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 VACisolated Plus CHARM, and LS DI 120 VAC isolated CHARM may not be able to detect when the contact on the device ispresently open. This occurs when very long cables are used in AC-actuated control circuits."
In reply to Ion Paidiu:
In reply to LaurentB:
In reply to Andre Dicaire:
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.
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