120V AC causing interference on DC circuits

[PhaseShift]

Were setting up a panel yesterday that had 24V DC controls inside the panel and well as a few microprocessors. Running through the panel we have 120V cables coming from the field passing through the cabinet in route to another location.

When setting up the panel we noticed that the processor inside the panel was giving a ground fault alarm. After tracing out each circuit, we determined that the ground fault alam was a nusience alarm and it must have been caused by something else. We thought maybe that 120V cables run through panel (3ft) may be causing this. So we changed the 120V cables in the panel to shielded cables and the problem went away. The 120V cables were serving switches in the field, so they had very little current.

Anybody ever seen this before? What do you think could have been causing this problem? Inductive or capacitive coupling between the 120V AC and 24V DC controls or something else?

 

[benaround]

I think you had a ground fault and when you changed the 120v cables it cleared.

 

[StephenSDH]

It is very common to have 120v and 24v in the same enclosure, and I have never heard of a problem. I have had problems with drives causing issues, but not a ground fault. Perhaps you have an intermitent fault that will come back.

 

[sgunsel]

It is very common to HAVE problems from inductive coupling if portions of the 24 vdc system are nearby and not properly shielded. Some systems are very sensitive to interference, others not so much. Depends. If you only use 24 volts to switch relays and lamps, you can get away with a lot. If you are making low level, measurements, i.e. millivolt or milliamp levels, increased separation distance and shielding are your friends.

 

[a.bisnath]

common problem

common problem

yes seen it many times ,especially in instrumentation applications ,please ensure that shield on your instrument/communication cable are grounded on one side of the cable only and a separate grounding system is used for them or you may get ground current loops

 

[PhaseShift]

yes seen it many times ,especially in instrumentation applications ,please ensure that shield on your instrument/communication cable are grounded on one side of the cable only and a separate grounding system is used for them or you may get ground current loops

The 120V control wires I was referring to were only discrete 120V signals and not an analog signals. Grounding systems should be seperate since 24V controls are grounded in panel, and 120V controls are grounded in a remote electrical room.

Once we put shielded cables in place of the non-shielded cables for the 120V signals the problem went away. Could there be that much interference with only the 3ft of 120V cable that ran through the 24V panel to cause this much issue. Since there is very little current on the discrete signal, I would think the issue would be more of a capacitance coupling issue?

 

[ELA]

Once we put shielded cables in place of the non-shielded cables for the 120V signals the problem went away. Could there be that much interference with only the 3ft of 120V cable that ran through the 24V panel to cause this much issue. Since there is very little current on the discrete signal, I would think the issue would be more of a capacitance coupling issue?

When you say you replaced the non shield cables with shielded was that over the entire length of the 120V signal cables (including cable lengths outside of the cabinet)?
If so then you are talking about more than 3ft of cable? The cable length outside the cabinet can pick up noise and then conduct that noise into the cabinet where it re-radiates into the other circuitry.
Just one possibility ...

To better understand , what does the ground fault alarm indicate (what signals are grounded that should not be)? Do you know what type of circuitry is used to sense the ground fault?

 

[mbeatty]

I also have seen this problem, on a few occasions, where the panel had AC control wires run in proximity to DC control wires. The problem was resolved by increasing the clearance between the different sourced wires or changing to shielded cable if clearance space was an issue.

Regards,

Mark