What are the methods for testing the dielectric strength of float level switches?

I. Power Frequency AC Withstand Voltage Test (Preferred for factory inspection and type testing, 50Hz sinusoidal AC)

1. Applicable Scenarios: Withstand voltage to ground of reed contacts, terminals, metal housing, and internal wiring; mainstream method for conventional industrial 220V products.

2. Test Points (3 mandatory tests):

1) Short-circuit all outgoing terminals ↔ ground the switch’s metal housing/guide rod/float metal body (main insulation withstand voltage);

2) Switch open state: Insulation between the two reed contacts (contact gap);

3) Power supply terminal of the built-in transmitter circuit board ↔ ground the housing (products with 4～20mA transmitters). 3. Test Parameters
Standard AC 1500V/1min; AC 220V; Reinforced Insulation AC 2000～3000V/1min;
Production Line Short-Line Sampling Inspection: AC 1500V/3～5s (Standard Permitted Rapid Withstand Voltage);
Leakage Current Trip Threshold: 5mA～10mA; Overcurrent, Sparking, Breakdown, or Smoke indicates Failure.

4. Operating Procedures
Voltage is increased uniformly from zero (≤500V/s), the voltage is stabilized, and the time is maintained. The voltage then returns to zero smoothly at the end of the test. The high-voltage end is fully discharged after the test.

II. DC Withstand Voltage Test (Specifically for Moist/Explosion-proof/Long Cable Switches)

1. Applicable Scenarios: Explosion-proof float switches, long-lead liquid level switches, PTFE/fluoroplastic insulation structures; DC withstand voltage test used when AC boost capacity is insufficient.

2. Voltage Conversion:
DC test voltage ≈ AC RMS value × 1.414, AC1500V corresponds to DC2120V, holding voltage for 60s.

3. Features: Low power consumption for capacitive load boost, no dielectric heat loss, easily detects latent moisture defects in insulation; longer discharge time after test (≥30s).

III. Lightning Impulse Withstand Voltage Test (Type Test, Explosion-proof/High-Risk Chemical Environment)

1. Applicable: Explosion-proof Exd, cryogenic liquid nitrogen, petrochemical field float switches; assesses the withstand capability of transient overvoltage caused by lightning strikes.

2. Methods A standard lightning impulse wave of 1.2/50μs was applied three times for both positive and negative polarities. No breakdown or flashover was considered合格 (qualified). The standard test voltage was 5kV to 10kV.

IV. Partial Discharge Dielectric Test (High-end Intelligent Transmitter Float, Sealed Encapsulated Products)

1. Applicable
For products with built-in PCBs, epoxy resin encapsulated junction boxes, and sealed reed assemblies, this test checks for internal insulation bubbles and encapsulation voids.

2. Test Method
Apply 0.75 times the power frequency withstand voltage for 30 minutes. A partial discharge of ≤100pC without continuous pulse discharge is considered合格 (qualified), allowing for early prediction of long-term damp-heat aging and breakdown risks.

V. Pre-treatment and Judgment Criteria

1. Before Testing: Disconnect the switch, remove non-voltage-resistant chips/capacitors, and use a 500VDC megohmmeter to measure insulation resistance ≥100MΩ before applying the withstand voltage; maintain a temperature and humidity of 15～35℃ and a humidity of 45%～75%.

2. Qualification Criteria: During the voltage stabilization period, there should be no arc flashover, no breakdown tripping, no overheating or smoke, and the leakage current should be stable and within limits.