and you are strongly advised to obtain
copies of the most recently issued regulations,
standards and guidelines.
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