Time Current Curve Generator

Protection coordination questions

What does the selectivity verdict actually check?

It compares the two bands already on the chart. Selectivity holds at a current when the downstream device's slowest possible clearing time is still faster than the upstream device's fastest possible tripping time, so the downstream breaker has to have finished before the upstream one can start. The lowest current where that stops being true is the selectivity limit current Is. Total means no crossing anywhere checked, partial names the limit, and none means the curves overlap from the downstream pickup upward.

Why does entering a fault current change the answer?

Because selectivity is only meaningful relative to a current. A pair whose curves cross at 4.5 kA is fully selective on a board that can only ever deliver 3 kA, since nothing on site can reach the crossing. Enter the prospective fault current at the downstream board and the check stops there. Leave it blank and the check runs to the downstream device's breaking capacity, which is the most conservative reading.

The curves cross below my fault level. Does that definitely mean no discrimination?

Not necessarily. Curve crossing is the conservative answer. Manufacturers publish verified discrimination tables for specific upstream and downstream pairs that can certify selectivity beyond the point the curves cross, because a real breaker's let-through energy is lower than the published envelope allows for. Where this page shows a limit below your prospective fault current, check the manufacturer's tables for that pair before changing the design.

Are these the manufacturer's published curves?

No, and the report says so. The curves are constructed from the tolerance envelopes in IEC 60898-1 and IEC 60947-2 combined with each device's catalogue record and the trip settings you choose. That is the right basis for design and coordination work, and it matches the shape and anchor points of the published curves, but for a final protection study the manufacturer's own curve sheets and discrimination tables remain the reference.

Why do some devices show a clearing time marked "type default"?

Because the catalogue does not carry a clearing time for that device. Electronic MCCB and ACB records do carry one and the report labels those "catalogue"; MCB and thermal-magnetic records generally do not, so a representative figure for the device type is used instead. The distinction is printed against every device in the PDF so a study can say which it relied on.

Which devices can I plot?

The Schneider low-voltage range currently in the ElecAS catalogue: Acti9 iC60, C120 and NG125 miniature circuit breakers in curves B, C and D, ComPacT NSX, NSXm and NS moulded-case breakers with thermal-magnetic, Micrologic and STR trip units, EasyPact CVS, and MasterPact MTZ1, MTZ2 and MTZ3 air circuit breakers with the MicroLogic X range. Fuses and RCDs are not plotted.

How do I improve discrimination when the check fails?

The usual levers, in order of how little they cost. Raise the upstream short-time pickup Isd so it sits clear of the downstream device. Add or increase the upstream short-time delay tsd so the downstream breaker has time to clear first. Switch the upstream instantaneous off where the trip unit allows it, since an instantaneous stage that reaches down into the downstream range is the most common cause of lost selectivity. Failing those, increase the upstream device rating or move to a trip unit with a genuine short-time stage.

What is a time-current curve?

A time-current curve shows how long a protective device takes to trip at any given fault current. Current runs along the bottom, tripping time up the side, both on log scales. The curve slopes down to the right because the bigger the fault, the faster the device operates.

Why is each curve drawn as a band rather than a single line?

Because a real breaker has a manufacturing tolerance, so it trips somewhere inside a range rather than at one exact point. The generator draws a fast edge and a slow edge from the tolerances in the product standards, and the device operates somewhere between them.

What do the B, C and D curves mean on an MCB?

They set how much current is needed before the magnetic element trips instantly. To IEC 60898-1 (adopted in Australia as AS/NZS 60898.1), curve B trips instantly between 3 and 5 times its rating, curve C between 5 and 10 times, and curve D between 10 and 20 times. Curve C is the general-purpose default; D is for equipment with a heavy switch-on surge such as motors and transformers.

What do long-time, short-time and instantaneous mean?

They are the three stages of an electronic trip unit. Long-time protects the cable against sustained overload. Short-time gives a deliberate delay so a downstream device can clear a fault first. Instantaneous trips with no intentional delay on a large fault. The generator draws all three on one chart, and the device follows whichever stage is fastest at that current.

What is protection coordination, or discrimination?

Coordination means that for a fault anywhere on the system, only the device closest to the fault opens, leaving the rest of the installation running. On the chart it means the downstream device's band sits fully to the left of and below the upstream device's band, with no overlap.

How do I read two curves together?

Plot the downstream device and the upstream device on the same chart. If their bands cross or touch at any current, both devices could trip together at that fault level and you lose discrimination. Where they overlap you either change a setting, change a device, or accept the loss and record it.

Does the curve prove my installation complies?

No. The chart is a coordination study aid. Compliance also depends on the cable passing its own checks (current-carrying capacity, voltage drop, short-circuit withstand and earth-fault loop impedance), which the Cable Selection calculator covers. Selecting the method and certifying the design remain with the engineer of record.