什么是接地故障？
A ground fault is an inadvertent contact between an energized conductor and ground or a grounded equipment frame. 故障电流通过的回路是接地系统以及任何因该故障而变为系统组成部分的个人或设备。 接地故障一般为绝缘击穿的结果。 设计和维护潮湿、多尘环境下的电气系统时，接地故障需要更加留意。 因为水为导体，所以会降低绝缘效果，增加发生危险的可能。
接地的目的是什么？
Almost all electrical enclosures are bonded or grounded, in an attempt to eliminate a hazardous potential that could exist on the device relative to the ground or floor that someone may be standing on. In other words, it reduces shock hazards. When we look at a grounded power system, we want to ensure that there is a low resistance grounding path from grounded devices back to the power source. This ensures that if there is a ground fault from an energized conductor to the frame of a piece of equipment, there is a low-resistance path for that fault current to flow back to the source. If the path became high resistance, the frame of the equipment could have a high voltage on it.
电气故障可分为两类：两相故障和接地故障。 Studies have shown that 98% of all electrical faults are ground faults (Source: Woodham, Jack, P.E. "The Basics of Grounding Systems" May 1, 2003). 在保险丝可以防止两相故障的场合，通常需要保护继电器等附加保护，以防止接地故障。
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故障主要因素
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占所有故障的百分比（%）
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接触潮湿环境
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22.5%
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器械、啮齿动物等造成的短路
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18.0%
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接触多尘环境
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14.5%
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其他机械损伤
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12.1%
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接触化学物品
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9.0%
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正常老化
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7.0%
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As an example, in the toaster circuit below, the black or hot wire is shorted to the metal casing of the toaster. 电路闭合后，所有或部分电流先是通过多士炉外壳，然后流过绿色地线。 当电流足够大时（一般为 6 x 15 A = 90 A），电路断路器将断开。 安装保护继电器后可以检测到低至5 mA的微弱电流，从而可以使电路保护器在非常低的电流下断开，它比传统电路断路器反应更迅速。
上例讨论的是直接接地单相电路，但后文讨论的三相电路的原理与其一致。 继电器和监控器是专门设计用来监测电流、电压、电阻或温度等的微弱变化，以发现表1中所示的主要隐患。
哪些问题会造成接地故障继电器意外跳闸？
Harmonics and higher-frequency noise, especially at the third harmonic, can be present in an electrical system and appear as fault current. 电噪声是一个日益严重的问题，因为使用变频设备、逆变器、蓄电池组/UPS以及LED照明的用户越来越多。 为避免误脱扣现象发生，请选择可从测量值中清除谐波频率与其他噪声的优质接地故障继电器。
相比不接地系统，使用接地系统有何优势？
不接地系统的一个主要问题是瞬态过电压的风险。 间歇或电弧接地故障会造成系统中的电压积压，造成应力增加，绝缘劣化，导致电压比标称系统电压要高出6倍。 接地系统的另一个优势是易于找到接地故障。 Ungrounded systems do not provide a path for ground-fault current to flow on the first fault, so current-based solutions are not used to detect a fault. Instead, voltage-based solutions or insulation monitors are used to detect presence of the fault.
However, because the voltage on the faulted phase will be reduced throughout the entire system, locating the fault can be very difficult. 接地系统可利用基于电流的接地故障继电器找到确切的故障点。
故障分为多少种？
There are 3 different types of faults: phase to phase faults, three phase faults and ground faults. 相间故障，即短路，是指过载电流流经电线并将其烧毁。 根据Dunki-Jacobs的教科书统计，95%为接地故障，4%为相间故障，1%为三相故障。
接地故障继电器有何作用？
在电路中，电流会回到其源头。 A current-based ground-fault relay may look for ground-fault current in one of two ways: 1.) Zero sequence. Here, the relay looks at the phase conductors to ensure that all current coming from the source returns on those same conductors. 如果部分电流通过不同的路径（通常为地面）返回源头，接地故障继电器将探测到这一偏差，如果其超出预定电量达到预定时长，接地故障继电器将会工作。2.) 直接测量。 接地故障继电器还可读取变压器中性点和地面连接中的电流（即使采用中性点接地电阻器）。 系统中任何区域的接地故障都会通过此路径返回电流。