Diesel Generator Protections. A Quick Check for an Inspector

Greetings! In this article, I recommend that you familiarize yourself with the main protections of diesel generators on a ship, which inspectors like to check. There are many protections on ship diesel generators and one article is not enough to consider them all, so only the main protections will be described, which are most often demonstrated when passing a particular check.

Using the example of the Daihatsu 6DE-18 diesel engine and the Taiyo FE 647A-8 brushless synchronous generator, we will consider the protection of marine diesel generators and how to test them. Other most common diesel engines (Himsen, Yanmar, MAN B&W) found in the merchant marine fleet will also be partially covered.

Usually, the inspector is interested in critical protections that stop the engine (do a shutdown). As a rule, these are three main protections:

OVERSPEED - exceeding the permissible speed of the diesel generator.
L. O. LOW PRESS - low engine oil pressure.
JACKET COOL. F.W. OUTLET HIGH TEMPERATURE - high temperature of the cooling water.

OVERSPEED

Checking overspeed (exceeding safe nominal engine speed) by inspectors is a rare occurrence. Usually, if they want to shutdown the engine with some protection, they give the engineers a choice. Well, the easiest and most harmless way to do it is low engine oil pressure. But nevertheless, you need to be able to demonstrate this protection.

Engine overspeed is a condition in which the engine shaft speed exceeds the permissible value set by the manufacturer. This can occur in both piston engines and turbine units, and is usually associated with a malfunction of the engine management system or external factors.

Causes of overspeed:

Failure of the control system (for example, the fuel supply system or electronic controller).
Reduced engine load, which can lead to it spinning above the rated speed.
Operating errors, such as improper use of braking or protective systems.
External influences, for example, with propeller engines - a sudden decrease in propeller resistance.

Consequences of overspeed:

Mechanical damage - exceeding the permissible speed can cause the destruction of the rotor, blades or other moving parts.
Engine overheating due to increased load.
Reduced engine life and accelerated wear of its components.
Risk of accidents and system failure.

Overspeed protection measures:

Variable speed governors are mechanical or electronic devices that control and limit the engine speed.
Alarm systems warn of deviations.
Fuel safety valves or shut-off valves that limit the fuel supply to the engine.

Daihatsu 6DE-18

There are several options for checking this protection. The simplest is to reduce the OVERSPEED setting of revolutions in the speed relay settings (Speed Relay). It can also be called Speed Switch or Speed Measuring Unit. This relay measures the rotation speed of the diesel generator using a pickup sensor, which reads the revolutions from the diesel flywheel.

Speed Switch MDP-REV Daihatsu

Speed Measuring Unit KOMECO ESP-2000-B Himsen

Usually the procedure is as follows: with the engine running (without load), change the speed settings to trigger the overspeed.

Speed Switch MDP-REV Daihatsu

Using the F (function) button, select the speed setting (overspeed). In this case, 1020 (for Daihatsu 6DE-18).
Using the arrows, lower the overspeed speed setting below the current speed (in this case, below 900) and hold down the S button to save the setting.
An overspeed alarm and engine shutdown will occur.
Then restore the setting to 1020 and save according to the procedure described above.

Speed Measuring Unit KOMECO ESP-2000-B Himsen

Using the MODE (mode) button, select the speed setting (overspeed). In this case, 1035.
Press the MODE button and use the arrows to lower the overspeed RPM setting below the current RPM (in this case, below 900) and press the ENT button to save the setting.
An overspeed alarm and engine shutdown will occur.
Then restore the setting to 1035 and save using the procedure described above.

Video of the OVERSPEED test on a Daihatsu engine

Video of the OVERSPEED test on a Himsen engine (available in the TikTok channel).

There may also be an option to use the "TEST" button, as on the Speed Relay of the Yanmar engine. On the Speed Relay, you can find potentiometers for changing the speed settings, but try not to touch them, use the test button.

Overspeed activation on Yanmar engine

Overspeed triggering on a Yanmar engine. AMS BEMAC

Video of an OVERSPEED test on a Yanmar engine

Another option, which is also more or less simple, is to drive (raise) the fuel rail on the speed controller. This option is suitable if there is no special speed relay, and the speed reading system involves a pickup sensor in conjunction with the PLC of the engine control system.

Video of an OVERSPEED test on the fuel rail of an ANQING Daihatsu engine

The third option is to change the speed setting on the speed controller. Usually, regulators try not to touch, and I also do not recommend doing this. It is better to drive the rail than to change the speed controller settings.

Speed controller (governor) Woodward UG-10D

There may also be another option, when a special limit switch for OVERSPEED RPM is installed, when activated, the protection is triggered. I met such protection on old MAN B&W diesel engines. Checking the overspeed here is simple, it is enough to activate the limit switch on the working engine.

Important! Any checks on engine shutdown should be done without load, i.e. the generator should not be on the buses.

Control post and engine speed controller (governor) Daihatsu

L. O. LOW PRESS (TRIP)

Low oil pressure is usually checked on the corresponding pressure switch. For the inspector, the exact setting of the pressure switch is not important here, it is enough to shut off the oil to the pressure switch and release the pressure. In this case, it is necessary to reduce the oil pressure on the pressure switch below 0.30 MPa (below 3 kg). In this case, the alarm should work and the engine should stop.

L.O. LOW PRESS (Low Oil Pressure) is an alarm (engine shutdown) that signals low pressure of the lubricating oil in the engine system. This situation can be critical, as lubricating oil plays a vital role in preventing friction and overheating of the engine.

Reasons for L.O. LOW PRESS to be triggered:
Insufficient oil level in the system.
Damaged or worn oil pump, resulting in low pressure.
Oil leaking from the system (e.g. due to breaks in the pipe or gaskets).
Clogged oil filter, restricting oil flow.
Problems with the oil pressure sensor or its wiring (false signal).
Excessive overheating of the oil, due to which its viscosity decreases and the pressure drops.
Faulty relief valves in the lubrication system.
Consequences of low oil pressure:
Insufficient lubrication of moving parts of the engine, which can cause their damage.
Increased friction and, as a result, overheating.
Damage to the crankshaft and camshaft bearings.
Risk of complete engine shutdown due to seizure.
Actions when L.O. LOW PRESS is triggered:
Immediately reduce the load on the engine or, if possible, stop it to avoid damage.
Check the oil level in the system.
Inspect the system for leaks or other visible problems.
Check the condition of the oil pump and filters.
Make sure the sensor and its wiring are in good condition (especially if a false signal is suspected).
Report the problem to the duty engineer or chief engineer and follow the instructions.
Important! Running the engine with low oil pressure is extremely dangerous and can lead to serious damage, so actions must be prompt and coordinated.

Low oil pressure switch (TRIP)

Video of low oil pressure switch test (Daihatsu 6DE-18 engine shutdown)

Video of low oil pressure switch test (ANQING Daihatsu engine shutdown)

JACKET COOL. F.W. OUTLET HIGH TEMPERATURE

High coolant temperature is usually checked with a calibrator. It is necessary to get the temperature sensor (thermostat) capillary and heat it to a temperature of 95 °C or higher. The corresponding alarm (shutdown) should be triggered and the engine will stop.

JACKET COOL. F.W. OUTLET HIGH TEMPERATURE is an alarm (engine shutdown) indicating that the fresh cooling water temperature at the outlet of the jacket cooling system of engine is too high. This situation requires immediate attention, as overheating can cause serious engine damage.

Possible causes:

1. Insufficient coolant level:
Leaks in the cooling system.
Incomplete filling of the system.
2. Faulty water pump:
Worn or damaged impeller.
Reduced pump performance.
3. Clogged heat exchanger:
Deposits on the heat exchanger walls.
Fouling on the sea side (if sea water is used for cooling).
4. Faulty thermostat:
Stuck in the closed position.
5. Engine overload:
Prolonged operation at high power levels exceeding the rated power.
6. Low cooling water flow:
Clogged filters.
Pump failure or wear.
7. Temperature sensor problems:

False signal due to faulty sensor or its wiring.

High Coolant Temperature (TRIP) Thermostat

Thermostat for high temperature of cooling water (TRIP)

Video of checking the Danfoss thermostat without a calibrator using a test (stopping the ANQING Daihatsu engine)

Emergency start of a diesel generator

Quite a rare occurrence among inspectors, but they can still check the emergency start of a diesel generator. Usually it is done on the starter using the emergency button on the solenoid.

Starting Magnetic Valve

On the Daihatsu 6DE-18 diesel engine, emergency starting is performed using a magnetic valve (Starting Magnetic Valve). Using a screwdriver, manually open the air supply valve to the starter.

Video of an emergency start of a Yanmar diesel generator

Video of an emergency start of a Yanmar diesel generator using a magnetic coil (part 1)

Video of an emergency start of a Yanmar diesel generator using a magnetic coil (part 2)

By the way, it is interesting that in the video above, the emergency start of the Yanmar engine is performed using a magnet coil, which is put on the solenoid core. This solenoid valve should have an emergency button, but apparently it was replaced with a new one without such a button. Therefore, it was necessary to use such a magnetic coil (this coil is taken from the Unitor kit for testing solenoids in refrigeration systems). This is the case when there is no emergency button, but a way to start was found. And this happens!

Now let's look at some auxiliary engine alarms. As a rule, they are of little interest to inspectors. Only in extreme cases can they check something from the list below, except for F.O. Leakage tank alarm.

F.O. Leakage tank alarm

High level of fuel leakage from the engine. This alarm can be checked on a running engine, it is enough to open the small tank with the float and activate the float manually or pour liquid into the tank.

Fuel leak tank with float

Correctly, it is necessary to pour liquid (diesel fuel) into a glass with a float to make sure that the cylinder (float level switch) is in good condition and not punctured.

Video of checking F.O. Leakage tank alarm on Himsen and ANQING Daihatsu engines (available in TikTok channel).

Auxiliary alarms of Daihatsu 6DE-18 engine:

L.O. Pressure low for T/C (0.20 MPa) - low pressure of turbocharger lubricating oil.
H.T. C.W. pressure low (0.15 MPa) - low pressure of cooling water.
F.O. Pressure low (0.35 MPa) - low pressure of fuel.
L.O. Pressure low (0.35 MPa) - low pressure of engine lubricating oil.
Starting air pressure low (1.5 MPa) - low pressure of starting air.
Control air pressure low (0.60 MPa) - low pressure of control air.

The alarms listed above do not stop the engine, but only send a signal to the local control panel and to the alarm monitoring system (AMS) in the engine control room (ECR).

Checking Daihatsu engine protections on AMS JRCS

LO filter differential pressure switch

LO filter differential pressure switch - engine oil filter pressure difference alarm.

Exhaust gas temperature sensors

Pressure sensor (F.O. Pressure low) and limit switch with cover removed from flywheel

Reverse Power Protection

Perhaps the most basic generator protection that inspectors check is reverse power protection.

Generator Reverse Power Protection is a system designed to prevent reverse current from the power system to the generator. Reverse power can occur when the generator, instead of producing power, begins to consume it from the system. This can lead to generator overload and equipment damage.

How the protection works

1. Reverse power occurs when:
the generator is idling;
the generator engine loses power or stops, but is connected to the bus;
the generator is not synchronized correctly with the bus.
2. The protection system monitors the direction of power flow. If power begins to flow into the generator, the protection is triggered, disconnecting the generator from the bus.

Main components of the system
Reverse power relay - measures the power and determines its direction.
Current transformers - provide measurement of the current in the circuit.
Circuit breaker - opens the circuit when the protection is triggered.
Protection process
When reverse power is detected that exceeds a specified threshold (usually 2-10% of the generator's rated power), the relay generates a signal to turn off the generator.
The shutdown delay time is adjustable to prevent false tripping, for example, during short-term peak loads.
Benefits
Prevents generator overload and damage.
Protects the generator mechanical system.
Reduces the risk of power grid failures.
Setting
The reverse power setting must match the generator characteristics.
The power threshold and delay time are adjusted.

To test the generator for reverse power, the load is usually removed from the generator being tested manually and driven to -10% of the nominal load. To do this, the diesel fuel rail is affected (Position - Less) using the speed controller. When the reverse power threshold is reached, the relay is triggered and the circuit breaker is tripped.

Terasaki TemP2ower AR212S Generator Circuit Breakers

Video of Taiyo FE 647A-8 Generator Reverse Power Test on JACOM JRCS PMS

The video above shows how to test a generator for reverse power on the Reverse Power Protection (JACOM-55) system.

Checking the Standby Diesel Generator

In a typical main power plant design, most ships have three diesel generators. Depending on the operating conditions of the ship, one or three diesel generators can operate in parallel. If one diesel generator is operating, then the second and third are in standby. All this is controlled by the PMS (Power Management System), and it is this system that determines how the generators operate and the standby modes.

Video of PMS BEMAC in operation

Video of PMS TERASAKI GAC21 in operation

Video of PMS JACOM JRCS in operation

Video of PMS DEIF in operation

The operation of the Standby diesel generator can also be checked by inspectors, especially during the "Blackout" scenario simulation.

In my practice, there were such checks. The following happens: one engine is tripped by protection (for example, due to low oil pressure), the ship is completely in blackout mode, except for the systems that operate from emergency batteries. In this case, the emergency diesel generator should start and automatically take over the buses and supply power to the emergency consumers.

On board a vessel, the emergency diesel generator must start and supply power to the emergency buses within 45 seconds in the event of a blackout, as required by the International Convention for the Safety of Life at Sea (SOLAS) and the International Maritime Organization (IMO) standards.

This period includes:

Automatic blackout detection time.
Starting of the emergency generator engine.
Generator stabilization and connection to the emergency buses.

It is important to note that critical systems such as emergency lighting, radio communications and safety management systems must be supplied with power from backup sources (e.g. batteries) until the emergency generator is connected.

At the same time, the first Standby of the diesel generator should be activated, if it does not work, then the second Standby of the diesel generator should be activated automatically. This mode may not include automatic taking over the busbars, so the operator must control this process and, if necessary, take the generator over the busbars.

Power Management System JACOM JRCS

You can also check the operation of the standby diesel generator in other ways without de-energizing the main generator. For example, you can try to start the first standby diesel by blocking the starting air to it, but it will not start. But the second standby should start. Such protection is provided in the PMS Kongsberg NORCONTROL Generator Control Unit GCU.

The second option is to switch the main engine mode to Standby, in which case the auxiliary blowers should start. In this case, the first standby diesel generator should start. You can simulate such a situation for a power plant, while switching the auxiliary blowers to manual mode so that they do not start. Such protection is provided in the PMS JACOM JRCS.

The third option is to check the standby using an additional load. Usually, the first standby starts at 80% or more engine load. For example, if the generator load is about 80% and, for example, a ballast pump or crane is started, then the standby diesel generator should start and automatically take over the buses, only after that the consumer will start. Such protection is provided in PMS DEIF or TERASAKI GAC21.

The fourth option is to check the generator for reverse power, leaving one diesel generator in standby mode. When the circuit breaker of the generator being tested trips, the standby diesel generator starts automatically. Such protection is provided in PMS TERASAKI GAC21.

There are many testing options here, but everything depends on the PMS system and the vessel's power plant. All of the above standby modes can work both in combination and separately.

Oil Mist Detector

Vessels may also have auxiliary diesel generators with oil mist detectors. These are usually installed on the main engines, but there may be exceptions and they depend on the type of vessel and the power plant.

Oil Mist Detector (OMD) is installed on marine diesel generators to prevent crankcase explosions and ensure safety. Its use depends on the following factors:

1. Engine power

OMD is usually installed on diesel generators with a power of:
500 kW and above.
This is due to the increased crankcase volume and higher risk of oil mist accumulation.

2. Classification societies

Many classification societies (e.g. DNV, ABS, Lloyd's Register, Russian Maritime Register of Shipping) require the installation of OMD on:
Medium- and high-speed diesel generators, if the power exceeds certain values (usually from 2.25 MW or higher).
3. Engine type
Medium- and low-speed engines. For these types of engines, OMD is mandatory, since the risk of oil mist explosion is increased due to the large crankcase volumes.
High-speed engines. Typically, OMD is not installed on engines with a capacity of less than 500 kW, since their crankcase volume and the risk of oil mist accumulation are relatively low.
4. International rules

According to the requirements of SOLAS (International Convention for the Safety of Life at Sea):
For main engines and diesel generators on ships carrying dangerous goods, OMD is mandatory.
For ships with engine power above certain limits (usually from 500-1000 kW).

Oil Mist Detector is another engine protection device that attracts increased attention from inspectors. How to check different OMDs, we will consider in the article on checking the main engine protection.

Let's summarize

In the article, we considered the main protections of diesel generators, which are most often checked by inspectors. As you understand, these protections are critical and based on their operation, a conclusion can be made about the safety of the ship's power plant. Normal practice is to check the operation of all diesel generator protections every three months according to the PMS (Planned Maintenance System), as well as before each major inspection in ports.

What interesting cases have you had with checking diesel generator protections on a ship? Any questions or anything to add? Write in the comments to the article!

Manuals! By the way, if you are interested in the manuals for diesel generators and PMS systems, which were described in this article, I recommend our closed telegram channel Marine Engineering Manuals.

Thank you for your attention! I hope this article was useful for you.

Marine Electrical Engineer

09/05/2025

Diesel Generator Protections. A Quick Check for an Inspector

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Diesel Generator Protections. A Quick Check for an Inspector