Abstract : 380V plant auxiliary power is supplied using all-plastic cables. The grounding current flows through a fixed cable support and a circuit formed by anchored flat irons with ferromagnetic properties. The rapid current decay causes incoordination in the protection time coordination between the circuit breakers in the cascaded power supply circuit, resulting in power outages in some busbar sections. To solve this problem, it is recommended to improve the protection device by using a time relay with a wide adjustment range to flexibly adjust the protection time coordination between the circuit breakers. Keywords : Plant auxiliary power, all-plastic cable, grounding current, protection time coordination 1. Problem Statement Currently, 380V plant auxiliary power feeders in power plants generally use all-plastic cables (insulation and sheath are all made of polyvinyl chloride). The grounding current flows back to the transformer through anchored flat irons on the fixed cable support. The flat irons have ferromagnetic properties and very high internal impedance, which varies with the current magnitude. When the grounding point is far from the transformer, the current decays rapidly, causing incoordination in the protection time coordination between the circuit breakers in the multi-stage series feeder. Figure 1 shows a typical 380V plant auxiliary power feeder in a power plant. When the grounding point in Figure 1 is also grounded, the grounding current is 290A, and the breaking time of circuit breaker TG-100B is 18s. Zero-sequence current transformer u-7 starts when the primary grounding current is 12A. The maximum delay of its associated time relay DS-32 is 5s, so the protection device associated with u-7 operates, and circuit breaker ME-630 breaks the circuit first, causing a complete power outage in the MCC section. [b]For details, please click: Analysis and Suggestions on Time Coordination of All-Plastic Cable Feeder Protection[/b]