Isolating shore power from on-board electrical systems is important to avoid excess corrosion and, even more importantly, to avoid risk of shock in and around the boat. Two common approaches to isolation are galvanic isolators and isolation transformers. Isolation transformers have many advantages and often are installed in Nordhavns, but have one potential downside though: when first plugged in, a substantial but very short current inrush can occur that can trip shore pedestal breakers.
It’s no big deal, as the problem may occur only once or twice in ten plug-in attempts with a 16A connection and rarely with higher-amperage shore power connections. Many boaters will never plug in an isolation-equipped boat to 16A or less and, for those that do, a breaker only rarely trips and a simple reset will clear problem without another thought.
The problem is intermittent in that it depends where in the shore-power sine wave the load is applied. If the load is connected near the zero crossing of the utility electrical sine wave, there is no problem. However, if applied further away from the zero crossing, the inrush currents can be very high and shore power breakers can trip. These inrush currents are short—far less than a second—but they can be up over 100A. In the old days of low-quality breakers, the short-duration high-current draw went unnoticed. But modern circuit protection devices will trip on even short-duration inrush currents.
For many boaters, this is a just a bit odd in that when plugging in they occasionally trip a breaker but, upon reset, it’s fine. Many attribute this to the act of plugging in causing an “unusual transient” and they don’t give it another thought. But this can be a big problem in some conditions. For example, if a short power interruption occurs when we are away from the boat on an extended trip, the breaker may pop when the power returns. Again it’ll be fine on reset, but in many of these conditions nobody is there to reset the breaker. What we want is a design where we can cycle the power hundreds of times and never see a breaker release.
The solution is a soft start and several very nice commercial solutions to this common problem are available that will work fine for many applications. For example the Charles 93 XFMRSOFT-A 120/240 VAC Soft Start Module and the Mastervolt Soft Start 13kW. Both are explicitly designed to eliminate this problem and both look to be well-engineered. We looked mostly closely at the Mastervolt solution since we have lots of Mastervolt equipment on the boat.
The Mastervolt soft start switches a 25-ohm resistor into the circuit for just under 20 msec on circuit energization and then removes it from the circuit. The resistor is there to slow the inrush and, as soon as that is done, it’s taken back out of circuit. Arguably Mastervolt is charging quite a bit for this simple device but, in the grand scheme of things, it’s not excessive and a packaged solution is by far the easiest to deploy.
We would have done this were it not for two aspects of the design we didn’t like for our specialized application: 1) we need to limit the inrush to not cause breaker release down at 6A or 8A (very low amperage shore services are the norm in some places in the world and we are wired such that we can run the boat from two independent shore services allowing these to work for us), and 2) the Schneider contactor they are using as part of the design is a 50hz rated part but we need to be able to support both 50hz and 60hz shore services. Solving the first problem requires using a slightly higher resistance resistor and the second problem requires a contactor rated for both 50 and 60hz. Both are easily available.
What we’ve built and installed on Dirona, pictured below, is a fairly simple device that puts a 40 ohm resistor in circuit for half second to slow the inrush from a maximum of up over 100A to no more than 6 to 8A. After a half second (500 msec), the resistor is taken back out of circuit.
Right at the location where shore power is brought into a boat, there needs to be an overload and fault protecting breaker before the power is routed to any other electrical device. A well-designed system also include a residual current protection device (RCD) at this point as well. In the pictures below the soft start is installed just above the shore power on-boat breakers. The power flows from the shore power pedestal to the boat through the shore power breaker and the RCD, and then through the soft start before heading to the on-board isolation transformer and the power distribution and breaker panel.
The softstart is installed immediately after the shore power breakers in the lower right hand corner of the pictures (visible in both
—click images for a larger view). From left to right in the lower junction box are the breaker and RCD for each of the three shore power systems supported on Dirona; 1) the 30A @ 120V shore power system, 2) the 16A at 240V second shore power system, and 3) the 50A at 240V primary shore power system. The soft start is connected between the right hand breaker and RCD on the 50A primary shore-power feed between the shore power breaker and the shore power isolation transformer. The top junction box is the soft start system that puts a 40 ohm resistor in series with the shore power for 500 msec (half second) before the soft start system switches the resistor back out of circuit.
If you look closely at the pictures, you’ll see the wires in and out of the soft start are unusually large. These are 6 gauge conductors (nearly 1/6″ of an inch in diameter) with heavy insulation. This seems needlessly large but the circuit is rated to carry up to 50A and can support loads of 12KW so the conductors need to be substantial to avoid excessive voltage loss and heating.
At the point in the project shown in the pictures above, the wires are hanging down below and to the right of the soft start system but they are not yet installed. The next step is to bring down the shore power and run the boat on batteries while the final wiring is completed.
The picture above shows the completed soft start system installed above the shore power breaker box. The City Marina in Amsterdam, where we installed the system, was a particularly good place to test its efficacy since the shore pedestal main breaker releases whenever we apply power. Once powered up, it’ll run fine indefinitely but during power up the breaker just about always pops. Fortunately the power is reliable here, but when some electrical service was done in the marina one day and shore power was shut down and re-enabled, each one caused the pedestal main breaker to trigger. We can now plug in and unplug at will, the power can be turned off and on repeatedly, and everything just works. Wonderful!