Cable Short-Circuit Rating and Protection Coordination

What fault cases must the cable check define?

Obtain the short-circuit study node, system voltage, frequency, earthing method, source configuration and study date. Record the fault types that matter to the circuit: three-phase, phase-to-phase, phase-to-earth or another project-defined case.

The maximum fault current usually governs thermal and mechanical stress. The minimum fault current can govern whether protection operates quickly enough. Each current must be tied to a duration and a network condition. A single value labelled "short-circuit level" is not enough, because source contribution, transformer configuration, generators, converter-based sources or network switching can change the result.

The IEC standard for calculating short-circuit currents in low- and high-voltage three-phase AC systems is listed in the References. Use it to establish the available current before any cable check is made.

How do you calculate the permissible short-circuit current?

The system study calculates the current that can flow. The cable check calculates the current the installed components can withstand for the clearing duration. Do not replace either step with a catalogue table whose assumed initial temperature, final temperature, duration or construction is unknown.

The thermal method in the IEC standard listed in the References uses two stages: calculate the adiabatic permissible current, then apply a modifying factor when non-adiabatic heat exchange is relevant. Whether that refinement is material depends on conductor size, duration, surrounding materials and the adopted design method. The project calculation should state the method rather than presenting an unexplained result.

For short faults, the adiabatic check is commonly written as S = I × √t / k, where S is the conductor cross-section in mm², I is the fault current in A, t is the duration in s and k is a constant for the conductor material and insulation. Equivalently, I²t ≤ k²S². The formula is valid for faults up to about 5 s. The k values below are those used in IEC 60364-5-54 and IEC 60364-4-43.

k values: IEC 60364-5-54 and IEC 60364-4-43. Temperature limits: commonly cited from IEC 60502-1 and manufacturer data. Confirm both against the approved datasheet for the final design.

Worked example: a 10 kA fault cleared in 0.4 s on a copper cable with PVC insulation gives S = 10,000 × √0.4 / 115 ≈ 55 mm² (basis: the adiabatic formula with k = 115). The next standard size is 70 mm². This is a minimum cross-section for the stated current, time and construction. It is not a design, and it changes when any input changes.

What temperature limit applies to my cable?

Permissible short-circuit temperature is not one universal number. It depends on conductor material, insulation, bedding, oversheath, metallic sheath or screen, connection method and accessories. It also depends on the voltage-class standard used for the project.

• For cables up to 0.6/1 kV, the applicable project standard should define the material and component temperature limits.
• For cables from 6 kV (Um 7.2 kV) to 30 kV (Um 36 kV), the IEC temperature-limit guidance listed in the References covers insulation, oversheath, bedding, conductor, metallic sheath, connections, accessories and installation influence.
• Above 30 kV (Um 36 kV), the corresponding guidance listed in the References applies within its stated scope and directs the permissible-current calculation to the thermal method described above.

The PVC and XLPE limits in the table apply to those insulation systems only. A copper/XLPE calculation is not evidence for an aluminium conductor, a different insulation system or a screen connection with another thermal limit. Use the actual cable datasheet and the approved construction.

Which metallic paths carry part of the earth-fault current?

For an earth fault, document how the current divides among metallic screens, sheaths, armour, protective conductors, neutral conductors, earth conductors and any parallel paths. Bonding arrangement and connection resistance both affect that division. A screen area that is adequate for one bonding scheme may not be adequate when the intended return path changes.

Check the full route through joints, link boxes, bonding leads, lugs and terminations. The cable supplier can confirm construction data and component limits, but the system designer remains responsible for the network fault-current distribution and earthing design.

How do you get the clearing time from the protection device?

Obtain the approved breaker or fuse type, rating, protection functions, current-transformer data and settings. Determine the total clearing time at each calculated fault current using the accepted coordination method and manufacturer data. For a breaker, total clearing includes relay or release operation plus interruption. For a fast fuse or breaker, use the manufacturer's let-through I²t rather than a nameplate rating, and use approved let-through data for a current-limiting device only where the method permits it.

Check both ends of the problem. The high-current case tests cable thermal withstand. A lower remote-end or earth-fault current may produce a longer operating time and can reveal a disconnection or sensitivity problem, even though its magnitude is smaller.

What should the procurement package contain?

The buyer does not need a copied standards table. The useful deliverable is a short-circuit schedule that makes the assumptions auditable:

• circuit identification and cable route;
• study node, fault type, maximum and minimum current and calculation standard;
• protection device, settings and clearing time used;
• conductor, insulation, screen, sheath and armour construction, with cross-sections;
• initial and final temperature assumptions, with their source;
• adiabatic or non-adiabatic method, and the calculated withstand;
• earth-fault current split and bonding arrangement;
• joint, termination and accessory confirmation;
• calculation revision, preparer, checker and unresolved deviations.

The review itself can be organised in four passes. A result should be held back from approval if any pass uses an assumed value that has no named owner.

• System pass: confirm the study revision, bus, fault types, maximum and minimum currents and network states.
• Protection pass: confirm the device, settings, total clearing time and any accepted current-limiting treatment.
• Cable pass: confirm material, cross-section, initial operating temperature, permitted final temperature, method and margin.
• Interface pass: confirm screens, sheaths, armour, neutral and PE conductors, joints, terminations, bonding leads and accessories.

Frequently asked questions