In short. A cable must be protected against overload by a device rated at or below its current capacity, and against short circuit by a device that clears the fault before the conductor overheats. On generators, lower fault current can slow the device, so the check must be done at both sources.
Overload protection
The device rating (or setting) must be at least the load current and not more than the cable’s current-carrying capacity after derating for installation, grouping and temperature. See cable sizing.
Short-circuit withstand
For short faults, heat cannot escape, so the cable’s thermal limit is expressed through the adiabatic equation.
I² × t ≤ k² × S²
Here I is the fault current, t the clearing time, S the conductor area and k a constant for the conductor and insulation material. The protective device’s let-through energy at the fault current must be lower than the cable’s withstand.
Earth-loop impedance and automatic disconnection
For safety, the earth-fault current must be high enough for the protective device to operate within the required time. This depends on the loop impedance of the line and protective conductor. Long cables raise impedance and reduce fault current, so check the longest circuits. On generator supply, source impedance is higher, so the check is stricter.
Voltage drop
Cable length also affects voltage drop under load and during motor start. See voltage drop.
Coordination checks
| Check | Question |
|---|---|
| Overload | Does the device protect the cable at its derated capacity? |
| Short-circuit | Does clearing time satisfy the adiabatic limit at maximum and minimum fault? |
| Earth-fault disconnection | Does minimum earth-fault current trip fast enough? |
| Selectivity | Does the device grade with upstream and downstream devices? |
| Voltage drop | Is the drop acceptable in running and starting? |
Illustrative example
A long feeder cable to a remote pump house satisfies voltage drop but, on generator supply, the earth-fault current is too low to trip the breaker in the required time. Solutions include increasing cable size, adding an earth-fault relay or residual-current device, or lowering the breaker setting.
Cable protection has two jobs
Protect the cable from overload and from short-circuit damage. Both depend on matching the protective device curve to the cable’s thermal capability.
Short-circuit withstand: I² × t ≤ k² × S², where S is conductor area in mm² and k depends on conductor and insulation
Worked check (illustrative)
For a 185 mm² copper XLPE cable, k is often taken near 143. Allowable energy = 143² × 185² ≈ 7.0 × 10⁸ A²s. If the fault current is 20 kA, the allowed time is 7.0 × 10⁸ ÷ (20,000)² ≈ 1.75 s. A breaker clearing in 0.3 s is comfortable. Confirm the k value from the cable standard and manufacturer.
Selection steps
- Choose the cable for load current with derating for ambient, grouping and installation method.
- Check voltage drop. See cable sizing.
- Check short-circuit withstand against the clearing time at the minimum and maximum fault level.
- Check that the protective device trips for an earth fault at the cable’s far end, taking account of loop impedance.
Generator supply considerations
Lower fault current on generator supply can lengthen clearing times for fixed-curve devices. Verify the far-end fault case.
Related
See overcurrent protection and coordination.
Common mistakes
- Checking only maximum fault current.
- Ignoring the derating factors.
- Using the same cable and device settings for utility and generator without checks.
What to send us
Send cable schedule and protective device data. Send your application details. See also coordination.
Questions engineers ask
What is the adiabatic check?
A check that the cable can absorb the fault energy without damage before the device clears it.
Why are generator checks stricter?
Fault current is lower, so devices may take longer to trip and earth-loop limits are harder to meet.
Can a larger cable fix a trip problem?
Larger cable lowers impedance and raises fault current, which helps the device trip.
What is a residual-current device?
A device that trips on small earth leakage, protecting people and equipment.
This guide gives general principles. Calculations should follow applicable standards.