The Difference Between a Status and a Command

Why this matters

A screen that says a piece of equipment is running is answering one of two completely different questions, and nothing on the screen tells you which. It is either reporting a decision the controller made, or reporting a physical consequence something downstream produced. The first is worth nothing as evidence and the second can be worth the whole diagnosis. Techs lose hours to this: the point says running, the customer says no heat, and everybody argues with the screen instead of asking what the point is wired to.

The useful skill is not knowing that status and command differ. It is knowing exactly how far downstream a given status point sits, because that distance is the only thing the point is actually proving.

One gate, asked of any point claiming to be a status

If the controller commanded the device on and the machine then did nothing useful, would this point still read true?

If the answer is yes, the point is a command echo wearing a status label, no matter what the graphic calls it. If the answer is no, the point is proof, and it is proof of exactly one thing: whatever physical consequence the sensing device responds to. Nothing further downstream than that.

That gate is worth asking out loud on site, because it forces you to name the sensing device, and naming the sensing device is most of the answer.

The ladder between the command and the consequence

Points that all get labeled status in the field sit on five different rungs. Each rung proves everything below it and nothing above it.

Rung 0 is common and it is the one that starts arguments, because a graphic can show a bright green running indicator that is literally a copy of the on command. It is not a lie by anyone; it is a display of the only information that particular point has.

Case one: the point that passed the gate

A supply fan with a differential pressure switch piped across the fan, wired to a binary input. Run the gate: command the fan on with the belt off, and the pressure switch sees no pressure rise, so the point reads false. It passed.

What did it prove when it read true? That there was a pressure difference across the fan above the switch's trip setting. That is genuine, physical and downstream of every electrical failure mode in the chain. It is also strictly bounded: it says nothing about volume, nothing about where the air went, and nothing about a closed damper further along the duct, because a fan running against a closed damper builds more pressure difference, not less. A point that proves work happened does not prove the work was useful.

Case two: the point that failed the gate and read true anyway

The same fan, but the status comes from an auxiliary contact on the starter. The belt snaps. The starter is still pulled in, the auxiliary contact is still closed, the point still reads true, and the graphic is still green while the building gets nothing.

Now move one rung up to a current-sensing switch on a motor lead. Better, and still not automatically sufficient. The motor with no belt on it is unloaded, not off, and an unloaded general-purpose polyphase induction motor typically still draws on the order of 30 to 50 percent of its full-load current, with the fraction running higher on small motors and on some single-phase designs. A current switch whose trip point sits below that band closes on a broken belt exactly as it does on a healthy one.

The gate resolved oppositely for these two cases with the same fan, the same command and the same fault. The difference was entirely which device the point came from and, for the current switch, where its trip point was set.

Worked example: setting a current switch so its status means something

A belt-driven fan. All values here are read on site, not assumed: the motor nameplate states 8.2 A full-load current, the clamp reads 6.8 A with the belt on and the fan doing its normal work, and with the belt removed and the motor running unloaded the clamp reads 3.0 A.

Those two measured currents define the entire usable window. The 3.0 A unloaded reading is 37 percent of the 8.2 A nameplate figure, which sits inside the ordinary no-load band for this class of motor, so the numbers are behaving.

A fixed-trip current switch in this position typically closes somewhere well below the unloaded current, because it is built to detect that a motor is energized at all. Against 3.0 A of unloaded current it stays closed with the belt off. It is a rung 3 device delivering rung 2 information, and the status is worthless for belt failure.

An adjustable switch has to be set inside the window between 3.0 A and 6.8 A. Set it too near 3.0 A and normal current variation with load and supply voltage will hold it closed on a broken belt. Set it too near 6.8 A and a light-load day drops it out and you have manufactured a nuisance alarm.

Split the window: 3.0 plus 6.8 is 9.8, halved is 4.9 A. Check both margins from that setting. Against the unloaded 3.0 A the setting is 1.63 times higher, so the belt-off case is clearly below trip. Against the loaded 6.8 A the setting is 1.39 times lower, so normal running has 39 percent of headroom before it would false-drop. Both margins are comfortable, which is what tells you the window was wide enough to be worth using at all.

What would flip this. If the measured unloaded current had come in at 5.5 A instead of 3.0 A, the window would be 5.5 to 6.8 A, a midpoint of 6.15 A, and margins of 1.12 times and 1.11 times. That is inside the range where day-to-day load and voltage variation lives, so current sensing would no longer be a defensible way to detect this belt, and the honest recommendation becomes a rung 4 device across the fan instead of a better-set rung 3 one. The window, not the switch type, decides.

What breaks it silently. Put this same motor on a variable-frequency drive and the trip point you just set at full speed becomes wrong everywhere else, because current falls with speed. A switch set at 4.9 A on a fan that spends its winter at 40 percent speed reports the fan stopped for most of the season, and after the third false alarm somebody disables the point rather than re-scaling it. On a drive, take the status from the drive's own run and speed feedback, or move to rung 4.

The failure mode. The common version is not a wrong setting, it is no setting: a switch installed at its factory position, commissioned by confirming the point turns on when the fan turns on, and never tested with the fan mechanically disconnected. It passes commissioning and it will never once detect the failure it was purchased for.

Every current reading above is taken with a clamp-on meter around one conductor, on a running machine, with the enclosure open. Do that only where the reading genuinely cannot be taken with the equipment de-energized, which is the narrow troubleshooting allowance at 29 CFR 1910.333(a)(1), and take it with meter and leads rated for the circuit's measurement category and available fault current plus the shock and arc-flash protection the assessment calls for under NFPA 70E-2021. Removing the belt is not part of that reading: isolate and lock the motor circuit under 29 CFR 1910.333(b)(2), prove dead by the live-dead-live sequence in NFPA 70E-2021, 120.5, and relieve any spring tension in the drive under 29 CFR 1910.147 before a hand goes near the sheaves. Where the status point is a wired-in metering current transformer rather than a clamp, its secondary is never opened while primary current flows, because an open secondary develops dangerous voltage; short the secondary at its shorting block first.

Where the ladder's ranking stops holding

The ranking assumes the machine's job is to move something and that a downstream consequence exists to sense. Two situations break that.

Where the consequence is slow. An electric heater's rung 4 evidence is a temperature rise that may take minutes to appear, so a controller cannot use it as a fast permissive. Here a rung 3 current point is the right choice, not a compromise, and rung 4 belongs in the alarm layer with a delay long enough for the physics.

Where the device serves a protective function. A status point on a safety device is telling you the state of a protection chain, not the state of a machine, and it is not on this ladder. If a status shows a limit or interlock open, the question is why it opened, and that is answered by measuring the quantity the device watches against its stated trip point. Swapping a device that operated correctly puts you at the same end state as jumpering it, one step slower.

How to verify a status point is telling the truth

Trace the point to its physical device before you trust it once. Not to the graphic, not to the point name, to the terminal and the device on the other end of the wire. Write the rung number in your notes. Everything else in this card is downstream of that one act.

Test it by breaking the thing it claims to prove, where it is safe to break. A status point is only verified by making the machine fail in the way the point exists to detect, with the machine isolated and locked out first. Never induce a failure on a combustion path, a pressurized or refrigerant-bearing path, or anything serving a relief or protective function; on those, verify by simulating at the sensing device instead.

Record the trip point and the two currents, not just the trip point. A setting written down alone cannot be re-judged. A setting written down alongside the loaded and unloaded readings it was chosen between can be checked by the next tech in one minute.

Ask what the point is used for as well as what it proves. A rung 2 point feeding an alarm is a minor annoyance. The same point used as a proving interlock, so that downstream equipment is allowed to fire because the fan reports running, is a different category of problem entirely, and it is the reason the rung matters.

References

• 29 CFR 1910.333(a)(1) - live parts de-energized before work, and the narrow conditions permitting energized troubleshooting
• 29 CFR 1910.333(b)(2) - lockout and tagging for work on electric circuits and equipment
• 29 CFR 1910.147 - control of hazardous energy for mechanical isolation and stored energy
• NFPA 70E-2021, 120.5 - process for establishing and verifying an electrically safe work condition
• Manufacturer documentation for the motor nameplate current and for the current switch's adjustment range and repeatability
• See related: What a Sensor Actually Reports; How to Verify an Actuator Reached Its Commanded Position; Interlocks and Why They Are Not Control