Verification of the withstand capabilities of cables under short-circuit conditions

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 I²t = k²S² characterizes the time in seconds during which a conductor of c.s.a. S (in mm ²) 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 Figure G52 below.

Fig. G52 – Value of the constant k according to table 43A of IEC 60364-4-43

		Conductor insulation
		PVC ≤ 300 mm2	PVC > 300 mm2	EPR XLPE	Rubber 60 °C
Initial temperature (°C)		70	70	90	60
Final temperature (°C)		160	140	250	200
Conductor material	Copper	115	103	143	141
	Aluminium	76	68	94	93

The method of verification consists in checking that the thermal energy I²t 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 Figure G53 below).

Fig. G53 – Maximum allowable thermal stress for cables I²t (expressed in ampere² x second x 10⁶)

S (mm2)	PVC		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	29.839	13.032	46.133	19.936

Example

Is a copper-cored XLPE cable of 4 mm² c.s.a. adequately protected by a iC60N circuit-breaker? (see Fig. G54)

Fig. G53 shows that the I²t value for the cable is 0.3272 x 10⁶, while the maximum “let-through” value by the circuit-breaker, as given in the manufacturer’s catalogue, is considerably less ( < 0.1.10⁶ A²s).

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.

Fig. G54 – Example of coordination table between circuit-breakers and busways (Schneider Electric)

Type of Canalis busbar trunking		KSA100
Isc max. in kA rms		25 kA	36 kA	50 kA
Type of circuit breaker	NG125	NG125N 100	NG125H 80	NG125L 80
	Compact NSXm	NSXm B/F/N/H 100	NSXm F/N/H 100
	Compact NSX	NSX100B/F/N/H/S/L
Type of Canalis busbar trunking		KSA160
Isc max. in kA rms		25 kA	36 kA	50 kA	70kA	90 kA
Type of circuit breaker	Compact NSXm	NSXm B/F/N/H 160	NSXm F/N/H 160	NSXm N/H 160	NSXm H 160
	Compact NSX	NSX100B/F/N/H/S/L	NSX100F/N/H/S/L	NSX100N/H/S/L	NSX100H/S/L	NSX100S/L
		NSX160B/F/N/H/S/L	NSX160F/N/H/S/L	NSX160N/H/S/L	NSX160H/S/L
		NSX250B/F/N/H/S/L	NSX250F/N/H/S/L	NSX250N/H/S/L
Type of Canalis busbar trunking		KSA250
Isc max. in kA rms		25 kA	36 kA	50 kA	70kA	100 kA	150 kA
Type of circuit breaker	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
Type of Canalis busbar trunking		KSA400
Isc max. in kA rms		25 kA	36 kA	50 kA	70kA	100 kA	150 kA
Type of circuit breaker	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
Type of Canalis busbar trunking		KSA500
Isc max. in kA rms		25 kA	36 kA	50 kA	70kA	100 kA	150 kA
Type of circuit breaker	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	NS630b N/H/L/LB		NS630b L/LB	NS630b LB
Type of Canalis busbar trunking		KSA630
Isc max. in kA rms		≤ 32 kA	36 kA	50 kA	70kA	100 kA	150 kA
Type of circuit breaker	Compact NSX	NSX400F/N/H/S/L		NSX400N/H/S/L	NSX400H/S/L	NSX400S/L	NSX400L
		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 LB
		NS800N/H/L/LB	NS800L/LB				NS800LB
	Masterpact MTZ1	MTZ1 06 H1/H2/H3/L1	MTZ1 06 L1
		MTZ1 08 H1/H2/H3/L1	MTZ1 08 L1