Switch Panels
Rule number one: Every circuit on a boat must be protected by a switch and fuse or circuit breaker combination. To wire a load direct from the battery is to invite a potential overload and possible fire.
It’s not uncommon to find installations where the switches and fuses have been randomly located as each new electrical or electronic item has been added. The net result is usually a highly confusing and illogical set-up resulting in a nightmare to troubleshoot. The answer is group switching and circuit protection on common panels. This rationalises wiring, as a single heavy duty feed is taken from the battery to the panel and thence distributed to individual loads.
Rule number two: Before embarking on any wiring, sit down and draw up a schematic diagram for the full circuit. This will come in handy later when someone has to trace individual wiring and will also ensure that you have enough logical switching for all loads.
A separate battery management panel is a prerequisite for larger installations, particularly with more than one battery. This panel will typically include: House on/off, Start on/off, Emergency Parallel, Anchor Winch, Bilge Pumps and Stereo Memory. The reason for including the bilge pumps and stereo memory is that they are circuits which are required to be connected to the battery at all times. The battery management panel will be located as close to the batteries as possible, to reduce high current cable loads. Keep in mind that it must still be in an accessible and dry location, preferably close to the navigation area.
Clearly, all the minor circuits such as navigation lighting will need to be grouped in a logical sequence. This is where a diagram preceding the purchase of the switch panel is invaluable. Also if you are running a solar array or wind generator, a separate panel including the regulators is desirable. If 240 volts is present on board then AC/DC separation is necessary with a dedicated panel. All AC switching must be adequately enclosed.
Ensure that the switches and protection are adequately rated according to the load current. Some panels specify not only maximum current for each individual switch, but also maximum load rating for all circuits. This probably means that the power feed to the switches is through a common link, which makes wiring easier, as only one connection has to be made to the battery supply. As always, take care selecting appropriate cable sizes for individual circuits and battery feed. Twin sheathed, tinned multi strand cable is preferred. The power feed typically requires 5mm² or 10mm² cable.
Generally, switch panels switch only the positive lead. A negative distribution bus or stud distributes current from the battery negative to the load negatives. Mount the negative bus close to the switch panel so that the negative wiring mirrors the route of the positive wiring. Since negative bussing can be a potential source of electrolysis, it is essential to focus on a common bus structure. Specify a much larger bus than required on day one, as you are bound to add more circuits in the future.
Desirable switch panel features
• Water or splash proof, depending on where it is to be mounted. Splash proof panels have rubber boots over the toggle switches. There are also more sophisticated panels that have a continuous rubber membrane covering the entire panel.
• Labels affixed to or engraved on the panel. It is a good idea to sit down and make a list of which labels you will require. Good panels come with lots of alternatives.
• Backlit labels. This means that the labels are illuminated so they can be viewed at night. The circuit for the backlighting should be routed via a switch so that it can be turned off when not needed.
• Panel lighting to indicate which circuits are switched on at any time. These are usually LED indicators drawing negligible current.
Final rule: make sure that the panels are easily accessible and have some means of hinging or removing them. Most importantly, label all the wires behind the panel and leave enough slack so the panel can be removed without tearing off all the connectors.
Circuit Protection Fuses
What happens if your boat has no protection for all the electrical circuits? An interesting exercise (not to be recommended) might be to connect a length of 2mm² cable directly between the positive and negative cables of a battery. The instantaneous result is that the cable will overheat, frying the insulation and resulting in the disintegration of the cable. It’s no coincidence that most boat fires are caused by electrical faults and the prime culprit is a short circuit. Remembering that every circuit in a boat must be protected by a fuse or circuit breaker, the first question to be answered is which of the two to choose?
There is no doubt that the cheapest option is to select fuses rather than circuit breakers. Fuses are very compact and have high accuracy of current rating, very often in smaller increments than available for circuit breakers.
The main disadvantages of fuses include susceptibility to corrosion which can cause voltage drops, and sensitivity to physical damage. In an emergency, many circuit breakers can be manually tripped, whereas removing a fuse in such conditions is often not practicable.
Perhaps the most significant disadvantage of fuses is trying to find a replacement when one of the damned things blows at the most inopportune time.
The subject of rating causes some confusion. It is commonly perceived that the fuse is there to protect the load only. In reality, the fuse is there to protect the complete circuit from the battery to the load. Therefore, the fuse must be rated to the load as well as the cable that it is protecting. The rating may not exceed the maximum current that the cable is able to carry. If you have a pump that normally draws up to 8A, requiring say a 15A fuse, but the weakest cable in the circuit is only rated 10A, than a 10A fuse should be used.
There will be a larger common cable back to the battery from the switch panel, which should also be protected, with a much larger fuse or circuit breaker. For maximum short circuit protection locate the fuse as close to the battery as possible.
Make sure that only the positive side of a DC circuit is protected, never the negative side, which should remain unbroken at all times.
There are many varieties of fuses to suit different applications. Fuses to protect sensitive electronics with little tolerance to over current situations will be fast blow type. Circuits with electric motor loads can experience up to six times normal run current during the start up phase. A slow blow fuse is therefore required, or the fuse will be blown every time the motor is started.
Fuses and matching holders are available rated up to 1000A and more, so there is no excuse not to protect high current circuits such as starter motors.
The most commonly used fast blow fuses found on boats for ratings up to 30A are the 3AG glass fuses and automotive blade fuses. A big advantage of the auto blade fuses is that they are widely available, even from filling stations.
For higher current ratings and heavier cables, Maxi blade and Midi fuses (slow acting, 30A-80A) Mega fuses (slow acting, 80A-300A) and lift truck fuses (eg: Bussman ANL, fast acting 80A-500A) are recommended.
I strongly recommend you use only high quality fuses and fuse holders.
The price of a fuse is negligible relative to the value of the load it is protecting, so it is silly to skimp and save a few cents by buying a fuse of dubious origin, in doing so compromising the protection of the expensive load. Brand names such as Bussman or Littlefuse are recommended
Bad contact between fuse and fuseholder can cause resistance and overheating. Make sure that there is good electrical contact between the fuse and the contacts on the holder, when first installing and during routine maintenance.
Circuit protection is the best line of defence against electrical overloads and possible expensive damage to your boat and equipment. Just remember that setting out in a boat without spare fuses is like leaving without fuel in your tank!
Circuit Breakers
The purpose of circuit breakers is the same as that of fuses, i.e. to protect circuits when overload or short circuit conditions occur. Unlike fuses, they are not destroyed when an exception occurs and can be reset.
The two types of circuit breakers most commonly used on boats are thermal and magnetic-hydraulic breakers.
A thermal circuit breaker includes a bimetallic strip, which is a strip of material that has two different metals fixed together. The two metals expand with heat, but each does so at a different rate. The heat generated by the electric current flowing through the circuit causes the bimetallic strip to bend.
At excessive levels, the strip is bent to an angle that pulls the breaker’s lever down and severs the electrical connection. They are cheap to produce. Their greatest disadvantage is that they are affected by ambient temperature – they will trip with a lower current flowing through them when it is hot than when it is cold.
A magnetic-hydraulic circuit breaker has an electromagnet whose magnetism increases with the current load. The moment the load exceeds that which has been prescribed, the electromagnet’s pull becomes powerful enough to force the circuit breaker’s lever.
The hydraulic element in these breakers allows the designer to vary how soon after an overload occurs the connection is broken. These breakers are more expensive to manufacture than thermal ones, but they are less affected by ambient temperature, shock and vibrations.
One important feature of some hydraulic magnetic breakers is ‘trip free’ action, which means the circuit will trip in the event of an overload even if the levers are held in the ‘on’ position.
Designers can vary the time delay between an overload occurring and the breaker tripping. This enables users to select breakers that trip quickly as soon as excessive current flow occurs e.g. for electronic equipment. However, when protecting a circuit that includes an electric motor, one would want a breaker to only trip if an overcurrent continues beyond the spike that normally occurs during motor starting. A breaker with a longer trip delay would therefore be selected.
Having said that, if a catastrophically high current or a short circuit were to occur, all breakers will trip almost immediately. Manufacturers publish characteristic curves that define what delay occurs at what percentage overload. These are often given names such as ‘standard curve’ or ‘motor start curve’.
Some circuit breakers can only be turned on. There is no way of switching them off other than creating an over current or short circuit situation. There are called ‘reset breakers’.
Others have a lever that enables them to be switched on and off like a switch, or they have a reset lever plus a manual trip button that allows the circuit to be broken (‘switchable’ and ‘manual trip’ breakers).
Most breakers are designed to be mounted through a panel, with the actuator protruding to the front of the panel and the body and terminals behind the panel. Others are designed for surface mounting on e.g. a bulkhead.
Breakers are available with different types and sizes of terminals. It is best to use breakers with screw connections and closed ring cable terminals. The physical size of terminal lugs and the weight of the cable must be taken into account when selecting breakers for high current applications.
Breakers are available in ratings up to 150A. For higher current ratings, fuses should be used.
Breakers are rated to protect either AC or DC circuits or both. Although most AC breakers do also work with DC, stick to the manufacturer’s specification.
For AC installations, double pole main circuit breakers that switch both the active and neutral must be used. Regulations require that a breaker that trips if the polarity is reversed (relay trip breaker) must be fitted to boats. This gives protection in cases where incorrectly wired shore power leads are used. A residual current device (RCD) is also compulsory. An RCD detects any leakage to earth and instantly interrupts the power supply.
Once again, choose only high-quality components for circuit protection. Your valuable investment and life could depend on it. You can rely on breakers from manufacturers such as Airpax, Bussman, Carlingswitch and ETA
*Gavin Sorrell works in collaboration with Aquavolt Electric Boat Parts.
Tel: 02 9417 8455 www.aquavolt.com.au