laptop power supply cables - fuses and ratings

[reverendo]

Hi there.

I have noticed that most laptop power supply cables have a 5A fuse in the plug, but a 2.5A rating on the clover leaf end of the cable. Is this safe? Surely there should be a 3A fuse in the plug at least, given the cable is rated for 2.5A. The whole industry seems to be happy to supply cables with 5A fuses though

 

[westward10]

Is it stamped on the side of the cable it's actual size like 3 × 0.75 or 3 × 1.0.

 

[reverendo]

Thank you. It says on the side of the cable "3G - 0.75mm²"

 

[westward10]

0.75 is rated at 6A so a 5A fuse is fine as it is there to protect the cable. The 'cloverleaf' socket is rated at 2.5A but in reality the charger will never come close to hitting this in normal operation.

 

[reverendo]

That is really good to hear thank you very much

 

[reverendo]

sorry, ive got another thought on this. If the charger goes wrong or shorts in a way that does pull more current...eg 4.5A, then won't the plug and fuse happily keep going and the clover leaf could melt. Once the plastic has melted then potentially it could catch fire or cause something else to catch fire.

 

[westward10]

Never say never but provided it came from a proprietary source, is used correctly (do not enclose or cover it) and is ideally not left on 24/7 it should be fine. The fuse it there to protect the cable so if something were to malfunction internally the fuse is not likely to save it.

 

[reverendo]

Thank you very much. I guess under normal circumstances with a single user should be fine. an issue becomes more likely if you have a fleet of 1,000 of them under varying conditions i guess

 

[westward10]

As soon as we introduce electricity and appliances which use this electricity to a building there is going to be a fire risk. To mitigate this all you can do is put sensible procedures in place, like ensuring your electrical installation is correctly installed and maintained and that your appliances are obtained from reputable sources and that they are used as per manufacturer instructions.

 

[Lucien Nunes]

sorry, ive got another thought on this. If the charger goes wrong or shorts in a way that does pull more current...eg 4.5A

The charger has another fuse internally to prevent this happening, matched to the charger consumption but often in the region of 1A. Even if that fuse were bypassed, most kinds of faults in the charger would blow the plug fuse instantly anyway, or blow some component in the charger and stop it consuming any current at all. E.g. if the main reservoir electrolytic shorts, this will blow the internal fuse. If one is unlucky it might simultaneously kill the rectifier and maybe even the plug fuse at the same time, but during this process the current will never be at a sustained level that could overheat the cloverleaf connector. In extreme circumstances with a very unlikely combination of component failures the PSU guts themselves might overheat, but the cloverleaf connector won't get a chance to overheat before the PSU is history.

As above therefore the fuse is to protect the flex and 5A is fine. We only have fused plugs in the UK because of our 32A socket outlet circuits. In other countries where the sockets are protected at 16 or 20A, there is no plug fuse and the flex and cloverleaf are still fine, because they are protected against short-circuit by that 20A device and against overload by the fuse in the charger.