Re: [foxboro] Power Supply Faults

Dear Dave,
For voltage outputs it is  wiser to use TCA with internal relays. Internal 
relays provides isolation in case of short circuit.
To prevent the analog outputs to go wrong in case of total failure it is 
better to configure intelligent positioners at the control valves to hold at 
the last position in case a signal loss is detected.It can provide you 
enough time to handle the situation as long as the process remains in stable 
condition In some installations I have seen an alternate control system for 
the critical loops with automatic or manual switchover circuit to prevent 
the total process downing.

Best Rgds,
Ali Ahmed Zahidi
DCS Controller
Pakistan Refinery Ltd.
Karachi, Pakistan.


----- Original Message ----- 
From: <dave.caldwell@xxxxxxxxxxxxxx>
To: <foxboro@xxxxxxxxxxxxx>
Sent: Tuesday, February 14, 2006 8:11 PM
Subject: [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.
>
>
>
> _______________________________________________________________________
> This mailing list is neither sponsored nor endorsed by Invensys Process
> Systems (formerly The Foxboro Company). Use the info you obtain here at
> your own risks. Read http://www.thecassandraproject.org/disclaimer.html
>
> foxboro mailing list:             http://www.freelists.org/list/foxboro
> to subscribe:         mailto:foxboro-request@xxxxxxxxxxxxx?subject=join
> to unsubscribe:      mailto:foxboro-request@xxxxxxxxxxxxx?subject=leave
>
> 
 
 
_______________________________________________________________________
This mailing list is neither sponsored nor endorsed by Invensys Process
Systems (formerly The Foxboro Company). Use the info you obtain here at
your own risks. Read http://www.thecassandraproject.org/disclaimer.html
 
foxboro mailing list:             http://www.freelists.org/list/foxboro
to subscribe:         mailto:foxboro-request@xxxxxxxxxxxxx?subject=join
to unsubscribe:      mailto:foxboro-request@xxxxxxxxxxxxx?subject=leave

Other related posts: