Which fuse should I use?

[Chris Crebbice]

Regarding a laptop charger that is marked 100-240V @ 3.2A input, and is rated 230W output, what fuse should I fit to this? I'm assuming the 3.2A is for the 100V input, and that the 240V end would be using more like 1.5A. So should I fit a 3A fuse to it? Here is a picture of the charger:

https://Upload the image directly to the thread.com/a/h0AfJIn

The plug cord itself (probably not the original one that came with the charger) is marked for 5A, which is one further consideration. I'm assuming it's OK to put a 3A fuse in a 5A rated plug lead, but can anyone confirm?

 

[bomjac]

3 amps is fine. Ultimately the fuse is for protection agains short circuit of the charger or cable.

Even if you were to hook it up to 100v, the 3a fuse would never trip. it requires around 5 amps to trip - and even that is not guarenteed.

 

[Wilko]

Hi - yes the 5A rated cable can be protected by a 3A fuse.

 

[GeorgeCooke]

In practice any fuse up to 13A can be used as the charger will be internally protected. In virtually every other country in Europe there would be no fuse and it would plug into a 16 A protected socket.

 

[Lucien Nunes]

You are correct about the highest current rating being at the lowest rated input voltage. However, I would normally fit a 5A fuse as the inrush may weaken a 3A over time. As mentioned, the charger is internally fused against overcurrent, the plug fuse is only required to protect the flex against short-circuit, and for any flex over 0.5mm² even a 13A fuse is acceptable (your flex will probably be 0.75mm²).

 

[l4urence]

General rule - below 750w then 3A fuse otherwise 13A

 

[pirate]

Maybe if just over 750W, a 5 or 10 amp fuse? depending on the actual load?

 

[l4urence]

@pirate good point - never really bothered with 5 or 10 - sadly my general rule was 'learned' in my previous life in the RAF - but probably because they didn't have any 5's or 10's

 

[GeorgeCooke]

More likely the "teacher" in the RAF was wrong. 749 watts (which comes under 750) would be 3.12 amps at 240v. No, it would not blow but running a fuse just above its rating for a long period of time would weaken it.

As has been said before the fuse is for protection of the cable not the load on the end of it which should be internally protected.

 

[pirate]

OK, just looking for some clarification here, so don't shout at me, ok?

In round figures, if it's say up to 700W and it's got a 3 A fuse and it's working fine...leave it alone (ISITEC rule). However, just because it is more than 700W doesn't mean a 13A fuse is the only option, right?

I recently fitted some ceiling mounted heating panels, and the 350W versions came without a plug, but recommended to fit one with a 3A fuse. The other ones were 700W and came fitted with a moulded plug fitted with a 13A fuse. What is the rationale there?
Some say, don't mess with the manufacturers stuff, fit it as it is...
I can't see why substituting say a 5A fuse would be wrong.

 

[Toneyz]

3A and 13A are widely available to the general public.

 

[GeorgeCooke]

I recently fitted some ceiling mounted heating panels, and the 350W versions came without a plug, but recommended to fit one with a 3A fuse. The other ones were 700W and came fitted with a moulded plug fitted with a 13A fuse. What is the rationale there?

Because as I said:
As has been said before the fuse is for protection of the cable not the load on the end of it which should be internally protected.

I can't see why substituting say a 5A fuse would be wrong.

Only 3 and 13 amp fuses are colour coded and officially recommended. The other sizes are black and not officially recommended.

 

[pirate]

Thanks for that, and I know 3 and 13 are readily available to the public, but so are 5 and 10. 5A are often fitted to electric drills, for obvious reasons.
The point about the fuse protecting the cable and not the load is fair enough, but you wouldn't recommend a table lamp being protected by a 13A fuse, as the lamp has no internal protection...nor do many appliances, so you want a fuse that will "blow" if the current exceeds the "design" current for the appliance, so if a fault occurs the appliance becomes isolated....it may be damaged, but won't continue to operate in a fault condition, causing heat damage to the supply cable.

 

[GeorgeCooke]

Thanks for that, and I know 3 and 13 are readily available to the public, but so are 5 and 10. 5A are often fitted to electric drills, for obvious reasons.
The point about the fuse protecting the cable and not the load is fair enough, but you wouldn't recommend a table lamp being protected by a 13A fuse, as the lamp has no internal protection...nor do many appliances, so you want a fuse that will "blow" if the current exceeds the "design" current for the appliance, so if a fault occurs the appliance becomes isolated....it may be damaged, but won't continue to operate in a fault condition, causing heat damage to the supply cable.

You still don't understand.The obvious reasons 5 amp are fitted to electric drills is because the manufacturer does not understand that the fuse is to protect the cable not the drill. Table lamps can be fitted with a 13 amp fuse. Incandescent bulbs have internal fuses as do CFLs and LED bulbs, as should other appliances. In most of the world plugs don't have fuses and the circuit is protected by a 16 amp MCB and tablelamps, drills etc survive perfectly well.

 

[DPG]

3A and 13A are the two recognised values of fuses for appliance plug-tops/plugs nowadays. Having said that I do occasionally use 5A fuses if I feel they are more suitable. And yes, I do realise they are protecting the cable.

 

[Toneyz]

Because as I said:
As has been said before the fuse is for protection of the cable not the load on the end of it which should be internally protected.



Only 3 and 13 amp fuses are colour coded and officially recommended. The other sizes are black and not officially recommended.

Why are they not recommended?

 

[GeorgeCooke]

Why are they not recommended?

Ask the IET, not me.

 

[Toneyz]

Only 3 and 13 amp fuses are colour coded and officially recommended. The other sizes are black and not officially recommended.[/QUOTE]
Why are they not recommended?

Ask the IET, not me.

Where have you taken this info from?

 

[DPG]

Anthony, I believe it is BS1362 that now recognises just the 3A and 13A values.

 

[Toneyz]

Anthony, I believe it is BS1362 that now recognises just the 3A and 13A values.

Yes, your right there, only those two are in BS7671 same with MCB's are manufactured in other ratings. I have the 5A and 10A fuses with BS1362 I thought that just because of the max Zs are not published in BS7671 they are not non-compliant and not recognised.