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Old 05-13-2010, 03:02 PM   #1
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Plug Fuse Question


Hi there

I was wondering if I could bounce something off you good folks.

I live in the UK and have just received a PC (for pro audio recording) from the US. The person I bought it from shipped it with a power cable which has a US three-prong plug with a label attached stating: 125v/10A, this obviously meaning the voltage used for this type of plug and the fuse fitted in it. This is fair enough as the company who built the PC don't deal with many people from the UK.

The PC's PSU is self switching between 90~264V. Great I hear you all say. Obviously, I'll be using a UK power cable with it.

I'm just a bit confused about fuses given I'm not all that clear on electrics.

If you scroll down on this page to the my PSU's specifications, it gives the Input Current for both US and UK AC Voltages - [email protected] and [email protected]

Is 5A the recommended fuse that I should have in the plug of my power cable or is this exactly how much current is drawn?

I mean, aren't you supposed to use a slighlty higher fuse than an appliance draws?

Any info would be greatly appreciated.


Regards

R.
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Old 05-13-2010, 03:30 PM   #2
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Fusing a switching power supply, even the soft-start type that you have, could be tricky.

At 700w output and 87% efficiency, actual input current at 240v would be (1/0.87)700w/240v = 3.4A so I guess a 5A fuse gives you some overhead.

BTW,
I can't believe they publish the failure rate

"MTBF >100,000hrs"

in the US this is a no-no, because then the consumer would be informed.

Another BTW about your country and ours. . .
There's a type of surgery that is done on women.
In the UK the number of these surgeries performed is proportional to the number of women in the UK. In the US it is proportional to the number of doctors who can do this type of surgery.

Last edited by Yoyizit; 05-13-2010 at 03:46 PM.
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Old 05-13-2010, 07:29 PM   #3
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Thanks Yoyizit

Interesting differences about our countries. These things never cease to amaze me.

Thanks for the info on the fuse. 5A it is then.

I'm not understanding where the 1.0/0.87 came into it the calculation. Can you please explain this? I like to learn stuff.

Regarding overhead, is it correct that the fuse inside a plug should be slightly greater than the input current rating, this being so that the appliance can operate normally without blowing the fuse. So that fuse only blows in the event of a surge?


Regards and thanks.

R.
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Old 05-13-2010, 07:38 PM   #4
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Quote:
Originally Posted by razorcut View Post
5A it is then.

I'm not understanding where the 1.0/0.87 came into it the calculation. Can you please explain this? I like to learn stuff.
1
So that fuse only blows in the event of a surge?
2
1 If 87% of the input makes it to the 700w output, then (700 watts)/0.87 = (X watts)/1.00. Solving for X gives you 700(1/0.87) = 805w of input power.

Checking, 805w x 87% = 700w output.

The remaining 105w heats up the PSU.

2 Yes, it shouldn't melt with normal current, and if the fuse value is too close to the actual current value you might get 'nuisance' outages.
Without soft start the current surge for a switch-mode power supply is enormous so sizing fuses in that case is a nightmare.

Last edited by Yoyizit; 05-13-2010 at 07:42 PM.
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Old 05-13-2010, 08:20 PM   #5
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If that cord is from the USA then it does NOT have a fuse in it. The 10 amp rating is just the maximum it is rated to handle.
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Old 05-13-2010, 08:53 PM   #6
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If the PSU has a switch to chose between 120 and 240 be sure to flip it accordingly!

One of my coworkers plugged a server into a new rack at work assuming it was 120v like everything else. Aparantly they decided to make that rack run on 208 (reads at 206). So yeah... I wish I would of been there to see when he turned it on LOL.
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Old 05-14-2010, 12:13 PM   #7
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Thanks

You folks have been great.

@Yoyizit - Thanks for explaining that calculation. I know that power = current x voltage. I just didn't know where your 1.0/0.87 came in. I undestand now though. I learn somethign new every day....lol

@joed - Thanks for that info. I thought the plug had a fuse in it. I never knew it didn't. In fact, I never knew that plugs had maximum ratings. I just bought what was supposed to be a 10A UK power cable but it turned out to be a 5A one (Amazon made a goof). I was merely going to exchange the 5A fuse in it for a 10A one. Now I won't bother as closer inspection of the plug indicates that its rated at [email protected] I needed a magnifying glass to read this info.

@Red Squirrel - Oops. I should have been more clear on the first post. The PSU is auto switching. I didn't need to do anything. Thankfully....lol. I hope any of your friend's gear did not blow?

In general is it best to have a couple of amps over-head in fuse capacity over the rated input current of the appliance in question?


Regards

R.
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Old 05-14-2010, 03:45 PM   #8
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Quote:
Originally Posted by razorcut View Post
In general is it best to have a couple of amps over-head in fuse capacity over the rated input current of the appliance in question?
It's a fine line; pick your fuse trip curve carefully

normal surge & fuse blows = nuisance
valid overload & fuse blows = OK
normal surge & fuse doesn't blow = OK
valid overload & fuse doesn't blow = big trouble
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Old 05-14-2010, 04:46 PM   #9
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Here in North America we only have a fuses(breakers) in the fuse panel. There are no fuses in the receptacles or cords. I believe in UK you have fuses at each receptacle.
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Old 05-14-2010, 05:17 PM   #10
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I'm not a computer geek but is it possible there is a fuse built into the power supply as extra protection.
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Old 05-14-2010, 05:26 PM   #11
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Quote:
Originally Posted by joed View Post
Here in North America we only have a fuses(breakers) in the fuse panel. There are no fuses in the receptacles or cords. I believe in UK you have fuses at each receptacle.
I don't know for a fact that they (receptacles, in the U.K.) do. But if it was wired so that each room or section has its own low/er amp. fuse, as opposed to the USA and Canada where we have fuses and breakers only in the panel. (Except for small size (in amps.) fuses built into appliances.) It is due to the fact that 220v. is used and a larger area would be protected by the breaker in the panel. Meaning, that if the protection device would blow it would knock out a large area (many rooms) at once.
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