7.1 Ladder Diagrams, Seal-In Circuits, and Safety Interlocks
Key Takeaways
- Read a ladder left to right: L1 through closed contacts to a coil on the right rail; series contacts are AND logic and parallel contacts are OR logic.
- Three-wire control puts Stop (N.C.) in series and Start (N.O.) in parallel with a N.O. seal-in of the same control relay so the coil drops on any series cutout and does not restart by itself after a control-power loss.
- When 1CR picks up, every 1CR N.O. contact in the drawing closes and every 1CR N.C. contact opens — search the whole ladder, not only the rung that holds the coil.
- A screw oil-pressure interlock proves differential oil pressure after a manufacturer start time delay; lost oil ΔP after that delay must drop the compressor.
- Trace a tripped breaker by identifying whether it feeds control rails or only the motor power circuit before predicting lamps, seal-ins, and restart behavior.
How to Read a Refrigeration Ladder Diagram
RETA BE-I and BE-II train operators to treat a ladder diagram as a logic map, not as a photograph of the panel. The left rail is the ungrounded control conductor (commonly L1). The right rail is the other side of the control source (L2, or a grounded neutral on some 120 V control transformers). Each rung is a horizontal path. You read left to right: control power must find a continuous path of closed contacts before it can energize the device at the far right — almost always a coil (control relay CR, motor starter, timer, or solenoid).
Series contacts on one path are AND logic: every contact must be closed or the coil stays de-energized. Parallel branches are OR logic: any closed branch can complete the path. A drafting rule you will see on BE worksheets and CIRO-style items: the coil sits last, against the right rail. Contacts drawn to the right of a coil are a drawing error, not a hidden circuit.
Contacts are shown in the shelf (de-energized) state. A normally open (N.O.) contact is drawn open and closes when its actuating device is true (coil picked up, pressure above setpoint, float raised — depending on the note on the drawing). A normally closed (N.C.) contact is drawn closed and opens when its actuating device is true. SPDT (single-pole, double-throw) devices have a common terminal plus both an N.O. and an N.C. path. Hand-off-auto (HOA) selectors and some float switches use SPDT so one mechanism can feed two mutually exclusive paths without both being live at once.
Neon indicating lamps are high-impedance lights. A lamp from L1 to L2 after the control fuse shows that control power is available. A lamp in parallel with a coil shows that voltage is present at that coil. A dark lamp means that portion of the ladder is dead or the lamp failed — confirm with a meter before you chase a coil. A glowing neon is not proof that the load is doing mechanical work; it only proves voltage is present. A burned-open coil can still show voltage at the terminals.
Overload (OL) heaters or electronic overload sensors live in the power circuit (the three motor leads). The OL auxiliary contact in the ladder is typically N.C. and sits in series with the starter or CR coil. A thermal or electronic trip opens that auxiliary and drops the coil even though the power fuses or breaker may still be closed. That is why a motor can sit dead with a "tripped overload" reset button while the disconnect still shows voltage on the line side.
Three-Wire Control: Stop in Series, Start in Parallel, Seal-In
Standard three-wire control is the CIRO default for compressors and large motors:
- Stop is a N.C. pushbutton in series with the entire run path. Pressing Stop opens the path and must drop the coil.
- Start is a N.O. pushbutton. It is in parallel with a N.O. auxiliary contact of the same coil — the seal-in (holding, latching) contact.
- Momentarily pressing Start energizes 1CR (or the starter coil). 1CR-1 closes and keeps the coil energized after Start is released.
- Any series cutout that opens — Stop, e-stop, OL, high-pressure, high-level, failed oil ΔP after the start delay — breaks the seal. The machine stays off until a person starts it again.
That last point is why plants use three-wire instead of two-wire maintained contacts: loss of control power drops the seal-in. When power returns, the compressor does not restart by itself. Two-wire (maintained HOA in Hand, or a maintained pressure switch feeding a coil with no seal-in logic) can restart on power restoration — treat that as a hazard, not as "normal automatic."
What 1CR actually does — the method, not a memorized figure: find the 1CR coil. Then search the entire drawing for every contact labeled 1CR (1CR-1, 1CR-2, 1CR-3). When the coil is energized, every N.O. 1CR contact closes and every N.C. 1CR contact opens. Typical jobs for those contacts: (1) seal-in the run circuit, (2) pick a power contactor or enable a VFD run permissive, (3) light a run lamp, (4) interlock another machine by opening a N.C. 1CR contact in that machine's start path so two compressors or a compressor and a pump cannot violate a sequence.
Tracing Method for CIRO-Style Ladder Items
The official CIRO Study Guide uses plant-style ladders (vessels, level switches, breakers, control relays). Do not memorize a copyrighted figure. Use this trace order on any unfamiliar drawing:
- Identify the rails and the control-circuit protective device (fuse or breaker). If that breaker trips, every coil and neon fed from those rails goes dark, seal-ins drop, and three-wire loads stay off until Start is pressed after power is restored.
- Identify motor-circuit breakers separately. If only the power breaker trips, the motor stops. If the control seal-in is still made, resetting the power breaker can restart the motor without a new Start — unless an auxiliary contact or undervoltage relay also dropped 1CR. Always ask: which circuit did this breaker feed?
- For a named relay (1CR): write the pick-up path (what must be closed) and list all 1CR contacts and their jobs. The exam question "what does 1CR do" is answered by that list, not by the coil symbol alone.
- For a level switch: read the tag (LSL, LSH, float) and the actuation note. "Closes on low" means the contact makes when level falls to the setpoint — often a N.O. low-level switch used to start a transfer pump or sound an alarm. A low-level cutout that stops a pump or compressor is usually a contact that opens on low (adequate level holds it closed). A high-level cutout in a compressor run circuit is a contact that must stay closed to run and opens on high liquid so the machine cannot ingest liquid.
- Do not assume N.O. versus N.C. from the process name alone. The drawing note wins. If two floats are shown, determine which one closes on low by the note, then trace what that closure energizes.
Compressor Safety Interlocks
Interlocks in the run path are the electrical expression of "this machine is not allowed to run." They belong in series with 1CR, not merely on a horn. An alarm-only contact that does not drop the coil is not a cutout.
| Interlock | Typical run-circuit contact | Drops the compressor when |
|---|---|---|
| Emergency stop | N.C., often through a master control relay | Operator hits e-stop; plants also drop refrigerant pumps and N.C. automatic refrigerant valves |
| Stop | N.C. pushbutton | Normal stop is pressed |
| High-level cutout | Closed to run; opens on high | Liquid in a separator, suction trap, or vessel threatens carryover/slug |
| High discharge pressure | N.C. pressure cutout | Discharge pressure reaches the cutout |
| High discharge temperature | N.C. | Discharge temperature reaches the cutout (failed oil cooling, high ratio, no flow) |
| Motor overload | N.C. OL auxiliary | Sustained overcurrent heating |
| Oil-pressure failure | Proves differential oil pressure after a start time delay | Oil ΔP is lost after the delay |
Oil-pressure interlock on screws — exam core. Lubrication is differential: oil-pump or injection pressure minus the OEM's reference (often suction or crankcase). On start, ΔP has not built, so a manufacturer time delay (industrial packages commonly use a delay on the order of tens of seconds up to about two minutes — use the package setting, not a number you invent) bypasses or ignores the oil-pressure cutout long enough for the oil system to prove. After that delay, the oil-pressure contact must show adequate ΔP. If ΔP is lost while running, the interlock opens the run circuit and the compressor stops. That is the whole point of the device. Jumpering the oil-pressure contact to "get it running" destroys bearings and rotors and is a mechanical-integrity failure, not a troubleshooting step.
E-stop is not a substitute for these cutouts, and these cutouts are not a substitute for e-stop. IIAR 2 also expects emergency ventilation to remain available when the refrigerant e-stop drops compressors and pumps. Do not land emergency-fan motors on the same "kill everything" contact that you use for the screw.
In a standard three-wire compressor ladder, the start pushbutton is momentary. After the operator releases Start, what keeps the control-relay coil energized?
After the manufacturer start time delay has expired, what should an oil-pressure failure interlock do on a rotary-screw compressor if differential oil pressure is lost?
A control-circuit breaker that feeds the L1 rail of a three-wire compressor ladder trips. Using proper tracing method, what happens?
A high-level cutout is wired in series with the compressor control relay. Which description matches a correctly applied cutout?