Are mains plug fuses really needed now?

JohnB47

A bit of an academic question, seeing as you always have to have one in plugs anyway but if you've got modern consumer units and they have those circuit breaker thingies, surely a 3A, 5A or 13A fuse in plugs isn't really protecting anything? I suppose there are still old fuse boxes in some houses, making fuses in plugs necessary.

casper_gutman

In my house the ring main for the sockets is connected to a 32A circuit breaker. The little puny flex on my desk lamp can't carry 32A, so having a lower-rated fuse in the plug is still a useful safety feature in case a fault causes the lamp to draw a large current.

Yes, there are also RCDs in the consumer unit but they work by looking for an imbalance in the current in the live an neutral. They are meant to improve safety when current is finding an alternative route - e.g. through you - but will do nothing to stop an excessively large current overloading some part of a plugged-in appliance. This could cause overheating and lead to a fire.

CliveOfIndia

I know what you mean - these days a fuse never seems to blow, I guess the circuit breakers are far more sensitive and will trip long before the fuse blows.  Speaking as a layman, I suspect they're not strictly necessary now - though of course, an extra layer of protection is no bad thing.  And of course, you have to have a fuse in there anyway to complete the circuit, and I can't see them redesigning every plug and appliance in the country just to do away with the fuse.

I guess a valid reason for retaining the fuse is if, like you say, you visit a friend's house or something that has an old fuse-box and plug your hair-dryer in, that sort of thing.  Or if you take anything electrical with you on holiday abroad where their standards might be different.  I suspect there may be places where a fuse is still very much needed to protect you.

Section62

JohnB47 said:

A bit of an academic question, seeing as you always have to have one in plugs anyway but if you've got modern consumer units and they have those circuit breaker thingies, surely a 3A, 5A or 13A fuse in plugs isn't really protecting anything? I suppose there are still old fuse boxes in some houses, making fuses in plugs necessary.

Yes, they are necessary.

The BS1362 fuse in a plug top is there to protect what is downstream of that point.  MCB's or fuses in the consumer unit are primarily there to protect the wiring between the consumer unit and the socket outlet, and would typically be rated higher than appropriate for what is plugged in to the socket.

There are some specialist applications where a BS546-type '5A' unfused plugtop might be used, in which case the fuse/MCB in the consumer unit (or a fuse elsewhere in the circuit) is what provides the downstream protection.

But if you rely on the MCB/fuse in the consumer unit rather than having fuses in plugtops, you increase the potential inconvenience (and possibly some risk) involved when a fault in one appliance trips out everything on the same circuit.

Section62

CliveOfIndia said:

I know what you mean - these days a fuse never seems to blow, I guess the circuit breakers are far more sensitive and will trip long before the fuse blows.

It depends on the type of appliance and the circuit it is plugged into, and the type of fault.

@sujsuj could testify that 13A plugtop fuses can and do blow with frustrating frequency in some circumstances.

sk2402005

The fuse in your plug, and mechanical circuit breakers in your consumer unit serve two different purposes.

The MCB is protecting the entire circuit for the ring main for example, it will be rated at a bit less than the circuit can provide (example 32A)

The fuse is there to protect the item you’re plugging in, so that it can’t draw more power than the flex can provide (example 13A)

A plugged-in item if it tried to pull 31A (i.e. before the MCB tipped) things would get very melty very quickly and cause a fire.

And as casper_guman has suggested, the RCD is there to stop electrocution, not a circuit overload.

MrsStepford

When I lived in Canada, the main itself was earthed, so none of the power sockets had switches or fuses, you just plugged them in. I was told that this is safer but I remain unconvinced. 

twopenny

Should you get a fire from one I guess the insurance company would take a dim view of no fuse.

Since a lightening strike blew all the routers in the area, was said by the phone line, but the fuse in the surge socket for my laptop plug seemed to keep it safe I'm not taking any chances

JohnB47

All very interesting and informative. Thanks everyone.

Risteard

MrsStepford said:

When I lived in Canada, the main itself was earthed, so none of the power sockets had switches or fuses, you just plugged them in. I was told that this is safer but I remain unconvinced. 

It's not necessary there or throughout most of the world because they don't have ring final circuits.

flo22

I had a fuse blow in an extension not long ago, the consumer unit did not trip