fuse

[Phil1884]

Hi, stupid question alert. All my 12 v appliances will run to a 6 gang fuse breaker panel. My pumps came with in line fuse assemblies which I fitted despite them being connected to the fuse breaker.

 

My question is, I will be connecting up a radio and 12 V TV soon, is it worth protecting with an additional in line fuse or is it acceptable to just have them running to an appropriately rated fuse in the breaker panel?

[by'eck]

With the fuses at the switch/breaker you will only need the one of appropriate rating. Adding extra fuses near the device will only add to potential unreliability even if supplied with such, and achieve nothing.

[b0atman]

Can you not fit an additional fuse block as I did when down rating fuse amperages to take account of the LED lamps I am now using.

[Slim]

What type of radio do you intend fitting? If it's a car type radio you may well find that it needs a permanent 12v feed to retain the memory.

 

 

Frank

[Phil1884]

What type of radio do you intend fitting? If it's a car type radio you may well find that it needs a permanent 12v feed to retain the memory.

 

 

Frank

Yeah Car, not sure if I will bother with the permanent feed, its not a livaboard, will just need to remember the security key!

[Tony Brooks]

Fuses protect the cable, not the appliance but if the appliance needs protecting it should be built in or supplied with the equipment. Much like the line fuse most car radios seem to use.

 

The fuse in the fuse/breaker (whatever that may be) should be specified to protect the thinnest cable that it feeds. If the fuse is then too small to supply the full loading on the circuit the thinness cable should be rewired with thicker cable that will be protected by the fuse. Otherwise one needs to fin another fuse to protect the thinner cable as per car radios.

 

Just make sure that any cable you use is thick enough to eb protected by the existing fuse/breaker.

[by'eck]

DC fuses or circuit breakers can protect both cable and device if rated to comfortably cover the maximum current draw, assuming as should be the case, each device has a dedicated positive feed and return. Exceptions include lighting where it makes sense to have several lights connected to one feed. Even then its worth considering splitting into separate say fore and aft circuits which will aid fault finding and could actually simplify wiring. Although it may be tempting to string extra devices from the same circuit, a little redundancy in the switch/fuse panel will allow future devices to be added with their own circuit.

 

The fuse or breaker (which in latter case could a combined switch) should be near the battery feed such that it protects the majority of the cable run from battery to device. Again there are exceptions where there is a power source at each end of the cable such as a long charge cable from alternator to bowthruster batteries, where a fuse each end is required.

[NMEA]

Fuses protect the cable, not the appliance

I am surprised, very surprised! an overcurrent device is not there to protect the cable, it is there to protect the circuit, that obviously includes, but is not limited to the cable. The best way to think of it for a non professional is to think of the weakest link in a chain, the weakest "downstrean" link should be protected, and that is usually not the cable.

Edited December 12, 2014 by NMEA

[ditchcrawler]

I am surprised, very surprised! an overcurrent device is not there to protect the cable, it is there to protect the circuit, that obviously includes, but is not limited to the cable. The best way to think of it for a non professional is to think of the weakest link in a chain, the weakest "downstrean" link should be protected, and that is usually not the cable.

Thats why ring mains have 30 amp fuses and table lamps that plug into the ring main 3 amp ones

[Tony Brooks]

OK, so why are domestic lighting circuits protected by 6 amp fuses then? They need that rating to ensure when all the lights are on the fuse does not blow even though each individual lamp draws far, far less than 6 amps. The 6 amp fuse is to protect the cabling from damage cause by excess current flow.

 

There is no way a 6 amp fuse can protect a bulb or any other device drawing a typical bulb current.

 

I suspect the majority of domestic circuits on a boat use one fuse for a number of different appliances but I would also expect a wtaer pump to be the only thing on that circuit. Now as far as by'eck is concerned I agree with him on single appliance circuits but it gets more complicated when a single fuse supplies several pieces of equipment. The fuse will have to be specified for the total current that may be drawn and as long as the cable is specified for that current AND the same cable is used to supply all the equipment on that circuit all will be well. You can not then specify a fuse for an individual piece of equipment. In any case if there is a danger of a piece of equipment drawing too much current then it should be fitted with its own protection device like the fuses in our LED bulbs and the thermal cut-outs on things like macerators and anchor winches - bow thruster too I would imagine.

 

If a TV shorted out inside it is immaterial that the fuse blows to protect the TV the TV was already faulty. It blew to prevent the TV short setting fire to the cable. If a motor seizes up and draws a large current it is again immaterial that the fuse blew to protect the motor. It blew to prevent the motor current setting fire to the cable. It matters not if excess current flow is caused by a faulty appliance, shorting to negative in a switch, or a cable fault, once that fault occurs all that matters is preventing the cable catching fire. Hence fuses protect cables.

 

It is far more likely that an ordinary boater will end up with a safe system if they follow the "fuses protect cables" thing and "Cables must be specified for minimum voltdrop and the maximum current that may flow during normal operation (plus a bit in most cases).

 

I am surprised at NMEA. If we follow his advice we are likely to have fuses blowing or breakers tripping left right and centre may multi-appliance circuits because the cable and fuse needs to carry the full current when all are in use. Not just the current when one is in use.

 

If for some reason a cable has to reduce in size as it may well have to when connection to the cables supplied with car radios the that cable will be a weaker link that the "main" cabling so it should have its own protection or a dedicated circuit using a fuse suitable for the smaller cable.

 

If the OP is running a dedicated circuit for just his TV then a main fuse will be fine as long as its a suitable size. If however he may also be running an aerial amplifier, DVD etc then depending upon the TV makers recommendations he may have to fit another fuse. It all depends upon cable size and current draw.

 

That's it beleive what you want. I have had my say.

[by'eck]

I am not in the least surprised by NMEA's post. He was commenting on a subject that is his profession. Maybe he put it more eloquently than I, but a single fuse/breaker can and should protect the cable and device, being part of a complete circuit. Unnecessary double fusing adds unreliability not to mention frustration when something fails. Even in the bad practice situation of several devices being connected to the same circuit, a faulty device placing a dead short on the circuit as a whole is going to blow the fuse. The fact that individual devices can't be adequately protected just adds weight to avoiding such. Individual circuits will be more reliable due to having fewer connections, potentially in difficult to access places.

 

You pick up on the point regarding multiple lights on one circuit but I already stated that was one of the exceptions. In any case a light generally works or it doesn't even with LED's, failing open circuit. In either case its only a low cost replacement. More expensive devices such as pump motors can be saved from overload by suitable fusing. Your argument that a permanently failed device is not worth protecting may be judged as reasonable but what of a near stalled Sealand toilet vacuum pump when a pipe gets blocked - the correctly spec'd breaker tripped and saved me over £100.

[Phil1884]

Hi there,

 

Thanks for the comments, a good debate being had with the ultimate aim of making these circuits as safe as possible. It's great for someone like me with not much experience to hear these comments from people with far greater knowledge than to just go into it blindly and assume all will be well.

 

I did think to myself, if I have the radio, aerial amplifier and TV on the same circuit then they probably can't simply rely on a single fuse at the breaker, they will need individual fuses. This makes me wonder whether I should give them their own dedicated circuits, but I am running out of fuse breakers!

[Murflynn]

this one will run and run.

........ now if only we could find ChrisW and Gibbo .........

[NMEA]

OK, so why are domestic lighting circuits protected by 6 amp fuses then? They need that rating to ensure when all the lights are on the fuse does not blow even though each individual lamp draws far, far less than 6 amps. The 6 amp fuse is to protect the cabling from damage cause by excess current flow.

 

There is no way a 6 amp fuse can protect a bulb or any other device drawing a typical bulb current.

Bulbs are a fuse, and disposable, many other loads that are part of a circuit are expensive and easily damaged at less than the fuse provided to protect that circuit if it is rated as simplistically as that required to protect the cable alone. I maintain an over current device should be there to protect the circuit downstream of it at the rating required to protect that circuit, of course if the cable is the only part of the circuit, for instance supplying a distribution board where further fuses are present to protect the circuits downstream of it. Think about the system holistically.

[weeble]

I am not in the least surprised by NMEA's post. He was commenting on a subject that is his profession. Maybe he put it more eloquently than I, but a single fuse/breaker can and should protect the cable and device, being part of a complete circuit. Unnecessary double fusing adds unreliability not to mention frustration when something fails. Even in the bad practice situation of several devices being connected to the same circuit, a faulty device placing a dead short on the circuit as a whole is going to blow the fuse. The fact that individual devices can't be adequately protected just adds weight to avoiding such. Individual circuits will be more reliable due to having fewer connections, potentially in difficult to access places.

 

You pick up on the point regarding multiple lights on one circuit but I already stated that was one of the exceptions. In any case a light generally works or it doesn't even with LED's, failing open circuit. In either case its only a low cost replacement. More expensive devices such as pump motors can be saved from overload by suitable fusing. Your argument that a permanently failed device is not worth protecting may be judged as reasonable but what of a near stalled Sealand toilet vacuum pump when a pipe gets blocked - the correctly spec'd breaker tripped and saved me over £100.

Yes, let's take the fuses out of all those 13A plugtops and stick in a nail. Make things much more reliable.

[by'eck]

Yes, let's take the fuses out of all those 13A plugtops and stick in a nail. Make things much more reliable.

Please keep up, we are discussing fusing DC circuits.

[Chalky]

Fuses protect the cable, not the appliance but if the appliance needs protecting it should be built in or supplied with the equipment. Much like the line fuse most car radios seem to use.

 

The fuse in the fuse/breaker (whatever that may be) should be specified to protect the thinnest cable that it feeds. If the fuse is then too small to supply the full loading on the circuit the thinness cable should be rewired with thicker cable that will be protected by the fuse. Otherwise one needs to fin another fuse to protect the thinner cable as per car radios.

 

Just make sure that any cable you use is thick enough to eb protected by the existing fuse/breaker.

 

Correct. The fuse is to protect the cable, not the load. The cable should be rated so that it can handle the maximum current that the load can draw under fault conditions before the fuse blows. In the design the cable is the limiting factor and the fuse is there to protect it.

I am not in the least surprised by NMEA's post. He was commenting on a subject that is his profession.

 

It's also my profession. The design rules that the harness designers work to for the products that we produce is that the fuse protects the cable not the load.

[smileypete]

Best to always use the manufacturers recommended fuse size where specified, this comes up from time to time, and on this one I'm with NMEA.

 

If there are multiple items as such on a single circuit, it's not ideal, but each could have a local fuse with another fuse protecting the wiring.

 

OK it means the local fuse is likely to fail and less convenient to replace perhaps, but it does give diversity and makes fault finding easier.

 

For example a couple of 12V sockets supplied by 20A cable, IMHO it's quite nice to have each socket protected by an adjacent 10A breaker, plus a 20A breaker or fuse back at the panel.

 

cheers, Pete.

~smpt~

Edited December 12, 2014 by smileypete

[John V]

I would go along with Chalky (it also is/was my profession)

The primary purpose of a fuse in a multiple device circuit is to protect the cabling, lighting being the most common circuit of this type

if you are using car cigarette lighter 12v sockets then they should be fused as the circuit/cable requirement.......all the plugs I have come across for these are internally fused which should then be fused for the device concerned. If you are using a different 12v socket that does NOT have fused plugs then the circuit should be fused at the maximum rating of a SINGLE socket irrespective of how many sockets are on the circuit, (a socket in this case being the weakest point of the circuit)

Single high draw devices such as toilets etc should always be on an independent circuit fused at the distribution point.

[by'eck]

Correct. The fuse is to protect the cable, not the load. The cable should be rated so that it can handle the maximum current that the load can draw under fault conditions before the fuse blows. In the design the cable is the limiting factor and the fuse is there to protect it.

 

It's also my profession. The design rules that the harness designers work to for the products that we produce is that the fuse protects the cable not the load.

 

Why can't fusing be rated to cover the maximum current the load device will draw under non-fault conditions, since fault conditions are unpredictable, typically the rating suggested by the manufacturer? This offers a greater degree of protection for cable and device.

[John V]

Why can't fusing be rated to cover the maximum current the load device will draw under non-fault conditions, since fault conditions are unpredictable, typically the rating suggested by the manufacturer? This offers a greater degree of protection for cable and device.

 

Maximum operating current of most electromechanical devices is not a precise point.......the exact conditions at that time effect the current draw (exact load, ambient temperature, exact voltage applied i.e. are the batteries on charge or not) all this means that even the lowest level fault condition is almost impossible to calculate. On top of this you then will have lots of nuisance tripping.

Fuses and circuit breakers are fairly crude devices and even the fast acting ones are slower to blow than the internals of a sophisticated device.

[by'eck]

Maximum operating current of most electromechanical devices is not a precise point.......the exact conditions at that time effect the current draw (exact load, ambient temperature, exact voltage applied i.e. are the batteries on charge or not) all this means that even the lowest level fault condition is almost impossible to calculate. On top of this you then will have lots of nuisance tripping.

Fuses and circuit breakers are fairly crude devices and even the fast acting ones are slower to blow than the internals of a sophisticated device.

Agreed which is why the manufacturer adds a margin to the recommended fuse/breaker rating of any external one. Its also explains why a manufacturer would shy from fitting an internal one for many applications. My last boat built to LLoyds approval standards had a builders fit of 50 circuit breakers on the main DC panel. All but four were in use. I can count on one hand how many times I had a trip over 12 years, and extremely thankful for the money they saved me when they did. Not to diminish the safety aspect of course.

[Radiomariner]

The fuse at the distribution board must be rated lower than the current rating of the cable. That is essential. (BSS failure if otherwise)

If there is only one appliance using that circuit and the manufacturer states a low fuse rating only then it is feasible to fit a fuse to that value. Multiple appliances on one circuit, and circuits with power sockets (which may be used by anything from a telephone charger to a hair dryer) it is normal to rate the fuse as highly as practical still below the cable capacity. In this case, any equipment that requires a fuse of a lower rating then do so at that appliance.

 

Appliances don't have to be defective to draw excessive current, a pump struggling to pump sludge instead of water for instance.

 

Light bulbs can not always be considered to be 'self fused'. Once at sea we had on board a complete set of navigation lamps that would short circuit when the filament failed. (This is what happens when you buy cheap goods in foreign parts.) The reason was that the molten broken filament would fall to the bottom of the lamp causing a short circuit. Because the fuse ruptured, the alarm to indicate that the lamp had failed together with the system that should automatically switch over to the 'stand by' light also failed.

 

LED lighting circuits. A boater, refurbishing his boat, decided on LED lighting and was delighted to find that a good bit of the extra cost was offset by the lower cost of the much lower rated cable. Unfortunately, there was not enough light at a few locations. so he replaced a few LED lamps with halogen ones. As the breaker kept tripping, he re-calculated the current requirement and fitted a higher rated MCB. It was the smoke detector that alerted him to the overheating cable.

Be warned, if you replace LED's with Halogen make sure that the cable is of sufficient size, not just the breaker

[Iain_S]

Yeah Car, not sure if I will bother with the permanent feed, its not a livaboard, will just need to remember the security key!

 

But be aware that many car radios use the permanent feed as the power source, with the "ignition" feed only used for switching, so the permanent feed will still need to be connected, i.e. both "ignition" and "permanent" connected to 12V+.

 

Iain

[Phil1884]

The fuses at my breaker are all KUOYUH Reset Circuit Breaker Series 88 rated at 5Amps which has been fine so far for the LEDs and water pumps, but now I'm after a 15 Amp fuse (I'm using 4.5mm squared 2 core wire which can handle the current).

 

I can only seem to find a source fromt eh states for these fuses, does anyone else have these fuses and if so, know where I can get them??