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Protection When Changing CSA

Les Spiteri
I have always went by the rule " when changing (lowering) csa of cable/wire then insert protection device", and further to that the second rule was "if its only a small run perhaps <2M then first rule need not apply". 


What is experience of everybody on this?


This particular application in design phase has an incoming 95mm cable in through a a switch disconnector which requires distribution to many smaller branch circuits. usually this application for me in control panels is simple as small currents (<30A) are in use but this larger requirement means the terminations get tricky having many different wire sizes.


Thanks Folks!33eaed845ccf5afaa1f80feb683c2254-huge-distribution.png
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    Ideally the cable needs to be projected against 2 distinct cases 

    1) an overload  that over heats the thin cable but not the fat one supplying it - that protection may be at the load end of the thin cable, perhaps the motor oveload relay.or outbound breaker of some sort.

    2) damage to the thin cable should operate a protective device (fuse/breaker MCCB etc)  to remove power to it, and perhaps the whole lot, without failing spectacularly and showering the user or the inside of the box with hot metal.

    This depends on the let-through energy of the next MCB or fuse upstream, which may be the thing covering the 90mm incoming cable, and rated at 400A or whatever. Here the adiabatic approach is needed to tell you if you need a local fuse down or not.

    You may decide not to do that 2nd one if the link is short (3m is the normal distance), and the chance of such damage is improbable / contained . (cable well supported and enclosed)

    The classic must be the wire to the light switch in the substation - it is not normally done in 185mm2  in the hop from the busbars to the 5A fuse holder ?

    Mike.

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  • 0
    Ideally the cable needs to be projected against 2 distinct cases 

    1) an overload  that over heats the thin cable but not the fat one supplying it - that protection may be at the load end of the thin cable, perhaps the motor oveload relay.or outbound breaker of some sort.

    2) damage to the thin cable should operate a protective device (fuse/breaker MCCB etc)  to remove power to it, and perhaps the whole lot, without failing spectacularly and showering the user or the inside of the box with hot metal.

    This depends on the let-through energy of the next MCB or fuse upstream, which may be the thing covering the 90mm incoming cable, and rated at 400A or whatever. Here the adiabatic approach is needed to tell you if you need a local fuse down or not.

    You may decide not to do that 2nd one if the link is short (3m is the normal distance), and the chance of such damage is improbable / contained . (cable well supported and enclosed)

    The classic must be the wire to the light switch in the substation - it is not normally done in 185mm2  in the hop from the busbars to the 5A fuse holder ?

    Mike.

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