Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 49)

Trusted
TM
AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 49
Floating Earth Systems and Loop Isolation
ICS Triplex Technology recommends that system zero volt references are well grounded. This
simplifies fault finding; the first earth fault will have a single effect on the field loop on which it occurs.
Whilst floating earth systems can survive a single earth fault, they are very unlikely to indicate the
position of the fault. Multiple faults can cause random scattered effects which are often impossible to
diagnose.
However, at many sites there is no possible secure earth, since the ground is too dry. If an earth point
cannot be reached, the alternative is to isolate all I/O individually. If each field loop is isolated, a fault
will only affect the loop on which it occurs.
Each loop should be isolated at the system end. If the loops are only isolated in the field, then a fault
on one loop can still affect other loops. Inputs may be isolated using separate isolating devices or by
using isolated input modules. Output modules may be considered as isolated per power group, i.e.
each group of 8 outputs can have a separate supply and zero volt reference, as long as the power
wires are separated. A possible future module T8473 has isolated outputs and may be considered a
volt-free contact output module, although its diagnostics will not recognise low supply voltages. Safety
relays may be used to reliably isolate individual outputs. A common supply will connect isolated outputs
together and should be avoided.
Hazardous Area Protection
System installation
The Trusted system modules are not certified for installation in hazardous areas. The system chassis
should be fitted in safe areas with temperature and humidity control. Installation in sealed EEx ‘d’
enclosures is not advisable because:
• No visual front panel health check is possible.
• System maintenance requires access to modules. This requires the enclosure to be opened.
This is only possible if the area is made safe which would probably require a site shutdown and
purge.
•
Ventilation from the enclosure is not possible, and overheating will occur rapidly.
EEx certification
System I/O is not certified for direct connection to hazardous areas. Appropriate zener barriers or I.S.
isolators should be fitted on all field loops to hazardous areas. Zener barriers need a reliable intrinsic
safety earth with an impedance of less than 0.1 ohms, which should be separate from all other earths.
Non-incendive certification
FTAs T8801 and T8831 may be used for direct connection of inputs from hazardous areas where non-
incendive specifications are used (typically in the Americas). Their application is described in the
sections above on digital and analogue inputs. Outputs require external power limiting devices.
Lightning Protection
Trusted system modules and FTAs do not provide protection against lightning strike, power surge or
severe induced serial noise. The modules include 2.5kV isolation between field circuits and system
circuits (and between channels for isolated modules), but this will not protect field circuits against high