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The thermal and electrical behaviour of the current buses in the chimney of the D0 solenoid during upset conditions is modeled to guide the selection of trip levels for magnet protection circuits which discharge the magnet if abnormal conditions are detected. The current buses in the chimney are designed to operate safely without likelihood of loss of superconductivity as long as normal cooling conditions are maintained. Helium liquid level probes, helium flow instrumentation, and thermometry all are provided to certify that proper cooling conditions exist in the subcooler and chimney at all times. Rising temperatures in any portion of the system, excessive voltage drops on the vapor cooled leads, or decreasing liquid level in the subcooler or flow rate in the system, will each cause the fast discharge system to be triggered. Postulated failures of the helium flow system, somehow undetected by any and all of the aforementioned instrumentation, can in principal eventually lead to loss of superconductivity in the buses. Quenching in one bus will rapidly lead to quenching in the other. Potential taps on the buses and magnet coil halves connected to voltage-detection bridges external to the system provide at least dually redundant signals which will unambiguously trigger the magnet rapid discharge system. The conservative design of the bus system ensures that it will not be damaged during such incidents, however improbable they may be. The transition leads in the subcooler are equally conservatively designed, and would not be damaged if they were operated in a fully non-superconducting state for several minutes. The loss of liquid helium in the sub cooler required to cause this condition would imply that helium flow from the magnet had stopped, which in turn would imply that flow to the magnet had also stopped. The lack of flow into the sub cooler would result in insufficient flow to the vapor cooled leads. Any or all of these conditions would be detected, as would easily detected spurious voltages on the potential tap system, before damage to the transition leads occurred

Thermal Safety of the Current Buses inthe Chimney of the D0 Solenoid