Isc at the receiving end of a feeder as a function of the Isc at its sending end

The network shown in Figure G41 typifies a case for the application of Figure G42 , derived by the "method of composition" (mentioned in chapter Protection against electric shocks and electrical fires). These tables give a rapid and sufficiently accurate value of short-circuit current at a point in a network, knowing:

• The value of short-circuit current upstream of the point considered
• The length and composition of the circuit between the point at which the short-circuit current level is known, and the point at which the level is to be determined

Fig. G41 – Determination of downstream short-circuit current level Isc

It is then sufficient to select a circuit-breaker with an appropriate short-circuit fault rating immediately above that indicated in the tables.

If more precise values are required, it is possible to make a detailed calculation or to use a software package, such as Ecodial. In such a case, moreover, the possibility of using the cascading technique should be considered, in which the use of a current limiting circuit-breaker at the upstream position would allow all circuit-breakers downstream of the limiter to have a short-circuit current rating much lower than would otherwise be necessary (See Coordination between circuit-breakers).

Method

Select the c.s.a. of the conductor in the column for copper conductors (in this example the c.s.a. is 50 mm²).

Search along the row corresponding to 50 mm² for the length of conductor equal to that of the circuit concerned (or the nearest possible on the low side). Descend vertically the column in which the length is located, and stop at a row in the middle section (of the 3 sections of the Figure) corresponding to the known fault-current level (or the nearest to it on the high side).

In this case 30 kA is the nearest to 28 kA on the high side. The value of short-circuit current at the downstream end of the 20 metre circuit is given at the intersection of the vertical column in which the length is located, and the horizontal row corresponding to the upstream Isc (or nearest to it on the high side).

This value in the example is seen to be 14.7 kA.

The procedure for aluminium conductors is similar, but the vertical column must be ascended into the middle section of the table.

In consequence, a DIN-rail-mounted circuit-breaker rated at 63 A and Isc of 25 kA (such as a NG 125N unit) can be used for the 55 A circuit in Figure G41.

A Compact rated at 160 A with an Isc capacity of 25 kA (such as a NSX160 unit) can be used to protect the 160 A circuit.

Fig. G42 – Isc at a point downstream, as a function of a known upstream fault-current value and the length and c.s.a. of the intervening conductors, in a 230/400 V 3-phase system

Copper 230 V / 400 V
c.s.a.of phase conductors (mm2)	Length of circuit (in metres)
1.5														1.3	1.8	2.6	3.6	5.2	7.3	10.3	14.6	21
2.5												1.1	1.5	2.1	3.0	4.3	6.1	8.6	12.1	17.2	24	34
4											1.2	1.7	2.4	3.4	4.9	6.9	9.7	13.7	19.4	27	39	55
6											1.8	2.6	3.6	5.2	7.3	10.3	14.6	21	29	41	58	82
10										2.2	3.0	4.3	6.1	8.6	12.2	17.2	24	34	49	69	97	137
16								1.7	2.4	3.4	4.9	6.9	9.7	13.8	19.4	27	39	55	78	110	155	220
25						1.3	1.9	2.7	3.8	5.4	7.6	10.8	15.2	21	30	43	61	86	121	172	243	343
35						1.9	2.7	3.8	5.3	7.5	10.6	15.1	21	30	43	60	85	120	170	240	340	480
50					1.8	2.6	3.6	5.1	7.2	10.2	14.4	20	29	41	58	82	115	163	231	326	461
70					2.7	3.8	5.3	7.5	10.7	15.1	21	30	43	60	85	120	170	240	340
95				2.6	3.6	5.1	7.2	10.2	14.5	20	29	41	58	82	115	163	231	326	461
120		1.6	2.3	3.2	4.6	6.5	9.1	12.9	18.3	26	37	52	73	103	146	206	291	412
150	1.2	1.8	2.5	3.5	5.0	7.0	9.9	14.0	19.8	28	40	56	79	112	159	224	317	448
185	1.5	2.1	2.9	4.2	5.9	8.3	11.7	16.6	23	33	47	66	94	133	187	265	374	529
240	1.8	2.6	3.7	5.2	7.3	10.3	4.6	21	29	41	58	83	117	165	233	330	466	659
300	2.2	3.1	4.4	6.2	8.8	12.4	17.6	25	35	50	70	99	140	198	280	396	561
2x120	2.3	3.2	4.6	6.5	9.1	12.9	18.3	26	37	52	73	103	146	206	292	412	583
2x150	2.5	3.5	5.0	7.0	9.9	14.0	20	28	40	56	79	112	159	224	317	448	634
2x185	2.9	4.2	5.9	8.3	11.7	16.6	23	33	47	66	94	133	187	265	375	530	749
3x120	3.4	4.9	6.9	9.7	13.7	19.4	27	39	55	77	110	155	219	309	438	619
3x150	3.7	5.3	7.5	10.5	14.9	21	30	42	60	84	119	168	238	336	476	672
3x185	4.4	6.2	8.8	12.5	17.6	25	35	50	70	100	141	199	281	398	562
Isc upstream (in kA)	Isc downstream (in kA)
100	93	90	87	82	77	70	62	54	45	37	29	22	17.0	12.6	9.3	6.7	4.9	3.5	2.5	1.8	1.3	0.9
90	84	82	79	75	71	65	58	51	43	35	28	22	16.7	12.5	9.2	6.7	4.8	3.5.	2.5	1.8	1.3	0.9
80	75	74	71	68	64	59	54	47	40	34	27	21	16.3	12.2	9.1	6.6	4.8	3.5	2.5	1.8	1.3	0.9
70	66	65	63	61	58	54	49	44	38	32	26	20	15.8	12.0	8.9	6.6	4.8	3.4	2.5	1.8	1.3	0.9
60	57	56	55	53	51	48	44	39	35	29	24	20	15.2	11.6	8.7	6.5	4.7	3.4	2.5	1.8	1.3	0.9
50	48	47	46	45	43	41	38	35	31	27	22	18.3	14.5	11.2	8.5	6.3	4.6	3.4	2.4	1.7	1.2	0.9
40	39	38	38	37	36	34	32	30	27	24	20	16.8	13.5	10.6	8.1	6.1	4.5	3.3	2.4	1.7	1.2	0.9
35	34	34	33	33	32	30	29	27	24	22	18.8	15.8	12.9	10.2	7.9	6.0	4.5	3.3	2.4	1.7	1.2	0.9
30	29	29	29	28	27	27	25	24	22	20	17.3	14.7	12.2	9.8	7.6	5.8	4.4	3.2	2.4	1.7	1.2	0.9
25	25	24	24	24	23	23	22	21	19.1	17.4	15.5	13.4	11.2	9.2	7.3	5.6	4.2	3.2	2.3	1.7	1.2	0.9
20	20	20	19.4	19.2	18.8	18.4	17.8	17.0	16.1	14.9	13.4	11.8	10.1	8.4	6.8	5.3	4.1	3.1	2.3	1.7	1.2	0.9
15	14.8	41.8	14.7	14.5	14.3	14.1	13.7	3.3	12.7	11.9	11.0	9.9	8.7	7.4	6.1	4.9	3.8	2.9	2.2	1.6	1.2	0.9
10	9.9	9.9	9.8	9.8	9.7	9.6	9.4	9.2	8.9	8.5	8.0	7.4	6.7	5.9	5.1	4.2	3.4	2.7	2.0	1.5	1.1	0.8
7	7.0	6.9	6.9	6.9	6.9	6.8	6.7	6.6	6.4	6.2	6.0	5.6	5.2	4.7	4.2	3.6	3.0	2.4	1.9	1.4	1.1	0.8
5	5.0	5.0	5.0	4.9	4.9	4.9	4.9	4.8	4.7	4.6	4.5	4.3	4.0	3.7	3.4	3.0	2.5	2.1	1.7	1.3	1.0	0.8
4	4.0	4.0	4.0	4.0	4.0	3.9	3.9	3.9	3.8	3.7	3.6	3.5	3.3	3.1	2.9	2.6	2.2	1.9	1.6	1.2	1.0	0.7
3	3.0	3.0	3.0	3.0	3.0	3.0	2.9	2.9	2.9	2.9	2.8	2.7	2.6	2.5	2.3	2.1	1.9	1.6	1.4	1.1	0.9	0.7
2	2.0	2.0	2.0	2.0	2.0	2.0	2.0	2.0	12.0	1.9	1.9	1.9	1.8	1.8	1.7	1.6	1.4	1.3	1.1	1.0	0.8	0.6
1	1.0	1.0	1.0	1.0	1.0	1.0	1.0	1.0	1.0	1.0	1.0	1.0	1.0	0.9	0.9	0.9	0.8	0.8	0.7	0.6	0.6	0.5
Aluminium 230 V / 400 V
c.s.a. of phase conductors(mm2)	Length of circuit (in metres)
2.5														1.4	1.9	2.7	3.8	5.4	7.6	10.8	15.3	22
4												1.1	1.5	2.2	3.1	4.3	6.1	8.6	12.2	17.3	24	35
6												1.6	2.3	3.2	4.6	6.5	9.2	13.0	18.3	26	37	52
10											1.9	2.7	3.8	5.4	7.7	10.8	51.3	22	31	43	61	86
16										2.2	3.1	4.3	6.1	8.7	12.2	17.3	24	35	49	69	98	138
25								17.1	2.4	3.4	4.8	6.8	9.6	13.5	19.1	27	38	54	76	108	153	216
35							1.7	2.4	3.4	4.7	6.7	9.5	13.4	18.9	27	38	54	76	107	151	214	302
50						1.6	2.3	3.2	4.6	6.4	9.1	12.9	18.2	26	36	51	73	103	148	205	290	410
70						2.4	3.4	4.7	6.7	9.5	13.4	19.0	27	38	54	76	1.7	151	214	303	428
95					2.3	3.2	4.6	6.4	9.1	12.9	18.2	26	36	51	73	103	145	205	290	411
120					2.9	4.1	5.8	8.1	11.5	16.3	23	32	46	65	92	130	184	259	367
150					3.1	4.4	6.3	8.8	12.5	17.7	25	35	50	71	100	141	199	282	399
185				2.6	3.7	5.2	7.4	10.4	14.8	21	30	42	59	83	118	167	236	333	471
240	1.2	1.6	2.3	3.3	4.6	6.5	9.2	13.0	18.4	26	37	52	73	104	147	208	294	415
300	1.4	2.0	2.8	3.9	5.5	7.8	11.1	15.6	22	31	34	62	88	125	177	250	353	499
2x120	1.4	2.0	2.9	4.1	5.8	8.1	11.5	16.3	23	33	46	65	92	130	184	260	367	519
2x150	1.6	2.2	3.1	4.4	6.3	8.8	12.5	17.7	25	35	50	71	100	141	200	280	399
2x185	1.9	2.6	2.7	5.2	7.4	10.5	14.8	21	13	42	59	83	118	167	236	334	472
2x240	2.3	3.3	4.6	6.5	9.2	13.0	18.4	26	37	52	74	104	147	208	294	415	587
3x120	2.2	3.1	4.3	6.1	8.6	12.2	17.3	24	34	49	69	97	138	195	275	389	551
3x150	2.3	3.3	4.7	6.6	9.4	13.3	18.8	27	37	53	75	106	150	212	299	423	598
3x185	2.8	3.9	5.5	7.8	11.1	13.7	22	31	44	63	89	125	177	250	354	500	707
3x240	3.5	4.9	6.9	9.8	13.8	19.5	28	39	55	78	110	156	220	312	441	623

• Note: for a 3-phase system having 230 V between phases, divide the above lengths by √3