Rockwell Automation AADvance Controller System Build - Manual (Page 38)

2-8 Document: 553632
(ICSTT-RM448_EN_P) Issue: 08:
System Build Manual (AADvance Controller)
PELV (protected extra-low voltage) is an extra low voltage circuit with a
protective partition from other circuits which has a protective earth connection.
Thus 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 galvanically 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 to meet the system load and for any
foreseeable load requirements and load transients.
Ensure that the power supplies have a minimum 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.
The power supplies used shall conform to IEC61131 Part 2, EN61010-1 and
EN 60T950 and shall be of adequate capacity for the system.
IMPORTANT TIP: 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.