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newSchoolWorks.com › VoTechWorks › Building & Construction › Elevator Mechanic (pre-apprentice) › Elevator Controls: Circuits, Controllers, and Signal Systems › Lesson 3
BLD-ELEV10-04 · Lesson 3 · 14 clock hours · lab

Elevator Safety Circuits: Devices, Sequence, and Verification

Objective. Students will identify every required safety-circuit device on a trainer or cutaway controller, trace the series safety chain in the correct sequence, and demonstrate that removing one simulated device opens the circuit and prevents car movement.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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What the Safety Circuit Does

An elevator safety circuit is a series electrical loop that must be complete—fully closed—before the controller will allow the car to move. Think of it as a chain: every link must be intact. If any single safety device opens its contact, the chain breaks, current stops flowing through the circuit, and the car remains stopped. This design is intentional. A single failed or defeated device is enough to halt the system. ASME A17.1, the Safety Code for Elevators and Escalators, requires that certain devices be wired in series in this manner so that no combination of other faults can bypass a safety function.

Devices in the Series Safety Chain

Although exact configurations vary by manufacturer and installation, the following devices are commonly found in the series safety chain on a traction elevator:

The exact sequence of these devices in the chain is defined by the equipment wiring diagram. You must always trace from the diagram for the specific unit you are working on—do not assume sequence from memory.

Why Series Wiring Matters

If safety devices were wired in parallel, a fault in one device would not stop the car because current could flow through another path. Series wiring guarantees that every device must confirm its safe condition before motion is permitted. This is sometimes called a "fail-safe" or "normally-open" philosophy: devices are closed (passing current) in the safe state and open when a hazard is detected. Removing the device entirely also opens the circuit and stops the car—you cannot accidentally defeat a safety by disconnecting it.

Lockout/Tagout Before Any Testing

Before touching any terminal, conductor, or device inside a controller, you must apply lockout/tagout (LOTO) in accordance with your employer's energy-control program and OSHA requirements for the control of hazardous energy. The procedure for this lab is:

1. Notify affected personnel that the equipment is being taken out of service.

2. Identify all energy sources (electrical mains, control transformers, stored energy in capacitors if present).

3. Operate the disconnecting means to the off position.

4. Apply your personal lock and a hasp if multiple workers are involved; each worker places their own lock.

5. Apply a tag identifying your name, date, and reason.

6. Verify zero energy: test with a calibrated meter at the terminals you will be working on before touching them.

Failure to apply LOTO before touching terminals is an automatic unsatisfactory grade for this session, regardless of your electrical results. This is not a paperwork rule—it reflects real consequences in the field where contact with energized elevator control circuits can cause serious injury or death.

Using a Multimeter to Verify Continuity

With LOTO applied and zero energy verified, set your digital multimeter to the continuity or resistance (Ω) function. Place the probes across each device junction in the safety chain, one test point at a time, and record the reading on your lab log. A closed (healthy) contact reads near zero ohms. An open contact reads overrange (OL) or infinite resistance. Work systematically from one end of the chain to the other, recording your readings at each labeled junction. This gives you a baseline of the circuit in its healthy, de-energized state.

Fault Simulation and Voltage Testing

For the fault-simulation portion, the instructor will open one device using a simulated toggle switch—mimicking a failed or open safety contact. After LOTO is properly released and the instructor supervises the re-energization, you will use a voltage test to locate the open point. Working from the power-supply side of the circuit, measure voltage between each junction and the reference (neutral or common). The last junction that shows supply voltage before the next junction shows zero volts identifies where the circuit is broken. Document the device name, its location in the sequence, and write a brief explanation of why the code requires that device.

Key Safety Reminders

Key terms10

Series safety chainA wiring arrangement in which all safety-circuit devices are connected end-to-end so that opening any single device breaks the entire circuit and prevents car movement.
Hoistway door interlockA combined mechanical-electrical device on each landing door that prevents the car from running unless the door is fully closed and mechanically locked.
Governor overspeed switchAn electrical contact on the centrifugal speed governor that opens the safety circuit when the car reaches an overspeed condition, before the mechanical safety is set.
Safety-circuit contactorThe output relay or contactor that de-energizes the drive and brake circuits whenever any device in the series safety chain opens.
Final limit switchA switch positioned beyond the normal terminal stopping zone at the top or bottom of the hoistway; it opens the safety circuit if the car travels past its allowed travel limits.
Lockout/tagout (LOTO)An energy-control procedure in which a worker isolates equipment from all energy sources, applies a personal lock and tag, and verifies zero energy before performing any work on the equipment.
Continuity testA multimeter measurement, performed on a de-energized circuit, that checks whether a complete electrical path exists through a device or conductor; a near-zero resistance indicates a closed, healthy contact.
Fail-safe designA circuit philosophy in which a device's safe state keeps its contact closed (current flowing), so that any failure—including a disconnected device—causes the circuit to open and the system to stop.
Pit stop switchA manually operated stop switch located in the elevator pit that maintenance personnel can open to prevent car movement while working in the pit area.
ASME A17.1The Safety Code for Elevators and Escalators, published by the American Society of Mechanical Engineers, which establishes the minimum safety requirements for elevator design, construction, and operation in the United States.

Worked examples4

1. During the continuity lab, your multimeter reads 0.4 Ω across the hoistway door interlock at Landing 3 and OL (overrange) across the car door contact. What do these readings tell you about each device, and which one would prevent the car from running?
  1. A reading of 0.4 Ω across the Landing 3 interlock indicates a closed contact with a small but acceptable wire and contact resistance. This device is healthy—it is passing current.
  2. A reading of OL (overrange or infinite resistance) across the car door contact indicates an open contact—no current path exists through that device.
  3. Because all devices are wired in series, any open device breaks the entire chain. The car door contact reading OL means the safety chain is broken at that point.
  4. The Landing 3 interlock, though it reads 0.4 Ω rather than exactly 0 Ω, does not prevent circuit completion by itself; contact resistance of less than 1 Ω is generally acceptable. The car door contact is the device preventing car movement.
Answer: The Landing 3 door interlock is closed and healthy (0.4 Ω). The car door contact is open (OL). The open car door contact breaks the series safety chain and prevents the car from running.
2. You are performing a voltage test on the energized safety chain (under instructor supervision, LOTO released). Supply voltage is 115 V AC. Probing from the supply side, you measure 115 V at Junction A, 115 V at Junction B, 115 V at Junction C, and 0 V at Junction D. Between which junctions is the open device located, and how do you know?
  1. In a series circuit with one open point, voltage from the supply will appear at every junction from the supply side up to and including the junction just before the open point.
  2. Junctions A, B, and C all read 115 V, meaning the circuit is complete from the supply through those junctions.
  3. Junction D reads 0 V, meaning no supply voltage reaches that point.
  4. The open must be located between Junction C (last point showing supply voltage) and Junction D (first point showing 0 V). The device wired between those two junctions is the open device.
Answer: The open device is located between Junction C and Junction D. The device between these two junctions is open, blocking current and dropping all supply voltage across the open gap.
3. A safety chain has 8 devices in series. The control transformer supplies 115 V AC to the chain. If every device is closed (healthy), what is the ideal voltage measured across any single closed contact, and what is the ideal voltage measured from the load side of the last device to the neutral reference?
  1. An ideal closed contact has zero resistance. By Ohm's Law, voltage drop across a zero-resistance element is V = I × 0 = 0 V, regardless of current.
  2. With all 8 contacts closed, the full 115 V supply appears at the load side of the last device (at the safety-circuit contactor coil), because all voltage is dropped across the coil load, not across any of the contacts.
  3. Therefore, the voltage measured across any single healthy closed contact is 0 V (ideal), and the voltage from the load side of the last device to neutral is 115 V (the full supply, present at the coil).
Answer: Ideal voltage across any single closed contact = 0 V. Voltage from load side of the last device to neutral = 115 V (full supply voltage appears at the safety-circuit contactor coil).
4. Before probing terminals in the pit junction box, you want to verify zero energy after applying LOTO. Your multimeter is set to AC voltage. You probe between the two line conductors and read 0 V, then between each line conductor and the grounded enclosure and read 0 V. Is it safe to proceed? Describe the reasoning.
  1. Reading 0 V between the two line conductors confirms no AC voltage difference exists between them at this point.
  2. Reading 0 V between each line conductor and the grounded enclosure confirms neither conductor carries voltage to ground.
  3. Both measurements together indicate no energized AC source is present at these terminals, consistent with a properly locked-out circuit.
  4. You should also confirm your meter is working correctly by testing a known live source before and after the zero-energy test (this is the 'test before touch' practice). If the meter is confirmed functional and both readings are 0 V, it is safe to proceed with work on these terminals.
Answer: Yes, it is safe to proceed. Both measurements show 0 V, indicating no energized AC voltage is present at the terminals. Confirming meter function on a known source before and after the test adds certainty to the zero-energy verification.
In class: Instructor walk-through of a full-size controller trainer or cutaway panel; students label a blank safety-circuit diagram with device names (door locks, limit switches, governor switch, safety-circuit contactor, etc.) as each is pointed out, cross-referencing ASME A17.1 safety-circuit requirements discussed at a general level. · Small-group lab: each group uses a multimeter to verify continuity through each safety-circuit device on the trainer while the circuit is de-energized under lockout/tagout; groups record measured resistance at each device junction on a lab log template. · Fault-simulation exercise: instructor opens one device in the safety chain (simulated by a toggle switch); students must identify the open point using voltage testing (panel energized only after LOTO is released and instructor supervises), document the finding, and explain in writing why that device is required.
Materials: Full-size elevator controller trainer or cutaway panel, digital multimeters, LOTO station (personal locks, hasps, tags), NEIEP safety-circuit reference diagram, lab log templates, ASME A17.1 safety code (general reference copy)
Check: Completed and accurate lab log with continuity readings at all required test points; student must also demonstrate, with the instructor observing, the correct LOTO sequence before touching any terminal — failure to apply LOTO is an automatic unsatisfactory for the session regardless of electrical results; pass = accurate log + correct LOTO procedure.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Elevator safety systems and code requirements
1. Why are elevator safety-circuit devices wired in series rather than in parallel?
Electrical testing and troubleshooting
2. During the continuity lab, a student measures OL (overrange) across the governor overspeed switch while the circuit is de-energized under LOTO. What is the most likely explanation?
Electrical safety and lockout/tagout
3. A student is about to probe terminals inside the controller to verify zero energy after LOTO. Which action must be completed BEFORE touching any terminal?
Electrical testing and troubleshooting
4. In a voltage-trace fault-finding test on an energized safety chain (115 V supply), you measure 115 V at the junction before the pit stop switch and 0 V at the junction after it. What does this indicate?
Elevator safety devices and their functions
5. Which device in the series safety chain provides electrical confirmation that the car's mechanical safeties are in the reset (un-tripped) position?
Elevator safety systems and code requirements
6. A final limit switch at the top of the hoistway is designed to open if the car travels beyond its normal terminal stopping zone. This is an example of which circuit design philosophy?
Electrical safety and lockout/tagout
7. During the lab, each student must apply their own personal lock to the hasp when multiple workers are on the same equipment. What is the primary purpose of this requirement?
Elevator safety devices and their functions
8. After completing a fault-simulation exercise, a student writes: 'The hoistway door interlock at Landing 2 was open. This device is required because it prevents the car from moving when a landing door is open, which could allow a person to fall into the hoistway.' This explanation is best described as:
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