Estimation of Short Circuit forces inside Multicore cables 1

[Power0020]

Cables with multicores will have high SC forces acting on live phases, the cables may survive that as part of their design. I need to select fastners/cleats for mutlicore cables, I am familiar with single core cables where the forces can be easily estimated and cleat spacing accordingly.

How to find the forces acting external to the multicore cable during a short circuit? cleats?

 

[Marmite]

With multicore cables the forces are contained within the cable and do not have any bearing on the selection of cleats. The cable manufacturer designs the cable with sufficient strength to avoid bursting under fault current scenarios. As long as you specify a cable that is adequately rated for fault current magnitude and duration, the internal short circuit forces will be safely contained within the cable.

Regards
Marmite

 

[Power0020]

Great. I have the same understanding.

I saw a SC test video on youtube for cable cleats, it shows a test for multicore cable:

https://youtu.be/4RnmIfdsC_0?t=375

The cable is trying to "expand" under repulsion forces, I think cleats must be able to withstand some force out of these forces internal to the cable.

Not sure how to estimate that figure.

 

[7anoter4]

I think oversheath standard thickness takes into consideration the maximum short-circuit possible.
If the three-phase short-circuit location it is at 3000 kVA 400 V transformer 7% short-circuit impedance [the largest power transformer I ever came across] terminals then:
Ik3=3000/sqrt(3)/400/0.07=61.86 kA
For instance for 3*240 mm^2 copper conductor 0.6/1 kV the permissible thermal current for XLPE insulation will be 61.86 kA for 0.27 sec.
The electromagnetic force for 61.86 kA will be 25113 N/m it[IEC 60865-1 2.3.2.1 Characteristic dimensions and parameters] and the PVC oversheath -according to IEC 60502-1-of 2.2 mm thickness minimum and in this case the stress effort is 4.25 N/mm^2.
If the minimum stress effort for PVC it is 12.7 N/mm^2 according to Table 18 of IEC 60502-1 then the force has to be at least 72351.56 N that means the current has to be 105 kA.

 

[7anoter4]

By the way, the cable clamps are required for single cores cables where no oversheath envelope is provided. See-for instance:

http://www.id-technik.com/en/start/