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Rockwell Automation Publication ICSTT-RM448J-EN-P - April 2018
Chapter 4
Before You Begin
System Power Requirements
A controller's system power should be supplied from two different 24 Vdc
(Nominal) power supplies with a common return path; that is, the 0 V return
will be the same between the power feeds. Each controller also requires an
external field power source for the field loops.
This means that a controller must be supplied with system power from a power
source that complies with SELV and PELV standards.
• SELV (safety extra-low voltage) is a voltage which is no larger than 30
Vrms, 42.4 Vpeak and 60 Vdc between conductors, or between each
conductor and earth in a circuit which is isolated from the line voltage
by a safety transformer.
• PELV (protected extra-low voltage) is an extra low voltage circuit with a
protective partition from other circuits which has a protective earth
connection.
To satisfy SELV and PELV requirements the power source must have a safety
transformer with a protective partition between the primary and secondary
windings so that the windings are galvanic and electrically isolated.
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.
WARNING:
A controller system must be installed with a power network that
is designed to meet over voltage Category II
