fuses/protective devices

[chicken001]

can anyone explain per-arcing i2t, arcing i2t, total let through energy. in relation to fuses/protective devices

 

[telectrix]

https://www.google.com/url?q=http:/...ds-cse&usg=AFQjCNH873f6PCqFmCvXQCk4sDDAx80RZg

 

[MarkieSparkie]

can anyone explain per-arcing i2t, arcing i2t, total let through energy. in relation to fuses/protective devices

Pre-arcing I[SUP]2[/SUP]t is the energy required to take the fuse element to the point where it starts to melt.
Arcing I[SUP]2[/SUP]t is the energy required to melt the fuse element.
Total operating, let through I[SUP]2[/SUP]t is the total energy until the arc is quenched.
The arc is caused by the fuse element melting and breaking the fault current.

Although protective devices may easily discriminate with each other under overload conditions, they may not under fault conditions, so a check is necessary to determine discrimination under fault conditions.
In order to achieve fault discrimination between two fuses:- the total operating, let through I[SUP]2[/SUP]t of the lower nominal rated device should be less than the minimum pre-arcing I[SUP]2[/SUP]t of the higher nominal rated device.

Since the relevant British Standards allow a certain manufacturing tolerance within which the design of the fuse characteristic can be made by the fuse manufacturer, therefore it is important to refer to specific manufacturers data when determining discrimination under fault conditions. It is considered prudent to allow a margin of safety between the two values to avoid nuisance blowing which may occur between fuses of different manufacturers.
As a rough guide (Rule of Thumb), fault discrimination will usually be achieved if the higher nominal rated fuse is twice the nominal rated value of the smaller fuse.

 

[D Skelton]

In order to achieve fault discrimination between two fuses:- the total operating, let through I[SUP]2[/SUP]t of the lower nominal rated device should be less than the minimum pre-arcing I[SUP]2[/SUP]t of the higher nominal rated device.

You're quite right, and this is something which many sparks either know nothing about or just don't pay attention to!