ORIGINAL · LOW-VOLTAGE ELECTRICAL TECH NOTES
AC Contactor LC1: What Exactly Separates NO from NC? How to Read the Model, Where to Use It, and How to Wire It
Schneider Electric's LC1 (TeSys) is the most common AC contactor on the industrial floor. This article covers in one pass the real difference between Normally Open (NO) and Normally Closed (NC) contacts, what every letter and digit in the model number means, typical application scenarios, and how to wire the main and control circuits — with an interactive demo and wiring diagrams. After reading, you can select and wire it on your own.
What is LC1 — know the family first
"LC1" is the model prefix of Schneider Electric's (formerly Telemecanique) TeSys-series AC contactors — found everywhere in factories and switchgear. It uses a small current (the coil) to switch a large current (the main contacts), enabling remote motor start/stop, reversing, star-delta reduced-voltage starting, and general load switching.
Within the same LC1 family there are different series suffixes. Identify the family first, then you won't buy the wrong one:
| Series | Current range | Positioning & features | Common codes |
|---|---|---|---|
| LC1D (TeSys D) | 9–150 A | General-purpose workhorse covering most motors (4–75 kW); expandable aux contacts, mechanical interlock, surge suppressor modules | LC1D09…LC1D150 |
| LC1F (TeSys F) | 115–800 A | Heavy-duty: Ag-graphite main contacts, double-break arc extinction, withstands frequent AC-4 reversing/jogging, longer life | LC1F115…LC1F780 |
| LC1N (EasyPact D3N) | 6–95 A | Economy, high cost-performance; fans/pumps/lighting/small automation | LC1N06…LC1N95 |
| LC1G (TeSys Giga) | 185–2600 A | Ultra-high power; AC/DC wide-voltage coils (BEE/EHE/KUE/LSE) | LC1G185… |
| LC2 / LC3 | — | LC2 = reversing contactor assembly (mechanically interlocked F/R); LC3 = star-delta contactor assembly | LC2D, LC3D |
One-line memoryWhen people say "LC1 contactor", 90% means the
LC1D branch; this article focuses on LC1D, but the other series follow the same naming logic.Core difference: Normally Open (NO) vs Normally Closed (NC)
This is the heart of the article. The "closed" (开闭) usually refers to Normally Closed (NC) — many electricians verbally say "closed contact" versus "open contact". The single criterion that separates them is the contact state when the coil is de-energized (no power):
NO (Normally Open)OPEN when coil de-energized, CLOSED when energized. Like a switch that "closes only when pressed".
NC (Normally Closed)CLOSED when coil de-energized, OPEN when energized. Like a switch that "is closed by default, opens once energized".
▶ Try it yourself (click to simulate coil energization)
Click the button to energize / de-energize the electromagnetic coil and watch the two contacts move in opposite directions: the NO "closes only when pulled in", the NC "opens when pulled in".
Normally Open (NO)
Coil de-energized: OPEN ✗
Normally Closed (NC)
Coil de-energized: CLOSED ✓
▶ One table for all the differences
| Item | Normally Open (NO) | Normally Closed (NC) |
|---|---|---|
| Default (coil de-energized) state | Open (no conduction) | Closed (conducting) |
| Coil energized state | Closed (conducting) | Open (no conduction) |
| Electrical symbol | Two broken lines (with a gap) | A slash across the contact (indicates normally closed) |
| Terminal numbering | Ending in 13-14, 23-24, 33-34 (even 4 = NO) | Ending in 11-12, 21-22, 31-32 (even 2 = NC) |
| Position on contactor | Main contacts (all NO); some aux also NO | Only on auxiliary contacts |
| Typical use | Start, self-hold (seal-in), run feedback, indicator | Stop, electrical interlock, protective interlock, fault alarm, safety circuit |
Terminal numbering rulePer IEC: terminals ending in 1-2 are NC, ending in 3-4 are NO; odd numbers (1/3/5…) are common terminals, even (2/4/6…) are outgoing. E.g.,
21-22 is NC, 13-14 is NO.▶ Main contacts vs auxiliary contacts
- Main contacts (1/L1-2/T1, 3/L2-4/T2, 5/L3-6/T3): all 3 poles are Normally Open. They carry the motor's large current (a few amps up to over a hundred) and sit under the arc chute. When the coil is energized, all three close and the motor runs; when de-energized, all open.
- Auxiliary contacts (13-14 NO, 21-22 NC): very small capacity (~10 A thermal), used only in the control circuit. A standard 3-pole LC1D already builds in
1NO + 1NC; when insufficient, snap the LA1DN expansion module (2NO / 2NC / 1NO+1NC) onto the top.
Common pitfallMain contacts are always NO — don't expect to use them as "conducting by default". For logic that must "conduct when de-energized and open when energized", you must use an auxiliary NC contact.
How to read the LC1 model (decode it)
Take the most common LC1D0910M7C (or the newer LC1D09M7C) and break down every segment:
| Segment | Meaning | Notes |
|---|---|---|
LC1 | Contactor product | Schneider TeSys AC contactor prefix (DC is LP1) |
1 | Structure type | 1 = single; 2 = reversing assembly (F/R); 3 = star-delta assembly |
D | Series | D = TeSys D (general); F = TeSys F (heavy); N = EasyPact (economy); G = Giga (ultra) |
09 | Current rating | Max operating current at AC-3, 440 V; 09 = 9 A |
10 | Aux contact code (legacy) | Older: 10 = 1 NO, 01 = 1 NC; current 3-pole standard builds in 1NO+1NC, segment often omitted |
M7 | Coil voltage + frequency | M = 220 V AC; 7 = 50/60 Hz universal (also 5 = 50 Hz, 6 = 60 Hz, D = DC coil, L = low-consumption DC) |
C | Origin | C = domestic (China); omitted = imported |
Add "00" above 115 AFor the LC1D115–170 range, you must insert
00 after the current, e.g., LC1D11500M7C; otherwise the model is invalid.▶ Current rating ↔ motor power cheat-sheet (AC-3, 380/400 V)
| Current (A) | Model | Motor power | Typical load |
|---|---|---|---|
| 9 | LC1D09 | 4 kW | Small pumps, small fans |
| 12 | LC1D12 | 5.5 kW | Conveyors, machine-tool aux |
| 18 | LC1D18 | 7.5 kW | Compressors, grinders |
| 25 | LC1D25 | 11 kW | Mid-size fans/pumps workhorse |
| 32 | LC1D32 | 15 kW | Air compressors, hoists |
| 38 / 40 | LC1D38 / LC1D40 | 18.5 kW | Crushers, large pumps |
| 50 | LC1D50 | 22 kW | Large fans, injection machines |
| 65 | LC1D65 | 30 kW | Production-line main drive |
| 80 | LC1D80 | 37 kW | Central AC, chillers |
| 95 | LC1D95 | 45 kW | Large pump stations, refrigeration |
| 115 / 150 | LC1D115 / LC1D150 | 55 / 75 kW | Heavy large motors (above 75 kW, go LC1F) |
Size down, not to the maxThe rated current is the AC-3 upper limit. For motors with frequent reversing (>30 jogs/hour), heavy long-acceleration starts (centrifuges, crushers), or cabinets running >40 °C long-term, step DOWN one size — e.g., an 11 kW motor goes from LC1D25 to LC1D32; contact life differs a lot while the price gap is small.
▶ Coil voltage codes (most often ordered wrong)
| Code | AC coil voltage | Typical scenario |
|---|---|---|
B7 | 24 V AC | From a safety isolating transformer |
F7 | 110 V AC | Imported / legacy equipment control |
M7 | 220 V AC | Most common in domestic equipment |
P7 | 230 V AC | Euro-standard equipment |
Q7 | 380 V AC | Directly from two mains phases |
Common DC coils: BD = 24 V DC, FD = 110 V DC, MD = 220 V DC; wide-voltage low-consumption coils use L or the Green series BEE/EHE/KUE/LSE. When a PLC transistor/relay output drives a small contactor directly, prefer 24 V DC (e.g., LC1D09BD) — but note that large DC coils may need an interposing relay.
▶ How to tell apart from the physical unit
- Read the side nameplate: model, current rating, and coil code (e.g.,
M7) are printed on the white side label — copy it when replacing; don't just remember "0910". - Read the terminals: top
1/3/5(or L1/L2/L3) is line-in, bottom2/4/6(T1/T2/T3) is load-out; coilA1/A2(A2 often has two holes for easy wiring); aux13-14is NO,21-22is NC. - Arc chute: the plastic lamellae above the main contacts; do not run energized with it removed.
Use cases: what NO and NC each handle
Once you grasp the "de-energized state" rule, the scenarios fall into place:
| Scenario | Contacts used | Logic |
|---|---|---|
| Jogging (machine tools, cranes, electric valves) | Main (NO) + Start button (NO) | Runs only while held, stops on release; no self-hold |
| Continuous self-holding (pumps, fans, conveyors) | Main (NO) + auxiliary NO for seal-in | After start, its own NO keeps the coil energized |
| Stop / E-stop | Stop button (NC) | Default conducting; press to open the coil; de-energized = safe |
| Reversing interlock (hoists, feeds) | The other contactor's NC in series with this coil | One pulls in and cuts the other, preventing phase short |
| Star-delta reduced-voltage start | Main (NO) ×2–3 + NC interlock + timer | Start in star (reduced V), switch to delta after delay |
| Fault alarm / safety interlock | NC as monitoring contact | On abnormal de-energize, NC resets closed and triggers alarm |
| Run indicator / PLC feedback | Auxiliary NO | After pull-in, NO closes and lights the indicator or feeds the PLC |
Design principleAny logic that "must act on de-energization to ensure safety" (stop, interlock, protection, alarm) uses NC; any logic that "acts only on energization to execute a command" (start, self-hold, feedback) uses NO. This also follows the Fail-Safe philosophy: when control power drops, the NC contact returns to closed and the system enters a safe state.
How to wire (main + control circuits)
① Main circuit: the path of three-phase power
The main circuit carries only large current, and the order must not be reversed (line-in at the bottom, load-out at the top would leave the top live when open — dangerous for maintenance).
② Control circuit: self-holding (start-hold-stop) circuit
Example with 220 V control (if using 380 V control, connect A2 to another phase instead of neutral). This is the most classic contactor logic — master it:
- Press SB2 (Start, NO): the control loop closes, KM coil energizes, main contacts close → motor runs; meanwhile the auxiliary NO
13-14closes. - Release SB2: current reroutes through the now-closed
KM 13-14and the coil stays energized — this is "self-holding" (seal-in). - Press SB1 (Stop, NC): the loop is cut, KM releases, main contacts and
13-14open together, motor stops. - Thermal relay FR trips: its NC contact (95-96) should be in series in the control loop, opening the coil on overload → protection.
③ Jogging circuit
Simply remove the KM 13-14 holding branch from the diagram above (do not parallel the self-hold contact): hold SB to run, release to stop. Common for machine-tool positioning, crane jogging, and valve fine adjustment.
④ Reversing interlock (the most critical safety measure)
Forward/reverse uses two contactors KM1 (FWD) and KM2 (REV) to swap two phase sequences. If both pull in at once, it causes a phase-to-phase short, so interlock is mandatory:
High-voltage warningReversing control must have double interlock: ① electrical interlock (as shown, the other's NC in series); ② button interlock (compound buttons that break the opposite loop when pressed); ③ a mechanical interlock module. Using only one risks a short-circuit from contact welding.
⑤ Star-delta reduced-voltage start (brief)
Large motors (generally >15 kW) draw 6–7× rated current on direct start, so star-delta is used: three contactors KM1 (main), KM△ (delta), KMY (star) plus a timer KT. KMY and KM△ also use an NC interlock; KT delays the switch from star to delta. The wiring logic is the same as above — just more main-contact combinations; design it against the specific motor nameplate.
▶ Golden wiring notes
- Coil voltage must match the nameplate: a 220 V coil on 380 V burns instantly; a 380 V coil on 220 V won't pull in. This is the most frequent field fault.
- Main contact in/out order: line-in on top, load-out on bottom (L1/L2/L3 up, T1/T2/T3 down) so the lower side is dead when open.
- Aux contacts only in the control circuit: tiny capacity — never carry the main load or a high-power indicator.
- Refit the arc chute: the main-contact arc lamellae decide breaking capacity; missing them causes arcing and burnt contacts.
- Inductive-load protection: across a DC coil or long control wire, add an RC snubber / varistor / flyback diode to suppress switching spikes and extend contact life.
- De-energize and verify before work: all wiring and maintenance must be done after power-off and verified dead (licensed electrician).
Selection cheat-sheet & troubleshooting
Four-step selection mnemonic
① Match power to ratingMotor power → current rating (see cheat-sheet above)
② Copy the coilRead the old unit's side nameplate coil code (M7/Q7/BD…)
③ Add contactsAux insufficient → LA1DN module; interlock needs a separate mechanical interlock
④ Step down oneFrequent/heavy/hot duty → step current down one notch
Common fault quick-reference
| Symptom | Possible cause | Troubleshoot |
|---|---|---|
| Coil buzzing / won't pull in | Control voltage too low; iron-core face dirty or rusted | Measure A1-A2 voltage ≥85% rated; clean the core face |
| Coil burned | Voltage mismatch (too high/low long-term); frequent operation | Verify nameplate coil code; confirm control supply |
| Contacts welded, won't release | Breaking current too high; arc chute missing; AC-4 overload | Re-select by AC-3/AC-4; refit the arc chute |
| Motor won't run but coil pulls in | Main contact in/out reversed; a phase loose (single-phasing) | Check 1-2/3-4/5-6 wiring and tightening torque |
| Self-hold fails | Aux NO (13-14) not paralleled or poor contact | Multimeter: check 13-14 closes with pull-in |
| Both FWD/REV pull in | Interlock NC welded or not wired | Check KM1/KM2 21-22 and the mechanical interlock |
FAQ
Is the LC1 "closed" the NC? How do I tell NO from NC at a glance?
Yes — the electrician's "closed contact" is NC (Normally Closed). The most direct method: with the coil de-energized, use a multimeter buzzer on the auxiliary contacts — beeping means NC, silent means NO. The terminal number also works: ending in 1-2 is NC, ending in 3-4 is NO.
Is the "10" in LC1D0910 1-open or 1-closed?
This is legacy numbering: in old models
10 = 1 NO and 01 = 1 NC. Current 3-pole LC1D already builds in 1NO + 1NC; newer numbers (e.g., LC1D09M7C) often omit this segment. So don't overthink "0910" — it still comes with one NO and one NC auxiliary contact.Can the domestic CJX2 replace LC1D?
Dimensions and mounting (35 mm rail) are basically compatible;
CJX2-0910 corresponds to LC1D0910 with the same pinout, so it can substitute. But for applications where quality, life, and certification matter, prefer the original Schneider.Coil 220 V or 380 V?
It depends on the control supply. Most domestic equipment control is 220 V (line + neutral) →
M7; if control is taken from two mains phases at 380 V → Q7; PLC/safety loops are often 24 V → BD. Always follow the old unit's nameplate.Why is the stop button NC and the start button NO?
This is the Fail-Safe principle: the stop button is normally conducting, so if control power drops or the button wire falls off, the motor stops immediately — it won't "fail to stop". The start is NO, so de-energized defaults to not starting, which matches safe habits.
