A breaker closes, the feeder comes alive, and for a fraction of a second the current that flows is not the current the design calculation assumed. Then it settles, and the circuit behaves exactly as expected.
Some energisations produce that first cycle quietly. Others draw enough current to reach a protection setting that was chosen for a steady load, and a healthy feeder trips before it has carried anything at all.
Understanding inrush current is mostly a matter of separating contributions that behave very differently, and then deciding which of them the protection is allowed to ignore.
What the First Cycle Looks Like
The current drawn at the instant of closing is not a single, tidy quantity. It is a sum of several effects that decay at different rates and peak at slightly different moments.
One contribution is capacitive. A long cable stores charge along its length, and the supply has to fill that capacitance before any load current can flow.
Another contribution comes from anything magnetic downstream. A transformer core that starts a cycle with residual flux and saturates draws a large unidirectional current for several cycles before settling.
A third contribution belongs to the motors on the feeder, which draw their own starting current whenever they are switched on. Only the last of these is a load, and only the last of them lasts.
An inrush current is the surge of current that flows when a feeder is energised and magnetic circuits saturate before settling into steady operation.
What Adds to the Inrush Current
Four sources behave differently enough that they should not be treated as one number.
| Source | Nature of the current | How it decays |
|---|---|---|
| Cable capacitance | Capacitive, roughly symmetrical | Immediately, once the wave has travelled |
| Transformer core | Unidirectional, offset from zero | Over a few cycles, faster when the core stays in flux |
| Motors | Inductive, lagging | Over seconds as the machine accelerates |
| Fault or partial discharge | Resistive or capacitive, not a transient | Not at all until the cause is removed |
Only the last row describes something the protection is meant to act on, and it usually looks different from the other three.
Charging current is the steady capacitive current a long cable draws because its insulation behaves as a capacitor, and it has to be distinguished from a switching transient.
What an Inrush Current Study Needs
Five inputs decide whether the result can be trusted or is only a number on a page.
The real route length
Cable capacitance scales with length, so a route figure taken from a preliminary drawing can mislead the study before it starts.
The transformer details
Rating, connection group and core information determine how large the magnetising inrush is and how long it lasts.
The device settings in service
The study is only useful if it is checked against the settings that will actually be applied on site rather than the ones in a design note.
The switching arrangement
Closing onto a loaded feeder is a different event from closing onto an unloaded one, and the study needs to say which is expected.
The records of past trips
An existing feeder that trips once and then holds is evidence worth more than an assumption about what the current probably did.
Protection coordination is the exercise of setting devices so they ride through the first cycle while still clearing a genuine fault.
Why the Cable Is Rarely the Cause of Inrush Current
When a feeder trips on energisation, the cable is the item that gets blamed first, usually because it is the newest and most expensive element in the circuit.
In practice an insulation fault behaves quite differently from a transient. A fault current is sustained until the device clears it, and it reappears when the feeder is closed again. A capacitive or magnetising contribution decays and behaves differently on the second attempt.
That difference is the most useful diagnostic available at site, and it costs nothing to observe.
It also explains why the answer to a first-cycle trip is often a setting adjustment or a change in the switching procedure rather than anything to do with the cable construction.
Magnetising inrush belongs to the transformer at the end of the feeder, not to the cable itself, and a note that confuses the two sends the investigation in the wrong direction.
Signs That Inrush Current Is Tripping a Healthy Feeder
A handful of patterns point towards coordination rather than a defect.
- The feeder trips on closing, then holds on the second attempt.
- Trips cluster in the hours when the network is lightly loaded and the voltage rides high.
- A second feeder of the same design and length behaves identically.
- The trip appears when a transformer is energised from the far end, and nowhere else.
- The event log shows a current that was high but short, with no sustained rise afterwards.
None of those patterns settles the question on its own, and together they point firmly away from the cable.
Understanding a switching transient helps explain why a healthy feeder can trip once and behave normally on the next attempt.
A Substation That Tripped Twice and Then Settled
Situation: A new transformer feeder at an industrial substation tripped on the first two closing attempts and held on the third, which was initially recorded as an intermittent cable problem.
Finding: The event records showed a short, high current that decayed within a few cycles, with no repeat during steady operation. Charging current and transformer magnetising inrush together reached the instantaneous element of the relay.
Decision: The protection settings were reviewed against the recorded waveform rather than against the design note, and the instantaneous element was adjusted while the feeder was confirmed to be sound.
Result: The third attempt, which had appeared to fix the problem, was explained by the change in transformer residual flux. The feeder has since closed cleanly on every attempt.
Ways to Live with Inrush Current
The options are engineering choices with different costs, and most projects use more than one.
| Approach | What it does | What it costs |
|---|---|---|
| Adjust the settings | Allows a short transient to pass without losing selectivity | A coordination study, and a review of upstream devices |
| Delay the close pattern | Avoids energising the transformer at the worst point on the wave | A change in procedure and in operator routines |
| Change the switching sequence | Reduces the number of magnetic circuits that start together | Coordination between the network operator and the site |
| Add damping or reactance | Reduces the peak that the supply sees | Capital cost and additional equipment |
The cheapest option is usually the one that starts with the recorded waveform rather than with an assumption.
Charging Current on Long Cables
A long cable behaves partly as a capacitor, and a small current flows into it continuously while it is energised, even with no load at the far end.
That charging current is not a transient and does not decay. It is a standing figure that the design has to carry, and it appears on the load side of a feeder even on a quiet night.
It matters for two reasons. It sets a floor on the current a relay sees at all times, which limits how low a setting can sensibly go, and it produces reactive power that the network has to absorb.
Where several long feeders are energised together, those two effects are large enough to change the switching plan rather than only the relay data.
Checks at Commissioning
Four things to complete before the feeder is treated as proven.
Record the first close
Capture the event log and, where a recorder is available, the waveform of the first energisation, since it may never be repeatable.
Confirm the settings in service
Compare what the device holds against the approved study, and write the comparison down while both parties are present.
Repeat the close deliberately
A second close after the machine has been at rest gives the residual flux a chance to change and separates the causes convincingly.
Agree who may change a setting
The practice of adjusting a device on site to stop an unexplained trip is how coordination studies quietly become fiction.
What to Put in an Inrush Current Enquiry
Cable suppliers cannot set protection, but they can supply the data that removes the guesswork from an inrush current study.
- The route length of each feeder, together with any parallel runs.
- The capacitance figures needed for a charging current estimate.
- The transformer rating and connection at the far end.
- The expected switching arrangement and who operates it.
- The test data that the protection study will need as input.
With those five items attached, the study starts from measured data and the first close is far less likely to become an event.
RFQ Inputs for Energisation Checks
These details let the cable data and the inrush current study meet before the feeder is switched for the first time.
- the route length of each feeder and any parallel runs
- the cable construction and insulation, with the capacitance data
- the transformer rating, connection group and switching arrangement
- the protection devices and the settings expected in service
- the earthing and bonding arrangement at both ends
- the test records expected at commissioning
- the switching procedure the network operator will follow
- the point at which the first close will be witnessed
Technical references such as IEC 60502, IEC 60228 and IEC 60332 can align cable construction and test terminology. The approved project specification, applicable local rules and qualified design authority still govern the final system decision.
JINCHUAN Cable Support for feeders with high energisation current
JINCHUAN Cable supplies the construction and length data an energisation study needs, including the capacitance figures that go with the insulation being ordered.
Review the cable range and the manufacturing profile, then send the feeder lengths so the study starts with real figures.
Record the first close and keep the waveform, because the only chance to capture a first energisation is the moment it happens.
FAQ
What is inrush current?
It is the elevated current that flows for a short time when a circuit is energised, produced by capacitance, magnetic saturation or motor starting rather than by load demand.
Does the cable cause it?
The cable contributes capacitive charging current, which is real but steady. The large, brief peaks usually come from magnetic circuits at the far end of the feeder.
Why does the feeder trip once and then hold?
The second close happens with a different magnetic state in the downstream transformer, so the current is genuinely different rather than intermittent.
Is charging current a fault?
No. It is normal behaviour of an insulated cable, and it is present continuously while the feeder is energised.
How is it assessed?
From the route length, the cable construction and the equipment downstream, compared against the settings that will actually be in service.
Can the cable be changed to reduce it?
Construction changes do alter capacitance, but the cause is usually better addressed in the settings, the switching sequence or the network arrangement.
Who sets the protection?
The protection engineer or the responsible network authority. The cable supplier provides the data the study needs.
What data should be requested?
Route lengths, insulation type, capacitance figures, the earthing arrangement and test records that correspond to the cable actually supplied.
What records matter most?
The event log and waveform from the first close, together with the settings held in the device at the time.
How can JINCHUAN Cable support an energisation check?
JINCHUAN Cable supplies length and construction data for the study and can confirm the insulation and screen details the calculation depends on.
