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Injected 24V Power Measurement

Twice in one week I have had an issue with power from a fuse on the 24 V power that is injected to Charms on a DeltaV backplane.  I understand that this power does not go through the DeltaV system as such and thus i can't follow it the same way that I can follow the 24 V power that goes to the CIOCs.  I was wondering if there is a way that a Charm on that backplane might happen to be able to see this 24V power in a way that I could monitor it.  The Charms and terminal blocks are the only parts of the system that I can think of that see the power and coudl possibly report the value to me.  I know that this is a very long shot but if there is anyone that knows how this might be done, it is someone in this group.  From what I understand of the system, that is the only hope that I have for being able to do this.

Thanks for the help on this!

Kent

11 Replies

  • RickV
    Kent,
    Unlike a CIOC that has some parameters you can monitor, the CHARM does not. (At least to my knowledge.) A typical cabinet design would have two power supplies that go through a diode then through a series of fuses or breakers to feed power to the injected terminal blocks of a CHARM Baseplate. The power supplies would be monitored through their alarm contacts, normally series together then on a cabinet alarm CHARM. To get an alarm from the injected power CHARM you would have to run this to a CHARM to get an alarm for loss of power. Keep in mind that each injected CHARM terminal block also has a fuse so even a good power supply fuse alarm you could have a fuse bad for an individual CHARM on a baseplate.
    If this continues to be a problem and you continue to blow fuses for power to an injected baseplate I would start looking at the power being used per baseplate. Remember that the limit is 10 amps per baseplate of injected power.
  • Lun.Raznik

    Have you tried looking at Power CHARMs?

    www.emerson.com/.../deltav-se4302t05

    You have to inject power into the CHARMs baseplate and you would use this CHARM to monitor the voltage.

  • Kent

    In reply to RickV:

    Thank you. In one case it was a blown fuse and in the other it was a loose connection. Then again, this was the first time I've seen this since we put our first charms in many years ago.
  • Kent

    In reply to Lun.Raznik:

    Thanks. I hadn't heard of this type of Charm. It looks like it indicates good at above 10V. I'll investigate this further.
  • Kent

    In reply to Lun.Raznik:

    I have the information from your link. I'm trying to find information in Books Online about installation etc. Can you point me to where I would find information on Power Charms in Books Online? Thanks again for the help!
  • Matt Stoner

    In reply to Kent:

    Hardware Installation -> DeltaV S-series and CHARMs Hardware Reference -> CHARMs I/O subsystem specifications -> Power Output CHARMs specifications -> 24 VDC Power CHARM
  • Kent

    In reply to Matt Stoner:

    Thanks.
  • Andre Dicaire

    In reply to Kent:

    Rick, Seems to me that the issue is not really a CHARMs issue, but a power distribution issue. The CHARM Baseplate has no active circuitry that consumes power from your 24V Source. The CIOC monitors the 24 VDC power on Primary and secondary. If you are using these two supplies via a redundancy module for injected power, the CIOC reports loss of a power supply in diagnostics. If the issue is with each individually fused circuit, does it make sense to add a CHARM to each baseplate? I've never heard of systems adding 24vdc fused power monitoring of this nature.

    The Power CHARM was intended to provide power to 4 wire devices, allowing it to be installed next to an AI CHARM to provide power and signal connections. It is equipped with diagnostics to monitor the presence of 24 VDC from the injected power bus, after the Fuse. It also provides a disconnect for the power to the field device. on a Fuse failure, or loss of 24 VDC, the Power CHARM will indicate a red LED and also report to Diagnostics an integrity signal. Again the intent was to provide remote diagnostics of the fuse condition. The device would report a loss of signal, impacting operations. The Power CHARM Diagnostic was to facilitate identification of the issue. It does not need to be referenced in a module to serve that purpose.

    I'm not sure about this use case. Checking all terminal screws are tight seems to be in order. Are there existing signals that use the injected power that could serve to infer a loss of power to the baseplate?

    Andre Dicaire

  • Kent

    In reply to Andre Dicaire:

    This is definitely a power distribution problem. We don't have this on every backplane but where are quite a few that have injected power for DOs that go to valves that pull enough power that they need injected power. Checking connections is certainly in order but that doesn't help with blown fuses. It is very infrequent but I'd hate to be relying on a valve operating and it fails to operate because of one of the infrequent fuse failures. What I'm looking for is something that will give me that indication for the back planes that have infused power. At least the back planes that have what we consider critical valves on them. I knew that this was a long shot and I really appreiciate all the help that you've given me.
  • Andre Dicaire

    In reply to Kent:

    I understand your situation. When we designed CHARM's, removing the need for fuses was a priority. Fusing of Isolated CHARMs would be expected to be applied at the source of the circuit power. The Fused Injected Power Terminal block therefore has a fuse when we provide the power to the circuit. Since the injected power is outside of the CIOC IO power distribution and connected via isolated CHARMS, the injected power is not "visible" to the CIOC.

    For monitoring the presence of the injected power, you have two options. You can install a CHARM in one slot, such as the Power CHARM or a DI 24V Isolated (with a local jumper). The DI approach may prove less costly if you plan on pulling the state of the channel into a module.

    The other approach would be to use the second set of terminals on the Address Plug terminals and drive a relay. You could then wire these contacts in series to create a fault signal and use a single input to set an alarm. If you have the available spare CHARM Slots, you can easily accomplish monitoring without any extra hardware and installation costs. You also avoid adding more wiring and connections to monitor the wiring and connections.

    I would be interested to know more about the spurious Fuse trip and what that circuit has in the way of End Devices. What is the total steady state current load on the injected power to this baseplate? How many devices? Do they have inrush current? And are you using a slow blow fuse on the distribution terminal block? What rating is the fuse? And finally, have you tested if a short circuit condition on a CHARM causes the main fuse to blow? Did the event occur as a result of turning one or more channels on? Monitoring the presence of the power to the baseplate will not indicate if your design is too close to the edge and won't prevent future events. You either have extremely bad luck or there is some root cause for your failures.

    The DO 24 VDC Isolated CHARM has a FET to drive the circuit and a PTC resettable fuse. In a short circuit condition, the FET closes the circuit, but the PTC detects the abnormal current and opens. As it cools, it may close but reopen right away. This protects the field wiring and restores normal operation when the Short Circuit condition is cleared. As a result, the 2 Amp Fuse on the Injected Power terminal block will not blow. Neither should the distribution fuse to the baseplate. Since the distribution fuse sees the cumulative current from all the channels on the baseplate, it is important to understand what the possible current draw will be and whether there are conditions when the combination of energized devices and inrush currents may result in higher current. A slow blow fuse should allow for inrush current dynamics.

    Anyway, good luck and hope you find a solution.

    Andre Dicaire

  • Kent

    In reply to Andre Dicaire:

    Thanks for the feedback. I hate to write things off to bad luck, but I suspect that is the best I will be able to do here. The back plane that had the fuse fail go out to nothing but interposing relays. They have been in service since 2016 and this is the first problem that they have given us. However, one of the questions that I try very hard to avoid having to answer is, "If you knew there was a problem, why didn't you do anything about it?". I'll look into your suggestions and talk with operations about at least doing something with critical valves. Thanks again to everyone for your help on this. This was a very long shot question and you guys came through big time.

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