Read what the symbols actually say
Read a diagram legend and its stated resting condition, distinguish relay coil and contacts, and predict an expected open circuit without declaring a failed part.
The short version
- A symbol shows function and connections; it is not a photograph or a measured state.
- Normally open describes the specified normal state, not a promise that a contact is always open.
- A shared label or junction establishes a connection only according to the drawing legend.
Start with this drawing legend
Our original DC trainer uses B1 for source, F1 for fuse, S1 for request switch, K1 for relay and L1 for indicator load.
Those identifiers belong to this drawing. A familiar letter or color on another machine is not enough to identify its function.
The legend says a filled junction joins wires; a crossing without a dot does not. Read that convention before tracing.
| Drawing mark | Declared meaning |
|---|---|
| B1 / F1 | Source / fuse |
| S1 / K1 / L1 | Request / relay / load |
| Filled junction | Connected in this example |
| Crossing, no dot | Not connected here |
Ask what state the page represents
The title block says supply off, request released, relay coil not energized. That is the drawing state, not a measurement of a working unit.
K1 has a normally open contact. In the stated resting condition it leaves the load path open.
A closed-contact symbol elsewhere may describe a normally closed device. Check the legend and the operating state instead of treating every open symbol as damage.
| Supplied record | Meaning |
|---|---|
| Drawing state | Supply off / request released |
| K1 coil | Not energized |
| K1 contact | Normally open |
| Meaning | Expected open in this state |
Keep the command and load paths separate
K1 appears twice: its coil in the request path and its contacts in the lamp path. The shared identifier associates them.
The association does not draw a wire between the coil and contacts. Trace each circuit back to its own stated return.
OMRON explains the mechanical relay relationship. This training drawing supplies its own terminals and circuit; it is not an OMRON product pinout.
| Supplied record | Meaning |
|---|---|
| Command path | B1, S1, K1 coil, return |
| Load path | B1, F1, K1 contact, L1, return |
| Shared K1 label | Associated functions |
| Not implied | Coil wire connected to lamp |
Predict before opening a reading
The supplied state is request off. Predict coil voltage, contact state and indicator behavior before looking at the records.
Our ideal trainer predicts no voltage across the coil, an open contact and an unlit indicator.
A quiet lamp in that condition is consistent with the drawing. It says nothing about operation after a valid request.
Separate expected behavior from a complete pass
The records match that prediction: request off, coil zero volts, indicator off. No failure is demonstrated in that condition.
You still need a different record to assess the requested-on state. A correct off-state does not prove the entire system works.
In our second supplied record, request on energizes the coil and the lamp lights. The conclusion covers these two recorded conditions only.
| Supplied record | Meaning |
|---|---|
| Record A | Request off / lamp off |
| Record B | Request on / lamp on |
| Supported | Two recorded states agree |
| Not supported | Every operating condition passed |
What changes with a closed contact?
A new training page shows an NC contact in a monitoring path. Its legend says normally closed with the actuator at rest.
Do not copy the normally-open lamp prediction. Write the new expected state from that page, then compare its supplied record.
If the page does not state the actuator or coil condition, mark the expected state unresolved. A familiar symbol alone cannot settle it.
| Contact description | Expected in this example |
|---|---|
| NO at stated rest | Open |
| NC at stated rest | Closed |
| State not supplied | Prediction unresolved |
Use the case without adding a fault
In the relay case, the complaint describes an off indicator. Collect the request state and legend before naming a failure.
If those records describe normal off behavior, choose that bounded conclusion. Do not order a relay to make every case end in replacement.
The related water-heater command activity uses a different original circuit. Follow its own request and return labels, not this trainer terminal map.
| Supplied record | Meaning |
|---|---|
| Case question | Was a request present? |
| Evidence | State plus matched legend |
| Decision | Expected behavior or unresolved |
| Next | Transfer to command activity |
A symbol starts a question
Read the legend, the normal state and the identifier. Separate a command circuit from the contact or load it controls.
Predict a condition, collect the corresponding evidence and limit the conclusion to that condition.
A simulated normal condition is still practice evidence, not a hands-on observation or equipment release.
Shop tip
Josh: Before blaming an open contact, ask whether the supplied command should have closed it.
Where this comes from
- OMRON General Purpose Relays: Introduction; Undated live manufacturer page; no revision shown; HTML sections What Is a Relay and Structure and Operating Principle; checked 2026-11-04. Scope: Mechanical relay coil/contact distinction only. No rating, pinout or replacement recommendation for an RV relay. https://www.ia.omron.com/support/guide/36/introduction.html
- OMRON General Purpose Relays: Explanation of Terms; Undated live manufacturer page; no revision shown; HTML Contact Section, Contact Symbols, Make Contacts and Break Contacts; checked 2026-11-04. Scope: NO/NC terminology and the need to read the drawing convention; the fictional connector and circuit legend are Rigprobe originals. https://www.ia.omron.com/support/guide/36/explanation_of_terms.html
- Original Rigprobe teaching examples, 2026-11-04. Identifiers, diagrams, connector layouts, timings and readings are invented problem data, not OEM drawings, specifications or universal acceptance limits.
Try it on a service call
Put the lesson to work on a hands-on troubleshooting case.