Sumitomo Electric has been developing conduction-cooled magnets using Bi2223 wire (DI-BSCCO). Due to high in-field critical current of the wire, the magnets can operate around 20 K. Consequently, iterative excitations and, central field of 10 T or R. T. bore of 300 mm in diameter have been achieved. With respect to reliability of the magnets, quench (thermal runaway) protection might be a problem generally for high temperature superconducting magnets such as BSCCO and REBCO magnets due to the small NZP velocity. Sumitomo Electric confirmed that a bridge circuit with a dump resistor, which detects a quench and dumps the stored energy into the dump resistor, can protect Bi2223 magnets. The experimental coil consists of epoxy-impregnated double-pancake coils between copper cooling plates. The coil was conduction-cooled and then heated around 30 K until it generated a certain voltage. Relationship among the decay time constants, the quench detecting voltages, and the heat loads was investigated. The comparison of different sized coils between inductance 0.4 H and 15 H indicates that the difficulty of the magnet protection depends on the heat production rate of the hotspot, but not on the size of the coil. The operating currents are 200 A, 250 A, and 300 A. The higher operating current, the lower the protectable detecting voltage is. If the decay time constant is long, the protectable heat production rates are almost the same. The results of the experiments have been reflected in the practical design of the magnets.