View source for Verification of the withstand capabilities of cables under short-circuit conditions

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{{Highlightbox|
In general, verification of the thermal-withstand capability of a cable is not necessary, except in cases where cables of small c.s.a. are installed close to, or feeding directly from, the main general distribution board}}

== Thermal constraints  ==

When the duration of short-circuit current is brief (several tenths of a second up to five seconds maximum) all of the heat produced is assumed to remain in the conductor, causing its temperature to rise. The heating process is said to be adiabatic, an assumption that simplifies the calculation and gives a pessimistic result, i.e. a higher conductor temperature than that which would actually occur, since in practice, some heat would leave the conductor and pass into the insulation.

For a period of 5 seconds or less, the relationship I<sup>2</sup>t = k<sup>2</sup>S<sup>2</sup> characterizes the time in seconds during which a conductor of c.s.a. S (in mm <sup>2</sup>) can be allowed to carry a current I, before its temperature reaches a level which would damage the surrounding insulation.

The factor '''k''' is given in {{FigureRef|G52}} below. 

{{tb-start|id=Tab1216|num=G52|title=Value of the constant k according to table 43A of IEC 60364-4-43 |cols=3}}
{| class="wikitable"
|-
! rowspan="2" colspan="2" |
! colspan="4" | Conductor insulation
|-
! PVC<br>≤&nbsp;300&nbsp;mm<sup>2</sup>
! PVC<br>>&nbsp;300&nbsp;mm<sup>2</sup>
! EPR XLPE
! Rubber 60&nbsp;°C
|-
| colspan="2" | Initial temperature (°C)
| 70
| 70
| 90
| 60
|-
| colspan="2" | Final temperature (°C)
| 160
| 140
| 250
| 200
|-
|rowspan="2"  | Conductor material
| Copper
| 115 
| 103
| 143 
| 141
|-
| Aluminium
| 76 
| 68
| 94 
| 93
|}

The method of verification consists in checking that the thermal energy I<sup>2</sup>t per ohm of conductor material, allowed to pass by the protecting circuit-breaker (from manufacturers catalogues) is less than that permitted for the particular conductor (as given in {{FigureRef|G53}} below). 

{{tb-start|id=Tab1217|num=G53|title=Maximum allowable thermal stress for cables I<sup>2</sup>t (expressed in ampere<sup>2 </sup>x second x 10<sup>6</sup>)|cols=3}}
{| class="wikitable"
|-
! rowspan="2" | S (mm<sup>2</sup>) 
! colspan="2" | PVC 
! colspan="2" | XLPE
|-
! Copper 
! Aluminium 
! Copper 
! Aluminium
|-
| 1.5 
| 0.0297 
| 0.0130 
| 0.0460 
| 0.0199
|-
| 2.5 
| 0.0826 
| 0.0361 
| 0.1278 
| 0.0552
|-
| 4 
| 0.2116 
| 0.0924 
| 0.3272 
| 0.1414
|-
| 6 
| 0.4761 
| 0.2079 
| 0.7362 
| 0.3181
|-
| 10 
| 1.3225 
| 0.5776 
| 2.0450 
| 0.8836
|-
| 16 
| 3.3856 
| 1.4786 
| 5.2350 
| 2.2620
|-
| 25 
| 8.2656 
| 3.6100 
| 12.7806 
| 5.5225
|-
| 35 
| 16.2006 
| 7.0756 
| 25.0500 
| 10.8241
|-
| 50{{tn|A}}
| 29.839 
| 13.032 
| 46.133 
| 19.936
|}
{{tb-notes
|A= For 50 mm<sup>2</sup> cables, the actual cross-section used for calculation is 47.5 mm²{{fn|1}}}}

=== Example ===

Is a copper-cored XLPE cable of 4 mm<sup>2</sup> c.s.a. adequately protected by a iC60N circuit-breaker? (see {{FigRef|G54}}) 

{{FigRef|G53}} shows that the I<sup>2</sup>t value for the cable is 0.3272 x 10<sup>6</sup>, while the maximum “let-through” value by the circuit-breaker, as given in the manufacturer’s catalogue, is considerably less ( < 0.1.10<sup>6</sup> A<sup>2</sup>s).

The cable is therefore adequately protected by the circuit-breaker up to its full rated breaking capability.

== Electrodynamic constraints ==

For all type of circuit (conductors or bus-trunking), it is necessary to take electrodynamic effects into account.

To withstand the electrodynamic constraints, the conductors must be solidly fixed and the connections must be strongly tightened, making traditional cable installations withstand level directly depending on the quality of the work executed by the electrical contractor.

For busways (busbar trunking systems), rails, etc. it is also necessary to verify that they will withstand the electrodynamic constraints during a short-circuit. But for busways the electrodynamic withstand performance is defined by construction, and verified by type tests according to IEC 61439-6, with a specified overcurrent protective device.

Manufacturers like Schneider Electric provide ready-to-use coordination tables between their circuit-breakers and their busways, making it quick and easy to select the optimal solution that guarantees the withstand strength of the system.

{{tb-start|id=TAB1259|num=G54|title=Example of coordination table between circuit-breakers and busways (Schneider Electric)|cols=5}}
{| class="wikitable"
|-
! colspan="2" | Type of Canalis busbar trunking
! colspan="6" | KSA100
|-
! colspan="2" | Isc max. in kA rms
!25 kA
!36 kA
!50 kA
| colspan="3" {{tb-HC2}} |
|-
| rowspan="3" width="100px" |Type of circuit breaker
|NG125
|NG125N 100
|NG125H 80
|NG125L 80
| colspan="3" {{tb-HC2}} |
|-
|Compact NSXm
|NSXm B/F/N/H 100
|NSXm F/N/H 100
| colspan="4" {{tb-HC2}} |
|-
|Compact NSX
|NSX100B/F/N/H/S/L
| colspan="5" {{tb-HC2}} |
|-
! colspan="2" | Type of Canalis busbar trunking
! colspan="6" | KSA160
|-
! colspan="2" | Isc max. in kA rms
!25 kA
!36 kA
!50 kA
!70kA
!90 kA
| {{tb-HC2}} |
|-
| rowspan="4" | Type of circuit breaker 
|Compact NSXm
|'''NSXm B/F/N/H 160''' 
|'''NSXm F/N/H 160'''
|'''NSXm N/H 160'''
|'''NSXm H 160'''
| colspan="2" {{tb-HC2}} |
|-
| rowspan="3" |Compact NSX
|NSX100B/F/N/H/S/L    
|NSX100F/N/H/S/L
|NSX100N/H/S/L
|NSX100H/S/L
|NSX100S/L
| {{tb-HC2}} |
|-
|'''NSX160B/F/N/H/S/L''' 
|'''NSX160F/N/H/S/L'''
|'''NSX160N/H/S/L'''
|'''NSX160H/S/L'''
| colspan="2" {{tb-HC2}} |
|-
|NSX250B/F/N/H/S/L 
|NSX250F/N/H/S/L
|NSX250N/H/S/L
| colspan="3" {{tb-HC2}} |
|-
! colspan="2" | Type of Canalis busbar trunking
! colspan="6" | KSA250
|-
! colspan="2" | Isc max. in kA rms
!25 kA
!36 kA
!50 kA
!70kA
!100 kA
!150 kA
|-
| rowspan="3" | Type of circuit breaker 
| rowspan="3" |Compact NSX
|NSX160B/F/N/H/S/L  
|NSX160F/N/H/S/L
|NSX160N/H/S/L
|NSX160H/S/L
|NSX160S/L
|NSX160L
|-
|'''NSX250B/F/N/H/S/L'''   
|'''NSX250F/N/H/S/L'''
|'''NSX250N/H/S/L'''
|'''NSX250H/S/L'''
|'''NSX250S/L'''
|'''NSX250L'''
|-
|NSX400F/N/H/S/L
|NSX400F/N/H/S/L
|NSX400N/H/S/L  
| colspan="3" {{tb-HC2}} |
|-
! colspan="2" | Type of Canalis busbar trunking
! colspan="6" | KSA400
|-
! colspan="2" | Isc max. in kA rms
!25 kA
!36 kA
!50 kA
!70kA
!100 kA
!150 kA
|-
| rowspan="4" |Type of circuit breaker
| rowspan="3" |Compact NSX
|NSX250B/F/N/H/S/L
|NSX250F/N/H/S/L
|NSX250N/H/S/L
|NSX250H/S/L
|NSX250S/L
|NSX250L
|-
|'''NSX400F/N/H/S/L'''
|'''NSX400F/N/H/S/L'''
|'''NSX400N/H/S/L'''
|'''NSX400H/S/L'''
|'''NSX400S/L'''
|'''NSX400L'''
|-
|NSX630F/N/H/S/L
|NSX630F/N/H/S/L
|NSX630N/H/S/L
|NSX630H/S/L
|NSX630S/L
|NSX630L
|-
|Compact NS
|NS630b N/H/L/LB
|NS630b L/LB
|NS630b L/LB
|NS630b LB
| colspan="2" {{tb-HC2}} |
|-
! colspan="2" | Type of Canalis busbar trunking
! colspan="6" | KSA500
|-
! colspan="2" | Isc max. in kA rms
!25 kA
!36 kA
!50 kA
!70kA
!100 kA
!150 kA
|-
| rowspan="3" |Type of circuit breaker
| rowspan="2" |Compact NSX
|NSX400F/N/H/S/L
|NSX400F/N/H/S/L
|NSX400N/H/S/L
|NSX400H/S/L
|NSX400S/L
|NSX400L
|-
|'''NSX630F/N/H/S/L''' 
|'''NSX630F/N/H/S/L'''
|'''NSX630N/H/S/L'''
|'''NSX630H/S/L'''
|'''NSX630S/L'''
|'''NSX630L'''
|-
|Compact NS
| colspan="2" |NS630b N/H/L/LB
|NS630b L/LB
| colspan="2" |NS630b LB
| {{tb-HC2}} |
|-
! colspan="2" | Type of Canalis busbar trunking
! colspan="6" | KSA630
|-
! colspan="2" | Isc max. in kA rms
!&le; 32 kA
!36 kA
!50 kA
!70kA
!100 kA
!150 kA
|-
| rowspan="6" |Type of circuit breaker
| rowspan="2" |Compact NSX
| colspan="2" |NSX400F/N/H/S/L
|NSX400N/H/S/L
|NSX400H/S/L
|NSX400S/L
|NSX400L
|-
| colspan="2" |'''NSX630F/N/H/S/L''' 
|'''NSX630N/H/S/L'''
|'''NSX630H/S/L'''
|'''NSX630S/L'''
|'''NSX630L'''
|-
| rowspan="2" |Compact NS
|NS630b N/H/L/LB
| colspan="4" |NS630b L/LB
|NS630b LB
|-
|NS800N/H/L/LB
| colspan="4" |NS800L/LB
|NS800LB
|-
| rowspan="2" |Masterpact MTZ1
|MTZ1 06 H1/H2/H3/L1
| colspan="4" |MTZ1 06 L1
| {{tb-HC2}} |
|-
|MTZ1 08 H1/H2/H3/L1
| colspan="4" |MTZ1 08 L1
| {{tb-HC2}} |
|}

{{footnotes}}
<references>
{{fn-detail|1|In fact, the cable (maximum) resistance values that should be used for precise calculation are the ones given by the IEC 60228 "conductors of insulated cables" standard. Calculation of cable resistance with the formula R {{=}} ρ L / S, as used for this table, provides values that are sufficiently close to these values (in the order of 1%), except for 50 mm² cables for which a "theoretical" cross-section of 47.5 mm² should be used.}}
</references>