What happens if a PLC is connected to an ungrounded transformer with floating voltage, if power is turned off and back on?

If a PLC is powered from an ungrounded/floating transformer secondary, turning the power OFF and back ON can make an intermittent problem disappear, reappear, or change even though the transformer itself is producing the correct line-to-line voltage.

The key is that the PLC needs two things: the correct supply voltage between its power terminals, and a reasonably controlled voltage relationship between its circuitry, chassis, shields, I/O commons, and ground.

Suppose the transformer secondary is 120 VAC:

The PLC may see a perfectly good:

and operate normally.

But because the secondary is floating, you might see:

or:

Those readings can change with wiring capacitance, filters, connected equipment, VFDs, power supplies, and the PLC's own EMI suppression components.

{SAFETY NOTE: If a voltage appears, DO NOT, DO NOT put a load across it just to prove the voltage goes away, it may be an actual voltage. Unrelated to PLC’s I have seen this happen with moisture in a panel feeding pumps. Although the installing electrician used a myers hub on the conduit feeding power to the box, it was loose. That allowed moisture to run down the conductors and into the top of the breaker. The input voltage was 480V, the measured secondary would vary from 0 to 480 between phases and 0-277 phase to ground.}

The entire circuit was main breaker feeding 4 branch breakers which fed motor starters for pumps. Had I put a load on the load sided of the breaker an arc flash could have quite possibly been triggered. (This was in the 2000’s before our plant used arc flash protocol).

What happens when you turn power OFF?

Inside a PLC power supply are capacitors and EMI-filter components. When you remove power:

At the same time, the floating secondary can lose whatever temporary ground reference it had acquired through connected equipment.

When power comes back:

That new reference does not necessarily settle exactly where it was before.

This can explain equipment that:

“Wouldn't run, but we shut the power off and turned it back on and now it works.”

The power cycle may have reset the PLC, but it may also have temporarily changed the common-mode or floating-voltage condition.

Where the trouble usually appears

The PLC CPU itself may run perfectly well because it mainly cares about the voltage between its supply terminals. Problems often show up instead in the I/O and communications.

For example:

If different devices acquire different references to earth or machine ground, you can get:

  • false digital inputs

  • inputs that fail to turn ON

  • analog signals that jump or drift

  • unexplained PLC I/O faults

  • communication dropouts

  • Ethernet/shield noise problems

  • intermittent module faults

  • PLC that operates after a reboot but faults again later

A particularly important case is the 24-VDC supply.

Imagine:

If the power supply's DC common is also floating, its voltage to machine ground may wander because of internal filter capacitance:

A high-impedance meter may show strange readings even though:

The PLC may continue functioning until some other device establishes an unintended ground path.

For instance:

Now the sensor unintentionally becomes the grounding path.

That can create intermittent behavior.

Why a restart can appear to "fix" it

A power cycle can do several things simultaneously:

  1. Reset the PLC CPU.

  2. Reset I/O modules.

  3. Reset electronic overloads or communication adapters.

  4. Discharge internal capacitors.

  5. Remove an accumulated common-mode charge.

  6. Re-establish the floating system at a different voltage relative to earth.

  7. Temporarily clear leakage or capacitive effects.

So a successful restart does not prove the PLC was the problem.

That is a very important troubleshooting lesson.

What I would measure

For a 120-V transformer feeding a PLC, I would check all three:

Then check the PLC supply:

If there is a 24-VDC supply:

Do these measurements before the power cycle if the fault is present, because once you shut the equipment off you may destroy the evidence.

That is one of the biggest mistakes in industrial troubleshooting:

A better sequence is:

One final distinction matters: a deliberately ungrounded control transformer isn't automatically wrong. Some control systems are intentionally isolated. But the entire grounding/reference strategy must be engineered correctly. If the PLC manufacturer, control-system design, or applicable electrical rules call for one secondary conductor or the DC common to be bonded, leaving it floating can create exactly the sort of intermittent troubleshooting nightmare you're describing.

“PLC Works After Power Cycle: Check for Floating Control Power Before Replacing the PLC.” That's a very practical field lesson for every breakdown electrician or technician ever.

A PLC system can operate for years from a floating transformer secondary because the PLC still receives the correct X1-to-X2 voltage. Over time, however, leakage paths, capacitive coupling, EMI filters, connected equipment, contamination, aging insulation, VFD noise, communication shields, or a developing ground fault can change the secondary’s relationship to earth ground enough that the PLC power supply or I/O circuitry begins having common-mode/reference problems.

For example:

The PLC may operate for years because:

even though:

Then something in the system changes.

At that point you might measure something like:

Those ground readings aren't necessarily mathematically complementary because the meter is interacting with high-impedance leakage and capacitive paths.

Now suppose you intentionally bond X2:

The references become:

The PLC power supply now has a stable common-mode reference, and the equipment may immediately operate normally again.

But this is the important diagnostic point

I would not conclude simply that “the transformer lost its ground reference.” If it was intentionally floating, it never had a fixed ground reference to lose.

A better diagnosis is:

The system previously tolerated a floating secondary, but a change in leakage, capacitance, grounding, connected equipment, or insulation caused the floating system's common-mode voltage to become incompatible with reliable PLC operation. Bonding X2 established a defined reference and stabilized the system.

And I would investigate why it changed after years of operation, especially if bonding X2 suddenly fixes the machine.

Check:

  • X1–X2 voltage before making the bond.

  • X1–ground and X2–ground with a high-impedance DMM.

  • Repeat ground measurements using an appropriate LoZ method.

  • PLC power-supply input.

  • 24 VDC + to 0 V.

  • 24 VDC 0 V to ground.

  • Insulation/leakage from X1 and X2 to ground.

  • Surge suppressors/MOVs.

  • EMI/RFI filters.

  • VFDs and servo drives connected to the control system.

  • Shields and communication grounding.

  • Any recently replaced sensor, power supply, PLC module, or other device.

  • Moisture, carbon tracking, contamination, or deteriorated wiring.

One scenario I would be particularly suspicious of is:

For years that unintended path may have effectively provided enough reference for the equipment to behave normally. When that path changes or disappears, the PLC system starts behaving strangely.

Then bonding X2 provides the stable reference the system was inadvertently getting elsewhere.

What could happen:

If I encountered that in a plant, though, I would treat the fact that grounding X2 cured a years-old intermittent failure as a major diagnostic clue, not merely call the transformer fixed. I'd look for the electrical change that caused the floating system to stop behaving satisfactorily before declaring the job complete.