5A Fuse Blows

I have just upgraded the graphics card on my pc to a XFX Geforce GTX280 XT and consequently the PSU, as my old one wasn't man enough, to a BFG ES-800W.

The problem is the fuse in the kettle lead keeps blowing. I plugged the kettle lead which came with the psu in and the pc worked fine, but the next time I tried to power it up there was no power at all. My first thoughts was the fuse so I change to my old kettle lead (also 5A) which worked fine the first time, but wouldnt start the next. It is definitly the fuse as I put a new 5A fuse in the lead and it worked, again for one time only. As for the cause I realy dont know.

One of my concerns is the graphics card isn't getting enough current, as the box states minimum power requirements for the card is 630W (non-sli) with a current rating of 40A****. Not sure what '****' means as it doesn't say anywhere. But the power supply has only a maximum of 36A on some of the leads. I dont realy understand this part so I could be wrong and it is ok.

My system is:
Operating System: Windows XP Pro
Mother Board: Asus A8N Sli-Premium (AMD 64bit)
Processor: AMD Athlon 4000+ 64bit
Graphics Card: XFX Geforce GTX280 XT
RAM: x2 512MB Corsair Twin-X
Sound Card: Soundblaster X-FI Music
Hard Drives: x1 Seagate SATA
Drives: x1 CD-RW and x1 DVD-RW
Case Fans: x2 Case Fans which came with (Silverstone Temjin Case)
USB Equipment: Mouse
PS/2: Keyboard (because if I use it in the USB it misses out key presses)

It is a double wall socket, I am plugging the PSU directly into its own socket and in the second socket I have a 4-Socket Multilead adaptor with a 32" Sony LCD TV, 22" Samsung LCD Monitor, D-Link Router and an Xbox360.

Please give oppinions and advise on what I should do.

Thanks

13A fuse...




A 5A fuse will blow at 5A (based on 240V)

Power = Voltage x Current

800W = 3.33 amps, which seems reasonable to fit a 5A fuse. But for some reason the fuse is blowing suggesting your rig is pulling more than 5A
240V


Edit: Don't worry about power requirement for the graphics card. 40A on a 12V system

Mains = 240V / AC
Inside the PC = 12V / DC

Power outside the PC (800W) = 240 volts x 3.33 amps
Power inside the PC (800W) = 12 volts x 66.6 amps (theoretically)

PSUs are typically 80% efficient = 66.66 x 80% = 53.3 amps available power, typically.


This is a very amateur diagnosis, and I'm not really sure why the fuse should be blowing, surely it would mean the PSU is over working?

Quote:

---

Originally Posted by GarethG

One of my concerns is the graphics card isn't getting enough current, as the box states minimum power requirements for the card is 630W (non-sli) with a current rating of 40A****. Not sure what '****' means as it doesn't say anywhere. But the power supply has only a maximum of 36A on some of the leads. I dont realy understand this part so I could be wrong and it is ok.

---

The *s will probably be refering you to different combinations and limitations on rail combinations - there should be a chart saying something like 460W combined for all the 12V rails or so.

Regarding the kettle lead I'm not sure your power supply is working correctly. 800W on a 240V supply is only 3.3A, so a 5A fuse SHOULD be enough. If it's pulling down more than that then I'd be very suspicious.

Nice ninja edit cptwhite ;)

The only thing I can think of is some sort of earth leakage or short circuit within the powersupply. I cant use my old one either as it doesn't have enough power for the graphics, and the reason I changed my old graphics card is because it suddenly stopped working while viewing e-mails.

If the 5A fuse is blowing, I'd say it was saving your system from some major badness.

Quote:

---

Originally Posted by Fraz

If the 5A fuse is blowing, I'd say it was saving your system from some major badness.

---

Agreed. Don't, whatever you do, be tempted to put a 13A fuse in! There is a problem somewhere you just have to find it.

Yeah im studying electrics and have already done electronics, I'd never think twice about using a 13A fuse in a pc.
As far as it goes to fault finding on a pc though i don't have a clue especially on power supplies. With my old graphics card suddenly breaking it makes me think there might be a problem with the motherboard. But im not sure if this would effect the primary winding current in the psu transformer.

Even if it was a problem with another component, there *should* be safety systems in the PSU to prevent it from either pulling down too much over the mains or over-supplying itself. That these aren't working is very worrying to me.

Quote:

---

Originally Posted by GarethG

Yeah im studying electrics and have already done electronics, I'd never think twice about using a 13A fuse in a pc.
As far as it goes to fault finding on a pc though i don't have a clue especially on power supplies. With my old graphics card suddenly breaking it makes me think there might be a problem with the motherboard. But im not sure if this would effect the primary winding current in the psu transformer.

---

IIRC, my corsair PSU lead was supplied with a 13a fuse...

Quote:

---

IIRC, my corsair PSU lead was supplied with a 13a fuse...

---

For what im about to say it applies only to a mains voltage of 230V (the UK standard).

Im not saying you shouldn't with all PSU's I mean with mine, it depends on the power and design of your PSU (in other words stick to manufacturors design). A 13A fuse would allow for a 2990W PSU. But for my 800W psu I could get away with using a 4A fuse and still it shouldn't blow under maximum load. This is because Power = Current x Voltage. You should also note that the cable's current carrying capacity should be >= the value of the fuse and never less. Thats why I shouldn't use a 13A fuse unless I want a fire to burn down my house when a fault occurs.

Correct me if im wrong :confused:

Quote:

---

Originally Posted by GarethG

A 13A fuse would allow for a 2990W PSU. But for my 800W psu I could get away with using a 4A fuse and still it shouldn't blow under maximum load. This is because Power = Current x Voltage.

---

Just remember though that your PSU won't pull 800W it will generate up to 800W DC at whatever it's efficiency is. If lets say the efficiency is 80% when drawing 800W that will try to pull 1000W from the socket and at 240V that would be 4.16A which would blow the fuse. The 5A one should be enough though and you'll never likely use that 800W :)

PS - Could it be that something inside your PSU isn't earthed properly and is causing the big current draw? I never realised that a 5A fuse could be used for it.

Couple of points - the plug fuse is designed to protect the connecting lead to the appliance - the PSU should be protected with its own fuse - probably internal.

The power supply will not be 100% efficient - so if it is running at 800Watts output at 80% efficiency - typical figure) it will be drawing about 1000 Watts input.

Although power is Voltage*current, this is an AC cct and so power factor has to be taken into account. Typical power factors are about 0.8, so power will be V*A*.8 - so assuming that the 800 watt power supply is delivering the maximum power (800 watts and drawing 1000W) the current drawen from the UK power supply (nominally 230V) will be 5.4 amps. (5.4*230*.8). This is worst case scenario - pessimistic power factor and PSU at maximum load, but you get the idea)

A typical plug top fuse will blow instantaneously at about 2.5 times its rated current (as I said earlier, it is designed to protect the cable - typically against catastophic failure - a short cct) but if it is overloaded continuosly, it will eventually fail - which is what is happening here. Futrther more, there will be a switch on surge when the PSU is powered up, again overloading the fuse. (Inrush current could be 10 amps for a brief perid)

Bottom line is that the minimum fuse value should be 7 amps - 10 would be the optimum, but it is unlikely that any harm would come from using a standard 13 Amp fuse.

it could be killing the fuse when you turn it off because of back EMF caused by the transformer coils..

also, mains voltage is variable: supposedly: 230 V +10% −6% but there are vspikes of 300v odd (all dependant on the step down transformers supplying the power)

and even if you did put a 13a fuse in, if there is a short then it would easily hit 13a and kill that fuse... but if it is just the PSU's operational parameters causingthe >5a current then the 13a would mean that you can turn it on more than once...

and TBH, 5a isn't really that much when we're talking about transformers...

Quote:

---

Originally Posted by TAKTAK

it could be killing the fuse when you turn it off because of back EMF caused by the transformer coils......

---

Er - no! Its a switched mode PSU so tthere is no transformer connected to the direct mains input - the transformer operates after the incoming mains is rectified! (But even if it was, there would be little back emf anyway as there would be little stored energy in the transformer under load conditions)

But as I said in my previous post, a 5A fuse is under-rated - change it - ideally for a 10A, but if you haven't got a 10A, use a standard 13Amp.

Quote:

---

Originally Posted by peterb

Er - no! Its a switched mode PSU so tthere is no transformer connected to the direct mains input - the transformer operates after the incoming mains is rectified! (But even if it was, there would be little back emf anyway as there would be little stored energy in the transformer under load conditions)

But as I said in my previous post, a 5A fuse is under-rated - change it - ideally for a 10A, but if you haven't got a 10A, use a standard 13Amp.

---

aaah, fair do's, i've never looked into AC-DC conversion (next week i am IIRC) TBH and just presumed it would do the step down first and then the ADC

I thought 13A was the norm for PSU leads?

Thank you for the replies, I do trust you but it's just a total contradiction from what I have read in other forums about similar power problems. They all say never to use a 13A fuse since that would be 2290W and my operational maximum for my psu is aroung 1000W been drawn from the socket and it also wouldnt be safe. I agree that is if i get an earth leakage or catastrophic short circuit the fuse will blow almost instantly, but will it blow fast enough to protect the circuitry in the psu been a 13A fuse. I will work with a 13A fuse but ill have to make sure the cable is rated higher than the fuse, but that wont be a problem.

If the short is in the cable, there will be no risk to the PSU - and that is what the plug top fuse is for. If the fault is in the PSU - the internal PSU fuse should fail (if fitted) but if it is a catastrophic failure, the plug top fuse will blow anyway - and speed of the fuse blowing is unlikely to be an issue.

As I explained in the previous posts, you need to take into account power factor, efficiency and inrush current - which is why a 5A fuse is inadequate. If it makes you feel happier, try a 7 amp (not easy to find - try rswww.com - packets of 10) or a 10 amp (same source).

http://uk.rs-online.com/web/search/s...=7A#breadCrumb

http://uk.rs-online.com/web/search/s...10A#breadCrumb

They are about £1.60 per packet plus VAT and P&P.

You have correctly identified that the fuse rating should not exceed the current rating of the cable it is protecting, but that is unlikely to be an issue. However you could replace the entire cord set to be certain.

Im still quite sceptical about using a higher rated fuse other than a 5A.

It is probably the inrush current causing it, but still to blow a 5A fuse from inrush current is abit high isn't it?

A 5A should be sufficiant enough even with the power factor in the equation.
The box states it has 80% efficiency (so 0.8 power factor), and using 230V for the supply we have the following equations.

I = P/V

So lets work out maximum drawn power if its an 800W psu with 80% efficiency, supplying maximum load.
800 x 1.2 = 960W power drawn at max load with 80% efficiency.
I = 960/230 = 4.17A
even if the supply voltage is 195V, a 5A fuse will still be sufficient

I don't think when a pc starts it will be using maximum load which will mean the current it draws at the start wont be very high. But then I dont know how PSU's work.

Quote:

---

Originally Posted by GarethG

Thank you for the replies, I do trust you but it's just a total contradiction from what I have read in other forums about similar power problems. They all say never to use a 13A fuse since that would be 2290W and my operational maximum for my psu is aroung 1000W been drawn from the socket and it also wouldnt be safe......

.

---

In that case they don't understand the principles of electrical protection - what justification do they give?

Quote:

---

Originally Posted by GarethG

Im still quite sceptical about using a higher rated fuse other than a 5A.

It is probably the inrush current causing it, but still to blow a 5A fuse from inrush current is abit high isn't it?

A 5A should be sufficiant enough even with the power factor in the equation.
The box states it has 80% efficiency (so 0.8 power factor), and using 230V for the supply we have the following equations.

I = P/V

So lets work out maximum drawn power if its an 800W psu with 80% efficiency, supplying maximum load.
800 x 1.2 = 960W power drawn at max load with 80% efficiency.
I = 960/230 = 4.17A
even if the supply voltage is 195V, a 5A fuse will still be sufficient

I don't think when a pc starts it will be using maximum load which will mean the current it draws at the start wont be very high. But then I dont know how PSU's work.

---

You are confusing power factor and efficiency. P=IV for unity power facor. The power factor for a SMPS will typically be .8 to point 9. In my earlier post I tool the worst case as 0.8 and efficiency as 80%. so power =V*I*0.8

Efficiency is typically calculated as output power/input power *100 - so if oupout power is 800 watts input power will be 1000Watts

(Your calculatiuon is only correct if you assume the efficiency is as a percentage of the output power)

So 1000=I*230*0.8

I=1000/(230*.8)

= 1000/184

= 5.4 amps.

Now I accept these are worst case conditions - so the steady state current will be less than that (however there will still be an inrush current which can be more than double the steady state current for a short time - milliseconds) but that is imaterial because the purpose of the plug top fuse is to protect against faults in the connecting lead - not in the appliance. (There are exceptions to that - electric kettles for example where the load is just a low resistance) but that is the principle. so provided the power cord set is rated at 13 Amps or more, you can use a 13Amp fuse. If it is rated at 10 amps, you use a 10 amp fuse.

Using your figures and a power factor of .9, the steadty stae current at full load will still be 4.6 amps - not much margin for the inrush current - and plug top fuses are designed to be fast blow. The data sheet for a BS1362 fuse shows that it will blow in 100ms at twice the rated current - oir 5 cycles of mains power .

However if you are still unhappy, I have given you a source for slightly higher rated fuses.

Who fitted the 5 amp fuse to the plug?

superb reply peterb thank you very much :)

Well I used two kettle leads both with 5A fuses, one came with the new BFG ES-800W PSU which had been fitted with a 5A fuse out of the box and the other was from my older Antec 430W? PSU which was also prefitted with a 5A fuse.