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Fault-tolerant distributed power

Apr 14, 2008
You’re on a long ocean passage, alone in a darkened wheelhouse of your power voyaging boat, watching the radar screen, checking the compass heading, trying to stay awake. Have you thought about what you’d do if all of a sudden, the radar screen went blank, or the autopilot died, or some other vital system failed? All those hundreds of feet of wire, running every which way through all those hard-to-get-at nooks and crannies, how would you go about finding the fault?

Perhaps you should consider retrofitting a simple three-wire distributed power electrical system — just three wires with a node (electronic circuit breaker) at each device, controlled from a panel in the wheelhouse. Troubleshooting is infinitely simple: if a node fails, you just go to that device, replace or bypass the node and you’re up and running again.

Traditional boat wiring systems start with a distribution panel of circuit breakers, located about midships. Heavy power cables, No. 4 AWG or so, one positive, one negative, are brought up from the battery bank to the panel. From the panel, two lighter gauge, typically No. 12 AWG duplex, again, one positive, one negative, are run fore and aft, side to side, up to the wheelhouse and the flybridge and down to bilge pumps and blowers, etc., to each and every electrical or electronic device on the boat. Now that’s a lot of wire. It wasn’t so bad when all you had on the boat was a bilge pump, a blower, a depth sounder, a VHF, perhaps an SSB and a few lamps. Nowadays, even a moderately sized power voyaging boat has all this plus GPS, radar, chartplotter, refrigeration, icemaking, TV, stereo and more, with multifunction displays and controls in the wheelhouse, flybridge, and of course, lots more wire.

While these systems start out neat and orderly, they lend themselves to some creative improvisation: a new piece of electronic gear to be added, a faulty circuit to be bypassed, whatever. It’s hard to justify time spent running around picking up the parts, installing another circuit breaker and running another pair of wires, all shipshape and Bristol fashion. The temptation is to pick up 12 volts at the nearest hot wire, splice in, and run the wires by the most direct route, and the result, ultimately, is not pretty or easy to troubleshoot.

The same dramatic growth in electrical and electronic systems and equipment took place in the automotive industry in the late 1980s and ’90s and led to the introduction of electronic control modules (ECMs) and digital switching systems. The goal was to make the vehicle more reliable, safer and more efficient while decreasing wiring harness weight, complexity and cost. Today this technology is widely used, not only in automobiles but also in trucks, buses, ambulances, RVs and even in naval ships, airplanes and spacecraft.
Multiplex power systems
A multiplex power distribution system in its simplest terms, is a three-wire system: a heavy-gauge two-wire bus (one positive wire, one negative wire), plus a light-gauge signal wire that carries digital control signals from one or more switch panels or touch screens in the wheelhouse or wherever.

Picture a bare hull with the two-wire power bus running fore and aft from the battery bank down the centerline of the boat. At a point close to the device you want to supply power to, you simply tap into the bus through a remotely operated “smart” circuit breaker. The signal wire runs from the switch panel along with the two-wire bus and is connected to the electronic circuit breaker or relay — three wires, remote controlled circuit breakers, and some software, and you’ve got a three-wire boat.

Multiplex power systems consist of three key functions: 1. remote switching, 2. control, and 3. intelligence. The remote-switching function is the device that actually turns the load on, off, selects its mode of operation or performs whatever action is indicated. Circuit breakers fulfill this role in traditional wiring systems, electromechanical devices with fixed-trip current levels at which they open the circuit. This is what they do, they do it very well, but that is all they do. Their trip levels are fixed and they are manually operated. New smart circuit breakers are solid-state (metal oxide semiconductor field effect transistor — MOSFET) devices. Smart circuit-protection devices open the circuit on overload currents just like conventional circuit breakers but can also provide current limiting and a programmable internal delay, which allows momentary overloads to be accommodated depending on the type of load — motors, solenoids, etc. They can provide even greater functionality such as remote trip-point setting such that one circuit-breaker model can be used for a wide variety of jobs, with the trip point programmed to the required current level at installation or during operation. This in itself is a great advantage in reducing the number of spares that must be carried. However, they can also provide feedback to the control panel about load currents, open wires or disconnected loads and even be programmed to take low-priority loads offline when the system senses low battery voltage, saving power for important functions like engine starting.

The control function is normally located in the wheelhouse and/or flybridge or on the center console on smaller craft. This may be a bank of digital switches, LCD touch panels or touch screens, programmed to perform the desired function at designated, remotely located smart switches. The intelligence function corresponds to the ECM in automotive systems and is a programmable microprocessor that can vary widely in its capabilities depending on the particular system and the sophistication of the vessels for which it is intended. The intelligence function receives commands from the control function, processes the commands in accordance with the instructions programmed into its microprocessor, and sends a digital signal to the appropriate smart switch.

These systems take different physical forms and degrees of sophistication depending on the particular system and its intended applications. Some manufacturers have based their systems on the controller area network (CAN) protocol, while others have chosen to use proprietary protocols of their own while providing gateways to CAN-based devices on the boat.

Some of these systems are very sophisticated, monitoring and displaying nearly everything on the boat, performing chores like switching equipment on and off, dimming the lights, adjusting the HVAC, measuring tank levels and interfacing with other vessel systems. Two such are Moritz Aerospace’s OctoPlex and MorPlex; and Airpax’s E-Plex systems
OctoPlex and MorPlex
OctoPlex and MorPlex are CAN-based systems that remotely monitor and control all AC and DC power distribution. The key element in the system is the power distribution module (PDM). The PDM contains up to 16 electronic circuit breakers (ECBs), each with a maximum current rating of 30 amps. Two ECBs can be connected in parallel for an output current of up to 60 amps, enough to power just about any load on the boat. After the system is installed, the various ECBs are programmed for trip level, trip delay, alarm, automatic shutdown for load shedding, automatic reset and much more via a laptop using software provided by Moritz. The programmed settings are retained in the PDM’s memory. In case it is necessary to replace an ECB, reprogramming of the replacement ECB is done automatically.

OctoPlex, for vessels in the 65-foot-plus range, and MorPlex, for smaller vessels, are sophisticated, full-featured — and costly — systems utilizing touch-screen displays at various locations on the vessel, providing visibility and control of the ship’s electrical systems from any screen.

Contributing to OctoPlex’s higher cost — and reliability — is the extensive use of redundancy. The system has two separate NMEA 2000 buses, a primary and a secondary, and two microprocessors, each of which can drive either bus, all connected to redundant touch-screen displays and control panels. Should the primary bus fail, the system automatically switches to the secondary bus. Similarly, if the primary microprocessor fails, control immediately shifts to the secondary microprocessor. In addition, should both buses and both data lines fail, both the AC and DC systems will continue to function. While there would then be no remote-status monitoring or remote control of the AC circuit breakers, the AC system, with input power from shoreside or a genset, would continue to operate normally in the manual mode. The DC system would also continue to operate normally, since each PDM has its own independent power supply and that will continue to supply power to the dual microprocessors in each PDM.
E-Plex is the product of ED&D, Inc., a small engineering design and development company based in Oviedo, Fla., specializing in the development of sensing and control systems for the power-generation industry. ED&D was acquired by Airpax, a large manufacturer of sensing, protection and control equipment, in 2005, and shortly thereafter signed a contract with Sea Ray Boats to put E-Plex on its 36-foot bridge sedan model.

E-Plex uses a proprietary networking protocol, and is comprised of a master control unit, and a number of different modules: AC and DC power distribution, interface modules for gensets, engines, HVAC, inverters, etc., and sensor modules for temperature, pressure, tank levels, and even an wireless remote module that can control up to eight devices. Each of these modules has its own programmable microprocessor that interfaces with the bus. Logic functions for the modules are stored in the master control’s memory so that if any module fails and must be replaced, its instruction set is automatically restored. User control is through a variety of displays, everything from touch switches and rocker switch panels to a full-color LCD touch screen that enables the user to interface with the E-Plex system including video and security monitoring.
Capi2, a Dutch company, entered the market in 2005 and quickly closed a deal with African Cats, a European builder of catamaran sail and powerboats, to put its power management system on all their boats.

Capi2 is a master/slave, proprietary protocol-based system in which the master is a push-button panel containing the microprocessor that manages all communication with the nodes each with its own microprocessor. Capi2 has three types of nodes: power nodes with trip limits of 3, 10 and 16 amps; a branch node with trip limits up to 10 amps; and a sensor node that reads temperature, “on/off” switch positions, “open/close” switch, tank levels, etc. Cabling consists of two wires: No. 6 AWG for power, and a control signal wire, No. 16 AWG; simple standard cables available in any port in the world. Capi2 is designed for boats from 35 to 80 feet.
Another approach to cost-effective multiplex power distribution is Cole Hersee’s MiniBus Digital Databus System. Cole Hersee, a long-time established supplier of electrical switches, circuit breakers and battery selector/disconnect switches to the marine industry, introduced the MiniBus in early 2005.

The basic MiniBus system consists of Cole Hersee Sigma rocker switches mounted on a switch panel, connected to a Switch Interface Module (SIM), which in turn is connected to a Power Distribution Module (PDM) by a dual twisted pair shielded cable, and the loads (motors, pumps, lights, etc.), plus the necessary programmable software. Simplicity, reliability and ease of installation make this an excellent system for low-volume and one-off builders, and for boatyards on retrofits and upgrades.

Several other excellent multiplex power distribution systems have established themselves in the market. These systems make great sense for power voyaging boats — they are the inevitable wave of the future. 

Ev Collier is an electrical engineer, an avid voyaging sailor, and powerboater and an amateur boatbuilder.