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Old 21-02-2021, 00:57   #1
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Circuit breakers and fuses

Gday everyone

Apologies if this has been done to death but I want to rebuild my electrical system that looks like a rat's nest. All the wire is nice tinned copper but the setup is pretty awful looking. I want to replace the switchboard and redo the fusing and bus bars.

My question is about setting the whole thing up. I would like a positive and negative bus bar. I am interested in why we have circuit breakers when we sometimes need very low load fuses to protect things like radios. I get that circuit breakers are there to protect the wire and fuses to protect the appliance but why do I need breakers if the fuse is sized lower to protect the appliance?

I don't know why breakers are needed if lower rated fuses are used as well. Am I missing something, I think I am? Fuses are easy to change but breakers are not, so would it not make more sense to have just switches leading to a fuse board with properly sized fuses.

Also if anyone has any links to really nice but simple set ups and how to s that would be great. I have Nigel Calders book but it doesn't really do the design of switchboards.

cheers

Phil
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Old 21-02-2021, 04:25   #2
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Re: Circuit breakers and fuses

Quote:
Originally Posted by catsketcher View Post
I don't know why breakers are needed if lower rated fuses are used as well. Am I missing something, I think I am? Fuses are easy to change but breakers are not, so would it not make more sense to have just switches leading to a fuse board with properly sized fuses.
Circuit breakers do not need to be replaced, they can be reset with the flick of switch. This instant reset facility is one of the major advantages, together with the incorporation of a switch,which reduces the failure points and simplifies the wiring.

Most boats have a limited number of circuit breakers. They are normally used to provide protection for branch wiring. For many devices no additional protection is needed, but if this is required it is normally done with fuses, primarily because circuit breakers would be expensive.

Low current circuit breakers are available and using these is often a better solution providing you can afford the cost. There are some other differences. Fuses have a slightly different time response to an overload situation, and they are generally less bulky.

There are some circuit breakers produced that can be used in ATO fuse holders:

https://www.bluesea.com/products/706...w_Profile_-_5A

The above devices are only thermal breakers, cannot be used as switch, and have a reasonably low interrupt capacity and voltage so they do not have all the advantages of most circuit breakers, but they can still be a useful option.

If planning a new installation, a circuit board incorporating a large number of circuit breakers is beneficial in my view. Modern boats are using fewer and fewer circuit breakers mainly to reduce costs.

See the photo below of the main circuit board on our new boat. Despite the large number of circuit breakers in this panel we still have a number of sub panels scattered throughout the boat with additional circuit breakers. However, we still have fuses protecting some devices as well as the main feeds from our battery bank.
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Old 21-02-2021, 06:40   #3
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Re: Circuit breakers and fuses

This is a worthwhile project. Once you've sorted out the wiring and have an idea of the design, you can have custom panels fabricated within a week or two that can re-use your existing spaces without carpentry work added in. When you examine your breakers, look to see if multiple inputs aren't made somewhere. This would be someone's attempt to convert your breaker into a bus which is a bad idea. Especially for stranded wire, screws will eventually work loose and the intended connection will be lost. This could be a fire hazard. Also, you might notice that you have wires coming out that don't actually go anywhere anymore. You will be glad to do this cleanup with the other work.
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Old 21-02-2021, 07:13   #4
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Re: Circuit breakers and fuses

Might want to consider a little of both. Provide a circuit breaker for a group of closely related electrical requiements, with individual fuses to each device/circuit. Example a running light breaker subdivided into bow light, stern light, compass light. Navigation breaker feeding autopilot, instruments, chartplotter, radar. Cabin lights feeding port side, stbd side, vberth, aft cabin, fans. The list goes on to suit your preference. The multiple feed/ fuses provides protection and circuit selectivity (when one device on a circuit goes south it does not take its neighbors).

Blue Sea makes a 6 circuit individual ATO block and you can jumper the inputs as desired. Also there are lots of devices that have on off switches that a fused circuit (always on is just fine). FM radio (you do have FM down under ? ), TV, CD player.

The big stuff, anchor windlass, inverter, bow thruster, solar charging, starter, and feed for above should be individually fused from a high amp bus near the feeding battery bank.


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Old 21-02-2021, 07:18   #5
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Re: Circuit breakers and fuses

Quote:
Originally Posted by catsketcher View Post
Gday everyone

Apologies if this has been done to death but I want to rebuild my electrical system that looks like a rat's nest. All the wire is nice tinned copper but the setup is pretty awful looking. I want to replace the switchboard and redo the fusing and bus bars.

My question is about setting the whole thing up. I would like a positive and negative bus bar. I am interested in why we have circuit breakers when we sometimes need very low load fuses to protect things like radios. I get that circuit breakers are there to protect the wire and fuses to protect the appliance but why do I need breakers if the fuse is sized lower to protect the appliance?

I don't know why breakers are needed if lower rated fuses are used as well. Am I missing something, I think I am? Fuses are easy to change but breakers are not, so would it not make more sense to have just switches leading to a fuse board with properly sized fuses.

Also if anyone has any links to really nice but simple set ups and how to s that would be great. I have Nigel Calders book but it doesn't really do the design of switchboards.

cheers

Phil
I have just finished the same type of project
see this link

https://www.cruisersforum.com/forums...ck-245760.html


Good discussion by lots of fellow cruisers
Cheers
Neil
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Old 21-02-2021, 07:51   #6
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Re: Circuit breakers and fuses

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Originally Posted by catsketcher View Post
I get that circuit breakers are there to protect the wire and fuses to protect the appliance but why do I need breakers if the fuse is sized lower to protect the appliance?
Typically the circuit breakers are at the beginning of the circuit, and the fuses close to the appliance. You need the breakers in case the wire gets shorted before where the fuse sits in the circuit.

So if the fuse is positioned right at the breaker, and is of lower amperage, you don't really need the breaker.
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Old 21-02-2021, 09:48   #7
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Re: Circuit breakers and fuses

I have both breakers and fuses. The breakers double as switches for circuits that need it such as nav lights, etc. and the fuses are used for circuits that are "always on" - at least when the Master Switch is on. For a very few circuits, such as for the bilge pumps, I use automatically re-setting thermal breakers for protection. These, when they blow (has never happened on Scorpius) will reset and try again after they have cooled down for a few minutes). Those devices are also fed from multiple batteries via HD diodes and bypass the Master Switch. Electronic devices generally have their own fuse right at the device and it is sized for the device, not for the wire leading to it (which the panel fuse is).
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Old 21-02-2021, 10:10   #8
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Re: Circuit breakers and fuses

[QUOTE Typically the circuit breakers are at the beginning of the circuit, and the fuses close to the appliance. You need the breakers in case the wire gets shorted before where the fuse sits in the circuit.QUOTE]


That observation has more to do with a surplus of lawyers than any electrical first principals.

A circuit can be protected at any point along the path for overcurrent, ground faults and short circuit are best protected at the power source. "Kill two birds ..." If you follow the latter.


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Old 21-02-2021, 10:27   #9
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Re: Circuit breakers and fuses

Quote:
Originally Posted by krid2000 View Post
Typically the circuit breakers are at the beginning of the circuit, and the fuses close to the appliance. You need the breakers in case the wire gets shorted before where the fuse sits in the circuit.

So if the fuse is positioned right at the breaker, and is of lower amperage, you don't really need the breaker.
^^^ This. The panel breaker protects the "downstream" circuit wire from burning up if there's a short along its length. So the breaker must be as close to the "start" of the wire as possible.


eg:

battery positive (-> battery fuse) (-> battery switch/selector) -> positive buss
-> panel breaker -> circuit wire (-> device fuse) -> device


( ) are optional components


To the OP - my preferred arrangement is some terminal strips behind the DC panel for terminating the rats-nest of circuit wires, then a neat harness from the other side of the terminal strips to the DC panel. Once everything's labelled and documented, it's very easy to troubleshoot and update.
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Old 21-02-2021, 10:42   #10
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Re: Circuit breakers and fuses

Neither fuses or breakers protect the appliances. Appliances shall be protected internally. Both fuses and breakers protect the wire between energy source and the appliance. Also breakers and fuses can be used as simple disconnect device for the maintenance /troubleshooting purposes. In this case size of the fuse or breaker depends where it is located in the circuit.
Personally I prefer the fuses over the breakers because they are more reliable than circuit breakers. The circuit breakers are electromechanical devices themselves and prone to failures especially those cheap the third shift knockoffs made in China. There is no guarantee the circuit breaker will trip when it has to. Or vice versa it becomes a nuisance and opens for no obvious reason.
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Old 21-02-2021, 10:53   #11
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Re: Circuit breakers and fuses

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Originally Posted by facciatosta View Post
Neither fuses or breakers protect the appliances. Appliances shall be protected internally. Both fuses and breakers protect the wire between energy source and the appliance.
This is often stated, but it should really be phrased that the circuit protection (fuse or circuit breaker) should not have a higher rating than is acceptable for the wire size.

Circuit protection that has a lower rating than is acceptable for the wire size is OK and this can be useful, as in some cases it can remove the need for an additional fuse installed to protect the device, simplifying the wiring and removing failure points.
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Old 21-02-2021, 11:09   #12
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Re: Circuit breakers and fuses

The previously linked BlueSeas website has lots of informative How To articles. There are also some excellent boat electrical system BOOKS that answer your questions.


With your plans to do such extensive work, it might be prudent to do some basic research and homework.


Good luck.
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Old 21-02-2021, 11:48   #13
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Re: Circuit breakers and fuses

Hereís my solution
24 breaker panel
20 15amp breakers
4 20 amp breakers
All breakers are connected to
individually fused terminal blocks
The breakers protect the wires to the fuse block.
Each individual circuit gets the appropriate sized
fuse to protect the wire to the load.
Complete flexibility and the umbilical to the
fuse blocks can be as long as necessary to
relocate them to be more accessible .
Positive goes to panel
Negative and Ground go to bus bars along side
fuse blocks
I hate rats nests
Cheers
Neil
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Old 21-02-2021, 11:55   #14
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Re: Circuit breakers and fuses

Quote:
Originally Posted by Time2Go View Post
Hereís my solution
24 breaker panel
20 15amp breakers
4 20 amp breakers
All breakers are connected to
individually fused terminal blocks
The breakers protect the wires to the fuse block.
Each individual circuit gets the appropriate sized
fuse to protect the wire to the load.
Complete flexibility and the umbilical to the
fuse blocks can be as long as necessary to
relocate them to be more accessible .
Positive goes to panel
Negative and Ground go to bus bars along side
fuse blocks
I hate rats nests
Cheers
Neil

My goodness: Circuit breakers and fuses in series on every circuit. That certainly sounds like overkill to me. Do you have provision for circuits like the bilge pump, VHF radio, and fan in the head that should NEVER lose power even with the master switch off or a dead battery?
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Old 21-02-2021, 12:23   #15
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Re: Circuit breakers and fuses

Quote:
Originally Posted by Time2Go View Post
The breakers protect the wires to the fuse block.
Each individual circuit gets the appropriate sized
fuse to protect the wire to the load.
I am confused why you do not (in most cases) size the wire to the load to be an adequate size for the circuit breaker? This would do away without the complication and extra failure points of the connections associated with the fuse blocks, as well as making troubleshooting easier.

Even wire as narrow as 16AWG (1.5mm squared wire) is adequately protected with a 15A circuit breaker (for most applications) so I expect you already meet this requirement. For the rare devices that require specific circuit protection below 15A you can swap out the 15A breaker for the appropriate value or use a fuse.
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