Correct Methods for Fusing, Protecting Circuits?

[jviss]

Hi MS,

I have two related questions.

1. What is the correct method of fusing devices on branch circuits? Say I have a 15 Amp breaker in the panel for "electronics," with multiple devices on this branch: VHF, AIS transponder, sailing instruments. Some of the devices are supplied with power cables with in-line fuses, rated to protect the equipment, not the circuit, e.g., 1 Amp or 5 Amp fuses.
   • are the inline fuses necessary?
   • should they be as close to the device as possible, or as close to the breaker as possible?
   • can the fuses be "ganged up" in a fuse block, like a Blue Sea Systems 5025?
2. I'm considering some solid-state, DIN-rail mounted low voltage disconnect circuits.
   • should they be installed before or after the breaker?
   • if after, is it O.K. that they are rated for less Amperage than the breaker?

Thanks very much,

jv

 

[LeslieTroyer]

Jives I went with the 5035 behind the breaker panel for devices that specd lower ratings. I wanted the ability to fuse more thanone breaker (ie Stereo, instruments, vhf,....) I did use the 5025 for 24x7 devices (bilge pumps, vessel monitor) it is behind a 30A MRBF directly off the house bank.

Not having 8 fuse holders spread all around make finding faults easier and the atc fuses I can read the rating easier than glass fuses

Les

 

[jviss]

I like that approach, Les. I assume you mean you've eliminated the supplied, in-line fuse holders in favor of the ones in the 5035? I would like that, even just for ease of just finding the fuses!
That said, I have three, unused 5015's which are the six-circuit, holders for glass-envelope fuses, with a negative bus bar. I'll use these and suffer with the glass fuses!

 

[LeslieTroyer]

correct - as I get bored with other tasks I'll find a fuse and remove it - and put the line to the 5035. The one I really need to move is the e7D fuse which is under the quadrant "guard" above the aft bunk in the C36 -- that one would be a royal pain to change!! I will have at least one of the BlueSea 5015 glass fuse holders (w Neg) surplus in a bit if you end up needing another.

 

[jviss]

if you end up needing another.

Thanks! Yea, my C36 has an e7 in a pod at the helm, I think the fuse is inside the pod. But I'm not sure. See? If it was blown, I'd be searching around for that thing.

 

[uncledom]

Technically you should fuse to the wire size and as close to the source as possible. Just a rule of thumb.

 

[Hankonthewater]

"Technically you should fuse to the wire size and as close to the source as possible. Just a rule of thumb" byUncledom,
I agree that one should rate the fuse to the wire used, as fuses protect mostly the wiring, not the device. High current devices, like winches might be different in this aspect: fuses do protect burnout of the (windings on the) motor. But I never understood the reason why the fuse needs to be close to the device itself.... even tough I am quite 'electrically' literate. Can anyone explain the technical reason? Some some other thoughts on this matter..... no light fitting is fused (that I have seen), they are all fused close to or at the switchboard.

Generally the fuse is placed in the positive wire. I can think of reasons why a fuse is needed close to the source of the power, ie battery/switchboard. When the fuse blows/disconnects, the entire circuit is then without power. If the fuse is close to the device ie VHF radio, then the positive lead from radio to battery/switchboard is still activated and carries 12 Volt. Therefore that part of the wire is NOT fused or protected.

Back to the original question. I do not know what the 'correct' way of fusing is. I always removed any in line fuses, and fused each single device separately at the switchboard, and never have to look behind panels etc for that elusive fuse.... and then finding that I haven't got that type of spare fuse. Now, I need to carry only one type of fuse or none at all when using re-settable fuses.
The advantage of separately fusing is that if there is a problem, ie short circuit or open circuit, I can switch off that circuit and only affecting one single instrument/device, all the rest of the boat remains operable.

 

[LeslieTroyer]

One reason manufacturers put fuses on the end of their pigtails is that it is cheap liability insurance. In lots of cases the wire they use in the pigtail is smaller than that going to the fuse block. If there is a fault and the unit shorts, and you have it connected to a 30A breaker. The first wire to burn up is that small 24 gauge wire in the pigtail. Who do you think will get sued?? By including a fuse they can ensure their pigtail won’t fry. They may want a specific fuse type also ( fast, slow, ultra fast)

Les

 

[Hankonthewater]

Thanks Leslie, that is is good explanation why manufacturers put in the in-line fuse. Hehehe, no techninal reason at all!

 

[Hankonthewater]

Thanks Leslie, that is a good explanation why there is an in-line fuse. Hehehe, no technical reason at all.
On edit: Oops, double post, sorry

 

[jviss]

One reason manufacturers put fuses on the end of their pigtails is that it is cheap liability insurance. In lots of cases the wire they use in the pigtail is smaller than that going to the fuse block. If there is a fault and the unit shorts, and you have it connected to a 30A breaker. The first wire to burn up is that small 24 gauge wire in the pigtail. Who do you think will get sued?? By including a fuse they can ensure their pigtail won’t fry. They may want a specific fuse type also ( fast, slow, ultra fast)

Les

30A breaker? The most common value for a branch circuit breaker, in my experience, is 15A.

That said, I'll assume there really is a technical reason, and go with that. Now, were to put them, if you relocate them?

 

[Hankonthewater]

Jviss, I agree that there is, on this tread at least, no plausible technical reason to have to fuse close to the device/appliance. If there is really no technical reason, then remove the in-line fuse and put the fuse/circuitbreaker on the switchboard. I mentioned in post #7 reasons for that location.
Or, if you really want to retain your in line fuse, make it a slow fuse, and the one on the switchboard a fast one (of the same or the same value). The latter may blow first. To have a line with 2 fuses... makes it more complicated, and every single connection is a point of possible failure, and for certain: a cause for additional voltage drop.

 

[jviss]

What I am thinking is that I'd have a branch circuit that starts from a 15A breaker in the main panel. I'll have 14 or 12 AWG wire from that breaker to fuse block, feeding each of my branches, through a fuse; the fuse, and the wire to the device, would be appropriate to the load, the length, and the device manufacturer's fusing recommendation.

I would probably not go smaller than 16 AWG for runs to the device, and then splice to whatever wire was a permanent part of the device, or the device's power cord, as close to the device as possible.

There may be instances where the fuse block is not near the main panel, but near a cluster of gear, such as the nav station.

 

[uncledom]

These are links to DC ampacity charts. You'll find that they don't coincide with 120v charts.
https://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity
https://www.bluesea.com/resources/1437
I personally have never seen a fuse protect a device or wire that was bad, it stops further damage in the circuit once a problem has occurred. Once your device fails it blows the fuse and protects the wiring from a high current draw, etc.... The only way to correct the problem is to fix or replace the defective part, whether that be the device or the wire feeding it.

 

[Davidasailor26]

One reason manufacturers put fuses on the end of their pigtails is that it is cheap liability insurance. In lots of cases the wire they use in the pigtail is smaller than that going to the fuse block. If there is a fault and the unit shorts, and you have it connected to a 30A breaker. The first wire to burn up is that small 24 gauge wire in the pigtail. Who do you think will get sued?? By including a fuse they can ensure their pigtail won’t fry. They may want a specific fuse type also ( fast, slow, ultra fast)

Les

That’s not really liability insurance, it’s just building the system appropriately. Using your example, if something goes wrong inside the device and it starts drawing 28 amps, the 30A breaker won’t trip, so the smaller wire burns. Fuses are installed to protect the wiring. If wire size goes down in a branch, it still needs to be protected.

 

[JimInPB]

A few specific individual devices may have special reasons for wanting a smaller fuse on the feeder, but in most cases that I am aware of, the smaller fuse at the device serves two purposes.

First, in the event that the device has an internal problem, it prevents that problem from having enough power available to damage larger & more expensive parts inside that device. If you are dealing with an expensive device that is worth sending to a repair shop when it fails, then this matters. If you are talking about something like a $200 VHF, then I would not worry about this reason so much.

The second reason to have the smaller branch fuse, is for the benefit of other devices. If your FM radio has a 2 amp fuse & 6 other devices are on the same 15 amp feeder, then a short in the radio will blow the 2 amp fuse & leave the other devices still working. This can be handy, especially if you have something important, like navigation equipment or a VHF, on the same feeder as your FM radio.

A down side to having the extra fuse, is that it adds an extra place for something to go wrong & those in-line fuses usually end up in a place that is not easy to access for troubleshooting.

That smaller fuse can be connected anywhere in the positive wire from the device to the feeder point. It does not need to be near the device. Your 15 amp feeder fuse or breaker needs to be within a short distance of the actual power source, but the secondary fuse can be anywhere in the wire to the device that is after the 15 amp feeder, assuming that all your wire before the small fuse is good for 15 amps.

When you use the ampacity charts that are listed above, please be aware that the lengths in the charts are “round trip” lengths. If you run a negative & a positive wire 10 feet to power a device, then that counts as 20 feet on the chart because you have 10 feet of positive wire going out + 10 feet of negative wire coming back.