In the testing procedures for substation maintenance, complete power distribution, and various factory electrical equipment, "reliable and good grounding" is always listed as the primary mandatory safety clause.

When reading the safety precautions for the HZGPNET-1KVA/3KV power frequency withstand voltage test bench of Baoding Huazheng Electric Manufacturing Co., Ltd., many careful electrical engineers will notice this particularly bold and clear technical prohibition:

The grounding terminal of the instrument must be reliably connected to the ground (not to the neutral line)

Why is it necessary to firmly connect the grounding wire to the real earth grounding network during the power frequency withstand voltage test, and it is strictly prohibited to connect it to the "neutral line (N line)" of the three-phase power grid nearby?

Seemingly an ordinary grounding lead, behind it lies the personal safety of the testing personnel and the integrity of the high-voltage measurement circuit:

1、 Zero line (neutral line) electrification and risk of zero disconnection

In power supply systems, it is often difficult to achieve absolute balance of three-phase loads. The neutral line (N-line) often flows unbalanced current, and there is a potential difference of several volts to tens of volts between the neutral line and the ground itself throughout the year; More seriously, once a single-phase short circuit, fuse melting, or virtual disconnection of the neutral line occurs at the front end of the power grid, the potential of the neutral line behind the breakpoint may even drift and rise instantly to 220V phase voltage! If the shell of the voltage withstand test bench and the grounding terminal of the low-voltage circuit are incorrectly connected to the neutral line, once the neutral line floats and becomes charged, the entire metal casing of the instrument and the surface of the test bench will carry a lethal high potential, making it easy for personnel to suffer serious electric shock injuries if touched.

2、 Instantaneous leakage to the ground at the moment of high-voltage breakdown

HZGPNET-1KVA/3KV can output power frequency high voltage up to 3KV (3000 volts). When the tested object (such as motor winding, transformer insulation bushing) experiences strong breakdown and flashover due to defects during the testing process, an extremely steep high-frequency discharge pulse will be generated internally:

If the grounding terminal is firmly connected to an independent grounding grid (grounding resistance is usually required to be less than 4 Ω), the breakdown transient current can quickly dissipate into the deep ground with extremely low impedance, and the casing potential is clamped within the safe voltage range;

If mistakenly connected to the neutral line, the overvoltage at the moment of high voltage breakdown will reverse into the workshop lighting or other single-phase distribution circuits, destroying sensitive weak current equipment such as computers and PLCs on the same circuit.

3、 Standard grounding operation guidelines

Bare copper wires with an area of 2.5 square millimeters or more must be selected on site and tightened onto the dedicated grounding copper bolts of the machine body;

The other end is firmly clamped onto the grounding flat steel or grounding grid pile of the substation with a dedicated grounding clamp;

The power supply socket must be a three hole reliable grounding socket that meets national standards.

Make grounding a habit and engrave the rules in your heart. Not borrowing the midline and not taking chances are the fundamental prerequisites for ensuring the safety of every high-voltage withstand voltage test machine!