Name: Covenant Systems
Address: US
Price range: $$

Question 1

Where do the relay curves come from?

Accepted Answer

They are the standard inverse-time equations, not a fit to a picture of someone's plot. The US families (U1 moderately inverse through U5 short-time inverse) follow IEEE C37.112, t = TD × (A + B / (M^P − 1)); the IEC families (C1 standard inverse through C5 short-time inverse) drop the constant term, t = TD × (B / (M^P − 1)). M is the current as a multiple of pickup, and both forms are asymptotic at M = 1, so an element sitting exactly at pickup never times out. The same functions are used by our event analyzer to check a relay's settings against what its own relay word actually did, where they reproduce SEL's published operating times to three figures.

Question 2

Why does it ask for secondary amps and a CT ratio instead of primary pickup?

Accepted Answer

Because that is how a relay is set and how the setting is argued about in a review. Entering the pickup in primary amperes means doing the division yourself before you start and again every time the CT ratio comes into question. The plot is drawn in primary amperes so devices at different ratios can be compared on one axis, and the editor shows you the primary equivalent as you type — but the number you enter is the number on the setting sheet.

Question 3

How is the coordination margin calculated?

Accepted Answer

Across the whole current range, not at the fault current you happen to be thinking about. For every adjacent pair the tool walks four hundred logarithmically spaced currents, computes both operating times at each, and reports the smallest gap it found and the current where it occurred. That matters because two inverse curves of different shapes can sit comfortably apart at one current and cross a decade further down. A pair whose order inverts anywhere is called out separately, because that is not a small margin — it is the upstream device tripping first and taking out more of the system than the fault required.

Question 4

What coordination time interval should I use?

Accepted Answer

The default of 0.30 s is the conventional figure for an electromechanical relay upstream: breaker interrupting time, plus relay overtravel, plus an allowance for setting and curve tolerance. Solid-state and numeric relays have no overtravel and are often coordinated at 0.20 s, and a fuse behind a relay is usually judged on the fuse's total clearing curve rather than a fixed interval. It is a setting here because it is a judgement your standards make, not one this tool should make for you.

Question 5

Why do I have to enter fuse curve points by hand?

Accepted Answer

Because we do not have the manufacturers' data and will not invent it. A fuse's characteristic is empirical — it comes from a published curve for that specific link, not from a formula — so guessing it from a rating and a letter would produce a plot that looks authoritative and coordinates against nothing real. What the tool does instead is exploit the fact that a fuse curve is very nearly a straight line on log-log axes: read two points off the manufacturer's sheet, and those two points fix the curve. The minimum melt and total clearing curves are drawn as a band, because coordination is judged against both edges.

Question 6

What is not modelled?

Accepted Answer

Transformer through-fault damage curves and cable damage curves are deliberately absent. Both are defined by standards — ANSI C57.109 and ICEA — and both would be straightforward to draw, but drawing a damage curve wrong is worse than not drawing it, because it is the line you are proving you stay left of. They will be added when we can check them against a study we already trust. Motor starting curves, ground-fault elements plotted on a separate residual axis, and arc flash energy are also out of scope for now.

Question 7

Is this a substitute for a coordination study?

Accepted Answer

No, and it is not offered as one. It plots curves and measures the gaps between them accurately, which is the arithmetic part. A study is the engineering around that: fault current calculations at every bus, the equipment ratings, the damage curves, the load and inrush the settings have to ride through, and a licensed engineer's judgement and seal. Use this to check an idea quickly, to see what a setting change does to the margin, or to understand a scheme someone else set — not to replace the study.

Question 8

Does anything I enter get uploaded?

Accepted Answer

No. The curve maths, the coordination check and the CSV export all run in your browser in local JavaScript. Nothing about your devices is transmitted — no names, no pickups, no ratios, no fault currents — because a settings sheet describes a real substation and the shape of your protection scheme is yours. This page carries the same anonymous usage analytics as the rest of the site, which records that a device of some kind was added and how many coordination pairs were checked. It cannot see what you set them to.

TCC Curve Plotter

The curve equations here are the ones our event analyzer grades relays against

Need it to do more?

How to read it

The curve families

Questions