In short. Overcurrent protection guards against overload (a modest excess lasting minutes) and short circuit (a large excess needing fast clearing). Inverse-time elements give long delays for small overloads, and instantaneous or short-time elements trip quickly for faults.
Elements
| Element | Purpose | Typical behaviour |
|---|---|---|
| Long-time (overload) | Protect cables and equipment from heating | Inverse time, trips in seconds to minutes |
| Short-time | Allow downstream devices to clear first | Definite delay, tens to hundreds of milliseconds |
| Instantaneous | Clear severe faults fast | No intentional delay |
Setting principles
- Pickup above the normal load and starting currents, below the cable and equipment limits.
- Time delays graded between levels for selectivity. See coordination.
- Instantaneous pickup above the largest downstream fault where selectivity is needed, and above transformer inrush on transformer feeders.
- Check that the minimum fault level exceeds the pickup with margin.
Generator overcurrent
Generator fault current decays, so a plain overcurrent element can drop out before it trips. Voltage-restrained or voltage-controlled overcurrent relays lower the pickup when voltage falls during a fault. Excitation that holds up fault current also helps. See generator protection architecture.
Motor starting and inrush
Starting current of motors and inrush of transformers can resemble a fault briefly. Settings must ride through these events. See motor starting.
Steps
- Determine load current, starting and inrush currents.
- Set long-time pickup and delay to the cable rating.
- Set short-time pickup and delay for coordination.
- Set instantaneous above downstream faults and inrush.
- Verify against minimum fault current and equipment damage curves.
Illustrative example
A 630 A feeder with a normal load of 400 A has its long-time pickup at 500 A with a time delay that allows a motor start. Its short-time pickup is set above the next downstream breaker’s instantaneous setting with a short delay. On generator supply the fault current is only about 3,000 A; the settings are checked to confirm that this is above the short-time pickup.
Overload versus short-circuit
| Condition | Magnitude | Protection response |
|---|---|---|
| Overload | Moderately above rated, sustained | Inverse-time (thermal) trip; takes seconds to minutes |
| Short-circuit | Many times rated | Instantaneous or short-time trip |
The inverse-time curve lets harmless brief overloads, such as motor starting, pass, while sustained ones trip before the cable overheats.
Setting the pickup
Choose the long-time pickup above the maximum continuous load and below the cable’s current-carrying capacity (corrected for installation). Choose the instantaneous setting above the maximum inrush and below the minimum fault current at the end of the protected circuit.
Maximum load current < Pickup < Cable capacity; Inrush < Instantaneous < Minimum fault current
Motor starting inrush
Motors draw several times their rated current at start. Settings must permit this without tripping while still protecting. Transformers also have magnetising inrush when energised. See motor starting.
Fuses versus breakers
- Fuses are current-limiting and fast on large faults; replacement is needed after operation.
- Breakers are resettable and adjustable but may be slower unless current-limiting.
- Combinations can give excellent protection when coordinated.
Generator-supplied circuits
Where the generator has low fault current, check that the short-circuit trip still operates. Some sites use a lower instantaneous setting or an alternative setting group in generator mode.
Testing
Verify pickup and timing by secondary injection or primary injection. See injection testing.
Common mistakes
- Instantaneous set too low for inrush.
- Pickup above the generator’s decayed fault current.
- Mixing up overload and short-circuit settings.
What to send us
Send the load list, relay or breaker model and fault data. Send your application details.
Questions engineers ask
What is inverse-time protection?
The higher the current, the faster it trips, matching the heating of cables and equipment.
Why does the generator need a special relay?
Its fault current decays, so a standard element may not operate, which voltage-restrained elements address.
Can instantaneous trips be disabled?
Sometimes, for selectivity, but then the arc-flash energy and equipment stress rise. Study before doing so.
How do I avoid nuisance tripping?
Set pickups above starting and inrush currents and test with real events.
This guide gives general principles. Settings need a study and manufacturer data.