Check your fuses!!! (1 Viewer)

[Ed Excel]

Nominally 12v equipment is usually designed for 11v - 16v.

V = IR
I = V/R
R = 6
I = 12/6 = 2
I = 10/6 = 1.66
I = 9/6 = 1.5

........... and the converse is true

I = 14/6 = 2.33
I = 15/6 = 2.5

Sorry about the bulb oh:

That's exactly what I was trying, perhaps badly, to explain. The equipment must be able to operate over a range of voltages/currents, not just the nominal :thumb:.

 

[Tootles]

Useful info on marine wiring, thanks.

I imagine cable sizes can be quite large on big vessels but I don't think it's for the reason you suggest. It is more likely to be because of voltage drop. If you consult BS7671, cable ratings, you'll find current carrying capacities of cables are the same for dc and single phase ac circuits. Current tends to flow in the outer skin of high frequency cables, such as with coaxials in tv installations.

Most big vessels use AC only, with no earth to hull connection. All the earth connections are connected to each other. Oil and LPG tankers use this system as well.

 

[tonyidle]

That's exactly what I was trying, perhaps badly, to explain. The equipment must be able to operate over a range of voltages/currents, not just the nominal :thumb:.

Agreed. But my point is that the OEM-fitted fuses are way in excess of requirements even allowing for the relatively small increase in the current drawn by nominally 12v devices operating up to near 14v. And that's fine for a simple filament lamp - but when LED lighting is installed such fuses offer very little protection in the event of an internal device fault. LED lighting (certainly the most efficient versions) may incorporate circuit boards, integrated circuits, resistors, etc, and offer much more potential for internal faults. Despite the fact that an internal fault is unlikely to cause serious damage or an increase in fire hazard a much smaller fuse is still a wise precaution.

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[Ed Excel]

Agreed. But my point is that the OEM-fitted fuses are way in excess of requirements even allowing for the relatively small increase in the current drawn by nominally 12v devices operating up to near 14v. And that's fine for a simple filament lamp - but when LED lighting is installed such fuses offer very little protection in the event of an internal device fault. LED lighting (certainly the most efficient versions) may incorporate circuit boards, integrated circuits, resistors, etc, and offer much more potential for internal faults. Despite the fact that an internal fault is unlikely to cause serious damage or an increase in fire hazard a much smaller fuse is still a wise precaution.

I'm still in the 'not neccassary' camp but If you feel more comfortable changing I see little wrong in what you suggest.

My point is, in LV/ELV lighting installations lighting points are not individually protected, unlike in power installations, and so luminaires need to be capable of operating from certain standard circuit arrangements eg 5A, 10A or 15A.

Consider the current drawn by a 1W LED from a 12V supply is 0.083A, many times smaller than any of the circuit protection devices above, but it must operate and not cause danger when connected to any of them. So little point in changing for me.

 

[tonyidle]

I'm still in the 'not neccassary' camp but If you feel more comfortable changing I see little wrong in what you suggest.

My point is, in LV/ELV lighting installations lighting points are not individually protected, unlike in power installations, and so luminaires need to be capable of operating from certain standard circuit arrangements eg 5A, 10A or 15A.

Consider the current drawn by a 1W LED from a 12V supply is 0.083A, many times smaller than any of the circuit protection devices above, but it must operate and not cause danger when connected to any of them. So little point in changing for me.

"luminaires need to be capable of operating from certain standard circuit arrangements eg 5A, 10A or 15A" is the bit I don't understand. Any device can be operated off any supply capable of supplying any current above that required. A 1W LED light unit will operate happily directly off the battery terminals without supply wires or a fuse in sight. An internal fault will however totally destroy it, melt the tracks off any internal circuit board, and fry the internal wires. It will do the same when fed from a 10A or 15A fuse. No manufacturer of any low current device takes any account of any aspect of the external supply other than voltage. The principle of using the lowest value fuse that meets the requirement is well established in every field of electrical engineering. Compromises do have to be made when supplying multiple devices with a common feed and the fuse chosen has to fit the fuseholder but it makes no sense whatsoever to use a fuse capable of carrying more than 10 times the required supply current.

 

[Ed Excel]

"luminaires need to be capable of operating from certain standard circuit arrangements eg 5A, 10A or 15A" is the bit I don't understand. Any device can be operated off any supply capable of supplying any current above that required. A 1W LED light unit will operate happily directly off the battery terminals without supply wires or a fuse in sight. An internal fault will however totally destroy it, melt the tracks off any internal circuit board, and fry the internal wires. It will do the same when fed from a 10A or 15A fuse. No manufacturer of any low current device takes any account of any aspect of the external supply other than voltage. The principle of using the lowest value fuse that meets the requirement is well established in every field of electrical engineering. Compromises do have to be made when supplying multiple devices with a common feed and the fuse chosen has to fit the fuseholder but it makes no sense whatsoever to use a fuse capable of carrying more than 10 times the required supply current.
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Perhaps the simplest way to put this is, fuses protect the cables not the equipment. A reduction in load, an equipment change, cannot effect the circuit protection.

The protective devices provided in any installation, at the point of distribution, are for final circuit protection not protection of equipment. The protective device ratings need to be sufficient to protect the cables of the installation whatever equipment is connected to those final circuits. So, my suggestion is, the fuse size makes no difference to the equipment, (as I think you actually suggested 'it will do the same when fed from a 10A or 15A fuse') be it 5A, 10A or 15A, as long as it's sized for protection of the circuit cabling, and therefore doesn't need changing if the load goes down, or up within obvious limits.

I would agree with what you say about selection of fuses but suggest they are considerations for a new design, not an existing installation, when cost is also an issue.

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[tonyidle]

"luminaires need to be capable of operating from certain standard circuit arrangements eg 5A, 10A or 15A" is the bit I don't understand. Any device can be operated off any supply capable of supplying any current above that required. A 1W LED light unit will operate happily directly off the battery terminals without supply wires or a fuse in sight. An internal fault will however totally destroy it, melt the tracks off any internal circuit board, and fry the internal wires. It will do the same when fed from a 10A or 15A fuse. No manufacturer of any low current device takes any account of any aspect of the external supply other than voltage. The principle of using the lowest value fuse that meets the requirement is well established in every field of electrical engineering. Compromises do have to be made when supplying multiple devices with a common feed and the fuse chosen has to fit the fuseholder but it makes no sense whatsoever to use a fuse capable of carrying more than 10 times the required supply current.
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Perhaps the simplest way to put this is, fuses protect the cables not the equipment. A reduction in load, an equipment change, cannot effect the circuit protection.

The protective devices provided in any installation, at the point of distribution, are for final circuit protection not protection of equipment. The protective device ratings need to be sufficient to protect the cables of the installation whatever equipment is connected to those final circuits. So, my suggestion is, the fuse size makes no difference to the equipment, (as I think you actually suggested 'it will do the same when fed from a 10A or 15A fuse') be it 5A, 10A or 15A, as long as it's sized for protection of the circuit cabling, and therefore doesn't need changing if the load goes down, or up within obvious limits.

I would agree with what you say about selection of fuses but suggest they are considerations for a new design, not an existing installation, when cost is also an issue.

I agree with you to a large extent. But sensitive equipment (read more expensive equipment) is often protected by its own individual fuse usually just after the cable or flex enters it. Domestic appliances have circuit protection via 30A ring main fuse/breaker followed by plug fuses protecting the device (and its flex). And probably only to save changing the ring fuse/re-setting the breaker every time. I've no doubt we can agree to differ but I would use a fuse adequate for the load rather than one rated at the circuit's potential maximum.

 

[Ed Excel]

Thanks for your interest Tony. It's good to debate stuff that would probably clear a dinner party. I usually get a look from my wife if I stray into technical detail.

Regards
Ed