A shed shutdown is a short window. The crew checks the protection settings once. The settings either grade or they do not. This example works a three relay chain. The downstream pair grades. The upstream pair does not, and the reason is worth reading.
The inputs
The fault current at the point of interest is 3000 A. Three relays see it.
- Pump feeder: IEC-SI curve, 60 A pickup, TMS 0.1, no instantaneous element
- Shed main: IEC-SI curve, 200 A pickup, TMS 0.25, no instantaneous element
- Farm incomer: IEC-VI curve, 400 A pickup, TMS 0.3, no instantaneous element
Each relay gets three settings. The curve type sets the shape. The pickup current sets where the curve starts. The time multiplier setting slides the whole curve up or down.
The clause
Cl 5.2 of IEC 60255-151 defines the inverse time characteristic. Annex A of IEC 60255-151 holds the constants for the standard inverse and very inverse curves. Cl 2.5.7.2 of AS/NZS 3000:2018 covers discrimination between devices in series.
The steps
The engine puts 3000 A through each characteristic. It then subtracts the trip times in pairs.
- Pump feeder trip time: 0.172 s
- Shed main trip time: 0.6289 s
- Farm incomer trip time: 0.6231 s
- Margin, pump feeder to shed main: 0.4569 s
- Margin, shed main to farm incomer: −0.0058 s
The result
The first margin holds. The pump feeder clears in 0.172 s. The shed main waits until 0.6289 s. That leaves 0.4569 s between them, and a fault on the pump circuit stays on the pump circuit.
The second margin is negative. The farm incomer trips at 0.6231 s. The shed main trips at 0.6289 s. The incomer therefore opens first.
That is a curve inversion, not a thin margin. A fault downstream of the shed main takes the whole farm supply out. The shed main never reaches its own trip time.
Two inputs drive the inversion. The incomer runs an IEC-VI curve and the shed main runs an IEC-SI curve. A very inverse characteristic falls away faster as the current rises. The TMS values then sit too close, at 0.25 and 0.3. The engine flags the pair, because the margin sits below the 0.3 s figure usually allowed between relays.
The inversion is current dependent. The same two settings can grade at a lower fault current. They cross over at a higher one. Check the grading at the fault current that applies at the relay location. A single convenient figure hides the crossover point.
Change one input at a time and read the two margins again. The curve type and the TMS are the two that move this pair. A pickup change moves the curve sideways instead.
Record the fault current used for each check. The setting sheet is only valid at that current. The next person to open it needs to know which figure it used.
This page is a design aid. Verify every value against the current edition of the standard.
See also: IDMT curve grading for a 100 A feeder, IEC 60255-151: IDMT curve constants and Arc flash basics.