Switchboards and Distribution Systems — Main, Emergency, and the Transfer Between Them

What the rule requires

The transfer sequence, and what triggers it

In normal operation the emergency switchboard is not being fed by the emergency generator at all. It is energized from a ship's service switchboard through the bus-tie — on a two-plant arrangement, the forward emergency board from the forward ship's service board and the aft from the aft NAVEDTRA 14104 Ch. 12 — Distribution Switchboards.

The operative trigger is loss of potential at the emergency switchboard, not at the main board, not at the generator. On that loss the bus-tie disconnects automatically 46 CFR §112.05-3, and the emergency generator is automatically placed in operation NAVEDTRA 14075 Ch. 9 — Emergency Power System. Shipboard practice is a normal and an emergency switchboard with automatic bus transfer, so vital loads — steering, navigation, emergency lighting, firefighting — stay powered when the main plant is lost DOE-HDBK-1011 Vol.4 §15-1.

Two further requirements on that bus-tie decide questions:

Parallel operation of the emergency source with any other source is prohibited, the only exception being interlock systems for momentary transfer of loads .
If the tie is arranged for feedback operation — emergency board feeding back toward the main board — it must open automatically on overload of the emergency source before that source is tripped off the line by the overload.

The reason for the feedback provision is the same reason the plant is stripped down: on most vessels the emergency generator does not have the capacity of the ship's service plant. The emergency switchboard can supply power to all parts of the ship, but when the emergency generator is set up in automatic all unnecessary circuits must be stripped from the board, or the generator will be overloaded and the breakers will trip or the diesel will stall .

You do not have to wait for a blackout to prove any of this. There must be a test switch at the emergency switchboard to simulate a failure of the normal power source and cause the emergency loads to be supplied from the emergency power source 46 CFR §111.30-29.

The main switchboard blacks out but the emergency board is still receiving potential through the bus-tie from the other ship's service board. Does the bus-tie open?

No. The automatic disconnect in is keyed to loss of potential at the emergency switchboard. While the emergency board still has potential, its trigger has not occurred. Read the condition at the emergency board, not at the machinery that failed.

What must be on an AC emergency switchboard

Every emergency generator must have an emergency switchboard, and for an alternating-current emergency generator that board must have a circuit breaker meeting §111.12-11, a disconnect switch or link for each emergency generator conductor, and an indicator light connected between the generator and the circuit breaker . The disconnect switch or link is not required where the board has a draw-out or plug-in type generator circuit breaker that disconnects each generator conductor and, if there is a switch in the generator neutral, each ungrounded conductor.

The required instrumentation is a short list worth knowing cold:

An ammeter with a selector switch that connects it to show the current for each phase.
A voltmeter with a selector switch showing generator voltage of each phase but bus voltage of one phase. The asymmetry is deliberate and it is examinable.
A frequency meter.
Ground detection meeting subpart 111.05 for the emergency lighting system.
An exciter field rheostat, and a voltage regulator with a voltage regulator functional cut-out switch.

Nothing in that list is a synchroscope. Compare the main switchboard, which houses the generator and feeder breakers, bus bars, protective relays, and the instruments used to operate and parallel the generators, including a synchroscope, together with the automatic voltage regulator controls, governor controls, and synchronizing equipment DOE-HDBK-1011 Vol.4 §15-2. Paralleling gear belongs where paralleling is done, and on the emergency board it is prohibited.

The generator field rheostat, double pole field switch, discharge clips, and discharge resistor are required only for an emergency generator that is not excited from a variable voltage or rotary amplifier exciter controlled by a voltage regulator unit acting on the exciter field . A direct-current emergency switchboard takes the equipment under §111.30-27(b) through (d) plus ground detection for the emergency lighting system.

Neutrals, overcurrent devices, and emergency lighting switches

Three prohibitions account for a disproportionate share of wrong answers.

The neutral conductor of the emergency-main switchboard bus-tie must not have a switch or a circuit breaker 46 CFR §111.05-37. More generally, a permanently grounded conductor must not have an overcurrent device unless that device simultaneously opens each ungrounded conductor of the circuit.

An emergency lighting system must not have a switch, with three exceptions: in a distribution panel, as required by §112.43-7, or in a circuit serving a hazardous space such as a paint room or cargo handling room where the switch itself is located outside the hazardous location 46 CFR §112.43-1.

Location and installation values

The emergency switchboard must be as near as practicable to the emergency power source, but not in the same space as a battery emergency power source . Where practical, emergency generators and switchboards are installed above the waterline to reduce the danger from flooding, and the emergency plant is sited as far from the ship's service plant as practical so a single casualty does not take out both .

For distribution panels and switchboards under 46 CFR §183.330: as dry a location as practicable, adequately ventilated, protected from falling debris and from dripping or splashing water; totally enclosed and of the dead front type; each switchboard fitted with a dripshield. Working space is at least 610 mm (24 in) in front and at least 455 mm (18 in) behind, and rear access is prohibited where the space behind is less than 455 mm. Nonconducting mats or grating go on the deck in front of every switchboard, and on the deck at the rear where it is accessible from the rear. Uninsulated current carrying parts are mounted on noncombustible, nonabsorbent, high dielectric insulating material. Boards accessible from the rear must be built so a person cannot accidentally contact energized parts, and door-mounted equipment must be constructed or shielded so no one contacts energized parts with the door open and the circuit energized. Sizing follows §111.30-19(a).

Ship's service 450-volt AC switchboards are generally of the dead front type, with no live connections exposed .

A switchboard has 20 inches of clear deck behind it. Is rear access permitted with a tag-out and a matting?

No. prohibits rear access outright where the working space behind is less than 455 mm (18 in), and 20 in exceeds that, so the space itself is adequate — the board may be accessed from the rear, and it then also requires nonconducting matting on the deck at the rear. Read the number against 18 in for the rear and 24 in for the front, and do not transpose them.

Panelboards and the shape of the distribution system

Each panelboard switching unit must be numbered, and each panelboard must have a circuit directory cardholder and a directory listing the circuit designation, a description of the load, and the rating or setting of the overcurrent protective device for each circuit 46 CFR §111.40-11. That last item is the one candidates leave out.

The system runs from generator and main switchboard through feeder breakers to distribution panels and load centers, and finally to branch circuits serving individual motors, lighting, and equipment, with loads connected in parallel so each receives full voltage . A radial layout gives one path to each load: cheap and simple, but a fault or outage upstream kills everything downstream. Ring or loop and selective secondary-network layouts feed critical buses from two directions so one failure need not black out the plant. Every circuit is selectively protected so the device nearest the fault clears it while the rest of the plant stays energized — which is the design intent behind the ship's service arrangement in which a faulty circuit is cut out automatically by the protective devices without interrupting supply to other circuits .

Switchgear is rated for continuous bus current, system voltage, and short-circuit interrupting capacity, and must safely interrupt the maximum fault current available at its location . The plant is documented on a one-line diagram showing generators, transformers, breakers, buses, and major loads with their protective devices — the reference used to plan switching and isolate equipment. Work on a board follows de-energizing, verifying zero voltage, and lockout/tagout of the correct breaker or disconnect, never operating switchgear beyond its ratings or with protection bypassed.

Telling it apart

The criterion is what source the board is designed to be energized from, and whether paralleling is permitted there. Get that right and the required equipment follows.

Main (ship's service) switchboard — energized from the ship's service generators it controls; paralleling is a normal operation, so it carries synchronizing equipment and a synchroscope, governor and AVR controls, and the feeder breakers for the distribution circuits DOE-HDBK-1011 Vol.4 §15-2. On a typical two-board destroyer arrangement each distribution switchboard controls two generators, and the forward board doubles as the control switchboard carrying instruments and controls for the aft generators so the sets can be paralleled and the load equalized NAVEDTRA 14104 Ch. 12 — Distribution Switchboards. Most often misfiled: candidates assume every switchboard on board looks like this one.
Emergency switchboard — normally energized through the bus-tie from a ship's service board, and from its own generator only after the tie opens on loss of potential; paralleling with any other source is prohibited except for momentary load-transfer interlocks 46 CFR §112.05-3. It carries the test switch, the generator indicator light, and the metering listed above 46 CFR §111.30-29. Most often misfiled: treating it as a second main board that can carry the whole vessel's load indefinitely.
Distribution panel / panelboard — energized from a feeder, with no generator control function at all; its requirements are numbered switching units and a circuit directory giving designation, load, and overcurrent device rating 46 CFR §111.40-11. Most often misfiled: forgetting that a distribution panel is precisely where a switch in an emergency lighting system is allowed 46 CFR §112.43-1.

Working a question

The main switchboard suffers a complete loss of power at sea. Take the sequence in order and the answer options separate themselves.

Identify where the trigger is read. Potential is lost at the emergency switchboard because its normal feed is the bus-tie from the ship's service board NAVEDTRA 14104 Ch. 12 — Distribution Switchboards. That is the condition §112.05-3 names.
The tie opens by itself. Automatic disconnect on loss of potential at the emergency switchboard 46 CFR §112.05-3. Any option requiring a hand on the tie is wrong.
The emergency generator starts automatically, and automatic bus transfer connects it NAVEDTRA 14075 Ch. 9 — Emergency Power System, DOE-HDBK-1011 Vol.4 §15-1. Options reading "must be manually started" or "the transfer device must be manually shifted" fail here.
It supplies the emergency bus, not the main bus. The emergency system is a distribution system separate from the ship's service plant , and the tie has just opened to keep the emergency source from paralleling with any other source. An option that has the emergency generator energizing the 450 V section of the main bus is the trap.
Strip the non-vital circuits. The purpose of the system is an immediate, automatic source of power to a limited number of selected vital circuits; leave unnecessary load on and the breakers trip or the diesel stalls.
If the installation is arranged for feedback, verify the tie opens automatically on overload of the emergency source before that source is tripped off the line . This is what stops a feedback attempt from taking the emergency plant down with the main plant.
Prove the whole chain at the board. Operate the test switch at the emergency switchboard to simulate failure of the normal source 46 CFR §111.30-29.

Where candidates lose the point

The emergency generator is answered as picking up the main bus. It attracts because the emergency switchboard can supply power to all parts of the ship NAVEDTRA 14075 Ch. 9 — Emergency Power System. What rules it out is the prohibition on parallel operation with any other source and the automatic disconnect of the tie 46 CFR §112.05-3.

A switch or breaker is accepted in the bus-tie neutral, because isolating every conductor sounds like good practice. The neutral conductor of the emergency-main bus-tie must not have a switch or circuit breaker, and a permanently grounded conductor takes no overcurrent device unless it simultaneously opens each ungrounded conductor 46 CFR §111.05-37.

A synchroscope is chosen as required emergency switchboard equipment, carried over from the main board. It is not in the required list at 46 CFR §111.30-29, and paralleling is prohibited.

The field rheostat, double pole field switch, discharge clips, and discharge resistor are read as universal. They apply only to an emergency generator not excited from a variable voltage or rotary amplifier exciter controlled by a voltage regulator unit acting on the exciter field.

"No switches in emergency lighting" is answered as absolute. Three exceptions stand: in a distribution panel, as required by §112.43-7, and in a hazardous-space circuit where the switch is outside the hazardous location 46 CFR §112.43-1.

The working clearances get transposed, or the rear-access prohibition is read as advisory. It is 610 mm (24 in) in front, 455 mm (18 in) behind, and rear access is prohibited below 455 mm 46 CFR §183.330.

The panelboard directory is answered as circuit numbers and load descriptions. The overcurrent protective device rating or setting is part of the directory too 46 CFR §111.40-11.

Check yourself

You are asked at an inspection to demonstrate that the emergency loads will transfer without blacking out the vessel. What do you operate, and where is it?

The test switch at the emergency switchboard, which simulates a failure of the normal power source and causes the emergency loads to be supplied from the emergency power source 46 CFR §111.30-29. It is a required fitting on the board, so an answer that you must open the main generator breaker is wrong.

The emergency generator is set up in automatic. Non-vital lighting and deck machinery feeders are still made up on the emergency board. What happens on a blackout, and what should have been done?

The generator is overloaded, and either the breakers trip or the diesel engine stalls. All unnecessary circuits must be stripped from the board when the emergency generator is in automatic, because the emergency plant does not have the capacity of the ship's service plant NAVEDTRA 14075 Ch. 9 — Emergency Power System.

A yard drawing shows the emergency switchboard mounted in the emergency battery locker, arguing it is as near as practicable to the emergency power source. Acceptable?

No. The board must be as near as practicable to the emergency power source but not in the same space as a battery emergency power source . The proximity requirement does not override the exclusion.

Working from the voltmeter selector switch on an AC emergency switchboard, how many phases of bus voltage can you read?

One. The voltmeter selector must show generator voltage of each phase and bus voltage of one phase . The ammeter selector, by contrast, shows current for each phase.

A branch circuit feeding a fan motor faults. The feeder breaker at the main switchboard trips and takes out the whole distribution panel. What design principle failed?

Selective protection. Every circuit must be protected so the device nearest the fault clears it while the rest of the plant stays energized DOE-HDBK-1011 Vol.4 §15-1, which is the same intent as the ship's service arrangement in which a faulty circuit is cut out automatically without interrupting supply to other circuits . The branch device should have cleared it.

An emergency lighting circuit serves a paint locker. May it be switched, and if so, where does the switch go?

Yes. A switch is permitted in a circuit serving a hazardous space such as a paint room or cargo handling room, provided the switch is located outside the hazardous location 46 CFR §112.43-1. A switch inside the locker is not permitted.

An emergency switchboard has a draw-out generator circuit breaker that disconnects each generator conductor. Are separate disconnect switches or links still required for the generator conductors?

No, provided the draw-out or plug-in breaker disconnects each generator conductor and, where there is a switch in the generator neutral, each ungrounded conductor . Absent that type of breaker, a disconnect switch or link is required for each emergency generator conductor.