to fuse or not to fuse

[Stu Jackson]

www.bluesea.com also has good material on this and fusing, but it's a PDF download and doesn't link. Here's the link to the page, scroll down for the PDF download of the Fuse & Wire Sizing Tables: https://www.bluesea.com/support/reference

[Thumbs Up]

Originally Posted by Thumbs Up

I think that you want the largest (fused from the panel) wire size that you can manage (within reason) going to the vicinity of the pump (to avoid voltage drop) then fuse it again at the pump lead according to the pump mfg. specs. (so that if you get a locked rotor, the pump won't melt while continuing to drain your battery). I use a 5 amp automotive spade type fuse in a capped holder. I can think of an improvement (untested by me): The secondary fuse could be a thermal circuit breaker of the type that resets itself after a few minutes. This way, instead of just blowing a fuse, the pump will make some last ditch efforts to restart itself after a few minutes. I can think of a few more improvements to the scenario as well such as a manual over ride switch so that you can quickly get it running again after you clear a blockage. And an alarm that tells you that the pump is blocked.

My post above is for a bilge pump. I don't think that a wash down pump needs to have a secondary fuse.

[RaymondR]

During my steel boat days I was not very particular about fusing circuits however I changed to GRP a couple of years ago and had an electrical fire a few months ago. I was horrified at how many circuits were affected once the insulation burned off the big one, which was the proximal cause, and then started shorting and burning other cables where they were bunched together. I'll be fusing a lot more circuits in future.

[sailorboy1]

Originally Posted by shadowdancer

just put in a salt water wash down pump and attached to my 15 amp panel. Do i need a fuse in between the panel and the pump ?

Bet you thought that would get a simple quick answer

[Frankly]

Keep in mind that while the primary purpose of overcurrent protection is protection from overcurrent, circuit selectivity also in the mix. Selectivity is when a breaker/ fuse opens, the only device left without power is the offending device/ circuit. If your compass light and chartplotter share a common feed somewhere upstream, don't want to be using a flashlight to read the compass if the problem is the chartplotter.

Circuit selectivity is usually pretty easy for overcurrent situations but more difficult for short circuit, ground fault, or high current conditions ( ie motor locked rotor).


Frankly

[waterman46]

Originally Posted by flightlead404

I beg to differ.

Properly size the circuit interrupting device at the buss (fuse, breaker) based on the gauge of the wire.

Properly size the gauge of the wire run based on device current draw and the voltage drop over the run, and whether its in a bundle or free air.

Don't add additional failure points in the line.

Exactly. There is only one point of interruption needed to break the circuit. If your wire is correctly sized, it will handle the 15A without overheating, and above that the breaker will trip. You want the highest reliability in the pump circuit - don't add fuses.

It doesn't matter if the high current was caused by a locked rotor or a wiring short circuit. Use the right rated breaker recommended for the pump and the right size wire and you'll be fine.

BTW, each bilge pump should have it's own dedicated breaker.



A valid use of fuses is where you want to protect wiring in a subcircuit where that wiring is smaller than the size rated for the breaker it is supplied from. For example, if you had one solar panel that can only supply 8A maximum and is wired with 12 gauge wire, and you want to connect it into a circuit that is supplied by a 20A breaker and 8 gauge wiring at the panel, you do want a 10A fuse in that solar panel output line.


Wire sizes in boats are usually governed by voltage drop considerations, and this normally results in use of larger wires than really needed to avoid overheating.

[GordMay]

Originally Posted by Thumbs Up

I think that you want the largest (fused from the panel) wire size that you can manage (within reason) going to the vicinity of the pump (to avoid voltage drop) then fuse it again at the pump lead according to the pump mfg. specs. (so that if you get a locked rotor, the pump won't melt while continuing to drain your battery). I use a 5 amp automotive spade type fuse in a capped holder. I can think of an improvement (untested by me): The secondary fuse could be a thermal circuit breaker of the type that resets itself after a few minutes. This way, instead of just blowing a fuse, the pump will make some last ditch efforts to restart itself after a few minutes. I can think of a few more improvements to the scenario as well such as a manual over ride switch so that you can quickly get it running again after you clear a blockage. And an alarm that tells you that the pump is blocked.

Auto-reset devices shouldn't be used with moving/rotating equipment, top prevent "surprise" re-starts.
I grant you, our typical bilge pumps aren't likely to cause much harm to wayward fingers.

[CharlieJ]

This thread has made this subject much more difficult than it needs to be.

To simplify:
1. In general, OCPD are installed in the B+ to protect the wiring in a circuit.
2. In the case of DC rotating equipment; e.g., pumps there is a possible failure mode of the equipment known as locked rotor as described up thread.
3. To protect against rotating equipment failure caused by locked rotor, marine equipment manufactures provide the recommended OCPD type and rating in their instruction manual.
4. Some of these specified OCPD ratings are not available OTS or are difficult to source; e.g., 4A, 7.5A, etc. in an "A" Series breaker typically found on USA panel boards by Blue Sea Systems, Paneltronics, etc.
5. And, quite frankly, disassembling a panel board to replace a standard 15A SPCB with a 5A SPCB can be a time-consuming job.
6. In general, fuses are available in fractional amperage ratings so to meet the requirements of #3 above and to be kinder to the client's wallet (#5 above), I install a fuse on the load side of the SPCB, and clearly label it.
> By placing the fuse in this location, it is in a better environment and it is not hidden.

And GordMay in #22 above is spot on wrt auto-resetting OCPD; they are not compliant with the ABYC Standards (voluntary) or with the ISO Standards (law).

[shadowdancer]

thanks for all the advice. i like the idea of a sub panel with smaller amp automotive fuses. all in one place and closer to the equipment

thanks again

[goboatingnow]

Personally I minimise stand-alone in-line fuses and ensure the supply wire is sufficient and protected by the appropriate breaker

[Bycrick]

I’ve gone to using the Blue Sea ATC fuse blocks everywhere. For example, the boat had a 15A circuit breaker on the main panel for the "Forward Cabin." A run of #10 wire. But up forward, there were 4 LED lights, maybe 200 ma/each and 2 Hella fans with maybe #22 duplex wire. The panel breaker might have protected the main #10 wire run from burning up, but did nothing for keeping the lights or fans (and their built-in wiring) from going up in smoke.

[S/V Illusion]

Originally Posted by Boatyarddog

The panel breaker/ fuse is adequate.
SV Cloud Duster

Yea but apparently simply giving a succinct and accurate answer here is forbidden.

[goboatingnow]

Originally Posted by Bycrick

I’ve gone to using the Blue Sea ATC fuse blocks everywhere. For example, the boat had a 15A circuit breaker on the main panel for the "Forward Cabin." A run of #10 wire. But up forward, there were 4 LED lights, maybe 200 ma/each and 2 Hella fans with maybe #22 duplex wire. The panel breaker might have protected the main #10 wire run from burning up, but did nothing for keeping the lights or fans (and their built-in wiring) from going up in smoke.

The vast majority of electrical faults cause a large short circuit current to flow , even small wires like 7:0.2mm can handle quite large currents for much longer then the breaker trip time.

It’s most unusual to get a fault that produces a large current but less then the breaker rating

You can’t protect yourself from every fault scenario ( that’s why you typically have insurance ).

I don’t like hidden or obscure in line fuses. I remove them from the likes of vhf radios etc, simply because in extremis I don’t want to running around trying to remember where the fuse is and what type it is.

[millsie]

Basically Yes!
electricity 101

[RaymondR]

It's very easy to place a blown fuse indicator at any fuse using a resister and a LED.

Every time I have to try to get another cable through a crowded conduit or try to trace out a circuit I ponder on pulling the whole mess out and replacing it with a house ring main and a nav ring main, fusing at the off take and switching each item using solid state switches and multi core, tinned data cable.