If we take some fuse-wire and pass through it different currents we could create a time current characteristic graph. The graph would show that the relationship between current flow and time for the fuse-wire to burn out is not linear. As the current increase the time for the fuse-wire to burn out decreases much faster giving a curved line.
Since current flow is proportional to resistance for a given voltage then a fault having a zero resistance would create the greatest current hence the fastest trip time. There is however additional resistance to consider and this is the resistance or impedance of the supply (Ze) the phase conductor and circuit protective conductor CPC or (R1+R2), these combined make up Zs.
The higher Zs is, then the longer it takes for the fuse to blow since current flow will be a less. It is possible that the value of Zs is so high that even when a short is placed at the furthest point there is insufficient current flow to cause the fuse to blow.
BS 7671 lays down the times that over-current devices must operate and this means that for a particular device there needs to be a maximum value of Zs that will allow the device to operate within this time.
Subscribe to:
Post Comments (Atom)
0 comments:
Post a Comment