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We have plugs and connectors in lead sets rated 2.5A and we use a 3A fuse and also rated 10A and we use a 13A fuse for example the kettle lead is rated 10A.
It is the same with 5A and 6A many lighting products like ceiling rose rated at 5A but we use a 6A MCB.
As you have pointed out fuses will take over the rated current and for it to be spot on the ambient temperature must also be spot on and the cooling must be carefully controlled i.e. all plugs must be in free air.
This clearly will not happen it is just approximate. Even the slight alteration in plug design to include finger guards will reduce the plugs capacity to remove heat from the fuse.
A magnetic trip with oil damper can be set to very fine limits but unless we need those fine limits then far cheaper to use a thermal device and it is a long time since I have seen motor overloads with dash pots.
Be it a fuse or a trip we want it to deliver the rated current for ever and a day without failing so they will always be designed to let through a little more than the rated value.
If we need a more accurate control for example motor overloads then we use a more expensive device able to be set more accurately. I have one job with two extractor fans which removed explosive gases and I was given the task to build a system which would close down the plant if both the extractors failed and I used a current switch to detect they were running and a very finely set overload which was tested to show it would trip with a stalled fan. But look at the price of a motor overload and compare to simple fuse and it is easy to see why we use a simple fuse in most cases.
It is the same with 5A and 6A many lighting products like ceiling rose rated at 5A but we use a 6A MCB.
As you have pointed out fuses will take over the rated current and for it to be spot on the ambient temperature must also be spot on and the cooling must be carefully controlled i.e. all plugs must be in free air.
This clearly will not happen it is just approximate. Even the slight alteration in plug design to include finger guards will reduce the plugs capacity to remove heat from the fuse.
A magnetic trip with oil damper can be set to very fine limits but unless we need those fine limits then far cheaper to use a thermal device and it is a long time since I have seen motor overloads with dash pots.
Be it a fuse or a trip we want it to deliver the rated current for ever and a day without failing so they will always be designed to let through a little more than the rated value.
If we need a more accurate control for example motor overloads then we use a more expensive device able to be set more accurately. I have one job with two extractor fans which removed explosive gases and I was given the task to build a system which would close down the plant if both the extractors failed and I used a current switch to detect they were running and a very finely set overload which was tested to show it would trip with a stalled fan. But look at the price of a motor overload and compare to simple fuse and it is easy to see why we use a simple fuse in most cases.