Ship current transformers CT which steps down the monitored current to a secondary (output) range of 0 to 5 amps (AC) to power the protective relay.
If the monitored current becomes excessive, the relay triggers a contact-switching auxiliary control circuitry to the breaker’s trip coil.

By using the selected time delay between 0.05 and 30s, short current peaks can be bridged.

Connecting the ship current transformer CT to the current monitoring relay terminals is depicted on the relay diagram’s symbol Terminals coded with the letter B represent connections to the current transformer.

Terminals B1 to B3 connect ship current transformer CTs with different step-down current ratios.

C shows another connection terminal, common for any CT ratio.

Example of a current monitoring relay and a related electrical diagram

The power is applied across A1/A2, and the current signal being monitored is applied across B1 and C. Depending on the measurement of the current ratio, the following connections are allowed:
B1/C: 3-30 mA, B2/C: 10-100 mA, B3/C: 0.1-1 A.
The relay operates with changeover contacts, the motor starter control circuits use NO (normally open) contacts in series with stop button contacts. The result Is that an occasional break in the power supply will stop the motor.

The relay can operate both contact groups, either with or without a time delay, depending on the rotary selector switch (Tv) position.
Whatever mode of operation is adjusted for the relay (either instant or delayed), only one relevant reference number will be used throughout a diagram.
Relay set for overcurrent detection (/>)
If the current Is greater than the set threshold, the output relay is energised, with or without a time delay. When the current returns to a value below the threshold, depending on the hysteresis setting, the relay Is Instantly de-energized.

How to set ship relay set for undercurrent detection

If the current is less than the setting threshold IS1, the output relay Is energized, with or without a time delay.

When the current returns to a value IS2 above the threshold, depending on the hysteresis setting, the relay is instantly de-energized.

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Ship monitoring relays

Ship monitoring relays are not intended to substitute fuses or motor protection switches.

Instead, they signal irregularities earlier before the corresponding protective devices are tripped.
Trips from a ship’s thermal relays are linked to:

• Ship power system current imbalance
• Abnormal temperature rise of the PTC (Positive Temperature Coefficient) thermistor probes
• cosfi measurement dropping too low
• Locked rotor and over-torque detection
• Underload
• Earth fault currents
• Phase rotation reversals
• Prolonged starting

Only a few of the trips listed in the control circuits are used onboard ships and are restricted to essential electrical equipment:

• Current monitoring relays
• Voltage monitoring relays
• Phase imbalance and phase sequence relays
• PTC relays

Some other terms commonly used for this subject are:
Starting override – Measured values outside the tolerated range are ignored during startup. This is when the starting override is active. The purpose is to tolerate brief fluctuations, such as motor inrush currents.
Response delay -Threshold value violations are accepted within the delay time. A signal is only generated if the threshold value violation continues beyond the delay time.
Threshold value – If the measured value passes the set minimum or maximum, the relay will trip.
Reset value -The value where the relay picks up again.
Hysteresis – The range between the threshold value and the reset value.

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The central freshwater cooling system with single-stage scavenges air­cooler and a separate HT circuit.

As freshwater is the standard cooling medium of the scavenge air cooler(s), this involves using a central freshwater cooling system.

The central freshwater cooling system comprises “low-temperature” (LT) and “high-temperature” (HT) circuits. Freshwater-cooling systems reduce the amount of seawater pipework and its attendant problems and provide for improved cooling control. Opttmizrng central freshwater- cooling results in lower overall running costs compared to the conventional seawater cooling system.

Cooling Water Treatment

The correct treatment of cooling fresh water- is essential for safe engine operation. Only totally demineralized water or condensate must be used. In the event of an emergency, tap water may be used for a limited period, but afterward, the entire cylinder cooling water system is to be drained off flushed, and recharged with demineralized water.

In addition, the water used must be treated with a suitable corrosion inhibitor to prevent a corrosive attack, sludge formation, and scale deposits. Monitoring the corrosion inhibitor level and water softness level is essential to prevent downtimes due to component failures resulting from corrosion or impaired heat transfer. No internally galvanized steel pipes should be used in connection with treated freshwater because most corrosion inhibitors have a nitrite base. Nitrites attack the zinc lining of galvanized piping and create sludge.

Pre-Heating

To prevent corrosive liner wear when not in service or during short stays m the port, it is important that the main engine is kept warm. Warming through can be provided by a dedicated heater, using boiler-raised steam or hot water from the diesel auxiliaries, or through direct circulation from the diesel auxiliaries.

If the requirement for warming up is from the cooling water systems of the diesel auxiliaries, it is essential that the amount of heat available at the normal load is sufficient to warm the main engine If the main and auxiliary engines have a cooling water system that can be cross-connected. it has to be ensured that when the cross-connection is made, any pressure drop across the mam engine will not affect the cooling water pressure required by the auxiliaries. If the cooling water systems are apart, then a dedicated heat exchanger is required to transfer the heat to the mam cylinder water system.

If the mam cylinder water pump is to be used to circulate water through the engine during pre-heating, the heater is to be arranged parallel with the cylinder water system, and on/off control is to be provided by a dedicated temperature sensor at the cylinder water outlet of the engine The flow through the heater is set by throttling discs but not by valves, to assure flow through the heater.

If the requirement is for a separate pre-heating pump, a small unit with 10% of tire mam pump capacity and an additional non-return valve between the cylinder cooling water pump and the heater are to be installed. In addition, the pimps are to be electrically interlocked to prevent two pumps from running at the same time.

The recommended temperature for stalling and operating the engine is 60 C at the cylinder cooling water outlet. If the engine has to be started below’ the recommended temperature, the engine power should not exceed 80% of CMCR until the water- temperature has reached 60 C.

The ambient engine room temperature and warm-up tune are the key parameters for estimating the heater-power capacity required to achieve the target temperature of 60 C.

The figure shows the warm-up tune needed in relation to the ambient engine room temperature to arrive at the heat amount required per cylinder. The graph covers tire warming up of the engine components per cylinder, also taking the radiation heat into account.

The readable figure is then multiplied by the number of cylinders to show the heater- capacity required for the engine. All the figures are related to the requirements of the engine and should be used only for the first rough layout of the heater capacity. During pre-heater- selection, however, the shipyard or ship designer must also consider other aspects, such as the heat losses in the external piping system, the water volume inside the system, the pipe lengths, and the volume of the ancillary equipment.

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The itemized overview of contactor accessories should help while sending routine ship technical inspections or ship spare orders and when ordering relays or contactors in an emergency.

Check basic ship contactor accessory

1. Basic unit
2. Side-mounted (lateral) auxiliary contact module
3. Pneumatic time module (On- and Off-delay), top-mounted
4. Two pole top-mounted auxiliary contact module
5. Four pole top-mounted auxiliary contact module
6. Mechanical latching module
7. Surge protector

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This addition provides the ordinary relay with time-delay actuation. Time-delay modules are constructed to delay armature motion on coil energisation, de-energisation or both.

A relay with such an accessory becomes a timer and is illustrated with a timer symbol.

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The resulting devices are called mechanically held relays.
Once the closing impulse has been given to the coil (A1-A2), the mechanical latch holds the contactor or the control relay in the closed position without the coil being continually energised.

Interruption of the contactor takes place through a further impulse, which releases the latch, NO (normally open) trip.

The latch reset coil terminals are assigned with the letter’E'(E1-E2).

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Even with a mechanical force against the magnetic system, the contacts of both contactors cannot close simultaneously.

An electrical interlock consists of two interlinked auxiliary NC contacts. While one contractor (eg 1KM1) is energizing, its interlocking contact (1KM1/21- 22) opens, preventing current flow. During this time current flows through the coil of the second contactor (1KM2).

The post What is Mechanical Interlock, Mechanical- Electrical Interlock on a ship’s electrical system appeared first on Electro-technical Officer (ETO).

Two position numbers adjacent to each contact terminal.

Modules can be from either side of the contactor on ship power system. This means that one of the two numbers is the right way up while the other is upside-down.

The post Ship Electrical Side-Mounted Auxiliary Contact Modules (Lateral contacts modules) appeared first on Electro-technical Officer (ETO).

Contacts, carrying such a load are called ‘Main Contacts’.

These are always NO (normally open) contacts and designated by single digits in pairs: 7-2, 3-4, 5-6 (for three pole contactors), and 7-8 (for four pole contactors). In addition to reference numbers, the following reference letter code is used: Li, L2, (.This is shown on the top of the contactor, representing three-phase power supply terminals and T1, T2, T3, from the bottom of the contactor, representing the load side terminals.

Ship electrical contactor arrangement

The main contacts marked with (1) are movable, but the main contacts marked with (2) are fixed.

The auxiliary contacts, (3) fixed and (4) movable, are only used in control circuitry for monitoring purposes.
If the contactor coil is designed to be energized with AC (alternating current), aluminum or copper (shading) rings are crimped to both ends of the magnetic core creating an out-of-phase current that increases the minimum pull on the armature during the AC cycle.

Maintenance of ship electrical contactors

Modern ship electrical contractors are extremely reliable but may have common faults. Worn main contacts due to the arc-effect is a known problem. The operating coil should be opened for internal inspection under recommended routine maintenance systems.
A high standard of maintenance on contactors is paramount to the efficient operation of a vessel. High standards of preventive maintenance will minimize ship inefficiency and related downtime.

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To avoid mistakes it is suggested that the relay order form should include the following information:
• Relay manufacturer
• type of relay
• coil voltage
• number of contact blocks
• contact reference numbers or contact reference letters.
The relay order should be similar to this table.

* Ship Automation for marine engineers and ETOs

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