Author Topic: Tripping circuit breakers  (Read 7488 times)

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Re: Tripping circuit breakers « Reply #30 on: February 09, 2017, 04:02:38 PM » Author: Ash
The point what I wanted to

Disappeared ?

Lets refinee my question. The breaker must protect the cable (trip in a way that prevents cable overheating above 160C, and prevents above 70 except for a short time). The worst case is either :

 - Progressive isolation failure over few seconds and then hard short

 - Heat impulse from the starting current of some futuristic load (normal startup) and then hard short

In the isolation case, what would be a current profile to take for a model ? 4 sec at 5In (for B breaker) / 2 sec at 10In (for C) and then hard short ? or maybe something worse is possible ?

In the load case, we can assume that the load must be designed so, it cannot make the cable exceed 70c in normal use. Lets asume this way :

 - The load takes Iw continuous in normal work

 - The other loads present on the circuit right before Iw is switched on, draw current Ip. Then Ip <= (In - Iw) where In = 16A

 - Based on this, and on some figure for ambient temp (say 35degC), we can figure out the initial temp of the cable after long time working at current Ip

 - And the cable with the sudden dt from switching on the new load must not exceed 70degC

But is there such "futuristic load that is the worst nightmare for cables" ? Dont present day motors with hard starting allready satisfy exactly that ?

And if we use the "futuristic load" model, what happens if the user got 2 such units and switches them on one after another on the same circuit ? (he can do that without explicitly overloading anything if Ip < In - 2Iw, or 3 such units if Ip < In - 3Iw and so on)
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