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Computed value for this example circuit, not a service limit.
Tap a part of the drawing, or move to it with the Tab key and press Enter, to read what it is.
Tap START or STOP, or hold one, to operate the circuit. Tap a test point ring, or move to it with the Tab key and press Enter, to place the selected lead. The meter reads as soon as both leads are on.
Pick a scenario, read the symptom, operate the circuit, take readings, then choose the cause.
A tap on a test point places the selected lead, and the meter reads as soon as both leads are on. Across a part means one lead on each side of it: across the control fuse is supply +24 V to fuse output.
Keep practising: motor control questions for your trade
This is a study simulation for trade exams, not a test or repair procedure. Readings are computed values for this example circuit, not service figures.
Meter an example 24 V DC start/stop circuit with a seal-in contact and an overload contact, then find the open or stuck part.
This free simulator puts a virtual meter on an example 24 V DC motor starter control circuit: a control fuse, an overload contact, STOP and START pushbuttons, a seal-in contact and the contactor coil. It is for 309A and 442A apprentices studying three-wire control and how to read a control circuit with a meter. It is a simplified teaching model, not any real starter, so every reading is a computed value for this example circuit.
A magnetic motor starter is a contactor and an overload relay: current in the contactor coil pulls the contacts closed, and a spring opens them when the coil current falls away. Three-wire control uses a magnetic contactor and a holding contact; in this example circuit the holding contact, called the seal-in contact, is wired across a momentary START pushbutton. Once the contactor picks up, the seal-in contact closes and keeps the coil energized after START is released. STOP is a normally closed pushbutton in series with the coil, so pressing it opens the circuit and the contactor drops out. The overload relay’s normally closed contacts are in series with the coil too, so when the overload relay trips they open and the motor stops.
After a power cut, a three-wire circuit does not restart by itself; someone has to press START again. This is called low-voltage protection. Low-voltage protection suits loads whose sudden restart could cause damage or injury; loads that should come back on by themselves suit low-voltage release instead.
Across a closed contact a voltmeter reads close to zero; across an open contact it reads the control voltage, as long as the rest of the path through the coil is complete. A control circuit path has one load, here the coil, and while the circuit works that load carries the full control voltage. In this example circuit, running, the coil reads 23.97 V and the closed STOP contacts read 0.00 V. At rest, in this example circuit, the open START pushbutton and seal-in contact read 24.00 V across them. Some faults show only while START is held: with START released, the open START and seal-in contacts also break the path, so an open fuse, overload contact, STOP or coil has nothing across it. In this simulator the ohmmeter works only with the supply off; in this example circuit the coil reads 60.2 Ω and a closed STOP reads 0.2 Ω, the resistance of the meter leads.
The motor and its power circuit run at line voltage and are only drawn here; the meter reaches only the 24 V DC control circuit of this example.
Symbol: A box marked 24 V DC, with its + side feeding the circuit and its other side the 0 V control common.
What it does: It powers the control circuit, separate from the line voltage that runs the motor; control circuits can run at a lower voltage than the motor they control.
If it fails: With the supply off, nothing in the control circuit is energized and the contactor cannot pick up.
Symbol: A narrow rectangle in the line from the supply.
What it does: It protects the supply, the coil and the control wiring, opening when too much current flows.
If it fails: An open control fuse breaks the whole control circuit, so START does nothing; with START held, the supply voltage stands across the fuse.
Symbol: A normally closed contact: the blade rests against its fixed contact until the overload relay trips.
What it does: It belongs to the overload relay and is in series with the coil, so when it opens the coil loses its current.
If it fails: While it is open, the contactor cannot pick up; resetting closes it, but the contactor stays out until START is pressed, and it is advisable to find what tripped the overload relay.
Symbol: A normally closed pushbutton: its contact stays closed until the button is pressed.
What it does: Pressing it opens the circuit, the coil loses its current and the contactor drops out.
If it fails: If its contacts stay open, START does nothing, and with START held the supply voltage stands across STOP.
Symbol: A normally open pushbutton: its contact closes only while the button is pressed.
What it does: A press energizes the coil, and once the contactor picks up, the seal-in contact keeps the coil energized.
If it fails: If its contacts do not close, nothing happens; if they stick closed, the contactor picks up as soon as the supply comes on.
Symbol: A normally open contact drawn across START and marked M, because the contactor moves it.
What it does: It closes when the contactor picks up and keeps the coil energized after START is released.
If it fails: If it does not close, the motor runs only while START is held.
Symbol: The coil of contactor M, on the 0 V side of the circuit.
What it does: Current in the coil pulls the contactor’s contacts closed; it is the one load in the circuit and carries the full control voltage while energized.
If it fails: An open coil has the full supply voltage across it with START held but draws no current, so the contactor never picks up.
Symbol: Three main contacts marked M feeding the motor, inside a dashed box.
What it does: The contactor’s main contacts switch the motor at line voltage; this simulator draws them but does not measure them.
If it fails: Power circuit faults are outside this simulator; finding them is work at line voltage.
| Fault | Symptom | What to test | What you would see | Cause |
|---|---|---|---|---|
| Control fuse open | Nothing happens when START is pressed. | Hold START and measure across each part in turn, from the control fuse to the coil. | In this example circuit, with START held, 24.00 V across the control fuse and 0.00 V across every other part. | Control fuse open |
| Overload contact open (tripped) | Nothing happens when START is pressed. | Hold START, then measure across the overload contact. | In this example circuit, 24.00 V across the overload contact while START is held, and 0.00 V once it is released. | Overload contact open (tripped) |
| STOP contacts open | Nothing happens when START is pressed. | Hold START and measure across STOP. | In this example circuit, 24.00 V across the STOP contacts while START is held. | STOP contacts open |
| START contacts do not close | Nothing happens when START is pressed. | Hold START and measure across START. | In this example circuit, 24.00 V across START even while it is held. | START contacts do not close |
| Seal-in contact does not close | The motor runs only while START is held. | Hold START, then release it and watch the contactor. | In this example circuit, with START held, the reading across each part matches a sound circuit (0.00 V across START, 23.97 V across the coil); dropping out on release is the evidence. | Seal-in contact does not close |
| Contactor coil open | Nothing happens when START is pressed: the contactor does not pick up. | Hold START and measure across the coil; then, with the supply off, measure the coil’s resistance. | In this example circuit, 24.00 V across the coil with START held while the contactor stays out, and the ohmmeter shows OL, an open, across the coil with the supply off. | Contactor coil open |
| START contacts stuck closed | The contactor picks up as soon as the supply comes on, and STOP holds it off only while pressed. | Turn the supply on with no button pressed; then, with the supply off, measure across START. | In this example circuit, the contactor picks up with no button pressed, so the circuit no longer gives low-voltage protection; with the supply off, START reads 0.2 Ω where a sound circuit reads OL. | START contacts stuck closed |
With START held, the open fuse is the one break left in the series path, so no current flows and the supply voltage stands across the fuse.
An open overload contact, STOP, START or coil gives the same symptom; the reading across each part with START held shows which one is open.
The overload contact is in series with the coil, so while it is open the coil gets no current.
In this example circuit, with START held, the fuse reads 0.00 V, where an open fuse would read 24.00 V; the supply voltage stands across the overload contact instead.
STOP is normally closed and in series with the coil, so contacts that stay open break the circuit just as holding STOP does.
With START held, the supply voltage stands across STOP, not across the fuse or the overload contact.
Until the seal-in contact closes, START is the only path to the coil, so contacts that never close leave the coil without current.
In this example circuit, with START held, the coil reads 0.00 V, where an open coil would read 24.00 V; the supply voltage stands across START instead.
Without the seal-in contact nothing holds the coil once START is released, so the contactor drops out.
Every reading with START held matches a sound circuit, so the clue is the contactor dropping out when START is released.
An open coil draws no current, so it cannot pull the contacts in, though with START held the full supply voltage stands across it.
A sound coil also carries the supply voltage while energized, so the clue is full voltage while the contactor stays out, then OL on the ohmmeter with the supply off.
Contacts that stay closed act like START held all the time, so the coil is energized whenever the supply is on, STOP is released and the overload contact is closed.
A sound three-wire circuit stays off when the supply comes on; that loss of low-voltage protection is the clue.
The circuit is generic and does not copy the wiring of any real product. Real control circuits and every motor power circuit are work for qualified persons, following the employer’s lockout and electrical safety procedures. On a real machine, pressing START runs the motor and whatever it drives.
Sources: an open trade-training textbook on basic motor control and the practice question banks on this site. Checked November 2026.
It is a normally open contact on the contactor, wired across START, that closes when the contactor picks up and keeps the coil energized after START is released.
With three-wire control the seal-in contact opens when the coil loses power, so the circuit waits for START: low-voltage protection.
Find what made it trip before relying on a reset: resetting closes the overload contact again but does not remove the cause, and START must then be pressed again.
Holding START closes the gap a sound circuit has at rest, so an open part becomes the only break and reads the control voltage; with START released, it can show nothing.