  In this paper, an overcurrent protection system is designed to ensure the
safety of the gyrotrons in the EAST ECRH system. Two overcurrent protection
systems were established, a fast one and a slow one. The fast one uses the
current transformers as the current sensors. The models of the current
transformers and the superconducting magnet were built to analyze the effect of
the environmental magnetic field on the current transformers using FI method.
The analysis results show that the magnetic induction at the position near the
current transformers must less than 0.002 T, i.e., the current transformers
should be placed at a distance greater than 2.2 meters from the magnet center
to ensure its normal work. The slow one uses the shunt to monitor the currents.
An anti-fuse FPGA and a timer is used to realize the signal processing in the
fast protection circuit and the slow protection circuit respectively. The
response time of the fast protection circuit is less than 100 ns, and the
response time of the slow protection circuit is less than 31 {\mu}s.
