W:\ELECTRICAL\INSTRUCT\FURNACE\FURNACE GENERAL INSTRUCTIONS 2011.DOC
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9 APPENDIX A - SAFETY EQUIPMENT DESCRIPTION & OPERATION
9-1
MANUAL RESET EXCESS TEMPERATURE LIMIT INTERLOCK (All)
This device will detect and be actuated when the temperature in the furnace work
space exceeds a preset level. On actuation, the manual reset excess temperature
limit interlock will open the control circuit to the main heat. In electrically heated
furnaces the circuit is opened to the heating element contactors (or SCR power
controller) and separate contactors. On gas heated equipment, the control circuit is
opened to the control and safety valves.
To restore operation, the operator must manually reset the excess temperature limit
interlock. This should be done only after determining the cause of overheating and
correcting it. The manual reset excess temperature limit interlock is normally factory
set at 50°F. above the maximum temperature of the furnace or, when known, 50°F.
above your maximum process operating temperature. See the manufacturers
operating instructions for adjusting the manual reset excess temperature limit
interlock.
9-2
SEPARATE CONTACTOR(S) (Electric heat only)
A separate contactor(s) is connected in series with the main control contactor(s) (or
SCR power controller) to open the circuit providing power to the heating elements.
These separate contactors are powered through a series of safety interlocks
including the manual reset excess temperature limit interlock. When any one of the
safety interlocks open, the control circuit to the separate contactors is opened. The
separate contactors provide additional protection which cannot be obtained with the
main heating element contactors (or SCR power controller) alone. The redundant
separate contactors provide a second cutoff device to the heating elements which
does not cycle to maintain temperature and for this reason is less subject to wear.
9-3
MAIN SAFETY SHUTOFF VALVES (Gas heat only)
Two (2) separate gas shutoff valves are connected in series with the gas control
valve to close the pipeline providing gas to the main burner system. When all other
safety equipment indicates proper operation and the gas burner pilot has been
proven ignited, these devices are electrically energized and allow flow of gas to the
main burner system. These gas shutoff valves are powered through a series of
safety interlocks including the manual reset excess temperature limit interlock. When
any one of the safety interlocks open, the control circuit to the gas shutoff valves are
opened. The gas shutoff valves provide additional protection, which cannot be
obtained with the gas control valve alone. The redundant gas shutoff valves provide
gas flow shutoff devices, which do not cycle to maintain temperature and for this
reason are less subject to wear.
9-4
PILOT SAFETY SHUTOFF VALVES (Gas Only)
On burners with a separate pilot burner, two (2) separate pilot gas shutoff valves are
connected in series with the pilot burner to close the pipeline providing gas to the
pilot. When all other safety equipment indicates proper operation and the gas burner
pilot has been proven ignited, these devices are electrically energized and maintain
flow of gas to the pilot burner. These pilot shutoff valves are powered through a
series of safety interlocks including the manual reset excess temperature limit
interlock. When any one of the safety interlocks open, the control circuit to the pilot
shutoff valves are opened.
