A mechanically held contactor cannot be made to fail to the closed position. Its armature is held by a latch, not by coil flux, so when control power disappears there is no stored energy to move it anywhere; it stays exactly where the last latch or unlatch pulse left it. A guaranteed fail-on lighting circuit requires an electrically held contactor whose load is carried by normally closed (NC) power poles: coil energized holds the lights off, and any loss of coil current lets the spring close the poles. The owner-furnished mechanically held units either get replaced or get an electrically held NC bypass contactor added in parallel.
Symptom: Last State Held, Not Fail-On
The number that matters is holding current. An electrically held contactor draws coil current continuously to keep its armature seated; remove that current and the return spring resets the poles within one or two cycles. A mechanically held (latched) contactor draws coil current only during the latch and unlatch pulses, typically a few tens of milliseconds, then sits at zero holding current. That zero is the reason it is chosen for lighting: no coil heat, no coil hum, no coil life consumed during a twelve-hour lighting shift. It is also the reason it cannot fail-on. With no holding current there is nothing to lose.
| Observed behavior on control-power loss | Cause | What decides it |
|---|---|---|
| Lights that were on stay on; lights that were off stay off | Mechanically held contactor, latch retains last position | Coil holding current reads zero in either state; two coils or a two-coil latch mechanism on the frame |
| All lights drop out | Electrically held contactor with NO power poles | Coil current present only while lights are on |
| All lights come on | Electrically held contactor with NC power poles (the fail-on topology) | Coil current present only while lights are off |
| NC pole discolored, pitted, or welded after a few months | NC pole rating below lamp load or inrush | Catalog NC pole rating versus measured load current and inrush |
| Coil burns open in the fail-on scheme | Coil energized for the entire lights-off period on a coil not rated for continuous duty | Coil duty rating and steady-state coil temperature |
| Lights flash on during control-voltage dips | Electrically held NC contactor drops out at its release voltage during the dip | Coil drop-out voltage versus control-bus sag under fault or motor start |
Mechanism: Spring Force Versus Latch Force
Every contactor has three forces in play: coil pull, return spring, and contact pressure. In an electrically held frame the coil overcomes the spring and holds the armature; the spring is the only actor once the coil is dead, and the spring always drives the armature to the de-energized position. Which poles are closed in that position is a matter of pole geometry, not control logic. A NO pole opens, a NC pole closes. Put the lamp circuit through NC poles and the physics of the return spring becomes the fail-on guarantee.
In a mechanically held frame the latch replaces the coil as the holding element. The latch is a mechanical detent that the spring cannot overcome; a second coil (or a second pulse to a single-coil mechanism) trips the detent. There is no electrical path from control power to the latch except through a deliberate pulse. That is what makes a mechanically held contactor immune to control-power loss in both directions: it will not drop out, and it will not close. Any scheme that tries to add a fail-on function to it has to introduce a separate spring-driven device.
Pole Rating Limits on NC Contacts
NC poles on ordinary block-type contactors are auxiliary-grade. On most block contactors above roughly 10-16 A frame size, the NC contacts remain rated around 10 A regardless of how large the NO power poles are. That 10 A has to carry the lamp load plus inrush. Electric discharge ballasts, LED drivers, and incandescent filaments all draw inrush well above steady-state current, and the make operation on a NC pole happens every time the coil drops. A 10 A NC pole switching a 20 A lighting branch does not fail on the first operation; it welds or erodes over months.
Two product classes solve this. Purpose-built lighting contactors from several manufacturers, including Square D and Siemens in the US market, use convertible power poles: the frame accepts NO or NC power poles interchangeably, stackable up to 12 poles, in 20, 30, and 60 A ratings. Each NC pole carries the same rating as a NO pole because it is a power pole, not an auxiliary contact. Above that range, bar-and-shaft contactors such as ABB's R-Line, historically Hubbell, and Telemecanique offer NC power poles at 400 A and above. They are large and expensive, but they are built for severe duty and carry AC-5 (lighting) and AC-6 (reactive load) utilization ratings, which is exactly the duty a large lighting bus imposes.
| Quantity | Limit or value | Where to read it |
|---|---|---|
| NC contact rating on block contactors above ~10-16 A frames | ~10 A, auxiliary grade | Contactor catalog page, auxiliary contact rating column |
| Convertible-pole lighting contactors | NO or NC power poles, up to 12 poles, 20 / 30 / 60 A | Manufacturer lighting contactor catalog, pole-kit selection table |
| Bar-and-shaft contactors with NC power poles | 400 A and above | ABB R-Line and equivalent severe-duty catalogs, AC-5 / AC-6 rating tables |
| Coil holding current, mechanically held | Zero after latch pulse | Coil data sheet: pulse duration and inrush VA |
| Coil holding current, electrically held | Continuous for the entire lights-off period in a fail-on scheme | Coil data sheet: sealed VA, duty rating, temperature rise |
| Lamp inrush | Load-type dependent, an order of magnitude above steady state for filament and capacitive driver loads | Lamp or driver data sheet; AC-5a / AC-5b category on the contactor |
Thermal Load of the Inverted Coil Duty
This is heat, not logic. Inverting the topology inverts the coil duty. In a normal NO lighting circuit the coil is energized while the lights are on; in a fail-on NC circuit the coil is energized while the lights are off. In an occupancy-controlled space that may be most of the day. The coil then runs at its sealed VA continuously, and the enclosure sees that dissipation continuously. Lighting contactors are often specified with intermittent-duty or reduced-duty coils precisely because a mechanically held frame never holds; a coil transplanted into continuous holding will exceed its insulation class temperature. Read the sealed VA and the duty rating on the coil data sheet, and confirm the coil is rated for 100% duty at the control voltage and ambient inside the panel. Add the coil dissipation to the enclosure heat budget.
The second thermal item is the NC pole itself. NC poles in a block contactor close under spring force only, with no coil assist, so contact pressure is lower than on a NO pole seated by the magnet. Lower contact pressure means higher contact resistance and more I²R heat at rated current. This is the physical reason NC auxiliary contacts carry a lower rating and why a convertible-pole lighting contactor, which uses a full power-pole spring stack in the NC position, is the correct part rather than a block contactor with NC auxiliaries.
Procedure: Converting to a Fail-On Lighting Circuit
- Define the failure set. List every event that must turn the lights on: control transformer loss, control fuse open, controller output loss, coil open circuit, control wiring break. A fail-on requirement that includes coil failure rules out any scheme where the coil has to work to close the lights; only a spring-closed NC pole covers a burned coil.
- Confirm what is installed. Identify the owner-furnished units as mechanically held by nameplate and by coil configuration (latch and unlatch coils, or a single coil with a mechanical latch). Measure coil current in the lights-on state; zero confirms a latched frame.
- Choose replace or supplement. Replacing with an electrically held convertible-pole lighting contactor fitted with NC poles is the clean solution. Supplementing keeps the mechanically held unit for normal switching and adds an electrically held NC contactor in parallel across its load poles as a bypass; that bypass coil is energized whenever control power is healthy, and its NC poles close the moment control power is lost. The supplement approach preserves the zero-holding-current advantage during normal operation on the main contactor but moves the continuous coil duty onto the bypass.
- Size the NC poles. Pole rating must cover the measured branch current at the AC-5 utilization category matching the lamp type, plus the inrush of the connected drivers or ballasts. For loads within 20, 30, or 60 A per pole, use convertible-pole lighting contactors; above that, move to a bar-and-shaft frame with NC power poles.
- Select the coil. Continuous-duty rating at the control voltage; drop-out voltage high enough that normal control-bus dips during motor starts do not cause nuisance closure. Read the release voltage on the coil data sheet and compare with the measured worst-case control bus.
- Wire the control. Controller output drives the coil through the control fuse. Coil energized equals lights off. Do not route the coil supply through any device whose failure should not turn the lights on unless that behavior is intended; every element in series with the coil becomes a fail-on trigger.
- Wire the power. Lamp branch through the NC power poles only. If a bypass contactor is used, its NC poles parallel the main contactor's NO poles; both share the same branch overcurrent protection.
Verification of Fail-On Behavior
- With the lights commanded off, open the control fuse or control transformer secondary. Lights must come on within the contactor's drop-out time, a few line cycles. Restore control power; lights must go back off as the coil re-seats.
- Lift the coil lead at the contactor terminal with lights commanded off. Lights must come on. This proves the coil-open failure case, which a control-power test alone does not.
- Hold the lights-off state for several hours and measure coil surface temperature and enclosure internal ambient. Compare with the coil insulation class and the enclosure rating.
- With the lights on, measure current per NC pole and pole terminal temperature under full lamp load. A pole running noticeably hotter than its neighbors indicates low contact pressure or an undersized NC pole.
- Cycle the circuit through at least a few dozen make operations under full lamp load and inspect the NC contact faces for transfer or pitting. Inrush damage shows early.
- Run a motor start or other known control-bus disturbance and confirm the lights do not flash on.
Recurring Pitfalls on Fail-On Lighting
The most common error is using the NC auxiliary block on a standard motor contactor as a lighting pole. It carries about 10 A and is built for pilot duty; under lamp inrush it welds, and a welded NC pole means the lights cannot be turned off at all. The second is retaining the original reduced-duty coil in a topology that now holds it continuously; the coil runs hot and opens, which does produce lights-on, but with no way to restore control until the coil is replaced. The third is forgetting that a fail-on circuit turns the lights on for every control interruption, including deliberate ones: a maintenance switch-off of the control transformer, a controller download that drops outputs, or a tripped control fuse in an unrelated circuit on the same bus all light the space. Segregate the lighting control supply so that only the intended failures reach the coil.
A hybrid bypass around a retained mechanically held contactor introduces its own trap. If the main contactor is latched on and the bypass closes on a control fault, both carry the load in parallel with unequal contact resistance; the pole with lower resistance takes most of the current. Size the bypass for the full branch current on its own, never as a share.
Stop and escalate when the branch current exceeds what convertible-pole lighting contactors carry per NC pole, when the coil data sheet does not state a continuous-duty rating at your control voltage, or when the required AC-5 or AC-6 category is not published for the NC pole configuration. Contact the manufacturer's technical support channel (Schneider Electric, Siemens, ABB, or Hubbell) with the measured load current, lamp type, and control voltage and request the NC pole rating table and coil duty data for the specific frame before ordering.
FAQ
Can a mechanically held lighting contactor be made to fail closed?
No. It holds position by a mechanical latch with zero coil holding current, so control-power loss leaves it in its last state. Fail-on requires a spring-driven electrically held contactor with the lamp load on NC power poles, either as a replacement or as a parallel bypass.
Does the NC auxiliary contact on a standard contactor work for lighting loads?
Not above small loads. On most block contactors larger than 10-16 A frames the NC contact is still rated around 10 A, and lamp inrush will weld it. Use convertible-pole lighting contactors with NC power poles at 20, 30, or 60 A, or bar-and-shaft contactors with NC poles at 400 A and above.
Can I keep the existing mechanically held contactors and still get fail-on?
Yes, by adding an electrically held contactor with NC power poles in parallel across the load terminals, coil energized whenever control power is healthy. Size its NC poles for the entire branch current, not a share, and rate its coil for continuous duty.
Does a fail-on scheme change the coil heat load?
Yes. The coil is energized for the entire lights-off period instead of the lights-on period, which can be most of the day. Confirm a 100% duty coil rating at the control voltage and add the sealed VA to the enclosure heat budget.
Can control-voltage dips cause the lights to flash on with NC poles?
Yes. Any sag below the coil release voltage drops the contactor and closes the NC poles. Compare the coil's published drop-out voltage against the measured control bus during motor starts and feed the lighting coil from a segregated control source.