Danger due to increased transformer grounding resistance

According to the power equipment test regulations, the grounding resistance of transformers below 100kVA is not greater than 10Ω, and the grounding resistance of transformers above 100kVA is not greater than 4Ω. However, due to defects in design and construction technology or damage from external forces, the grounding resistance of the transformer grounding point often increases, and the ground wire breaks, which brings certain difficulties to the operation and management of the power supply unit. For this reason, some measures must be taken to prevent the damage caused by transformer neutral wire and ground wire disconnection and ground resistance from rising.

The grounding wire and neutral wire of the transformer are disconnected and the grounding resistance increases;

Due to the non-standard burial of the working grounding body, sloppy installation and construction technology, loose joints between the grounding body and the grounding wire, and too dry ground, etc., all of which may cause the increase of the system grounding resistance.

Due to insufficient understanding of the function and importance of the grounding wire, too few selections of the neutral wire section, when the three-phase load is unbalanced, the neutral wire current is too large, resulting in burning. In addition, the ground wire or neutral wire may be disconnected due to external force or theft of the ground wire.

Elevated phenomena and hazards

1、The grounding resistance of the transformer grounding wire increases, and at the same time, it is grounded with the insulation damage of the phase wire. For example, if the B phase is grounded, a current will flow in the transformer grounding wire at this time, and the B phase voltage is added to the ground and the grounding resistance. At this time, if someone accidentally touches the ground wire or neutral wire of the transformer and the transformer shell, the human body will form a parallel connection with the ground resistance. If the ground resistance is large enough, the voltage applied to the human body will be very high, resulting in electric shock. If the human body accidentally touches phase A or phase C, the voltage applied to the human body will be a line voltage of 380V, which will pose a greater threat to personal safety. In addition, because some users connect the neutral line of the transformer to the shell of the electrical equipment for protective grounding, and the equipment is not insulated from the ground. When phase B is grounded, the ground is at the same potential as B, and the phase voltage is added between the ground and the neutral line. At this time, if the electrical equipment is powered by phase A or phase C, the voltage of the shell to ground will rise to 220V, and the voltage of the equipment to ground will be 380V. At this time, if people touch the shell of the electrical equipment, it will also cause electric shock. In addition, when phase B is grounded, a current will flow from the ground into the chassis and return to the neutral line, and some electrical equipment with higher requirements will not be able to operate.

2、If the grounding wire is disconnected or the grounding wire near the transformer is disconnected, it is as if the grounding resistance has risen to infinity. At this time, the earth potential is the ground phase potential.

3、When the neutral wire of the three-phase four-wire power supply transformer is disconnected, due to the imbalance of the three-phase load, the load ground point will shift, and the potential of the ground point will not be zero, which will cause some phase voltage to rise and burn out the electrical equipment. When the grounding wire breaks or the grounding resistance increases, the grounding wire of the transformer arrester is disconnected or the grounding resistance increases. When the overvoltage is struck by lightning, the arrester cannot discharge to the ground normally, resulting in damage to the arrester or transformer.

HZJD-470A Digital Ground Resistance Tester