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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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).

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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.

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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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In addition to basic unit contacts, top-mounted auxiliary contact modules offer two or four contact extensions.

To prevent duplicating terminal markings when NO and NC contacts are combined, the modules will have different position digits.

Picture iustrates the difference between early make (EM) and late break (LB) contacts and make (NO) and break (NC) contacts that are met at top-mounted auxiliary contact modules.

The contacts layout for the module illustrated is, from left to right: 53-54 NO, 61-62 NC, 75-76 NC/
LB and 87-88 NO/EM.

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Relays are used to controlling and regulate circuits for the indirect control of electric motors, valves and clutches.
A modular system is formed around functional basic units, consisting of an AC or DC operated drive (coil with armature) and auxiliary contacts.

The control relay’s basic unit consists of four contact blocks (poles) marked with reference numbers:
• 13-14 (NO contact)
• 21-22 (NC contact)
• 33-34 (NO contact)
• 43-44 (NO contact).

Reference numbers are also shown as a table in most control diagrams, usually below the relay’s coil symbol.
The manufacturer’s stamp should be marked on the unit and reference letters marked adjacent to the terminals (53NO, 61A/C, 75NC, 87A/0).These manufacturer’s numbers are an important reference for reordering or cross-ordering with other manufacturers components.

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Being a solid state there are no moving parts to wear out, no sparking on contact, and no contact corrosion and they can be switched on and off quicker than mechanical relays for massive ship electrical system sound great. However, they are expensive to build for very high current ratings where electromechanical contactors dominate.

Solid state relays will only open the AC circuit at a point of zero current as their inherent hysteresis (memory) maintains current continuity after the LED is de-energised, until It falls below the holding current’. Known as ‘zero-crossover switching’, this is the advantage of uninterrupted operation during the sine wave when using AC power and avoids large voltage spikes.
A problem associated with SCRs is that they are more likely to fail shorted (or closed) rather than open, as is the case of EM relays. As a ‘fail-open’ condition is deemed to be safer, EM relays are often preferred for certain applications.

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The reeds provide a magnetic operating gap and serve as a contact pair providing NO, NC and CO combinations.
Reed relays on ship electrical systems have the advantage of being hermetically sealed and as a result, are protected from atmospheric contaminants.
To make NC or CO contacts, reed switches will occasionally use permanent magnets for magnetic biasing. The current rating of the reed, normally up to one amp, depends on the size, type and amount of plating.

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