柴油发电机组使用常见问题（发电机组的接地与接零） Common problems in using diesel generator sets (grounding and neutral connection of generator sets)

柴油发电机组的接地与接零

为了人身安全和电力系统工作的需要，要求电气设备采取接地措施平常按接地目的的不同，一般分为工作接地保护接地和保护接零三种，接地体是埋入地中并且直接与大地接触的导体。

工作接地

电力系统由于运行和安全的需要，常将中性点接地，这种接地方式称为工作接地工作接地有下列目的：

（1）降低触电电压在中性点不接地的系统中，当一相接地而人体触及另外两相之一时，触电电压为相电压的1.732倍而在中性点接地的系统中，触电电压就降低到等于或接近相电压。
（2）迅速切断故障设备在中性点不接地的系统中，当一相接地时，接地电流很小（因为导线和地面间存在电容和绝缘电阻，也可构成电流的通路）不足以使保护装置动作而切断电源，接地故障不易被发现，将长时间持续下去，对人身不安全而中性点接地的系统中，一相接地后的接地电流较大（接近单相短路）保护装置迅速动作，断开故障点。
（3）降低电气设备对地的绝缘水平在中性点不接地的系统中，一相接地时将使另外两相的对地电压升高到线电压而在中性点接地的系统中，则接近于相电压，故可降低电气设备和输电线的绝缘水平，节省投资同时，中性点不接地也有好处第一，一相接地往往是瞬间的，能自动消除，在中性点不接地的系统中，就不会跳闸而发生停电事故；第二，一相接地故障可以允许短时存在，这样，以便寻找故障和修复。

保护接地 

保护接地就是将电气设备的金属外壳（正常情况下是不带电的）接地，宜用于中性点不接地的低压系统中我们可以分析一下电动机的保护接地。

（1）当电动机某一相绕组的绝缘损坏使外壳带电未接地的情况下，人体触及外壳，相当于单相触电这时接地电流（经过故障点流入大地的电流）的大小决定于人体电阻和绝缘电阻当系统的绝缘性下降时，就有触电危险。
（2）当电动机某一相绕组的绝缘损坏使外壳带电而外壳接地的情况下，人体触及外壳时， 由于人体的电阻与接地电阻并联，而通常人体电阻远大于接地电阻，所以通过人体的电流很小，不会有危险这就是保护接地保证人身安全的作用。

保护接零

保护接零就是将电气设备的金属外壳接到零线上，宜用于中性点接地的低压系统中。再以电动机为例，当电动机某一相绕组的绝缘损坏而与外壳相接时，就形成单相短路，迅速将这一相中的熔丝熔断，因而外壳便不再带电即使在熔丝熔断前人体触及外壳时，也由于人体电阻远大于线路电压，通过人体的电流也是极为微小的。同时注意，中性点接地的系统中不采用保护接地。

Grounding and Zeroing of Diesel Generator Sets

For the needs of personal safety and power system operation, electrical equipment is required to adopt grounding measures. Generally, according to the different grounding purposes, there are three types: working grounding protection grounding and protective grounding. The grounding body is a conductor buried in the ground and directly in contact with the earth.

Work grounding

Due to the needs of operation and safety, the neutral point of the power system is often grounded, which is called working grounding. Working grounding has the following purposes:

(1) In a system where the neutral point is not grounded, when one phase is grounded and the human body touches one of the other two phases, the electric shock voltage is 1.732 times the phase voltage. However, in a system where the neutral point is grounded, the electric shock voltage decreases to be equal to or close to the phase voltage.

(2) Quickly cutting off faulty equipment in a neutral grounded system, when one phase is grounded, the grounding current is very small (because there is capacitance and insulation resistance between the wire and the ground, which can also form a current path), which is not enough to activate the protective device and cut off the power supply. Grounding faults are not easy to detect and will continue for a long time. In systems where the neutral point is grounded due to personal safety hazards, the grounding current after one phase is grounded is large (close to a single-phase short circuit), and the protective device quickly activates to disconnect the fault point.

(3) Reducing the insulation level of electrical equipment to ground. In a neutral ungrounded system, when one phase is grounded, the ground voltage of the other two phases will rise to the line voltage, while in a neutral grounded system, it will be close to the phase voltage. Therefore, the insulation level of electrical equipment and transmission lines can be reduced, saving investment. At the same time, the neutral ungrounded system also has advantages. Firstly, the first phase grounding is often instantaneous and can be automatically eliminated. In a neutral ungrounded system, there will be no tripping and power outage accidents; Secondly, a single-phase ground fault can be allowed to exist for a short period of time, in order to locate and repair the fault.

Protective grounding

Protective grounding is the process of grounding the metal casing of electrical equipment (which is normally non electrified), and is suitable for use in low voltage systems where the neutral point is not grounded. We can analyze the protective grounding of electric motors.

(1) When the insulation of a certain phase winding of an electric motor is damaged, causing the shell to be charged but not grounded, and the human body touches the shell, it is equivalent to a single-phase electric shock. At this time, the magnitude of the grounding current (the current flowing into the ground through the fault point) depends on the human body resistance and insulation resistance. When the insulation of the system decreases, there is a risk of electric shock.

(2) When the insulation of a certain phase winding of an electric motor is damaged, causing the shell to be electrified and grounded, when the human body touches the shell, the resistance of the human body is connected in parallel with the grounding resistance. Usually, the resistance of the human body is much greater than the grounding resistance, so the current passing through the human body is very small and there is no danger. This is the role of protective grounding to ensure personal safety.

Protective grounding

Protective grounding refers to connecting the metal casing of electrical equipment to the neutral wire, which is suitable for use in low-voltage systems with neutral grounding. Taking the electric motor as an example again, when the insulation of one phase winding of the motor is damaged and connected to the casing, a single-phase short circuit is formed, quickly melting the fuse in this phase. Therefore, the casing is no longer charged. Even when the human body touches the casing before the fuse is blown, the current passing through the human body is extremely small due to the high resistance of the human body compared to the line voltage. Please note that protective grounding is not used in systems with neutral grounding.