introduction
Unlike indoor lighting devices, outdoor lighting devices are more susceptible to electric shock accidents because they are exposed to various harsh environmental conditions and exposed to the public who do not understand electrical safety. Moreover, the outdoor lighting device is usually in a place where there is no equipotential bonding, and under the same fault condition, the outdoor lighting device has a higher contact voltage than the indoor lighting device, thereby increasing the risk of electric shock. Therefore, how to improve the safety of outdoor lighting devices is the first problem that every electrical engineer must solve.
1. Power safety problems and solutions brought by grounding methods
Nowadays, the distribution of buildings is usually based on TN-S and TN-CS systems. However, outdoor landscape lighting is difficult to equipotential bonding. In this way, the electrical equipment often causes the equipment casing to be charged due to insulation failure. When the equipment casing voltage exceeds 50V (25V in rainy weather conditions), an electric shock accident may occur.
The measures to prevent such electric shock accidents in the TN system are usually to set the RCD that can cut off the faulty circuit within 0.4s, but if the 10kV power distribution system and the low-voltage power distribution system share a grounding device, and 10kV is a large current grounding system, when In the event of a single-phase earth fault, the fault voltage is conducted along the PEN line to the end distribution box (TN-CS system) and to the metal housing of the device via the PEPEN line. This voltage is far away due to the inability to do equipotential bonding in outdoor locations. Far above 50V, this will cause an electric shock. At this time, there is no “leakage†in the power supply circuit, and the RCD does not operate, and accidents cannot be avoided. To this end, we can take the following measures:
First, build a local TT system. That is, the line leading to the outdoor only contains the phase line and the neutral line, and an independent grounding pole is provided in the outdoor part without outdoor equipotential bonding, and another ground protection line is taken out.
The protective earth of this part of the electrical equipment, the fault voltage on the power line will not be transmitted to the outer casing of the outdoor equipment. This protective ground has no electrical connection with the system ground of the power supply. When a ground fault occurs, the fault current is small. Setting the instantaneous RCD at the outgoing end of the loop ensures that the power supply is quickly cut off when a ground fault occurs in the loop.
Second, power is supplied by an isolation transformer. The primary and secondary circuits of the isolation transformer can be completely isolated electrically. The secondary conductor of the transformer is not grounded. The metal casing of the equipment is not grounded and is not connected to the PE line. In this way, the ground fault in the secondary circuit When the fault current does not return to the conductor path of the power supply, the fault current and the expected contact voltage are small, and are not connected to the PE line of the power supply end, and the conduction path of the fault voltage of the power supply terminal is cut off, which is not enough to cause an electric shock accident.
Again, use Class II electrical equipment. Class II electrical equipment has double or reinforced insulation. Due to the perfect insulation, it is not necessary to use electrical measures to automatically cut off the power supply and connect the PE wire. Therefore, it is impossible to cause an electric shock caused by the fault voltage along the PE line, so there is no equipotential. Class II electrical equipment can be safely used in connected locations.