Shipboard Generators, Motors, and Electrical Safety

What the rule requires

Generator overcurrent protection: the 115 percent figure

Take protection first. Each generator must be protected by an overcurrent device whose set value does not exceed 115 percent of the generator full load rating 46 CFR §183.320. Subchapter J puts the same ceiling on the pickup of the longtime overcurrent trip: 115 percent of the generator rating for a continuous rated machine, or 115 percent of the overload rating where the machine carries a 2-hour or greater overload rating 46 CFR §111.12-11.

Cables read the same number as a minimum. Their current-carrying capacity must be at least 115 percent of the continuous generator rating, or at least 115 percent of the overload where a 2-hour or greater overload rating exists 46 CFR §111.12-9.

What shifts the base figure is the presence of that 2-hour-or-greater overload rating. Absent it, work from the continuous rating and nothing else.

Some questions ask for the subpart rather than the number. Overcurrent protection of generators sits in subpart 111.12; motors, motor circuits and controllers in 111.70; transformers in 111.20; appliances in 111.77 46 CFR §111.50-1.

The generator breaker's trips

Each ship's service and emergency generator is protected by an individual, trip-free, air circuit breaker, adjustable so that its tripping characteristics closely match the generator capabilities . Inside that breaker the trips are not interchangeable. Each answers to its own condition.

Longtime overcurrent trip or relay. Always required, set as necessary to coordinate with the trip settings of the feeder circuit breakers, pickup no higher than the 115 percent ceiling above.
Instantaneous trip. Normally prohibited. Two cases reverse that and make it mandatory: three or more alternating-current generators capable of being paralleled, or a breaker serving a direct current generator. Where fitted, it is set above the maximum asymmetrical short circuit available from any one of the generators that can be paralleled, and as close to that value as practicable.
Reverse-power or reverse-current trip. Required on every generator arranged for parallel operation.

Two behaviours are fixed whatever the trip fit. The breaker opens when the prime mover shuts down, and it does not reclose by itself afterwards.

Location gets tested as well. The ship's service generator overcurrent protective device belongs on the ship's service generator switchboard, and generator and switchboard share one space. A control room inside the machinery casing counts as part of the machinery space for that purpose.

A single 60 Hz ship's service generator, continuous rated. The switchboard breaker has a factory-fitted instantaneous trip. Acceptable?

No. An instantaneous trip is permitted only where three or more AC generators can be paralleled, or where the breaker serves a DC generator . One AC machine falls under neither case. Protection here rests on the longtime overcurrent trip, coordinated with the feeder breakers.

Two generators on one switchboard

Where two or more generators supply ship's service power, each needs an independent prime mover, and the generator circuit breakers are interlocked so the machines cannot be connected to the switchboard at the same time 46 CFR §183.322. The exception carries the weight of the paragraph. Breakers of a generator operated in parallel with another fall outside the interlock requirement, provided the installation meets §111.12-11(f) and §111.30-25(d) of subchapter J.

Read together, then: parallel operation is lawful where the plant is built for it, and a plant not built for it has to be interlocked against it. Reverse-power or reverse-current trips are the hardware that marks the difference.

Where the machine may be installed

Nothing electrical belongs in the bilge. Generators, motors, electric couplings and electric cable in or around the bilge area must be arranged or constructed so that bilge water cannot damage them 46 CFR §111.01-5, and generator cables are barred from the bilges outright . Every generator and motor goes in a location that is accessible, adequately ventilated and as dry as practicable, mounted above the bilges to avoid damage by splash and contact with low lying vapors . Starting motors, generators and any spark producing device are mounted as high above the bilges as practicable 46 CFR §182.410.

Dirt and vapour matter as much as water. Electric equipment must be arranged, so far as practicable, to prevent mechanical damage from accumulated dust, oil vapors, steam or dripping liquids. Apparatus liable to arc must be ventilated, or placed in ventilated compartments where flammable gases, acid fumes and oil vapors cannot collect, with skylights and ventilators arranged to prevent flooding of the apparatus 46 CFR §111.01-3.

Ambient temperature is the other installation figure worth committing to memory. The design figure is 50 °C (122 °F). A machine may instead be designed for 40 °C (104 °F) where the space will not exceed 40 °C under normal operating conditions, and a 40 °C machine may be used in a 50 °C location if it is derated to 80 percent of full load rating with the rating or setting of the overcurrent devices reduced accordingly .

Motors: disconnect, overloads, and what actually burns them

Begin at the disconnect, which is an externally operable switch or circuit breaker 46 CFR §111.70-1. Two motor circuits are lifted out of the general standards and sent elsewhere: steering gear motor circuits and protection to part 58, subpart 58.25, and propulsion motor circuits to subpart 111.35.

In ungrounded three-phase AC systems only two motor-running protective devices (overload coil or heater type relay within the motor and controller) need be used, placed in any two of the ungrounded conductors. That allowance falls away where a wye-delta or delta-wye transformer is used.

A controller has to be designed so that an accidental ground in a remote control circuit cannot leave the stop switches inoperative or start the motor 46 CFR §111.70-7.

Now the machine itself. The induction motor drives most auxiliaries: a three-phase stator sets up a rotating field at synchronous speed (Ns = 120f / poles), inducing rotor currents that drag the squirrel-cage rotor along at a slight slip DOE-HDBK-1011 Vol.4 §12-1. Slip is the difference between rotor speed and synchronous speed, and it never falls to zero in an induction machine, since without it no voltage would be induced in the rotor NEETS Module 5 — Glossary ¶2. Shipboard AC motors mostly run on three-phase, 60-Hz, 450-volt power NAVEDTRA 14104 Ch. 12 — Electric Motors.

Three distinct hazards are guarded against, and mixing them up costs marks. Short circuits are cleared by fuses or by the magnetic trip of a breaker, sized well above inrush. Sustained overload is caught by thermal overload relays set near full-load current. The third group is abnormal conditions: single-phasing, undervoltage, locked rotor. Single-phasing does the worst damage, since the motor keeps turning on two phases at greatly increased current; phase-loss protection is fitted to important machines for that reason. Insulation life halves roughly for every 10 °C of sustained over-temperature, and that figure is the justification for careful overload sizing.

Two other types show up in questions. The synchronous motor runs at exactly synchronous speed on a DC-excited rotor and, when overexcited, supplies leading reactive power to correct plant power factor, at the cost of a separate excitation source and a starting means. The wound-rotor induction motor accepts external rotor resistance for high starting torque and speed control.

DC machines: the commutator and the brushes

What makes a DC generator's output unidirectional is the commutator, standing in place of the slip rings of the AC machine. Its segments, contacted by the carbon brushes, reverse the external connections to the coil at the instant the induced voltage would reverse NEETS Mod. 5 §1-1. Output is regulated by varying the DC field current, not machine speed.

Load current in the armature windings raises a field of its own, and that field distorts the main field. This is armature reaction. It shifts the magnetic neutral plane in the direction of rotation, leaving the brushes to short coils that still carry induced voltage NEETS Mod. 5 §1-2. Interpoles set between the main poles and wound in series with the armature oppose it automatically at any load; large machines add compensating windings in the pole faces.

Sparking at the brushes is the first and most important symptom of trouble in a DC machine, and it wants correcting before the commutator burns. Have four causes ready: a worn or dirty commutator, wrong brush position, weak brush-spring tension, heavy armature reaction.

On DC motors the starter is a large resistance in series with the armature, removed gradually as speed builds. It is there because the armature draws excessive current until it develops counter emf NEETS Module 5 — Glossary ¶1.

An emergency DC generator sparks heavily at the brushes under load but not at no load. Where do you look first?

Sparking that appears only under load points to armature reaction shifting the neutral plane, and to brush position with it: the brushes are short-circuiting coils that still carry induced voltage . Look at commutator surface condition and brush-spring tension in the same visit, since a worn or dirty commutator and slack springs give the same symptom.

Grounding, and the check before the bus

The neutral of a grounded generation and distribution system is grounded at the generator switchboard, and the ground connection has to be accessible so that insulation resistance of the generator to ground can be checked before the machine is connected to the bus 46 CFR §111.05-17. Emergency power generation is arranged differently, and all three of its conditions are testable: no direct ground connection at the emergency switchboard; the neutral bus permanently connected to the neutral bus on the main switchboard; no switch, circuit breaker or fuse anywhere in the neutral conductor of the bus-tie feeder between the two boards.

Equipment grounding cuts shock exposure by holding motor and generator frames, metal bases and structural parts at ground potential NEETS Module 16 §1.5.6. On a steel hull that ground is usually inherent, the cases and frames being in contact with one another and with the vessel's structure. Equipment on shock mounts is the case calling for a separate flexible ground connection. Paint or grease at a bonding surface defeats it. Portable tools use keyed three-prong plugs and outlets, the ground pin should read less than 1 ohm to equipment ground, and no tool goes into service unless you are certain it has a properly grounded conductor.

A main line emergency disconnect switch accessible to an unauthorized person needs a means of locking it in the open-circuit position with a padlock or equivalent, and it must not lock closed 46 CFR §111.95-3. Any cabinet, panel, box or other enclosure fed from more than one source of power carries a sign warning of that condition and identifying the circuits to be disconnected 46 CFR §183.220. Instruments round out the switchboard: a voltmeter and an ammeter for any generator rated at 50 volts or more, plus a means of measuring frequency on each AC generator .

Telling it apart

Three protection methods turn up in hazardous-space questions. What separates them is the space in which the CFR accepts each one.

Explosion-proof. Accepted in both hazardous spaces: a space containing machinery powered by, or fuel tanks for, gasoline or other fuel with a flashpoint of 43.3 °C (110 °F) or lower, and a locker used to store paint, oil, turpentine or other flammable liquids 46 CFR §183.530. The equipment must meet §111.105.
Intrinsically safe system. Accepted in both of those spaces on the same footing, and held to §111.105 likewise.
Ignition-protected. Accepted only in the low-flashpoint machinery or fuel-tank space, where the requirement reads explosion-proof, intrinsically safe, or ignition protected for use in a gasoline atmosphere 46 CFR §182.410. It is not one of the two options offered for flammable-liquid lockers, and the paint-locker fitting is where candidates misfile it.

Working a question

A 40 °C ship's service generator, continuous rated, nameplate full-load current 120 A, is installed in a machinery space that reaches 50 °C in service. What is the maximum setting of its overcurrent device?

Read the nameplate for the rating basis: continuous rated, 120 A full load, designed for 40 °C. Nameplate content is required by Article 445 of NFPA 70, and a derated machine must show its derated capacity 46 CFR §183.320.
Compare space ambient with design ambient. A 50 °C space against a 40 °C machine means the installation stands only under the derating allowance.
Apply the derating, 80 percent of full load rating: 0.8 × 120 A = 96 A. That becomes the machine's rating for every decision downstream.
Apply the 115 percent ceiling to the derated figure, since the rating or setting of the overcurrent devices is reduced accordingly: 1.15 × 96 A = 110.4 A.
Answer 110 A. The 138 A alternative is 115 percent of the un-derated nameplate current, and it is the distractor the derating step exists to defeat.
While you are at the switchboard, confirm the instruments: voltmeter and ammeter, the machine being over 50 volts, and a means of measuring frequency for an AC generator.

Cable sizing under 46 CFR §111.12-9 runs in that same order. Establish the governing rating, then apply 115 percent, bearing in mind that for cable the figure is a floor.

Where candidates lose the point

Applying 115 percent straight to nameplate current and stopping, which yields 138 A in the worked case. Derating to 80 percent comes first when a 40 °C machine sits in a 50 °C space, and the overcurrent setting comes down with it.
Answering that a generator circuit breaker must have an instantaneous trip, on the reasoning that fast clearing is always better. It is prohibited outside the two cases: three or more AC generators capable of being paralleled, or a DC generator.
Reading the interlock requirement of 46 CFR §183.322 as a ban on paralleling. The interlock bars simultaneous connection where the installation is not built for parallel operation. A parallel-capable machine carries reverse-power or reverse-current trips, and the exception applies.
Placing the emergency switchboard neutral ground at the emergency switchboard, by analogy with the main plant. No direct ground connection goes there. The emergency neutral bus is permanently connected to the main switchboard neutral bus, and no switch, breaker or fuse may sit in the neutral conductor of the bus-tie feeder.
Choosing "ignition-protected" for a paint locker fixture because the phrase appears nearby in the regulation. For lockers holding paint, oil, turpentine or other flammable liquids the options are explosion-proof or intrinsically safe.
Answering "check insulation resistance after the generator is on the bus." The ground connection must be accessible for that check before connection.
Blaming the voltage regulator when a DC machine sparks at the brushes. Name commutator condition, brush position, brush-spring tension and armature reaction.
Treating the emergency disconnect padlock as locking either way. It locks open and must not lock closed.

Check yourself

A ship's service generator is rated 200 A continuous, with no overload rating. What is the minimum acceptable current-carrying capacity of its cables, and where may they not run?

Not less than 115 percent of the continuous rating: 1.15 × 200 = 230 A 46 CFR §111.12-9. That section also bars generator cables from the bilges, and cable in or around the bilge area generally must be arranged or constructed so that bilge water cannot damage it 46 CFR §111.01-5.

You are ordered to prepare the No. 2 generator to run in parallel with No. 1. What protective device must that machine's breaker carry that a single-generator installation need not?

Reverse-power or reverse-current trips, required on each generator arranged for parallel operation 46 CFR §111.12-11. That provision, with §111.30-25(d), is also what lifts the interlock against simultaneous connection to the switchboard 46 CFR §183.322.

A 460-volt, three-phase fire pump motor keeps running after one supply fuse opens. What is happening, and why is it more dangerous than a straightforward overload?

Single-phasing, the loss of one of the three supply phases DOE-HDBK-1011 Vol.4 §12-1. The motor runs on the remaining two phases at greatly increased current, so the winding cooks while the machine still appears to be in service. Insulation life halves roughly for every 10 °C of sustained over-temperature. Phase-loss protection is provided on important machines for exactly this reason.

The prime mover on the running generator trips out on low lube oil pressure. What must the generator circuit breaker do, and what must it not do afterwards?

Open upon the shutting down of the prime mover, and not close again automatically after tripping . Reclosing is a deliberate act once the cause is found.

You are fitting a light in a locker used to stow paint and turpentine. An ignition-protected fixture is on the shelf. Do you fit it?

No. Electrical equipment in lockers used to store paint, oil, turpentine or other flammable liquids must be explosion-proof or part of an intrinsically safe system, and that equipment must meet §111.105 46 CFR §183.530. Ignition-protected is offered for spaces containing machinery powered by, or fuel tanks for, low-flashpoint fuel, not for the locker.

You open a motor controller enclosure to work on the starter, having opened the motor's disconnect. What tells you the enclosure may still be live, and what does the regulation require of that arrangement?

The warning sign. Any enclosure containing more than one source of power must be fitted with a sign warning of the condition and identifying the circuits to be disconnected 46 CFR §183.220. Where a control, interlock or indicator circuit above 24 volts cannot be fed from the load side of the motor disconnect, the independent disconnect device sits adjacent to the main one, with a fixed sign on the exterior of the main disconnect door warning the operator to open both 46 CFR §111.70-7.

A portable grinder is drawn from the store for work in the shaft alley. Name the checks on the plug before it goes into the receptacle.

Pins firmly in place and neither bent nor damaged; wiring terminals and connections sound, with loose connections and frayed wires corrected and foreign matter removed; ground pin resistance measured at less than 1 ohm to equipment ground; and the plug aligned before insertion, plugs and outlets being keyed NEETS Module 16 §1.5.6. The tool stays in the store unless you are certain it has a properly grounded conductor.

Two machines are on the switchboard, one 40 °C-rated motor in a 40 °C engine room and one 40 °C-rated generator in a 50 °C engine room. Which nameplate must show something extra?

The generator's. A machine designed for 40 °C used in a 50 °C location must be derated to 80 percent of full load rating with the overcurrent device reduced accordingly, and the nameplate must show the derated capacity 46 CFR §183.320. The motor in a space that will not exceed 40 °C under normal operating conditions is properly rated as it stands.