What are fuses for?

pealy

May sound like a silly question but I've been having a debate with a colleague at work...

I have always thought that a fuse is designed to stop problems with an electrical item from affecting the rest of the system. So a fuse in my radio's mains plug is there in case the radio has a fault.

My colleague suggests that the fuse in a radio's mains plug is there to protect it from problems originating elsewhere in the circuit.

Who's right.

This arose from a question about why my household alarm has a single fuse box on it's power supply. I discovered that the alarm is wired to the downstairs lighting circuit so assume it's there in case the alarm develops a fault.

Tallymanjohn

It would need the radio to have a fault - ie drawing excess current - for the fuse to blow.

espresso

You are right. The fuse in the plug protects the appliance under fault conditions and will allow the rest of the items connected to that circuit or ring main to continue to operate without blowing that circuits fuse.

kat21

simply put, a fuse is a protection system employed to protect the device to which it is atached. different types and fuse ratings are dependant upon the device to which it is protecting
kat21

rygon

The fuse protects the system wiring. Ie if you have a faulty toaster the fuse in the plug protects the wiring to the toaster. However if the fault is to get through, the main house fuses will protect the ring main

Also for interest, why they should never be tampered with

say a "13 Amp." Mains Flex of 1.25mmx1m has a resistance of 0.014ohms

using the good ol equation of V=IR and that we know the voltage of uk home is 230v, if a fault occurs where you have a short circuit within that wire you get the current to be

I= V/R
I=230/0.014

I= 16428.57amps

im sure you agree thats quite a lot. Its scary to think that some people will replace a fuse with a nail "because the fuse keeps on blowing" or "beecause i havent got any spare fuses"

flang

http://en.wikipedia.org/wiki/Fuse_%28electrical%29

espresso

rygon wrote:

The fuse protects the system wiring. Ie if you have a faulty toaster the fuse in the plug protects the wiring to the toaster. However if the fault is to get through, the main house fuses will protect the ring main

Also for interest, why they should never be tampered with

say a "13 Amp." Mains Flex of 1.25mmx1m has a resistance of 0.014ohms

using the good ol equation of V=IR and that we know the voltage of uk home is 230v, if a fault occurs where you have a short circuit within that wire you get the current to be

I= V/R
I=230/0.014

I= 16428.57amps

im sure you agree thats quite a lot. Its scary to think that some people will replace a fuse with a nail "because the fuse keeps on blowing" or "beecause i havent got any spare fuses"

You seem to have forgotten about the fuse or circuit breaker protecting that socket! The main electricity supply company fuse will be 100A or less.

:rolleyes:

kat21

espresso
Actually rygon is spot on and its nice to see somebody using ohms law.
I posted a simplyfied explanation.
kat21
if you read rygons post in detail you will see that he does include the word house fuse or protection for that particular ring main. The actual type of fuse wasnt really of topic whether its an rcd, mcb, wired fuse,!
kat21
simply put as I posted earlier on
:rotfl:

djohn2002uk

What espresso points out is absolutely right. 16000 amps will never flow through the cable because even if a nail were put in the plug, the ringmain would be protected by either a 30A fuse or a 32A breaker so that would be the maximum current to flow, and only for milliseconds. And protecting that would be the main breaker in the consumer unit and then (if all that failed) would be the supply company fuse with a max rating of 100A and there are still loads around that are still only 60A.

kat21

djohn2002uk
I suggest you back track the thread and actually read what rygon has posted he quiet rightly as you do and I have too, pointed out that their is indeed a ring main fuse and also the boards fuse which as you say is between one hundered amperies and sixty amperies

please re read what rygon says and read towards the end of his posting. My own explination is alot simpler
kat21

rygon

With regards to my high current scenario
Yes im sure that the next inline fuse/breaker would trip if that happened, but with any safety device it will take time for it to react, and its that short time where the damage (all that current will cause a lot of heat damage).
It was also there to give people a bit of appreciation to electrical faults and why you should take care even if its only 230v.
My mate was once called out to a house where a guy had done some DIY and installed a few new sockets. When he turned the electricity on it had tripped the mcb. Couldnt understand why it would trip..so he tried it again..and again and then held it in place until the main mcb tripped as well. Mate had a look and he had wired it incorrectly and also due to his persistance had melted the contacts in the mcb