Maximum Fault-Clearing Time Calculator

Cable protection

Cable short-circuit calculator

Calculate fault-clearing time, permissible fault current or minimum conductor size using the cable short-circuit withstand equation.

Fault-clearing formula

t = (K² × S²) / I²

t time in seconds K conductor constant S area in mm² I fault current in A

Enter cable details

Conductor inputs

All fields required

What do you want to calculate?

t Clearing time I Fault current S Conductor size

Conductor material

Copper Aluminium

Insulation type Used to check the permitted final conductor temperature. Conductor size mm² Cross-sectional area of the conductor. Fault current A Enter 5,000 A for a 5 kA fault current. Normal use conductor temperature Temperature immediately before the fault. If not known, refer to Table 3.1 below Final conductor temperature Choose a table value after selecting the insulation. If not known, use the maximum values given in Table 5.2

The supplied Table 5.1 does not provide an aluminium K value at the 350 °C limit for high-temperature insulation.

Calculated result

Maximum clearing time

— seconds Complete all required fields

Automatic table lookup

K value

—

K appears automatically when the material and both temperatures match a combination in Table 5.1.

Next common conductor size

Reference only—confirm the final selection against all installation, derating and protection requirements.

Calculating clearing time (t): This calculator determines the maximum allowable fault-clearing (trip) time for the circuit breaker. The circuit breaker must disconnect within this time to prevent excessive heating that could damage or melt the cable insulation.

Temperature reference

Insulation limits used by the calculator

The insulation type determines the permitted final conductor temperature. The selected temperatures and conductor material then determine K.

160°C

Thermoplastic

For conductors up to and including 300 mm². The limit is 140 °C for conductors larger than 300 mm².

250°C

Cross-linked elastomeric

R-EP-90, R-CPE-90, R-HF-90, R-CSP-90, R-HF-110 and R-F-110.

250°C

Cross-linked polyolefin

X-90, X-90UV, X-HF-90 and X-HF-110.

350°C

High temperature

R-S-150 and Type 150 fibrous insulation.

Table 3.1 — Limiting temperatures for insulated cables (Normal use conductor temperature)

Type of cable insulation	1	2	3
Thermoplastic
V-75	75	75	0
HFI-75-TP, TPF-75	75	75	-20
V-90	75	90	0
HFI-90-TP, TP-90	75	90	-20
V-90HT	75	105	0
Cross-linked elastomeric
R-EP-90	90	90	-40
R-CPF-90, R-HF-90, R-CSP-90	90	90	-20
R-HF-110, R-F-110	110	110	h
R-S-150	150	150	-50
Cross-linked polyolefin (XLPE)
X-90, X-90UV, X-HF-90	90	90	h
X-HF-110	110	110	h
Mineral-insulated metal-sheathed (MIMS)
MIMS	100 (sheath)	250 (sheath)	–
Other types
PE, LLDPE	70	70	h
Type 150 fibrous or polymeric	150	150	–

Material	Temperature limit, °C
Thermoplastic
I.I.DPE, PE, V-75, IIHF-75-TP, TPE-75, V-90, IIHF-90-TP, TP 90 and V-90IIHT
≤ 300 mm²	160
> 300 mm²	140
Cross-linked elastomeric
R-FP-90, R-CPF-90, R-IIHF-90, R-CSP-90, R-IIHF-110 and R-F-110	250
Cross-linked polyolefin
X-90, X-90IIJV, X-IIHF-90 and X-IIHF-110	250
High temperature
R-S-150 and Typc 150 fibrous	350

Table 5.1 — Values of constant (K) for determination of permissible short-circuit currents

Initial °C	Copper						Aluminium				Lead		Steel
Initial °C	140	150	160	220	250	350	140	150	160	250	150	200	150	200
130	37.1	52.2	63.5	106.0	120.4	155.1	24.6	34.5	42.0	79.6	9.6	17.3	18.9	34.2
125	45.6	58.5	68.8	109.3	123.3	157.3	30.2	38.7	45.5	81.6	10.7	18.0	21.2	35.5
110	65.2	74.8	83.1	118.8	131.8	164.1	43.2	49.5	55.0	87.2	13.7	19.9	27.2	39.4
90	85.4	92.9	99.7	131.0	142.9	173.1	56.6	61.6	66.1	94.6	17.1	22.4	33.8	44.2
85	89.9	97.1	103.6	134.0	145.6	175.4	59.6	64.3	68.6	96.4	17.8	23.0	35.3	45.4
80	94.3	101.2	107.4	136.9	148.3	177.7	62.5	67.0	71.2	98.2	18.6	23.5	36.8	46.6
75	98.5	105.1	111.2	139.9	151.1	179.9	65.3	69.7	73.7	100.0	19.3	24.1	38.3	47.7
70	102.7	109.0	114.8	142.8	153.8	182.2	68.0	72.2	76.1	101.8	20.0	24.7	39.7	48.9
65	106.7	112.8	118.4	145.7	156.5	184.5	70.7	74.8	78.5	103.6	20.7	25.3	41.1	50.0
60	110.6	116.5	122.0	148.6	159.2	186.8	73.3	77.2	80.9	105.4	21.4	25.8	42.5	51.2
55	114.5	120.2	125.5	151.6	161.9	189.2	75.9	79.7	83.2	107.2	22.1	26.4	43.9	52.3
50	118.3	123.9	129.0	154.5	164.7	191.5	78.4	82.1	85.5	109.0	22.7	27.0	45.2	53.4
45	122.1	127.4	132.5	157.3	167.4	193.8	80.9	84.5	87.8	110.9	23.4	27.5	46.5	54.6
40	125.8	131.0	135.9	160.2	170.1	196.2	83.4	86.9	90.1	112.7	24.1	28.1	47.9	55.7
35	129.4	134.5	139.3	163.1	172.8	198.6	85.8	89.2	92.4	114.5	24.7	28.6	49.2	56.8
30	133.1	138.0	142.7	166.0	175.6	200.9	88.3	91.5	94.6	116.3	25.4	29.2	50.5	58.0
25	136.7	141.5	146.0	168.9	178.3	203.3	90.7	93.9	96.9	118.1	26.0	29.8	51.8	59.1

Final conductor temperature (°C) shown in header columns.

Note: The results represent the cable short-circuit withstand relationship only. Confirm the final cable and protective-device selection against all applicable installation, derating, voltage-drop and disconnection requirements.

Cable short-circuit calculator

Conductor inputs

Insulation limits used by the calculator

Thermoplastic

Cross-linked elastomeric

Cross-linked polyolefin

High temperature

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