Education · Start/Stop Commands

Start/Stop Commands Signals

Every fan and pump the BMS "starts" is really started by a chain of hardware the BMS only touches at one end. The controller's whole contribution is a binary output closing a small relay contact — everything after that click is copper, and most of the service calls this page is about live in that copper. One question, start to finish: when a BO commands equipment ON, what sits between the controller's relay and the motor starting — and why does Hand work when Auto doesn't? The BO itself — relay vs. triac, feeding BO-C from the hot leg — is the Controller Wiring lesson's territory, taken as given here. This page walks downstream: everything between the BO's click and the contactor's clunk.

The Command Path

Lay the chain out left to right, the way the current sees it. The BO is the decision: a dry contact inside the controller that closes when the program (or an operator override) says run. The BO doesn't switch the motor, and it doesn't even switch the motor's starter — its little pilot-duty contact drives the coil of an interposing relay, and that relay's heavier contact is what enters the starter's control circuit. Two reasons the middleman exists, and both are on the nameplate: contact rating — the BO is rated for a small 24 VAC load, while a contactor coil is a heavier, inductive load with real inrush — and voltage class — the starter's control circuit commonly runs at 120 VAC, and line-class wiring has no business landing on a DDC terminal strip. The BO switches a small relay it can handle; the relay switches a circuit the BO never could.

From the interposing relay's contact, the starter's control circuit runs through the HOA switch — Hand-Off-Auto, the human override, and the whole next section — with the BO's path entering on the Auto leg. Then comes the safety string: a series of hardwired vetoes, each a contact held closed in normal operation and opened by a trip — a freeze stat that opens when the air across a coil approaches freezing, a firestat or duct-detector shutdown contact, and the motor overload's auxiliary contact, which opens when the overload trips. Any one of them opens and the circuit is dead, no matter who is commanding — and the string sits downstream of the HOA on purpose, so that Hand does not bypass the safeties. Last in line is the contactor coil: the muscle. Energize it and the contactor's line-side contacts pull in, putting three-phase power to the motor. What lives inside the starter beyond that coil — contactor sizing, overload elements and trip class, the line-side power — is the electrician's territory; the controls side of the circuit ends at the coil.

Hold that picture and every element gets a one-breath failure signature, seen from the BMS seat. A BO that never commands, a dead interposing-relay coil, a broken wire on the controller side: Auto is dead, Hand still runs. An HOA someone left in Off: dead in both, and the fix is a glance, not a meter. An open safety: dead in Hand and Auto alike — the string outranks everyone. A dead contactor coil or lost control power: dead everywhere, and the meter goes to the coil terminals first. The diagram below is the whole chain with those signatures written under each link.

The start/stop command path from BO contact to contactor coil A starter control circuit drawn left to right. Control hot — 120 VAC, typically from the starter's own control transformer — enters at the left and passes through the interposing relay's contact, then the HOA switch's Auto leg, then a safety string of three series contacts labeled freeze, fire, and overload aux, then the contactor coil, and returns on the control neutral. A Hand leg branches off the hot ahead of the interposing relay and re-enters at the HOA's Hand terminal, hardwiring around everything the controller owns. At lower left, a DDC controller's binary output switches a separate 24 VAC circuit that energizes the interposing relay's coil — the controller's only touch on the whole chain. Beneath each element, a callout describes what a break at that point looks like from the BMS seat: a dead BO or relay leaves Auto dead while Hand runs; an HOA left in Off and an open safety are dead in Hand and Auto alike; a dead coil or lost control power is dead everywhere. CONTROL HOT 120 VAC typ. — from the starter control xfmr AUTO leg HAND leg — around the BO and relay INTERPOSING RELAY IR shown de-energized CONTROLLER BO1 BO dry contact 24COM BO-switched 24 VAC A H OFF opens both HOA — shown in AUTO SAFETY STRING FREEZE FIRE OL AUX C CONTACTOR COIL N control neutral coil in → contacts close → motor runs (line side) A BREAK HERE, SEEN FROM THE BMS SEAT point never commands: Auto dead, Hand runs — check the BO first coil or contact dead: Auto dead, Hand runs — listen for the click left in OFF: dead in Hand and Auto — look, then meter any safety open: dead in Hand and Auto — walk the string coil / control power: dead everywhere — meter at the coil

starter control hot (120 VAC typ.) BO-switched 24 VAC (controller side) common / neutral

HOA Semantics — Who Gets to Decide

The three positions of that selector switch are a complete statement about authority, and each one is worth saying precisely. Hand connects the coil circuit around the BO and the interposing relay entirely — the unit runs, and the BMS isn't overridden so much as bypassed: no software point, no priority, no command from any seat changes what the motor does. The safeties still stand — Hand enters the chain upstream of the string, not around it. Off breaks the circuit ahead of both legs: it beats the BO, and it beats Hand. Off has to win, because the person standing at the unit has to be able to make it stop — though an HOA in Off is not lockout/tagout; the starter's line side is still hot, and nothing about a selector switch protects anyone working inside. Auto is the position the drawings assume: the circuit routes through the interposing relay's contact, and the decision belongs to the BO — which is to say, to the program.

Now watch what each position does to the pair of points the BMS graphic shows. A command point records a request; a status point reports a fact. In Auto, on a healthy unit, the two agree — command ON, status ON; command OFF, status OFF. In Hand, they split by design: the unit runs while the BMS may well be commanding OFF — command OFF, status ON, and the status is the one telling the truth. That disagreement is the exact mirror of the broken-belt service call, where the BMS commands ON and the proof point reports that nothing is actually moving. How the status point earns the right to be believed — current switches, differential-pressure switches, the whole art of proving a thing ran — is its own lesson, told from the input side; here it's enough that the graphic can show you the disagreement, and that a command/status split is a message, not a glitch. The panel below is the three positions as the graphic sees them.

The HOA truth panel: what each position does to command and status A three-row panel, one row per HOA position. Hand: the unit runs, wired around the BO — the BMS is bypassed, not consulted; the graphic shows command OFF against status ON, a disagreement that mirrors the broken-belt case. Off: the unit is stopped, period — Off beats the BO and Hand alike; the graphic shows command ON against status OFF, and no BMS write can change it. Auto: the BO decides — the program is in charge; command and status agree, and when they split it is time to start hunting. A caption reads: status reports a fact, command records a request — Hand and Off make them split by design. POSITION WHAT THE UNIT DOES COMMAND vs STATUS H O A HAND runs — wired around the BO the BMS is bypassed, not consulted CMD: OFF STATUS: ON disagree — the mirror of the broken belt H O A OFF stopped, period beats the BO and Hand alike CMD: ON STATUS: OFF disagree — and no BMS write can change it H O A AUTO the BO decides the program is in charge CMD: ON STATUS: ON agree — when they split, start hunting status reports a fact; command records a request — Hand and Off make them split by design

One more thing Hand is for, before the troubleshooting section cashes it in: Hand answers a question no software point can. Flip it and the machine either runs or it doesn't — with the program, the network, the controller, and the BO all removed from the experiment. That's not an override so much as a diagnostic instrument, and it's the first move in the classic service call below.

Finding the Break

The call comes in as "the fan won't run," and the first move isn't a meter — it's the HOA, because one flip of that switch cuts the whole chain into two halves and tells you which half to hunt in. Runs in Hand, dead in Auto: Hand just proved everything downstream of the HOA — the safety string, the contactor coil, control power, the starter, the motor itself — in one move. The fault is upstream, in the Auto leg: the BO (is the point actually commanding? check it before touching a wire), the interposing relay (is the coil getting its 24 V? is it clicking? is the contact actually closing?), or the wiring between them. Dead in Hand and Auto both: now the upstream half is exonerated instead — nothing the controller does or doesn't do explains a unit that won't run in Hand. Hunt downstream: walk the safety string contact by contact until you find the open one (freeze stats are commonly manual-reset — a trip at 38 °F overnight stays tripped until someone presses the button), then the contactor coil, then control power itself.

The safety string deserves one more beat, because a fair question hides in it: the controller already knows the discharge temperature — why not let the program catch a freeze condition and stop the fan in software? Some sequences do exactly that, as a first line. But the hardwired string exists for every moment software can't be trusted with: the controller powered down, the program mid-download, the network dark, the point overridden by someone troubleshooting three floors away. A trip that protects people or destroys equipment when missed — the firestat, the freeze stat guarding a coil full of water — has to outrank all of that, and copper in series with the coil is the only authority that does. That's the dividing line: life-safety and equipment-destroying trips live in copper; software interlocks handle the polite coordination — sequencing, optimization, the interlocks that make the plant graceful rather than safe. The BMS gets told about a hardwired trip (ideally through a status or alarm point); it doesn't get a vote in it.

And when the equipment grows up and gets a drive, the shape survives. On a VFD, the contactor disappears — the drive itself switches the motor — but the run command is the same story landed on a drive terminal: a BO, an interposing relay where ratings call for one, an enable or safety circuit the drive honors before it will run. Drives replace the contactor; they keep the command path's shape.

That answers the question this page opened. Between the BO's click and the motor: an interposing relay to amplify the decision, a selector switch that says who's deciding, a string of hardwired vetoes, and a coil that does the actual work. And Hand works when Auto doesn't because Hand enters the chain downstream of everything the controller owns — which is exactly why it's the first thing a good tech reaches for: one flip, and half the suspects go home.

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