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4.2 Safety Testing

Safety testing is a mandatory, non-negotiable step in the final assembly process that verifies the product poses zero electrical hazard to the end user. These tests are the final audit that confirms the enclosure, grounding scheme, and insulation integrity comply with international regulatory standards (e.g., UL, CE, IEC). Failure to perform and log these tests results in non-compliance, legal liability, and safety risk.

4.2.1 Classification and Test Sequence Mandate

The product's safety class dictates the required test sequence and voltages.

A) Device Classification

Product standards require classification based on the protection method:

  • Class I (Earthed): Accessible metal parts are deliberately tied to Protective Earth (PE) via the main power cord.
    • Must-Run Tests: Earth Bond, Insulation Resistance (IR), Dielectric Withstand (Hipot).
  • Class II (Double Insulated): No Protective Earth connection; safety relies on two layers of reinforced insulation.
    • Must-Run Tests: IR, Hipot (Primary – Accessible Metal/SELV).

B) Test Sequence

The test sequence is mandated to prevent equipment damage and ensure reliable results:

Earth Bond – IR (Insulation Resistance) – Hipot (Dielectric Withstand) – (Leakage)

Rule: The system must stop immediately upon any failure. The operator must verify discharge to < 30 V before opening the test enclosure.

4.2.2 Ground Bond Test (PE Continuity)

The Earth Bond Test verifies the integrity and low resistance of the Protective Earth (PE) path from the power input pin to all accessible metal parts of the enclosure.

  • Setup: Test current must be applied from the PE pin to each accessible metal point (trays, lids, heatsinks). 4-wire measurement is preferred.
  • Test Current: A high AC current (typically 10 – 25 A for 1 – 2 seconds per specification) is injected to simulate a fault condition.
  • Mandate: Scrape pads/serrated washers must be used at the bond points. Failures are often caused by paint under washers or loose lugs.
  • Starter Limit: Resistance must be ≤ 0.10 Ω (including test lead compensation).

4.2.3 Insulation Resistance (IR) and Hipot Testing

These tests stress the insulation using different voltage types to find contamination versus structural breakdown.

A) Insulation Resistance (IR)

IR is a gentle DC test that finds contamination (e.g., moisture, flux residue) before heavy stress.

  • Starter Setup: Typically 500 VDC applied for 30 – 60 seconds dwell.
  • Test Pairs: Primary (L+N shorted) – PE/Chassis (Class I).
  • Starter Limit: Resistance must be high, generally ≥ 100 MΩ for benign devices.

B) Dielectric Withstand (Hipot)

Hipot applies high voltage (HV) to verify the structural integrity of the insulation.

  • Voltage Rule: Typically 1000 VAC to 2000 VAC or equivalent DC.
  • Control the Stress: The test must include a ramp-up time of 1 – 2 seconds and a dwell time of 2 – 3 seconds (production) to avoid nuisance trips from capacitive charging.
  • Failure Management: If EMI Y-capacitors or MOVs cause the AC Hipot test to trip: switch to DC Hipot, increase the ramp time, or use the product's designed test bypass (never snip components).
  • Acceptance: No trip at spec; leakage current must be within the defined window (mA scale for AC; µA for DC).

4.2.4 Station Safety and Data Traceability

Due to the high voltages, strict operator safety protocols and rigorous data logging are mandatory.

A) Station Safety Non-Negotiables

  • Interlocks: The test fixture must have an interlocked enclosure where the door locks during HV application. An E-stop must be immediately reachable.
  • Discharge: The system must enforce discharge to < 30 V before the door unlocks; a visible "SAFE" lamp is mandatory.
  • Leads: HV leads must be guarded; alligator clips on painted metal are prohibited.
  • Operator Protocol: The ESD strap must be OFF during HV tests. The "one-hand rule" (one hand only in the cabinet) must be enforced if testing without full enclosure.

B) Data and Audit Mandates

The test system must capture and link all test data to the unit's Serial Number (SN).

  • Data Fields: The log must include the Recipe ID, Class (I/II), timestamp, current/voltage applied, ramp/dwell times, and the resulting worst-case Ω for Earth Bond.
  • Log Integrity: Log results must be stored automatically in the MES; manual edits are prohibited.

Final Checklist

Mandate

Criteria

Verification Action

Earth Bond

Resistance ≤ 0.10 Ω; high current applied.

Scrape pad and serrated washer presence verified; high Ω indicates paint under the washer.

Hipot Integrity

No trip at spec voltage (AC or DC); controlled ramp/dwell time.

Audit confirms test system handles Y-caps without false tripping (e.g., using DC Hipot).

Safety Interlocks

Enclosure interlocked; discharge to < 30 V before access.

Operator protocol ensures ESD strap is OFF during HV tests.

Insulation Check

IR ≥100 MΩ verified (contamination/moisture check).

Test program ensures dry conditions before final IR test.

Traceability

All parameters (Voltage, Ω, current, ramp time) logged per unit SN.

MES must record the Recipe ID and version used for compliance.

Rework Rule

If Hipot fails due to paint under lug, surface must be scraped clean and re-tested.

Failure to zero the surface results in recurrence.


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