The hot high-voltage test

The solution is a hot high-voltage (hipot) test during the functional test. You bring the product into its operating state so that all relevant circuits, coils, and relays actually switch. The hipot is then applied while the product is galvanically isolated from the mains via an isolation transformer. This way, you test the insulation under realistic conditions, maximizing the chance of finding faults early.

Cold vs. hot testing: what’s the difference?

• Cold HV test
  The product is de-energized. You mainly test basic insulation between mains-powered parts and enclosure/PE, but circuits that close in operation remain open.
  Advantages: easy to automate, low risk of collateral damage.
  Limitation: not all paths are included.

• Hot HV test
  The product runs in a defined functional mode. Relays and auxiliary supplies are set as they operate in the field.
  Advantages: full circuits under test, higher hit rate on insulation faults, fewer field failures.
  Attention point: requires more test engineering and strict safety measures.

Why an isolation transformer is essential

The isolation transformer provides:

1. Galvanic isolation from the mains for the DUT (safety and noise reduction).

2. Control over reference points: you deliberately define where HV is applied and where you measure.

3. Limitation of fault currents in case of unintended earth faults.

Choose a transformer with:

• Power headroom for the maximum operating state in which you test, plus inrush peaks.

• Low leakage inductance and sufficient short-circuit strength.

• Optionally, multiple secondary taps for different product specifications.

Goal of the hot HV test

• Close all relevant paths that are closed in the field.

• Verify insulation between mains-powered/critical parts and accessible parts/PE under realistic load.

• Find wiring or assembly errors in relays/connectors faster.

• Detect latent defects: compression damage in harnesses, tight creepage/clearance, unintended earth contacts.

Practical workflow: step by step

1. Pre-check: visual inspection, PE continuity, insulation resistance if prescribed.

2. Function start: power the DUT via the isolation transformer, switch to the defined “hot” functional state.

3. Stabilization: wait until relays and supplies are stable (e.g., 1–3 s).

4. HV connect: check test clamps, close interlocks, activate warning.

5. Ramp-up: regulate to test level with a defined V/s.

6. Dwell: hold voltage, log voltage and leakage currents.

7. Evaluation: trip/no-trip, leakage current within limit.

8. Ramp-down and discharge: ensure controlled discharge of DUT capacitances.

9. Result log: write to database, generate label/QR.

10. Return to safe: HV off, DUT to idle.

Integrate in your line: smart automation

• Complete test program one product = one test plan, with all HV parameters and other safety and functional tests.

• PLC/fieldbus: connect your HV tester to line control (I/O, Profinet, Ethernet/IP) for start/ready/fault handshakes.

• Vision or AOI: confirm switch/LED states before enabling HV.

• Data storage: store the measurement data per product, so every test can be traced.

Safety and standards

• Shielded test area with clear signaling and access control.

• Documented work procedure and test risk analysis.

• Periodic calibration and verification of the measurement chain and interlocks.

• Training & authorization for operators and maintenance.

Always follow the relevant product and safety requirements (e.g., machine- and appliance-specific standards). The exact test voltage, leakage-current limits, and times come from the standard applicable to your product. Record the standard references in the recipe.

Design checklist for your hot HV setup

• Correctly size the isolation transformer (power, duty cycle, inrush current).

• HV tester with ramp, programmable limits, data logging, and external interlock.

• Fixture with mechanical interlock, clear contact points, and discharge circuit.

• Safety relay chain with guard, light curtain, or lid switches.

• Operator login with permissions.

Case: why this works in practice

A manufacturer performed a cold HV test for years. Occasionally, fault reports came in due to breakdowns with certain relay combinations. After introducing the hot HV test in a functional mode where other circuits are active, the failure rate dropped to nearly zero. The cause turned out to be a limited creepage distance that only surfaced during the hot HV test. Hot HV testing revealed it; cold testing did not.

Conclusion

If you really want to be sure your product can withstand real-world use, test it hot during the functional test. You close all paths that matter in the field, minimize latent defects, and boost first-pass yield without compromising safety. The key is a well-thought-out recipe, a solid safety chain, and flawless data logging.

The hot high-voltage test can be implemented using the Schleich GLP2-Modular, fully configurable to customer requirements with all desired measurements in a single test system.