Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 37)

Trusted
TM
AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 37
Caution
Devices that cannot be reliably line monitored when de-energised must not be used for safety
related energise-to-action applications.
State 6 may also be caused by external lamp tests, or otherwise when the outputs are powered from
an external source. It is recommended to configure lamp tests through the system application so that
there are no secondary connections powering the load. Module diagnostics may detect external
supplies as ‘stuck-on’ faults which will lead to a module shutdown.
High inrush current loads
As for dynamic loads above, release 3.5 firmware or later will improve handling of inrush currents; it will
pulse-drive a heavily capacitive load until the current falls within specification. There are no user-
replaceable fuses in a Trusted digital output module; instead a DSP algorithm dynamically monitors the
load. Before release 3.5, capacitive loads were often reported as short circuits. On detecting a short
circuit, the module de-energises the output.
Note that inrush currents can occur with small filament bulbs as well as bacons and sounders. This is
due to the very low cold resistance of the filament.
If inrush current is still a problem even with firmware from release 3.5, replace or refit the device to
smooth out its demand peaks. Some devices can be fitted with optional ‘soft-start’ modules to smooth
the current demand.
Power supply fluctuations
It is possible that a heavy load may drag the supply voltage down, either on energising/de-energising or
with a constant noise signal. Some module diagnostics routines rely on measurements of the field
supply voltage. If the voltage changes significantly during a test, the diagnostics may report a fault.
This may lead to a slice going offline.
Ensure that the power supply is able to sustain a stable voltage during heavy switching demand. At
release 3.5 output module firmware, the outputs will pass considerably more current whilst trying to
energise a load (for example, the ‘Field Fault’ switches on a demonstration kit will bring down the 24V
supply with this firmware).