Rockwell Automation Publication ICSTT-RM448M-EN-P - February 2021 61
Chapter 4 Before You Begin
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.
• Make sure that the power distribution cabling is sized to allow the
maximum prospective fault currents and tolerable voltage losses. This is
specifically important where floating supplies are employed and other
power sources can cause high prospective fault currents if multiple earth
faults occur.
Controller Power Supply Requirements
A controller requires the following power supply sources:
• A dual redundant power supply of + 24 Vdc with an operating range of 18
Vdc to 32 Vdc. The AADvance controller is designed to accept supply
transient and interference according to IEC 61131 part 2.
An over current fault in the controller must not cause the system to lose power.
Consequently, the power sources must be able to supply the peak current to
open any over current protection devices (such as fuses) without failing.
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