What Is Fault Transfer Voltage and What Causes It in Low-Voltage Systems？

Fault Transfer Voltage

In low - voltage distribution systems, there is a type of personal electric shock accident where the accident occurrence point and the system fault point are not at the same location. This kind of accident occurs because after a ground fault happens elsewhere, the generated fault voltage is conducted to the metal casings of other equipment through the PE wire or PEN wire. When the fault voltage on the metal casing of the equipment is higher than the human - body safe voltage, an electric shock accident will occur when the human body comes into contact with the metal casing of the equipment. This fault voltage is transferred from other places, so it is called the transfer fault voltage.

There are mainly two reasons why the transfer fault voltage causes the ground fault point and the accident point to be not at the same location:

• A ground fault in the medium - voltage system causes a transfer fault voltage in the low - voltage system;

• The casing of a device in the TN system fails and becomes live, causing the casings of all other electrical appliances to have the transfer fault voltage;

1. Transfer Fault Voltage from Low - Voltage System to Low - Voltage System

In the TN system, the casings of all electrical appliances are connected together. At this time, if one device fails and its casing becomes live, it will also cause a potential difference to the ground to appear on other devices, resulting in a transfer fault voltage.

The type of the low - voltage grounding system is the TN system. When a single - phase ground fault occurs in a low - voltage single - phase outgoing line circuit, the ground fault current passes through the ground fault point, the earth, and the grounding resistance of the distribution transformer and returns to the transformer to form a loop. Due to the large resistance at the ground fault point, the fault current is small and insufficient to make its circuit breaker operate. The fault current passes through the grounding resistance of the distribution transformer, and a fault voltage will be generated on its grounding resistance. This fault voltage will be conducted to the metal casings of the equipment along the PE wire, thus generating a transfer fault voltage and causing the electric shock accident point to occur;

2. Transfer of Fault Voltage from Medium - Voltage System to Low - Voltage System

A 10/0.4 kV distribution transformer should have two independent grounding devices: protective grounding for the transformer and working grounding for the low - voltage system. However, to simplify grounding and cut construction costs, the protective grounding of most medium - voltage distribution transformers shares a single grounding electrode with the working grounding of the low - voltage system. This means that if a tank - shell fault occurs in the medium - voltage part of the distribution transformer, a transfer fault voltage will be induced in the low - voltage system lines and even on the casings of all equipment.

This fault essentially stems from a single - phase ground fault in the medium - voltage system.

When a tank - shell fault occurs in the distribution transformer, a ground fault current is generated. If the low - voltage system uses the TN grounding method, the repeated grounding of the PE wire causes the fault current to split. One part flows back to the earth via the working grounding resistance of the transformer’s low - voltage system, while another part returns to the earth through the repeated grounding resistance along the PE wire before going back to the medium - voltage power source. The fault current passes through the working grounding resistance of the low - voltage system, creating a voltage drop across this resistance. This causes a potential difference between the neutral point of the low - voltage system power supply and the earth. This potential difference propagates to the low - voltage distribution lines, resulting in a transferred over - voltage. In a TN grounding system, this transferred over - voltage can even spread to the casings of all low - voltage equipment via the PE wire.

The magnitude of the fault current depends mainly on the grounding method of the medium - voltage system and the distributed capacitance current. The amplitude of the transfer fault voltage is closely related to the grounding methods of both the medium - voltage and low - voltage systems, with the medium - voltage system’s grounding method being decisive.

Fault transfer voltage amplitude ranking: Small - resistance grounding system > Ungrounded system > Arc - suppression coil grounding system;
A medium - voltage system with a neutral point grounded via a small resistance and a low - voltage system using the TN grounding method are more prone to electric shock accidents, posing a significant threat to users’ personal safety.

Conclusion

• The transfer fault voltage causes the ground fault point and the accident point to be disjoint in two main scenarios: 1) A ground fault in the medium - voltage system induces a transfer fault voltage in the low - voltage system; 2) A faulty, live device casing in a TN system causes transfer fault voltage on all other electrical appliance casings;

• For these two transfer fault voltage types, the ground fault point and the electric shock accident point do not coincide. The grounding point is hard to detect, and the root cause of the transfer fault voltage accident is difficult to analyze. With equipment metal casings charged by the transfer fault voltage, the risk of electric shock to people increases to some degree.