108 Rockwell Automation Publication ICSTT-RM447M-EN-P - July 2019
Chapter 6 Specifying a New Controller
• For digital and analogue outputs a field power supply of +24 Vdc within
a range of 18-32 Vdc is required.
Recommended field circuits are given for each type of I/O module later in the
section "Connecting Field Wiring".
Power Distribution Protection
The power distribution circuit for each field input and for each output module
must be protected, externally to the controller. Rockwell Automation
recommend that power distribution must meet national and local panel wiring
protection standards.
Digital Output Field Power
Special fusing arrangements are required for Digital Output field supplies for
UL, ATEX and IECEx approved installations, (see topic on field loops for
Digital Output Modules).
Power Supply and Power Distribution Requirements
The power supplies and power distribution, if incorrectly designed, are a
possible electrical or fire safety hazard and can contribute to common cause
failure. It is therefore necessary to:
• Establish the power philosophy, specific earthing philosophy, power
requirements, and the separation requirements where items of
equipment are separately supplied, for example system internal supplies
and field loop supplies.
• Make sure that the chosen Power Supply Units (PSUs) are compatible
with the power feeds supplied. Alternatively, measures must be put in
place to make sure that the power feeds stay within the specifications of
the PSUs.
• Define the power distribution requirements, together with the
protective philosophy for each distribution; for example, current limited
at source or protective devices. Where protective devices are used, it is
important to find out that sufficient current will be available to make
sure their protective action and the protective device can break the
maximum prospective fault current.
• Make sure that the power supplies are sufficient to meet the system load
and for any foreseeable load requirements and load transients.
• Make sure that the power supplies have a minimum hold up time of 10
ms.
IMPORTANT It is highly recommended that the negative side of the field supply be
connected to earth (ground). This will avoid possible fail danger conditions
that can be caused by some earth fault monitors used with floating power
supplies.
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