Energy let-through

[automationman]

Hi Ya'll... I read an interesting post on here a few days ago but can't find it now!! Did not get a chance to read it all but energy let-through came up in the conversation/debate. The main topic may have been about regs compliance of some sub-mains. I was going to join the debate at the time but had to go to work (sad)

My memory of energy let-through (which is now a very old memory) makes me think it is related only to the difference between the maximum PFC of the circuit VS the point at which the fuse disconnects within a half cycle. In other words as the AC phase angle increases from 0 towards 90deg (increasing voltage/current assuming unity PF) then disconnection occurs before the maximum PFC is reached.. (neglecting fuse rating)

So it seems we are essentially talking about what happens within a 5ms window of the whole 20ms period (assuming 50Hz)

Given a typical maximum disconnection time of 400ms, is it not an academic point? Since several other cycles may have occurred (during the pre-arcing period) prior to the point where the rate of current rise causes total disconnection. Does this not make that final area under the curve (what I remember as being the energy-let-through) almost irrelevant? That is not a criticism of the conversation, in fact I found the subject very interesting and hope someone can tell me where it is and educate my out of my poor memory and probable poor understanding of the subject



Cheers,

 

[automationman]

Just googled energy let-through and I see it is I2t. I guess that it refers to I2t over the entire period from fault current intiation to final disconnection time (melting time of fuse link)

I imagine the fuse link starts its "melting process" on the first fault cycle and stores heat from that cycle then responds to further (cumulative) heating effects from subsequent cycles where each cycle (from fault initiation to disconnection) carries increasing current (with rate of current rise determined by circuit PFC assuming zero/minimal impedance fault)

If that is the case then my first assumption/memory of energy let-through being related only within a single cycle is clearly wrong? (unless the PFC is extremely low resulting in rapid current rise and therefore possible disconnection within a single cycle)