For the self-powered system, protection from overspeed is provided by a mechanical overspeed switch. The gas shutoff valve (if equipped) is an energize-to-shutoff valve. The valve is activated by voltage from the ignition system.
There are two basic types of protective systems that are powered by an external source. Both systems are energize-to-run systems: manual start/stop and automatic start/stop. An external power source of 24 volts is required for these systems. A gas shutoff valve is also required. An electronic speed switch provides protection from engine overspeed.
For the automatic start/stop, the customer provides the cranking panel and the logic for starting and stopping.
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| Illustration 1 | g00776891 |
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Inside of a typical junction box (1) Terminal strips (2) Electronic speed switch (3) Emergency stop switch (4) Run relay (5) Circuit breakers | |
The junction box contains terminal strips (1) and emergency stop switch (3). An engine oil pressure switch is located at the back of the junction box.
The junction box may contain some other items according to the type of protection system that is used. The items may include the electronic speed switch (2) and run relay (4) for the gas shutoff valve. Circuit breakers (5) help to protect the electrical system from power surges.
The junction box also contains the specific wiring diagram of the engine's electrical system.
Self-Powered Protection System
This system requires no external power for electrical protection. The power is provided by the ignition system when the engine is running. The system is an energize-to-shutoff system. The engine is protected from low engine oil pressure, high water temperature, and overspeed. A gas shutoff valve (if equipped) will shut off gas to the engine when the valve is energized.
The junction box includes a bypass switch for starting, an emergency stop button, and an engine oil pressure switch.
When the engine is stopped, no electrical power is available to the system. The engine oil pressure switch, the water temperature switch, and the mechanical overspeed switch are in the normally open positions.
When the engine is started, the bypass switch must be held in the upward position until sufficient engine oil pressure is achieved. Otherwise, low engine oil pressure will prevent the engine from starting because the voltage from the ignition system will be grounded.
When the engine is running without any problems, the ignition system is not grounded. The engine oil pressure switch, the water temperature switch, and the mechanical overspeed switch are in the normally open positions. When the switches are open, voltage is not supplied to the gas shutoff valve (if equipped).
To stop the engine, turn OFF the gas supply. Press downward on the bypass switch in order to ground the ignition system.
When the engine is running, any of the following shutoffs will cause the ignition system to be grounded through closure of the respective switch:
- Engine oil pressure becomes less than the setting for the engine oil pressure switch.
- The coolant temperature exceeds the setting for the water temperature switch.
- The engine rpm exceeds the setting of the mechanical overspeed switch.
- The emergency stop switch is activated.
No spark is available to the ignition system when the ignition system is grounded. Voltage from the ignition system that flows to any of the closed switches will also energize the gas shutoff valve (if equipped) in order to shut off the gas supply.
This fully independent system requires external DC voltage of 24 volts for electrical engine protection. The engine is monitored for low engine oil pressure, high water temperature, and engine overspeed. The gas shutoff valve is an energize-to-run valve.
The junction box contains an electric speed switch, a relay for the ignition system, an engine oil pressure switch, a circuit breaker, a start/stop switch, and an emergency stop switch. The relay remains energized in order for the engine to run.
When the Start/Stop switch is moved to the START position, the pinion solenoid for the starting motor is activated. The starting motor's pinion engages with the flywheel ring gear.
The current will arm the relay for the ignition system and the gas shutoff valve. Because of insufficient engine oil pressure during start-up, the engine oil pressure switch is bypassed in order to allow the engine to start.
When the setting for crank termination is achieved, the contacts of the switch for crank termination open. The current flow to the starting motor circuit stops.
When the correct engine oil pressure is achieved, the engine oil pressure switch will close. Full protection for the engine is now provided for low engine oil pressure, high water temperature, and engine overspeed.
When the START/STOP switch is released, the spring loaded switch moves to the RUN position.
To stop the engine, turn the START/STOP switch to the STOP position. Current is removed from the gas shutoff valve in order to close the valve. The ignition system is grounded and the electronic speed switch closes.
In an engine that is operating properly, electric current for the gas shutoff valve and the relay for the ignition system flows through the following switches. Any of the following shutdowns will cause the ignition system to be grounded through opening of the circuit for the respective switch.
- Engine oil pressure becomes less than the setting for the engine oil pressure switch.
- The coolant temperature exceeds the setting for the water temperature switch.
- The engine rpm exceeds the setting of the mechanical overspeed switch.
- The emergency stop switch is activated.
No spark is available to the ignition system when the ignition system is grounded. Voltage from the ignition system will not flow to any of the opened circuits. This de-energizes the gas shutoff valve in order to shut off the gas supply.
This system (full system without switchgear) is not an independent system. This system is made for a customer that owns a switchgear. The switchgear is wired into the junction box. The switchgear monitors the engine oil pressure, coolant temperature, and speed.
The junction box contains an electronic speed switch, a relay for the ignition system, an engine oil pressure switch, a circuit breaker, and an emergency stop switch. The gas shutoff valve and the relay for the ignition system remain energized in order for the engine to run.
Procedure After an Engine Shutdown
If the engine is shut down because of an abnormal operating condition, perform the following steps before you restart the engine:
- Determine the cause of the shutdown and correct the condition.
- Make sure that the emergency stop button and the electric speed switch are reset.
- Make sure that the ignition system is grounded. Shut OFF the gas supply to the engine and crank the engine. Crank the engine for approximately 15 seconds in order to clear the unburned gas from the exhaust system.
- Manually reset the gas shutoff valve.