No universal formula converts contact size, gap, and material into a safe AC or DC breaking rating. A contact rating is a qualified operating envelope for a defined voltage, current waveform, load type, switching duty, environment, and service-life criterion. For undocumented legacy switches, assign a rating only from manufacturer data or controlled qualification testing.
Contact-rating boundary
The term breaking rating here means the current and voltage that a contact can interrupt without unacceptable arcing, welding, insulation damage, temperature rise, or loss of service life. A making rating addresses closing stress, including bounce and inrush. These are separate limits.
AC and DC ratings are not interchangeable. An AC waveform passes through current zero periodically, which helps extinguish an arc. DC lacks that natural zero crossing, so the arc can persist while the contacts separate. The acceptable DC voltage can therefore be substantially different from the AC value, but no conversion factor applies universally.
| Known characteristic | What it indicates | What it cannot establish |
|---|---|---|
| Contact material | Likely resistance to erosion, transfer, oxidation, or welding | Complete voltage and current rating |
| Physical contact gap | One influence on arc extinction and open-circuit isolation | Breaking capacity without opening speed and circuit data |
| Same envelope as an OEM part | Possible mechanical interchangeability | Electrical equivalence |
| Measured steady current | Thermal loading during conduction | Inrush, transient, or interruption stress |
Check 1: Circuit-duty classification
Classify each switched circuit before selecting or testing a contact. Dividing circuits into less than or greater than 5 A is useful for organizing an inventory, but it is not a contact-rating boundary. Load behavior determines the stress.
| Load | Dominant contact stress | Required reading |
|---|---|---|
| Resistive controller input | Low steady current; possible unreliable conduction if the contact lacks a suitable minimum switching load | Closed-circuit current and voltage drop across the contact |
| Relay or solenoid coil | Inductive opening transient and sustained arc | Coil current immediately before opening and voltage across the opening contact |
| Motor | Starting current, possible stall current, and inductive interruption | Current waveform at start, normal operation, and the actual opening condition |
| Capacitive input | High closing current while capacitance charges | Peak make-current waveform and pulse duration |
If the load type is known, continue to Check 2. If it is unknown, trace the conductors to the load and identify any suppression components before taking electrical measurements.
Check 2: Operating-waveform measurement
Record whether the circuit is AC or DC and measure at the contact terminals under the operating states that actually cause opening and closing. The described legacy population spans 12 V through 240 V, AC or DC, and currents from milliamps to tens of amps; those endpoints describe the application range, not a permissible rating for any individual contact.
For a resistive load, I = V/R predicts nominal steady current when resistance remains stable. For an inductive load, stored magnetic energy is E = 1/2 L I². Opening forces that energy into the contact arc, coil resistance, wiring impedance, and any suppression network. For a capacitive load, stored energy is E = 1/2 C V², while closing current depends on total series impedance and switching instant. None of these formulas produces a contact rating by itself.
- Check 2A: Measure open-contact voltage. Expect the normal operating voltage plus any switching transient to remain inside the candidate device's published rating for the same AC or DC duty.
- Check 2B: Capture closing current. Expect the measured inrush magnitude and duration to fall within a documented making capability.
- Check 2C: Capture current immediately before opening. Expect the value to remain within the documented breaking capability for that load category.
If any waveform exceeds a published limit, select another device or reduce the stress before continuing. If no applicable published limit exists, proceed to Check 3 as an unqualified legacy component.
Check 3: Construction and documentation match
Compare more than dimensions. Contact force, opening speed, bounce, wiping action, terminal temperature rise, insulation spacing, enclosure atmosphere, actuator travel, and arc-management features all affect performance. Two parts with the same size, gap, and material can have different electrical endurance.
Check the manufacturer documentation for separate AC and DC limits, resistive and inductive load classifications, making and breaking limits, minimum switching load, operating frequency, ambient conditions, and expected electrical life. Use only the row that matches the measured circuit.
If a matching published rating exists, continue to Check 4. If the rating covers a resistive load but the actual load is a relay, motor, solenoid, or capacitor, treat it as nonmatching. If the OEM part and proposed replacement both lack ratings, physical similarity supports fit only; qualification testing must establish electrical suitability.
Check 4: Arc-energy control
Place suppression at the load where practical so stored energy circulates locally instead of across the opening contact. A diode can suppress a DC coil, but it can lengthen coil release time; verify that timing against the control function. An appropriately selected voltage-clamping or resistor-capacitor network may suit other waveforms. Select every suppression component from its voltage, pulse-energy, repetition, and failure-mode data rather than from contact current alone.
Check the existing circuit before adding suppression. A controller input may already contain filtering, a coil may contain an internal suppressor, and a motor circuit may require a switching device specifically qualified for motor duty. If suppression changes release time or leakage current enough to alter operation, select a different method or a contact with adequate unsuppressed breaking capability.
When a contact performs an elevator protective or motion-related function, apply the governing equipment approval process. A bench result does not independently authorize a substitution in a safety-related circuit.
Resolving procedure
- Create one record for every distinct circuit: AC or DC, load type, normal voltage, open-contact voltage, steady current, make-current waveform, break current, switching frequency, ambient condition, and existing suppression.
- Search the switch or relay documentation for a rating matching all recorded conditions. Reject AC-only data for a DC circuit and resistive-only data for an inductive circuit.
- Select a device whose documented making, carrying, and breaking limits cover the measured waveforms. For milliamp controller inputs, also match the published minimum switching load.
- Add or revise load suppression only after checking its effect on coil release, controller leakage, and normal operation.
- Test representative assemblies at the worst measured operating condition and to the applicable qualification plan. Define acceptance limits for contact welding, missed operations, insulation condition, temperature rise, and contact resistance before testing.
- Record the approved circuit envelope rather than issuing one universal rating for the replacement part.
Verification readings
- Check 1: Measure voltage across the open contact during switching. Expect the complete waveform, including the transient, to remain below the documented limit for the applicable AC or DC load category.
- Check 2: Capture current at closing and opening. Expect both waveforms to remain inside the documented making and breaking limits.
- Check 3: Measure voltage drop across the closed contact at the working current. Expect a stable reading that remains within the qualification plan's acceptance limit.
- Check 4: Operate the circuit through the required qualification sequence. Expect no welding, missed operation, abnormal heating, insulation damage, or unintended delay caused by suppression.
Frequently asked questions
What happens if I use an AC-rated switch contact on DC?
The DC arc may persist because the current has no natural zero crossing. Use a published DC breaking rating for the measured voltage, current, and load type; do not derive it from the AC rating with a fixed factor.
What happens if a power contact switches only milliamps?
A contact intended for higher current may not reliably conduct a low-level controller signal if films or oxides are not disrupted. Check the published minimum switching load and verify closed-contact voltage drop under the actual milliamp load.
What happens if no OEM switch contact rating exists?
Treat the part as electrically unqualified, even when its size, gap, and material match the original. Select a documented replacement or run a defined qualification test; the final verification step is a successful operating sequence with acceptable contact voltage drop and no welding, missed operation, abnormal heating, insulation damage, or timing error.