What is Fuse/CB Protection For

[bodoo23]

What does actually fuse or circuit breaker protect in electrical cabinet ? When I search it on web, I found different answers. Some says it is for overload and short circuit protection of equipment. but I see that recommended fuse is 6A for 3A device or 1A for 0.25A device. I can understand it protects from short-circuit but I don't understand how 1A fuse will protect the 0.25A rated equipment in case of a 0.75A overload. if it is just for cable protection, the recommended fuses above may be logical but then it means we don't provide overload protection for equipment but just for cable ?

 

[Bit_Bucket_07]

To protect the wire and prevent it from starting fires.

 

[I_Automation]

Fuses and circuit breakers have "curves"


That tells you the overload rating and speed.


A 0.25A fuse of a time delay type WILL allow 1 amp for a short period of time, 0.75 amp for a little longer time, but will eventually blow.


There are electronics rated fuses that are very fast blow. Transformers have transformer rated fuses, and are not supposed to have circuit breakers.



Similar with circuit breakers, some will handle a little overcurrent for a longer period than others, but will eventually trip. Both cables and components can be protected and usually are.

 

[ASF]

As a colleague likes to say, "any piece of wire can carry any amount of current. The only variable is, for how long."

 

[EICS]

To protect the wire and prevent it from starting fires.

as above, other devices are there to protect equipment.

 

[rupej]

In general the fuse/CB is to protect the wire. However, with some devices such as transformers and motors, there is a maximum fuse size allowed for the size of the device, so I would argue the fuse plays a role in protecting those too.

 

[jdbrandt]

What does actually fuse or circuit breaker protect in electrical cabinet ? When I search it on web, I found different answers. Some says it is for overload and short circuit protection of equipment. but I see that recommended fuse is 6A for 3A device or 1A for 0.25A device. I can understand it protects from short-circuit but I don't understand how 1A fuse will protect the 0.25A rated equipment in case of a 0.75A overload. if it is just for cable protection, the recommended fuses above may be logical but then it means we don't provide overload protection for equipment but just for cable ?

IN the USA, there is a breakdown in two categories: Short Circuit Protection (i.e. stopping wires from melting), and Supplemental Protection (like your 0.25AMP fuse).
Supplemental protection can be for a variety of things, ... perhaps to isolate individual loads from the main feed, etc.

 

[goghie]

Hm...
When I was in high school I've been learned that one should always protect equipment, except when it is not know what will be connected on other side of wire/cable - then protection should be according to installed wire/cable.
Your protective equipment should be as selective as possible.

 

[Tom Jenkins]

As a colleague likes to say, "any piece of wire can carry any amount of current. The only variable is, for how long."

 

[JesperMP]

Fuses and circuit breakers protect against both overload and short circuit.
These are two different things, that is why in a circuit breaker the tripping time as a function of the current is made out of two curves.

The overload protection trips by bimetallic strips that bend when they get heated up. The heating/bending mimics the heating up of the connected cable or motor. The bimetallic strips dont trip instantanously when the current exceeds the set value, instead it is a gradual function, higher current makes the time shorter.

The short circuit protection trips basically a relay coil. So it acts in principle instantanously when the current exceeds the set value.

Here is an example of a circuit breaker tripping curve.
Notice that it is two different curves because it is two different functions.
https://electricalinstallation.net/wp-content/uploads/2020/08/Presentation1-1024x576.jpg

A fuse, i.e. a melting wire of usually silver, has a melting time that is a simple exponential-like function of the current. It can also protect against both overload and short circuit but the curve is simpler.

Both circuit breakers and fuses can be used to reduce the short circuit current, which is useful for reducing the short circuit current of an installation of a control panel. That is an entirely different and important topic.