Allen-Bradley AADvance T9110 Safety Manual page 108

Chapter 4 AADvance Functional Safety System Implementation
108
The 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 does not exceed 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 meet 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.
The power supplies and power distribution, if incorrectly designed, present a
potential 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.
Ensure that the chosen PSUs are compatible with the power feeds provided.
Alternatively, measures should be implemented to ensure that the power feeds
remain 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
establish that sufficient current be available to ensure their protective action
and that the protective device can break the maximum prospective fault
current.
Ensure that the power supplies are sufficient for the system load and for any
foreseeable load requirements and load transients.
Ensure that the power supplies have a minimum output hold-up time of 10ms.
Ensure that the power distribution cabling is sized to accommodate the
maximum prospective fault currents and tolerable voltage losses. This is
specifically important where floating supplies are employed and other power
sources may result in high prospective fault currents in the event of multiple
earth fault conditions.
ATTENTION: The power supplies used shall conform to the electrical
requirements and tests defined in IEC 61131 Part 2, EN 61010-1 and EN
60950 and shall be of appropriate capacity for the system.
NOTE 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.
Rockwell Automation Publication ICSTT-RM446N-EN-P - April 2018