Hi,
I want to fuse a circuit for safety but am unsure how to choose the proper fuse rating. The circuit runs basic 110V a/c (home/residential outlet). The circuit consists of FOUR 7.5 watt incandescent light bulbs running in parallel. That's it.
What is the proper amp rating for a typical glass fuse to handle this load? Also, how do you figure this out?
Thanks!
Current = power/voltage = 30/110 = 0.273 amps. A 0.5 amp fast blow fuse would be adequate.
[edit] If this is to be turned off and on very frequently, the 1/2 amp fuse might suffer from metal fatigue because a cold incandescent bulb filament has an inrush current that is higher than it's operating (hot) current. If it fails from this you might want to use a slow blow fuse or a 1 amp fuse. I am not an expert on fuses, though.
[another edit] If by safety you mean electrocution, a fuse will not be protection. You must design things so that a person can never be exposed to the voltage.
Thank you! If the bulbs were running in serial it would make no difference (from standpoint of the fuse), correct? The power consumption would still be the same, right? Not to sound like an idiot, but I know that voltage is additive in a serial circuit, such that if you have two DC 5V bulbs running serially on a DC 10V line with adequate amperage, then both bulbs will light %100 at 5V each. But on an A/C line, if, for example, you were to take TWO of my 7.5watt bulbs (rated at 110V), and ran them serially, would there be a %50 reduction in the brightness of the bulbs even though there is plenty of amperage? I assume so if the rules are the same for AC. So if I had a 1 amp breaker for that circuit I could theoretically run about fifteen 7.5 watt bulbs on that breaker before the blew, correct?
Off topic but just out of curiosity, can too much amperage damage a properly voltage-rated component? For example, let say you tried to run a 12V LED flashlight bulb on a 12V DC current that was rated at something crazy like 6 amps. Even though the LED draws, say .02 amps and voltage is correct, would the 6 amp current damage the LED?
First question: In the case of incandescent lamps in your example, AC is the same as DC. The bulb color will shift towards red and I believe the brightness will be somewhat less than half if two are in series because of characteristics of the filaments. The amperage will be about half.
Second question: If you have a properly sized current limiting resistor in series with the LED (a must!) it doesn't matter what the current rating of the power supply is. It is only the power supply voltage that is to be considered. Study Ohms law. This is a must if you are fooling with electricity.
To further clarify your 12 volt flashlight example, you could run that flashlight off of an automotive battery capable of 100's of amps.
Thank you for the info!
Mains fuses are really to protect the wiring, so rate them for that, not the load. Fuses are there
to prevent fires, not save people from shocks, which they cannot really do.
Low power equipment often has its own internal fuse to protect its innards from excessive damge/fire
(since typical mains wiring will supply many kW without difficulty). But just some lamps I think you
only need a standard lighting fuse (In the UK that's 5A, your territory will differ).
Is there protective earth? Is the equipment double insulated?
Incandescent lamps tend to act as their own fuse!
Something to bear in mind when choosing fuses is that fuses made to EU standards are designed to last indefinitely when carrying the rated current (well, for thousands of hours anyway). Fuses made to North American standards are designed to last for 4 hours at rated current so should only normally be operated at 75% of rated current.
Russell.
russellz:
Something to bear in mind when choosing fuses is that fuses made to EU standards are designed to last indefinitely when carrying the rated current (well, for thousands of hours anyway). Fuses made to North American standards are designed to last for 4 hours at rated current so should only normally be operated at 75% of rated current.
I'd really like to see a link or something to back up that claim; I'm not saying you're incorrect or anything, but I would think that if this were the case, then whatever statement you pointed to might also give a reasoning as to why.
To me, it would seem like a fuse rated for 5 amps should allow up to 5 amps to pass as long as necessary - not blow before then. Then again, maybe this is why you see so many idiots putting higher-rated fuses in places in their cars (then wondering why fire results)...
Lots of information out there from fuse manufacturers. See for example:
Documents | SCHURTER (page 5)
Another good guide is:
http://www.littelfuse.com/~/media/electronics/product_catalogs/littelfuse_fuseology_selection_guide.pdf.pdf
That quotes from the UL standard that a fuse carrying the rated current shall have an opening time of greater than 4 hours. That's why a 75% derating is recommended.
Russell.
but I would think that if this were the case, then whatever statement you pointed to might also give a reasoning as to why.
Basically the EU and UL have totally different ideas as to what a fuse is for.
UL are obsessed with fire and the EU is obsessed with electric shocks. This often leads to incompatible safety requirements each side of the Adlantic.
True, although you have to remember that fuses cannot prevent lethal shocks.
Many more recent UL and EU standards are virtually identical now as they are based on international (ISO) standards, perhaps with slight variations to suit local conditions.
Russell.
True, although you have to remember that fuses cannot prevent lethal shocks.
The point is that in the EU all things conductive that the consumer can touch is earthed. Therefore when a fault develops that makes anything that could be touched live, a fuse between live and ground will blow thus preventing a shock.