There are a number of monitor points which provide signals to the gas interlock panel and which can cause the supply of gas to be interrupted. The primary purpose of this system is to facilitate the safe handling of a flammable gas. A secondary but equally important function is protection of the detector hardware. Finally, this system serves to help insure the integrity of the data collected by Hall A experiments by alerting the experimenters on shift if a condition arises which might affect the detector gas quality. The conditions monitored are 1) flammable gas leak detection, 2) low main supply pressure, 3) high delivery pressure, 4) high exhaust line pressure, 5) forced airflow in gas shed, 6) over-temperature in gas shed or a shield house, and 7) house fire alarm. The "Kill Gas" buttons in the counting room and in the gas shed also feed into this interlock system.
When a fault condition occurs, power to the solenoid valves controlling gas flow into the gas shed is turned off, closing the valves. An audible alarm sounds in both the gas shed and the counting room, and one or more red lights on the interlock panels in both locations indicate the specific fault detected. The audible alarm may be silenced by pressing the "Alarm Override" button. Note that this does not restore gas flow or clear a fault.
After the fault is cleared it is necessary to activate the "Low Pressure Override" circuit by pressing the corresponding button on the interlock panel in the gas shed. This circuit temporarily disables the "Low Pressure" fault circuit, allowing the solenoid valves to open up and restore gas pressure to the inlet pressure switches. When pressure is restored this circuit automatically resets itself.
Note that the Excess Flow Valves will almost always trip immediately after the solenoid valves are re-opened. This is because of the sudden high flow rate which occurs when the pressure in the gas line downstream of a solenoid valve is low and the solenoid valve is opened with full inlet pressure. To reset the Excess Flow Valves refer to the section Resetting a closed Excess Flow Valve.