Symptom
An input in one panel driven from another panel never turns on, turns on with a meter reading of a few volts across it, flickers, or turns on when nothing is commanded; voltage from a DC common to ground reads a strange value that changes when things are touched; a signal that worked on the bench fails in the field.
The short answer
Floating Reference Between Panels
A signal wired from one panel to another only works if both ends agree on where zero volts is. When each panel has its own 24 VDC supply and the commons are not connected, or one common is bonded to ground and the other floats, a discrete signal from one panel has no return path into the other, and an analog signal reads an offset equal to whatever potential the floating common has drifted to. The reliable fixes are to bond each supply common to its panel ground at one point so the earth is the shared reference for discrete signals, to carry a dedicated common conductor with the signal wires, and to cross the panel boundary with interposing relays for discrete signals and isolators or single-ground current loops for analog signals.
Key points
- A discrete signal needs a return path. Two panels with separate supplies and no common connection have none.
- A floating DC common drifts to whatever leakage sets; half-voltages and phantom inputs are the result.
- Bond each supply common to the panel ground at one point, or bond none and monitor for ground faults; do not mix.
- Interposing relays are the standard way to cross a panel boundary with discrete signals.
- Analog signals cross the boundary as a current loop with one ground, or through an isolator.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| DC commons not connected between the panels | Measure resistance between the two DC commons with both supplies off. Open means no shared reference. Check each panel for a common-to-ground bond. |
| One supply common bonded to ground, the other floating | Measure DC volts from each common to ground. The bonded one reads zero; the floating one reads a wandering value, often several volts. |
| Signal wired without a return conductor | A single wire from a contact in panel A to an input in panel B, with no common wire in the same cable. The circuit relies on the earth or is open. |
| Shared common carried on an undersized wire that also carries load current | Measure DC volts between the two commons with the loads running. A volt or more means the common wire is carrying return current for something else and shifting the reference. |
| Wetting voltage from panel A into an input referenced to panel B | The contact in A switches A 24 V into B input, which expects B 24 V relative to B common. Check which supply each side of the input is referenced to. |
| Common conductor open in a multi-conductor cable | Continuity of the common conductor end to end; a broken common turns every signal in the cable into a floating one. |
| Ground path between panels of high impedance | Both commons bonded, but the panels are in different buildings or the bonding conductor is corroded. Measure resistance and AC volts between the two panel grounds. |
What floating means
A 24 VDC control supply has a positive and a common. If the common is bonded to the panel ground, every voltage in that panel is measured relative to earth, and two such panels share the earth as a reference. If the common is not bonded, the whole supply floats: the difference between positive and common is 24 V, but the voltage from either to ground is set by leakage through the supply, the loads, and cable capacitance, and it can sit anywhere from zero to a few tens of volts and wander. A floating system is legitimate and is used deliberately in some plants with a ground fault monitor, because a single ground fault does not trip anything. The trouble starts when one panel floats and the other does not, or when signals are wired between two floating panels as if they shared a reference.
Why the input does not turn on
A discrete input turns on when current flows through it from its positive terminal to its common. A contact in panel A that applies panel A 24 V to an input in panel B provides the positive side. The current has to return from panel B common to panel A common, and if there is no wire and no shared ground between them, there is no circuit. The input sits at a voltage the meter can see but that drives no current, or a tiny leakage current turns it partly on. The result is the classic symptom: 20 V across the input and the input is off.
Diagnostic procedure
- 1
Draw the circuit
Trace the signal from the contact or output in one panel to the input in the other, and note which supply powers each side. If the drawing does not show a return path, that is the problem.
- 2
Measure each common to ground
DC volts from each panel DC common to its panel ground. Zero means bonded. A wandering value means floating. One of each is a mismatch.
- 3
Measure between the commons
DC and AC volts between the two DC commons, with loads running. Zero means they share a reference. A steady offset means a return current is flowing through the connection. A wandering value means they are not connected.
- 4
Test the input locally
Jumper the input from its own panel 24 V. If it turns on cleanly, the input and the card are fine, and the problem is the reference.
- 5
Check the cable
Continuity of every conductor in the interpanel cable, including the common if one exists. A broken common looks like a floating panel.
- 6
Decide the fix
Bond the commons and add a return conductor, or add interposing relays, or add isolators, per the table below. Then re-measure the commons and confirm the input switches with a clean 24 V or 0 V across it.
Crossing a panel boundary
| Signal | Recommended method | Why |
|---|---|---|
| Discrete status or command | Interposing relay in the sending panel; its dry contact is wetted by the receiving panel supply | Each side stays inside its own reference; a supply fault in one panel cannot reach the other |
| Discrete, both panels bonded to the same ground bus | Direct wiring with a common conductor in the same cable | Acceptable within one building on one grounding system; the common wire carries the return, not the earth |
| 4-20 mA | One loop, powered from one panel, grounded at one point | Current is unaffected by a reference difference as long as there is one ground |
| 0-10 V or other voltage signal | Signal isolator, or convert to 4-20 mA | A voltage signal is measured against a reference, and two panels do not share one |
| Serial data | Isolated converter | Common-mode voltage between panels corrupts data and damages transceivers |
| Any signal between buildings | Fiber, radio, or isolators on every conductor | Buildings have separate grounds by design; copper between them carries the difference |
Bonding the common
Where the policy is bonded, connect the common terminal of each supply to the panel ground bus with a short, sized conductor, once. Do not bond it again at a terminal strip or at a load; a second bond puts return current on the ground conductors. Mark the bond on the drawing so the next person does not remove it as a mistake or add another. Where several supplies share a panel and feed common circuits, bond them at the same bus so their commons are at one potential.
Frequently asked questions
- Is it wrong for a 24 VDC supply to be ungrounded?
- No. A control circuit at that voltage may be grounded or ungrounded. Ungrounded systems tolerate one ground fault and need a ground fault monitor so that the fault is found before a second one causes trouble. The fault is inconsistency: one panel grounded and another not, with signals wired directly between them.
- Why did it work on the bench?
- On the bench both panels were fed from one supply or sat on one bench ground, and the earth or the shared supply provided the return. In the field each panel had its own supply and its own ground, or a ground path of high impedance, and the return disappeared.
- Can I just run a common wire between the panels?
- For discrete signals within one building on one grounding system, yes, if the commons are bonded at one point each and the wire carries only the signal returns, not load current. The cleaner answer that also protects each panel from the other is an interposing relay per signal.
- The input reads 12 V and is off. What is that?
- The signature of a floating reference or an open return: the input sees a voltage through leakage but no current flows through it. Measure from the input common to the sending panel common; the wandering or offset voltage you find is the missing reference.
Related topics
- Ground Loop SymptomsHow a ground loop shows itself: hum on analog signals, offsets that change when large loads switch, serial links that error, and readings that differ by location. The mechanism, the measurements that prove it, and fixes that never lift a safety ground.
- Panel Power SuppliesThe 24 V DC supply behind everything in a modern panel: sizing with real inrush and duty, single versus redundant with diode modules, how a switch-mode supply behaves under a short, distribution and fusing per branch, monitoring, and the failure modes that take out a whole panel at once.
- Control RelaysThe interposing and logic relays in a control panel: why a controller output drives a relay instead of a starter coil, contact ratings and the difference between resistive and inductive loads, coil suppression, ice cube relays versus terminal relays versus solid-state, and the failure modes that make a relay the first suspect in a dead circuit.
- Inputs Not ReadingA switch closes in the field and the program never sees it. How the wetting voltage, the common, sinking and sourcing, and the wiring each lose the signal, and how to find which one did.
- How to Size a Control Panel Power SupplyAdd up the real load, account for inrush and simultaneity, add headroom, and check the heat you just added to the enclosure.
- Missing Equipment GroundAn enclosure, motor, or device with no effective equipment ground: how it shows up as a tingle, a fault that does not trip, surge protection that does nothing, or instrument noise; how to test the path safely; and why a ground rod is no substitute.
Direct contact
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