Reasons for internal arc faults

According to incomplete statistics, there are over 200 cases of internal arc faults in medium voltage switchgear every year, causing a large number of casualties and property losses.

Although our switchgear technology and production level continue to improve, and safety measures are constantly strengthened, arc faults still occur and need to be taken seriously.

The GB/T3906 standard lists the causes of internal arc faults

9.103.2 Causes and preventive measures

Experience has shown that :certain areas inside the casing are more prone to malfunctions than others. As a guideline, Table 102 lists the most prone areas to faults based on experience, the causes of internal faults, and possible measures to reduce the probability of internal faults occurring. If necessary, what  requirements should users fulfill for installation, delivery for use, operation, and maintenance.

Table 102  Locations, Causes, and Examples of Measures to Reduce the Probability of Internal Faults

The aging and disrepair of switchgear. Long term condensation and contamination of insulation components can lead to the formation of discharge paths on the surface or internal breakdown. Moisture and dust accumulation on the insulation board can increase local field strength, resulting in breakdown or surface discharge. Switches may refuse to open, causing the circuit to be unable to withstand short-circuit currents and resulting in arc faults.

The contact of the circuit breaker handcart is not aligned properly, not fully engaged, and long-term operation leads to increased temperature rise, resulting in insulation damage and arcing.

Quality issue, the quality of the transformer used is too small, causing arcing due to phase breakdown. Transformers can also cause short circuits and generate electricity due to poor quality.

Foreign object intrusion, such as an explosion in the switchgear of a certain substation, the cable room door being pushed open, and an arc spraying out, did not cause any personal injury due to nighttime. After on-site inspection, it was found that a dead mouse was lying between two phases. The mouse entered the high-voltage chamber, causing electrical breakdown between the two phases and resulting in an arc fault.

The accident reflected insufficient sealing of the switchgear and lack of sealing in the primary and secondary cable trenches, resulting in the entry of small animals. In addition, the bolts of the switch cabinet door lock were not tightened as required, and all bolts of the switch cabinet cover plate and door plate that meet the internal arc IAC AFLR need to be tightened in order to meet the requirements of internal arc faults. If the operator is lazy and does not tighten each bolt, it will cause damage and even personal injury during arc faults.

Operation error: In the past, there were incidents of operators placing the grounding switch handle horizontally on the circuit breaker contact arm of an old switchgear, and then pushing the circuit breaker into the cabinet. Due to the live busbar side, a three-phase arc short circuit quickly formed, causing significant personal injury accidents.

Therefore, the operation of closing the switch cabinet door, that is, only when the compartment door is closed properly and all bolts are tightened in place, can the circuit breaker handcart be shaken into the working position. This is an important safety guarantee measure. During the shaking process, arcing will occur between the contacts. If the insulation is not met, such as when the valve plate is tilted and not fully opened, arcing faults may occur. Therefore, the valve mechanism should be reliable, and the contact arm needs to be insulated.

During the maintenance of the switchgear, if the busbar or incoming side is electrified, and the valve is opened incorrectly, the head or hand may enter the contact box, causing a short circuit discharge of the high-voltage static contact to ground and causing personal injury. This kind of thing often happens. It is necessary to install mechanical locks on the valve, which can only be opened by a circuit breaker or a dedicated handcart.

It is common to accidentally enter the live interval during maintenance, and safety measures such as key interlocking are needed to prevent incorrect entry into the high-voltage live interval.

Interlocking failure, if the circuit breaker is in the working position and the grounding switch is closed, if the grounding switch does not have the ability to close the short-circuit current, it will be unable to close the short-circuit current, causing erosion and arc fault. If the switch is quickly opened again at this time, it may cause the air arc to elongate, expand the arc energy, and amplify the accident. Therefore, if a misoperation is found, the action needs to be completed and not changed, relying on the switch to complete the fault removal.