5 amp fuses

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DaveHerns

New Member
The fuse on my cigar lighter extension lead blew and it's marked 250V 3 amp . My question is does any 3 amp fuse the right size do the job ?

I know excess current blows fuses but what puzzles me is that at 250 volts a 3 amp fuse will take 750 watts but at 12 volts only 36 watts .So is the voltage rating of the fuse irrelevant ?
 
Yes and no. You are right that it is current - or to be more precise heat - that blows the fuse. Basically the fuse has a certain resistance and once the current rating is exceeded the heat generated causes the fuse to open circut. So there is no problem using a 240V rated fuse in a 12V system (provided the fuse seats properly in the holder). But a 12V rated fuse may not actually break the circuit in a 240V system - the higher voltage might be sufficient to strike an arc across the broken terminals of the blown fuse. Also there is a minor googly in that the 12V is DC and the 240V is AC. This does change the behaviour of the arc, so a 240V AC rated fuse might not break 240V DC. So the two ratings are not directly comparable.

Chris
 
Basically, what the voltage rating means is that the fuse BODY INSULATION (NOT the fuse wire itself) can safely handle/withstand 250V (RMS AC) potential difference at one end cap to the other without arcing/flash over. Those fuse types are used in mains-powered electronic power supplies (Monitor, TV, Sky Boxes, Computers, etc), as well as low voltage applications. I think they are mostly quick-blowing (though not in all applications).

http://en.wikipedia.org/wiki/Fuse_(electrical)#Rated_voltage

The fuse will blow if a current much greater than 3A flows. Rule of thumb, a quick blowing 3A needs 6A (2 x 3A) flowing for like 0.25-0.5 seconds to blow. Ambient temperature is one variable which affects the exact value of "blow" current.

A mains plug fuse is not that quick blowing, that is designed for use in spiky and transient current flows (welders et al). That's why electronic supplies have quick blow fuses internally.

Certainly I would not recommend using a mains plug fuse in place of a quick blow!
 
chrisyork said:
...so a 240V AC rated fuse might not break 240V DC. So the two ratings are not directly comparable.
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They aren't directly comparable, that is correct

If you mean a 240V AC fuse might get arc'd by a 240V DC (rectified and smoothed/flat) voltage, then I would seriously doubt that. 240V AC means the RMS value is 240V, the peak or highest swing of 240V is ~340V

(Peak value for sinusoidal Alternating Voltage = RMS value x 1.414 (square root of 2)). I think it's more likely that 240V (RMS) AC will arc than 240V (flat) DC.

For the record; Although the voltage wave swings from -340 to +340V, the average value of power (or 'heating effect' if you like) dissipated by a resistor connected across the source is the same amount of (constant 'flat') power as if 240V flat DC were connected across it. That's where the 240V value comes from!
 
DaveHerns said:
Chap in Maplins didn't have a clue when I asked him the same question !
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Cor blimey!

Maplins should (or rather, they did!) have some of the fuses you are after. I ordered (via the web, I admit) some from them not that long ago!
 
I'll clarify that - I found them in the catalogue and they were in stock but he didn't know why fuses for car systems were 250V rated
 
DaveHerns said:
I'll clarify that - I found them in the catalogue and they were in stock but he didn't know why fuses for car systems were 250V rated
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Oh, I see! Hmm... :?

As Chris and I stated, its to make sure there's no arc and effectively bridge the gap when the fuse has blown!
 
Oh dear this takes me back to school! Are these the correct formulae?

1. Amps (current) = Watts (power) divided by volts (which push the current around the circuit).

ie 3 amps = 750 watts divided by 250 volts in the house

and 3 amps = 36 watts divided by 12 volts in the P6

2. Volts divided by amps = Ohms ( the measurement of how much current will flow through the resistance).

ie 250 volts divided by 3 amps = 83 ohms

but 12 volts divided by 3 amps = 4 ohms

so the answer is use only the correctly rated fuse for 12 volts except in an emergency and then be very careful as you could damage the equipment being protected with too high a rating or cause a fire with too low a rating.

However it is getting on for 50 years since I passed O level physics and I have never used that knowledge since so I might have remembered the ohms bit incorrectly? Someone please correct me if I am wrong.

Cheers

Tony Bunting
 
Sorry Tony. You don't drop the whole voltage across the fuse - otherwise there'd be nothing left to run the headlights or whatever that it's feeding, and it might well be 250V rated, but it's resistance (very small) doesn't change just because you've dropped the feed voltage. The thing that blows the fuse is heat. Heat is created by the dissipation of power, ie VI where V is the voltage drop across the fuse. We already said that V=IR, so time for the fuse to blow is proportional to I x I x R (I can't see how to do a sqared simbol), which makes the fuse very sensitive to current changes, as you want it to be!

Chris
 
Thanks chris. You either did A level Physics or I have forgottten some of my O level physics but probably the latter as I think we did half the current A level course as part of O level syllabus from what I remember from when my son did A level physics.

Cheers

Tony
 
Chris has explained it very well!

There is a very slight volt drop in the fuse itself, and it does increase (albeit slightly) with more current (and therefore creates more heat), and when a certain current is exceeded, the heat is such it blows the fuse. The voltage rating is there to ensure that the maximum voltage the fuse might be subjected to does not cause an arc within the fuse itself after blowing.
 
That's one of the reasons why we have very high voltage power lines, so the current can be (relatively) low, and the cables don't need to be ridiculously thick. Very roughly speaking, amps are amps regardless of voltage, so you need the same thickness wire to carry 10 amps at 12 volts as you do at 200 volts.

A 3kw heater at 240 V, is about 13 amps and needs a fairly thin mains cable, something like 1.5sq mm, yet 3kw at 12 v (Tony's formulae are correct) is about 250 Amps for which you need something like a very heavy duty jump lead !
 
webmaster said:
That's one of the reasons why we have very high voltage power lines, so the current can be (relatively) low, and the cables don't need to be ridiculously thick. Very roughly speaking, amps are amps regardless of voltage, so you need the same thickness wire to carry 10 amps at 12 volts as you do at 200 volts.
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If the conducting material used (i.e. the metal used as the wire's conductor (more often than not copper, but aluminium is used in some applications)) is the same, then yes, the conductor thickness (or more precisely, surface area (which is related to thickness)) is the same for x amount of current.

BUT!

The insulation material on a wire designed for 240V is thicker and/or a different material (but likely both) than a wire deisigned for 12v. This is to ensure that the insulation doesn't get broken down by the higher electric stress/tension on the wire.

The National Grid transmits electricity at higher voltages because they lose less power in the power transmission lines.

A 3kw heater at 240 V, is about 13 amps and needs a fairly thin mains cable, something like 1.5sq mm, yet 3kw at 12 v (Tony's formulae are correct) is about 250 Amps for which you need something like a very heavy duty jump lead !
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Yes, thats why starter cables from the battery to the solenoid are so thick!
 
DaveHerns said:
Does the same apply to wiring ?
Will flex designed to take 10 amps at 250V take 10 amps at 12 volts ?
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Yes, it will take it in terms of being able to carry the current.

One possible consideration is that the insulation of domestic wiring/flex may not be as resilient under the bonnet and/or on the outside. (Though I have seen some cars with mains flex used to power a horn and washer bottle under the bonnet)

But in terms of carrying current, it will do it, no question at all.
 
Domestic electric cables with a single solid copper core are susceptible to fatigue failure of the conductor under the vibration environment in a car. With a multi strand core they should be no problem though.
 
chrisyork said:
Domestic electric cables with a single solid copper core are susceptible to fatigue failure of the conductor under the vibration environment in a car. With a multi strand core they should be no problem though.
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To be precise:- Yes the single solid (well, even the stranded but stiff) cable/wire (Usually 'twin and earth' cable, but not always) that is used to supply sockets, cookers, etc is way too stiff and would definitely fail (and possibly even if they were inside the car, where there is little movement of them). That type of cable is for internal wiring and are securely fixed in position

I assumed (yes that word again!) he meant flex as in stranded flexible mains cable (kettle lead, radio (figure of 8 ) type mains cable, hoover supply cables, computer cables, etc. Basically cables that are on portable devices and that have a 13A mains plug at their end!)
 
I did indeed mean " flex " . I notice on E bay there's "thin wall " automotive cable which seems very thin for it's claimed current carrying capacity .I suppose that's thin compared with the nice thick cable you get on P6's

I don't intend using household flex on my car but I did use it on a 12volt lead lamp
 
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