[foxboro] Power Supply Faults

To All,

First time caller. Long time listener. Enjoy the list.

I am interested in knowing what, if anything, you can do to avoid a
process downing event when the 24vdc power supply for the 200 series
base plates, FBMs, and Field Termination Assemblies is faced with a
short circuit fault.

Recently, we experienced a significant event in our plant utilities area
when a loop was incorrectly wired to a 241c, switched output (external
source), with power distribution (PO916QQ) FTA. The wiring error allowed
fault current to pass through the fuse of the termination assembly
starving the 200 series baseplates, FBMs, and FTAs of current. The
technician heard relays dropping out and immediately re-opened the knife
switch he had just closed. This was too late however to prevent the loss
of all service water wells, and two steam boilers.

In investigating this incident we determined that the 15A fuse on the
FTA never blew, as the 15 amp / 24VDC Acopian power supply (redundant)
reduced its voltage to current limit and protect itself. According to
the fuse's time-current curve it would take about 30 seconds to blow
this fuse at 30 amps current. So, even if the Acopian power supply could
source 30 amps (which it cannot), we'd still have shutdown our wells and
boilers. In fact, we now believe that the relays on the 241c termination
assemblies are probably the first piece of hardware to drop out. We
think the ride through of such small relays is extremely short (10 to 50
ms range).=20

We experienced a similar event several weeks ago when some technicians
doing demo work cut through a pair of wires to a 24vdc standard rated
solenoid. As this output was not current limited we again experienced
fault current.

While these may be isolated incidents, it got us thinking. Should we
have non-current limited outputs connected to our power supply? Can we
get fault protection that would act quickly enough to allow the FBMs
(particularly those with relay outputs) to ride through? If not, is it
even worth doing? What are other users doing?

In conclusion... I'd be pleased to hear from anyone on the list
regarding how they manage the 24VDC supply to FTAs-particularly those
which are non-current limited (such as, a 241c, switch output, external
source, with power distribution to a Standard rated output). Moreover,
if anyone has any experience with preventing, mitigating, or responding
extremely quickly to power supply faults in order to prevent a process
downing event, I'd like to hear from you.

Thanks in advance,
Dave C.

 
 
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