The hipot test is done, and HV is off. Can I walk up and touch the test piece now?
Absolutely not. Stored energy in the specimen can be lethal: E = ½ × C × V². Example: 50 nF at 100 kV = 250 Joules (lethal threshold ≈ 10 J).
Mandatory 7-step discharge procedure (IEEE 510 §5.3):
|
Step |
Action |
Duration |
Rationale |
|
1 |
Reduce HV supply to zero (ramp down) |
2–3 s |
Prevents voltage overshoot |
|
2 |
Turn OFF HV supply, disconnect input power |
Immediate |
Prevents re-energization |
|
3 |
Wait (capacitive self-discharge) |
≥ 60 s |
Initial bleed through the bleeder resistors |
|
4 |
Apply a grounding stick with a resistor to HV |
Hold 10 s |
Controlled discharge limits current spike |
|
5 |
Move the stick to the nearest ground point |
2–3 s |
Confirms at ground potential |
|
6 |
Short-circuit & ground HV + LV together with copper braid |
Leave in place |
Prevents recharge from absorption |
|
7 |
Wait 2 min → verify with non-contact detector |
≥ 2 min |
Dielectric absorption recovery voltage |
Grounding stick specs:
• Insulating pole rated for test voltage. Discharge resistor: 5–20 kΩ.
• Ground cable: 16–35 mm² copper braid, length ≤ 5 m.
Dielectric absorption (recovery voltage):
• Paper/oil insulation re-absorbs charge → terminals can re-energize to 20–40% of test voltage within 1–2 min.
• Discharge → wait 2 min → check → if voltage recovered, discharge AGAIN.
• Repeat until voltage stays < 50 V after 2 min.
NEVER:
❌ Touch the HV terminal immediately after the supply is off.
❌ Use a screwdriver/bare wire to short-circuit HV (uncontrolled arc + shockwave).
❌ Discharge without series resistor (damage specimen + arc flash).
❌ Assume cable/TX self-discharged after 30 s (long cables: 5–30 min).