One of the first questions anyone new to megohmmeter work asks is: *"Which voltage should I actually set?"* Pick too low and the test is meaningless. Pick too high and you risk puncturing insulation that was perfectly fine before you touched it. The good news is, with a **Huazheng insulation resistance tester** — the 10 kV model — you have seven preset ranges and a continuously adjustable 50 V–10 kV custom mode to dial in exactly what the equipment calls for.

## The Seven Preset Voltages — And Where They Fit

Here's the quick field guide most crews end up taping to the side of the case:

- **100 V / 250 V** — control wiring, telecom cables, low-voltage signal circuits. Full-scale resistance up to 500 GΩ.

- **500 V** — standard for equipment rated under 250 V AC. Think house-service panels, small control transformers. Full scale 1.0 TΩ.

- **1000 V** — the workhorse for equipment rated 250 V to 600 V: three-phase motors up to a few hundred horsepower, LV switchgear, factory wiring. Full scale 2.0 TΩ.

- **2500 V** — MV motors and generators up to roughly 5 kV class, distribution-class cables. Full scale 5.0 TΩ.

- **5000 V** — 6 kV–11 kV rotating machines, MV cables, dry-type transformers. Full scale 10.0 TΩ.

- **10 000 V** — 15 kV and 35 kV class power cables, large generator stators, oil-filled power transformers. Full scale 20.0 TΩ.

The underlying formula is simple: **RFS = 2×10⁹ × UN**, meaning full-scale resistance scales linearly with test voltage. That's why you can't just crank everything to 10 kV — you lose resolution on anything mid-range.

## Rule of Thumb from IEEE 43

The most commonly referenced standard, IEEE 43, recommends a test voltage roughly equal to the equipment's rated line-to-line voltage, with a minimum of 500 V for low-voltage gear and 5 kV for machines rated above 12 kV. For acceptance testing on new equipment, many OEMs specify 1.5–2× rated voltage — which is exactly where the Huazheng's 10 V-step custom mode pays off.

## When to Use Custom Mode

The single-button shuttle lets you nudge the voltage in 10 V increments, or fly through in 100 V / 200 V steps depending on how fast you spin. Typical cases:

- A nameplate calls for 600 V — set it to 600 V exactly instead of rounding up to 1000 V.

- Step-voltage (SV) tests where you ramp in defined stages.

- Customer QA documents written to non-standard voltages.

## Don't Forget the Short-Circuit Current

Test voltage is only half the picture. A long 20 kV cable with significant capacitance can pull the voltage down during charging if the meter's short-circuit current is anemic. The Huazheng's **6 mA adjustable output** (1–6 mA in 1 mA steps) solves that — pick 1–2 mA for small samples, bump it up to 5–6 mA when you're testing a full drum of cable.

Get the voltage right, and your PI and DAR numbers will actually tell you something.

## Frequently Asked Questions

**What test voltage should I use for a 400 V motor?**

500 V for routine maintenance; 1000 V is also common and within IEEE 43 guidance. Use 1000 V for acceptance testing on new motors rated up to 600 V.

**What test voltage should I use for an 11 kV motor or generator?**

5000 V (5 kV) for routine testing per IEEE 43. Many utilities apply 2.5 kV for quick DAR checks and 5 kV for full PI tests.

**What test voltage should I use for a 15 kV or 35 kV cable?**

10 kV. The Huazheng tester's 10 kV range with 20 TΩ full-scale resistance is specifically built for this class of MV/HV cable.

**What does RFS = 2×10⁹ × UN mean?**

It is the full-scale resistance formula. Full-scale resistance in ohms equals 2×10⁹ multiplied by the test voltage in volts. At 1000 V, full scale is 2 TΩ; at 10 000 V, full scale is 20 TΩ. Readings above full scale lose resolution.

**Why does short-circuit current matter during insulation testing?**

Capacitive loads like long cables draw significant charging current. If the tester cannot supply it, the output voltage sags and your reading is wrong. The Huazheng's 1–6 mA adjustable output keeps voltage stable even on long MV cable runs.