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26-06-2023, 19:32
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#46
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Registered User
Join Date: Feb 2008
Location: Liveaboard
Boat: Switch 51 #10
Posts: 566
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DC fusing guidelines
I found this nice presentation, DC Fusing Guidelines, somewhere on the Internet years ago. There is no author mentioned, and I could not find it back on the internet.
Therefore I could not share the link, only attach the article itself.
Hopefully the unknown author does not mind me sharing it.
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27-06-2023, 01:50
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#47
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always in motion is the future

Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 20,161
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Re: DC fusing guidelines
Quote:
Originally Posted by Jef & Marin, Netherlands
I found this nice presentation, DC Fusing Guidelines, somewhere on the Internet years ago. There is no author mentioned, and I could not find it back on the internet.
Therefore I could not share the link, only attach the article itself.
Hopefully the unknown author does not mind me sharing it.
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It is a well intended document by its unknown author, but it’s flawed.
Attached is the relevant part from ABYC E11. It gives an absolute maximum at 150% of conductor ampacity and this is okay for motors. If the circuit is not for a motor then the fuse or breaker should be a maximum of 100% of the ampacity.
When there is no such fuse or breaker available, then a larger value may be used as long as it’s not more than 150% like for motors.
Also, the rule of thumb used by me and all other EE’s I know is indeed to start with an expected current but then multiply that value by 1.5 and then look at the tables for ampacity and voltage drop for the circuit to be installed, then decide on the size of the wire. After that, the branch circuit is protected by a breaker or fuse that protects that size wire.
This is where all the 15A breakers come from. Most circuits don’t need more than 15A and even 18AWG can handle 15A so it almost always fits.
__________________
“It’s a trap!” - Admiral Ackbar.
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27-06-2023, 03:15
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#48
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Senior Cruiser

Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 51,858
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Re: Dumb questions about breakers
Quote:
Originally Posted by catsketcher
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The hydraulic magnetic breakers used in most Blue Seas systems are pretty pricey. I can find thermal Carling breakers that are much cheaper and could use them as switches...
Phil
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Quote:
Originally Posted by s/v Jedi
... I just default to recommending to change nothing to the original design.
Not only did they have many technical reasons for doing things the way they did, but also compliance with standards is only guaranteed without any modifications.
For cost: if you find the toggle breakers too expensive, you can use other breakers. I also use these and they are only a couplebucks each: https://www.bluesea.com/products/cat...ton_Reset-Only
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A circuit breaker is an electrical device, designed to automatically detect, and eliminate [switch, “trip”, or rapidly open], short circuits and overloads.
In an overload condition, there's a temperature buildup between the insulation and conductor. If left unchecked, the insulation's life will drastically reduce, ultimately resulting in a short circuit. This heat is a function of the square of the rms current (I squared), the resistance in the conductor (R), and the amount of time the current flows (t).
Usually, a short circuit occurs when abnormally high currents flow as a result of the failure of an insulation system. This high current flow, termed short-circuit current, is limited only by the capabilities of the distribution system. To stop this current flow quickly, so that major damage can be prevented, the short circuit [or instantaneous] element of an MCCB is used.
Blue Sea “21xx” series Circuit Breakers - Push Button Reset-Only, as linked by Jedi, are a thermal trip type.
Thermal Trip: Made up by a bi-metalic strip, which actuates the opening of a circuit breaker, with a delay, depending on the overcurrent value [& ambient temperature]. This trip unit is intended for protection against overloads.
Magnetic Trip: Made up by an electromagnetic coil & solenoid, with a fixed threshold, which actuates an instantaneous trip (about some tens of milliseconds) of the circuit breaker, on a pre-determined overcurrent value (multiple of the In). This trip unit is intended for protection against short circuit.
Hydraulic-Magnetic Trip: These protectors are combined with a hydraulic delay, to make them tolerant to inrush current. They are wired in series with the breaker contacts, and used magnetics to actuate a piston, slowed by an oil “dashpot”, thus providing an “inverse time” trip response.
Thermal-Magnetic: Most common type of trip unit in use. Uses a bi-metallic element to sense [the heat, “L”] overloads, and an electromagnet to sense [high current, “I”] short circuits.
The magnetic element of a thermal magnetic circuit breaker is a coil of wire wrapped around a plunger [solenoid]. The magnitude of current, through
the coil, causes the plunger to release the trip bar thus tripping the breaker. The magnetic element is an instantaneous trip element, as it is not delayed and will only operate at a fixed level of current.
The thermal element in a thermal magnetic trip breaker is a bi-metallic strip which bends in proportion to its temperature. When the temperature becomes high enough, the bending strip will actuate a “trip bar”, which releases the opening springs of the circuit breaker, and opens the breaker
contacts.
__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"
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27-06-2023, 07:41
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#49
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Registered User
Join Date: Sep 2014
Location: Laporte, Texas
Boat: Allied 42 XL2
Posts: 75
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Re: Dumb questions about breakers
I have not seen anything discussing electrical equipment in a marine environment. If the OP decides to go with toggles and fuse blocks, he needs to ensure he purchases pieces and parts intended for use in boats. Automotive style toggles are neither going to be water resistant nor corrosion resistant. Fuse blocks, even some of those sold by Blue Sea are going to have the same potential problems. Then there are the extra connections needed to tie the toggles and fuses together, all targets for corrosion and increased impedance. My boat was built with numerous terminal blocks scattered around the boat to connect various devices, both AC and DC. None of them were protected from corrosion and almost all of them had to be replaced and the crimp-ons redone to remove corroded connections and reduce impedance in the circuit.
The inline fuses supplied by instrument manufacturers are there to protect the smaller gauge wire provided by the manufacturer and to protect the device if the device does not have internal protection.
It is a boat, water will get inside. The fewer the number of connections; especially, the exposed connections, and the more water resistant the equipment, the better off you will be in the long run.
One last comment and I will be done with my two cents worth. Battery voltage in a boat is not constant and can vary depending on usage and charging equipment. That said, the power supplies for newer instruments automatically adjust for a wide range of voltages; for example, the Raymarine Smart Pilot can accept 10 to 32 volts, the Raymarine p70 controller will accept down to 11 volts, and a Garmin 743 chartplotter will accept down to 10.5 volts before it shuts off automatically. As wattage or power required by a device will remain constant, when battery voltage goes down, amp draw goes up and, conversely, a higher voltage will reduce amps. The point being that the amp draw of a device can vary depending on the state of the battery, so designing an electrical system to the minimum required is not necessary, the manufacturers have already addressed this situation.
Cheers and good luck with your project.
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27-06-2023, 10:33
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#50
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Registered User
Join Date: Jul 2020
Location: Virginia, USA
Boat: Hunter 340
Posts: 1,471
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Re: DC fusing guidelines
Quote:
Originally Posted by Jef & Marin, Netherlands
I found this nice presentation, DC Fusing Guidelines, somewhere on the Internet years ago. There is no author mentioned, and I could not find it back on the internet.
Therefore I could not share the link, only attach the article itself.
Hopefully the unknown author does not mind me sharing it.
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The diagrams are useful and may actually help some people learn concepts but the advice to use an OCP of 125% of rated wiring capacity on page one is very poor advice.
Breakers and fuses don't instantly trip at 100% of rated capacity instantly So put 100A or even 115A amps on a 100A breaker/fuse for 30 seconds and it will not trip. There already is a margin built in due to the nature of OCP. There is a time vs over current graph for all fuses/breakers. Rapid trips like <1 second involve not 100% of rated capacity but usually 300% to 500%+ of rated capacity.
Putting a 125% margin on top of that could greatly increase the trip duration in a overload and in fact it could increase the trip duration to infinite as in the breaker will never trip.
Someone getting a nuisance trip on a 15A breaker on a 15A circuit with normal loads around 10A is very likely not due to the rating of the breaker. It could be high contact resistance or damaged equipment or the breaker has simply worn out (it happens). Yes switching the 15A breaker with a 20A breaker may make the issue go away but you are just hiding the real problem and it is still there and now the wiring is under protected.
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27-06-2023, 12:22
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#51
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Registered User
Join Date: Sep 2014
Location: Laporte, Texas
Boat: Allied 42 XL2
Posts: 75
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Re: Dumb questions about breakers
When I was learning electrical wiring practices, the following rule was standard practice:
To determine the size of a circuit breaker, you need to follow these steps12:
1. Calculate the amp draw using the basic electric power equation.
2. Multiply the amp draw by 1.25 to account for the 80% breaker rule. The resulting amps are the minimum ampacity a correctly sized circuit breaker should have.
To further confuse the issue:
The 80%/100% rule is reflected in the NEC codebook, where it states specifically that unless the breaker is tested and labeled for use at 100% of its continuous current rating, it may have only a maximum of 80% of its labeled continuous current rating flowing through it in normal operation. Continuous is considered 3 hours at rated load.
In other words, a 10 amp breaker, that has not been tested and certified and so marked for continuous use should only be loaded to 8 amps under continuous operation; 10 amps if it has been certified for continuous use.
To keep it simple, if you have a 7 amp continuous load, use a 10 amp breaker, a 13 amp continuous load use the next size up or 15 amp, with respective appropriately sized wire. For loads with motors; such as, autopilots, watermakers, A/C units and similar loads that start and stop motors, factors may have to be applied to address the instantaneous motor startup loads which can be 4 times higher than normal running loads. This surge load is partially addressed with the normal, built-in time lag to trip in a circuit breaker. Fortunately, newer A/C units are being built with soft start controls and staged startup of the three major components in order to reduce the initial surge.
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27-06-2023, 12:45
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#52
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Registered User
Join Date: May 2015
Location: Muskegon, Mi
Boat: Columbia 36
Posts: 1,299
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Re: Dumb questions about breakers
Quote:
Originally Posted by rslifkin
If it's going to be a single device circuit, I see no reason not to down-size the breaker closer to the expected draw, that way it can offer some protection to the device as well as the wire.
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It is acceptable to fuse/breaker a circuit to the needs of the only device on that circuit. But... You can then never add an additional device to the circuit or switch to a new device with higher current rating. I'm with the others saying breaker the circuit at 10 or 15 amps with appropriately sized wires and use fuses at the devices to protect them. Most of these devices come with a fuse holder in the positive feed wire for a reason.
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27-06-2023, 13:37
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#53
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Moderator

Join Date: Jul 2007
Boat: Bestevaer.
Posts: 15,388
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Re: Dumb questions about breakers
Quote:
Originally Posted by capt jgw
It is acceptable to fuse/breaker a circuit to the needs of the only device on that circuit.
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In the "good old days” this was a very common way of wiring boats.
Modern boats have electrical switchboards with a minimum number of circuit breakers, despite the greater electrical sophistication. This means that one circuit supplies many, many devices.
I do not think this is a good development. In some cases it means a single failure point can disable all navigation equipment, or bilge pumps, or navigation lights etc etc.
A fewer number of independent circuits reduces costs (circuit breakers are expensive) and modern boat buyers do not seem to noice these details. We do not perhaps need a return to days when a circuit supplied only one device, but it would be much better if boats had more than handful of independent circuits.
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27-06-2023, 13:48
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#54
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Registered User
Join Date: Sep 2014
Posts: 51
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Re: Dumb questions about breakers
I went with Blue Sea 360 series panel. Able to custom spec how many and what size breakers for both AC and DC circuits. Not cheap, but good gear snd works well.
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27-06-2023, 13:58
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#55
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Registered User
Join Date: Jun 2019
Location: Rochester, NY
Boat: Chris Craft 381 Catalina
Posts: 6,994
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Re: Dumb questions about breakers
Quote:
Originally Posted by noelex 77
In the "good old days” this was a very common way of wiring boats.
Modern boats have electrical switchboards with a minimum number of circuit breakers, despite the greater electrical sophistication. This means that one circuit supplies many, many devices.
I do not think this is a good development. In some cases it means a single failure point can disable all navigation equipment, or bilge pumps, or navigation lights etc etc.
A fewer number of independent circuits reduces costs (circuit breakers are expensive) and modern boat buyers do not seem to noice these details. We do not perhaps need a return to days when a circuit supplied only one device, but it would be much better if boats had more than handful of independent circuits.
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Agreed. There are certain things I group (like lights, or a fuse block for a given set of helm electronics), but plenty of important things (like nav lights, horn, etc.) each have their own circuit. My boat was mostly done this way to start with and I've maintained that. In the case of a the fuse blocks for some of the helm electronics, I have 2, each on their own breaker. These feed things like VHFs and chartplotters. Each one gets a sized to the device fuse on the fuse block (and things are split up so the 2 VHFs are fed as 1 from each of the fuse blocks, not both on the same).
When a circuit is intended to be for a single device (and won't have additional things added to it later), I see no reason to oversize breakers. The wire may be good for 30 amps if it's a long run and sized based on voltage drop, but if the device only draws 7 amps, I'd use a 10 amp breaker as there's no advantage to the larger one, but there are potential downsides to it. Factory wiring on my boat followed this philosophy as well. The breakers for the small maintenance pumps in the bilges are 7.5 amps, for example.
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27-06-2023, 14:39
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#56
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Moderator

Join Date: Jul 2007
Boat: Bestevaer.
Posts: 15,388
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Re: Dumb questions about breakers
This is the type of circuit boards that I think is a problem. This is is on a ocean going cruising yacht. There are are only 9 independent DC circuits. This is not unusual in modern yachts.
If we focus on just one “navigation”. All navigational instruments are likely be wired to this circuit. If this circuit becomes defective. For example a fault, an overload in one device, the circuit protection will kick in disabling all navigation equipment. Likewise corrosion, or a bad connection in this circuit will disable all equipment on this breaker.
Navigation equipment as well as other essential systems such as navigational lights, bilge pumps etc should be be spread over several circuits. A single point of failure that can disable essential systems should be avoided as much as possible. This is tough to do with only 9 independent DC circuits.
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27-06-2023, 15:04
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#57
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Registered User
Join Date: Aug 2009
Location: Lake Macquarie NSW
Boat: Chamberlin 11.6 catamaran
Posts: 991
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Re: Dumb questions about breakers
Quote:
Originally Posted by rslifkin
Agreed. There are certain things I group (like lights, or a fuse block for a given set of helm electronics), but plenty of important things (like nav lights, horn, etc.) each have their own circuit. My boat was mostly done this way to start with and I've maintained that. In the case of a the fuse blocks for some of the helm electronics, I have 2, each on their own breaker. These feed things like VHFs and chartplotters. Each one gets a sized to the device fuse on the fuse block (and things are split up so the 2 VHFs are fed as 1 from each of the fuse blocks, not both on the same).
When a circuit is intended to be for a single device (and won't have additional things added to it later), I see no reason to oversize breakers. The wire may be good for 30 amps if it's a long run and sized based on voltage drop, but if the device only draws 7 amps, I'd use a 10 amp breaker as there's no advantage to the larger one, but there are potential downsides to it. Factory wiring on my boat followed this philosophy as well. The breakers for the small maintenance pumps in the bilges are 7.5 amps, for example.
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Cheers for this. This is what I was thinking - one device per circuit, so no need to have an ovespecced breaker and then a hard to find inline fuse. And no one else is going to use this boat in the next couple of decades, it is my forever boat. So no more devices will be added to any circuit but the wire will be capable of increased current loads with higher fusing and reduced voltage drop.
I get Jedi's warnings about fast blow fuses but I think it will be a huge improvement if I can find the fuses near the switch board rather than hidden near some devices back end. My boat is relatively simple and I don't like multi cause electrical issues, hence the desire for single use circuits, and testable bus bars/terminal blocks so I can more easily trace issues. I have had some magical electrical things happen sometimes and want each circuit to be very simple and easily testable and fixable offshore. At the moment when something goes wrong I have to pierce wiring insulation to determine voltage - hence the desire for bars or nice terminal blocks.
cheers
Phil
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27-06-2023, 22:59
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#58
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always in motion is the future

Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 20,161
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Re: Dumb questions about breakers
Both examples are the wrong thing to do. There have been countless cases of fires after people had altered appliances because they think they know better and we say duh and face palm… only to go ahead and tamper with appliances ?! I am sure that the manual will state to “only use specified type and value fuse” and only use the supplied power cord with it’s fuse.
Don’t be that guy we have to face palm
The example of that panel with a couple of switches is wrong too… of course it is just cutting corners and make it cheap.
If you want to approve things then here is a start that can’t be beat… I’ll give two examples around the main DC panel I recently designed, see attached picture. Please ignore the two missing battery monitors… they are in use aboard and will be mounted later. This picture with every LED on the switches lighted is just after I wired all that up to test the LED’s and the dimmer.
In the lower left corner you see two double pole breakers and two little switches. This is for a hardened, fully isolated electronics power supply. There are two isolated dc-dc chargers that can be turned on/off with the little switches and whose outputs are switched via the double pole breakers. Their inputs are connected to the 24V battery bank and the 12V battery bank.
After the breakers there is an independent AGM battery that powers all electronics except radios. Think NMEA2000, autopilot, plotter, radar etc.
Then there are three DC surge suppressors installed: one for the 12V bank, one for the 24V bank and one for the electronics power supply.
The isolated system means that the DC negative of the electronics is isolated from all other DC power, as is the DC positive. This means that a transient surge, be it from starting the engine, switching on an A/C or a (near) lightning strike, is very unlikely to affect this power supply.
In normal operation the 24V charger is active and keeps the AGM battery in float at 13.2V. The other breaker is off. When there is lightning in the air, you can shut both chargers off and both breakers off and simply run from battery.
The DC distribution panel has two electronics breakers that distribute this power, as well as a radio breaker that sends regular 12V to the VHF radios.
Second item is the navigation lights to the right of the panel. At the lower right corner there are two thermal breakers and two dimmers. One set for 12V and the other for 24V. The dimmers are used for LED’s in switches not just on these switch panels, but throughout the boat. These breakers also provide power to the switches on this panel incl. the navigation light switches. This seems pretty boring as far as a navigation lights switch panel but all this is just a backup. The primary means of switching navigation lights is with distributed switching over NMEA2000 from plotter screen or even a smartwatch etc.
Items like these make a system better. Removing a fuse from a radio does not make it better. If you don’t want to hunt for a fuse then simply leave enough slack for the cord at the radio end so you can simply pull it out. Also, in the 45 years that I have my marine radio operators certificate, I have never seen one of those fuses blown!
__________________
“It’s a trap!” - Admiral Ackbar.
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27-06-2023, 23:24
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#59
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always in motion is the future

Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 20,161
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Re: Dumb questions about breakers
Here is the diagram. At the time I was drawing this, I planned to isolate the VHF antenna from the mast but later I decided that risks were too high and the VHF shouldn’t be on this power at all, so please ignore it
During lightning storms we shut off the VHF, take the coax cable out of the radio and use a handheld VHF instead.
The diagram shows the setup with isolated chargers, double pole breakers and surge suppressors.
__________________
“It’s a trap!” - Admiral Ackbar.
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28-06-2023, 04:35
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#60
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Moderator and Certifiable Refitter
Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: C.L.O.D.
Posts: 21,524
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Re: Dumb questions about breakers
Quote:
Originally Posted by noelex 77
In the "good old days” this was a very common way of wiring boats.
Modern boats have electrical switchboards with a minimum number of circuit breakers, despite the greater electrical sophistication. This means that one circuit supplies many, many devices.
I do not think this is a good development. In some cases it means a single failure point can disable all navigation equipment, or bilge pumps, or navigation lights etc etc.
A fewer number of independent circuits reduces costs (circuit breakers are expensive) and modern boat buyers do not seem to noice these details. We do not perhaps need a return to days when a circuit supplied only one device, but it would be much better if boats had more than handful of independent circuits.
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^^ THIS
One of the basic tenets of safety systems is that a failure of one system should not affect any other system, especially so when it comes to critical equipment.
While the primary function of a fuse / circuit breaker is to protect wiring, this should not be at the expense of a degraded safety case. The architecture of a well designed electrical network should ensure the failure of one critical circuit does not impact other critical circuits. The necessary redundancy begins at the power sources and extends right through to critical loads; at no point should a single point failure affect anything else but a single critical system.
Of course the recreational cruising sailboat power distribution does not need to be as sophisticated as say the systems on board a multiengine rescue helicopter (the one that is going to take you home when it turns to custard) but the further you move away from best practice, the more likely you will be swimming in that custard bowl.
FWIW, that machine will have multiple generators, a hot (always on) bus, essential buses, main buses, auxiliary buses, emergency buses, bus isolation, bus ties with each bus having it's own protection and then every circuit is individually protected.
On your off-shore cruising boat, each critical device really should be individually protected, preferably fed from a dedicated essential bus with provision to tie to a general bus. Anything else is rather sloppy and second third rate.
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
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