120 VAC DI Isolated Charms not changing state- Always On

We have a new install with a few hundred 120VAC DI Isolated charms. We started commissioning and the first one we tested stayed on all the time no matter what you did at the device. It is wired correctly and field device is functional. If we open and close the inputs at the Charms term block it changes state.  I found a Knowledge base article NK-1200-0069 that states you can put a 15K resistor in the circuit or buy 120 VAC DI Isolated PLUS Charms modules.  It sounds like it is a problem for long cable lengths. I guess long is relative since this cable length is about 100'. Doesn't seem long to me but it sure seems to have the symptoms described in the NK.

Has anybody seen this before?  Are there alternatives to buying all new charms or installing 300 resistors?

Here is a snip from Knowledge Base Article NK-1200-0069-

"This Knowledge Base Article, NK-1200-0069, documents an issue in which the DI 120 VAC isolated CHARM, DI 120 VAC
isolated Plus CHARM, and LS DI 120 VAC isolated CHARM may not be able to detect when the contact on the device is
presently open. This occurs when very long cables are used in AC-actuated control circuits."


Scott Brown

7 Replies

  • You can use an intermediary relay with lower coil impedance to increase the current through remote switch and thus the current due to cable impedance will become negligible
    Ion Paidiu
  • In reply to Ion Paidiu:

    resistor as the drawing is my prefered solution !!! that will discharge the low current detection induced by impedance ( capacitance and inductance) of long cable
  • In reply to LaurentB:

    cost of 300 resistors + wiring is always the cost effective way
  • Are you using the 3 wire TB?

    Original DI isolated 120VAC with Injected power placed CHARM on high side. By wiring the Isolated DI on the low side, it was found to address this behavior. This was first reported on a migration of Provox to Deltav. The Provox IO was low side sensing. Once Deltav was wired low side, the signals worked correctly.

    This led to the 3 wire DI terminal block.

    Emerson also modified the DI plus to use a higher threshold for on/off state detection, helping the VAC signal to be more robust.

    The KBA shows a third mitigation that adds a 15K resistor in parallel with the CHARM to reduce the circuit resistance and drain induced voltage to neutral.

    If you are already wired with 3wire TB as low side sensing, installing the resistor as noted should allow you to avoid need for DIPlus or interposing relays.

    Andre Dicaire

  • In reply to Andre Dicaire:

    Thank you all for the info. We do have 4 wire term blocks with different fused 120 feeds for the various instruments. Our vendor said the DI Plus Modules are 8+ weeks out so it seems the most appealing is to add resistors.
  • In reply to s_brwn:

    4 wire or 3 wire Terminal Block?  Resistor solution places resistor across CHARM to Neutral, If you have 4 wire, you still want to place CHARM on low side of contact and connect resistor across the CHARM to Neutral.

    The original Injected Power terminal block delivers the High Side sense circuit, with fuse in the TB, pulling power from Injected Power distribution in baseplate.  With the 4 Wire standard TB, you can wire the CHARM in either of these arrangements.  With the 3 Wire TB, you can wire the Low side sense using the Fused AC power to the contact and return to the CHARM.  

    The circuit drawing from the KBA that you included is based on the low side sense wiring of the 3 Wire TB.  If your circuit power is being provided from the location of contact, i.e. an MCC bucket, the fuse should be at the source to protect wiring.  a pair of wires per channel means you can require a high side sense to be low side.  This connects the CHARM to Neutral and avoids islanding the CHARM while it's connected to the Hot.  This helps against induced signals.  By adding the resistor, any induced voltage/current is divided with the resistor allowing a lower Impedance path to ground, but only if the CHARM is connected low side.  

    Andre Dicaire

  • Andre,
    Thanks for the info.
    I have a 4 wire term block.
    It appears that ours are installed in a high side config.
    We have fused power coming in on 1, Neutral on 3, and the contact closure across 2 and 4.
    Scott Brown