AUWiring

Protective device energy check calculator

Compare entered protective-device energy with an entered withstand limit and a fault-current energy cross-check for Australian project records.

  • Calculator
  • Protection
  • Australia
Use the board, circuit, protection schedule or project row reference.
Enter the device reference from the schedule, manufacturer record or project worksheet.
A2s
Enter the protective-device energy value from the source record.
A2s
Enter the withstand limit from the cable, equipment or manufacturer basis.
s
Enter the clearing time used for the current-time cross-check.
kA
Enter the current basis that belongs to the same clearing-time point.
device_energy_margin_A2s = entered_withstand_limit_A2s - entered_device_energy_A2s; device_energy_utilisation_percent = entered_device_energy_A2s / entered_withstand_limit_A2s x 100; prospective_current_energy_A2s = (prospective_fault_current_kA x 1000)^2 x clearing_time_s; prospective_energy_margin_A2s = entered_device_energy_A2s - prospective_current_energy_A2s
  • Device energy and withstand limit are entered by the user.
  • Prospective-current energy is a cross-check from entered current and clearing time.
  • The calculator does not read manufacturer curves or choose equipment.
  • Keep the source of the energy and limit with the exported record.
Formula variables
VariableMeaningUnitUse
I2t_deviceEntered device energyA2sProtective-device energy entered from the manufacturer or project source record.
I2t_limitEntered withstand limitA2sWithstand limit entered from the cable, equipment or manufacturer basis.
IfaultProspective fault currentkAFault-current basis entered for the same comparison point.
t_clearClearing timesClearing-time basis used for the current-time cross-check.
I2t_faultProspective current energyA2sCalculated from entered prospective current and clearing time.
device_energy_margin_A2sEnergy marginA2sEntered withstand limit minus entered device energy.
device_energy_utilisation_percentEnergy utilisation%Entered device energy divided by entered withstand limit times 100.
prospective_energy_margin_A2sCross-check marginA2sEntered device energy minus prospective-current energy.
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Protective device energy check calculator technical guide

Compare entered protective-device energy with an entered withstand limit and current-time energy cross-check for Australian project records.

Entered device energy

Use this worksheet when a protective-device record already has an energy value and the task is to compare that value with an entered withstand limit. It supports switchboard records, cable review notes, equipment checks and protection worksheets where the energy source must stay visible.

The page is intentionally narrow. It does not calculate conductor withstand from cable size and k value, and it does not choose a device. It records the entered source value, compares it with the entered limit, then shows a current-time energy cross-check from the same fault-current and clearing-time basis.

Device energy fields
FieldUse in this worksheetMust come from
Device energyCompared with the entered withstand limit.Manufacturer data, project schedule, trip-unit record or competent-person worksheet.
Withstand limitThe limit used for the comparison.Cable, equipment, manufacturer or project basis.
Prospective fault currentCurrent basis for the arithmetic cross-check.Short-circuit calculation, project record, utility information or engineering basis.
Clearing timeTime basis for the current-time energy value.Manufacturer data, protection setting record or project note.

Energy source workflow

  1. Identify the worksheet row and protective-device reference.
  2. Enter the device-energy value from the source record.
  3. Enter the withstand limit being compared with that device-energy value.
  4. Enter prospective fault current and clearing time for the same comparison point.
  5. Review energy margin and energy utilisation.
  6. Review prospective-current energy against the entered device-energy source value.
  7. Keep the manufacturer or project source record beside the exported worksheet.

If the current, time or device setting changes, create a new worksheet row rather than stretching the old result into a different basis.

Formula basis

The worksheet uses four visible relationships:

Formula summary
OutputFormulaInterpretation
Energy margin`entered_withstand_limit_A2s - entered_device_energy_A2s`Positive means the entered device energy is below the entered limit.
Energy utilisation`entered_device_energy_A2s / entered_withstand_limit_A2s x 100`Shows how much of the entered limit is used by the entered energy value.
Prospective current energy`(prospective_fault_current_kA x 1000)^2 x clearing_time_s`Cross-checks the entered current and clearing-time basis.
Cross-check margin`entered_device_energy_A2s - prospective_current_energy_A2s`Negative means the current-time arithmetic is above the entered source value.

The cross-check is not a device curve. It is only a transparent arithmetic comparison so the worksheet can reveal when entered source values do not align.

Worked Australian examples

A board record compares 900000 A2s entered device energy with a 1322500 A2s withstand limit. With 3 kA and 0.1 s, prospective-current energy is also 900000 A2s, so the energy margin is 422500 A2s.

A review row uses 1500000 A2s entered device energy against the same 1322500 A2s limit. The energy margin is -177500 A2s, so the device-energy record and withstand source need review.

A cross-check row keeps the entered device energy at 900000 A2s but changes clearing time to 0.2 s at 3 kA. Prospective-current energy becomes 1800000 A2s, which is above the entered source value and needs source-record review.

Boundary with related calculators

Use this page for an entered protective-device energy value. Use the I2t cable withstand calculator when the task is conductor thermal withstand from current, time, conductor area and k value. Use the fuse let-through comparison calculator when the record is specifically a fuse let-through worksheet.

Route boundary matrix
TaskUse this page?Better route when not this page
Compare entered protective-device energy with an entered withstand limit.Yes.Not applicable.
Calculate conductor withstand from cable area and k value.No.I2t cable withstand calculator.
Estimate prospective fault current.No.Short-circuit current calculator.
Compare fuse-specific let-through energy.No.Fuse let-through comparison calculator.
Compare rating and breaking-capacity values.No.Breaker rating comparison calculator.
Compare upstream and downstream time-current points.No.Device coordination worksheet calculator.

Manufacturer data remains controlling

Protective-device energy depends on the exact product, curve, setting, clearing time, fault-current basis, installation context and tolerance. Australian project records also sit inside current standards, local authority expectations, DNSP conditions where relevant and switchboard documentation.

This worksheet is useful because it keeps the entered energy, entered limit and current-time cross-check visible. It is not useful when the energy source is unclear, the withstand source cannot be traced, or current and clearing time do not belong to the same comparison point. Keep the manufacturer record and reviewer basis with every exported row.

Stop points

  • The device reference cannot be matched to the project record.
  • The entered device-energy source is missing or unclear.
  • The withstand limit source is missing or unclear.
  • Clearing time and prospective fault current do not belong to the same point.
  • The current-time cross-check is higher than the entered device-energy value.
  • The row is being used to choose products rather than record an entered comparison.

Device energy record

A distribution board record compares an entered protective-device energy value with a cable or equipment withstand limit.

Worksheet reference
PDE-DB-1
Protective device
MCCB-DB-1
Entered device energy
900000 A2s
Entered withstand limit
1322500 A2s
Clearing time
0.1 s
Prospective fault current
3 kA
  1. Energy margin+422500 A2s
  2. Energy utilisation68.1%
  3. Prospective current energy900000 A2s
  4. Cross-check margin0 A2s
Energy margin+422500 A2s

Entered device energy is below the entered withstand limit.

The entered device energy remains below the entered withstand limit, and the current-time cross-check matches the energy source value.

  • The device energy value is entered by the user from the project source record.
  • The withstand limit comes from the applicable cable, equipment or manufacturer basis.
  • The calculator does not interpret manufacturer curves.

Above entered limit

A worksheet row compares a larger entered device-energy value with the same withstand source.

Worksheet reference
PDE-REVIEW
Protective device
MCCB-DB-1
Entered device energy
1500000 A2s
Entered withstand limit
1322500 A2s
Clearing time
0.1 s
Prospective fault current
3 kA
  1. Energy margin-177500 A2s
  2. Energy utilisation113.4%
  3. Prospective current energy900000 A2s
  4. Cross-check margin+600000 A2s
Energy margin-177500 A2s

Entered values need source-record review.

The entered device energy is above the entered withstand limit, so the source record and project basis need review.

  • The limit is user-entered rather than selected by the calculator.
  • The row remains a worksheet comparison.
  • Manufacturer and project records remain controlling.

Fault-current energy cross-check

A current-time basis is checked against the entered device-energy value to expose a source-record mismatch.

Worksheet reference
PDE-CROSS-CHECK
Protective device
MCCB-DB-1
Entered device energy
900000 A2s
Entered withstand limit
1322500 A2s
Clearing time
0.2 s
Prospective fault current
3 kA
  1. Energy margin+422500 A2s
  2. Energy utilisation68.1%
  3. Prospective current energy1800000 A2s
  4. Cross-check margin-900000 A2s
Energy margin+422500 A2s

Entered values need source-record review.

The current-time arithmetic is higher than the entered device-energy value, so the row needs a source-record review.

  • Prospective current and clearing time refer to the same point.
  • The cross-check is arithmetic only.
  • The page does not replace manufacturer data.

Questions

Does this worksheet choose a protective device?

No. It compares values entered by the user and keeps equipment choice outside the calculator.

Is this the same as I2t cable withstand?

No. I2t cable withstand calculates conductor thermal withstand from cable area and k value. This page compares an entered device-energy value with an entered limit.

Where should the device energy come from?

Use the manufacturer data, trip-unit record, project schedule or competent-person record that applies to the device and setting.

Why show prospective-current energy?

It gives a visible arithmetic cross-check from the entered current and clearing time so a source-record mismatch is easier to see.

What if the energy margin is negative?

Review the device-energy record, withstand source, clearing-time basis and project requirements before relying on the row.