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ETAP Short-Circuit Analysis and Protection Coordination

Once you can build a model and run a load flow study, the next two studies every power system engineer must master are short-circuit analysis and protection coordination. They answer two different but connected questions. Short-circuit analysis asks: if a fault happens here, how much current will flow, and can the equipment survive and interrupt it? Protection coordination asks: when that fault happens, will the right device trip — and only that device — so that the rest of the plant keeps running? Getting either answer wrong has real consequences, from damaged switchgear to whole-plant blackouts caused by a single faulty feeder.

This guide assumes you already know the basics from our ETAP basics and load flow guide: building a one-line diagram and entering equipment data. It explains short-circuit concepts and protection coordination in plain language, in the order you would actually work through them on a project. As with all our software guides, this is generic educational material written from standard engineering practice. It does not reproduce any software manual or proprietary content, and for hands-on practice you need a licensed copy of the software, an authorised trial, or an academic licence from the vendor. Always work to your project specifications, the applicable standards for your region, and the manufacturer's data for the actual devices installed.

Why fault current matters

When a short circuit occurs, current rises to many times the normal load current in a fraction of a second. The equipment in the fault path must withstand two things: the mechanical and thermal stress of that current until it is cleared, and, for switching devices, the duty of actually interrupting it. A breaker rated below the fault current at its location may fail to clear the fault, which can destroy the breaker itself and extend the damage far beyond the original fault. That is why fault levels are calculated at every bus, not just at the main switchboard: fault current falls as you move further from the source through transformers and cables, so a device that is adequate at a remote panel may be hopelessly underrated at the main bus, and vice versa.

Fault current also changes with the operating arrangement. With two transformers in parallel, the fault level at the low-voltage bus is much higher than with one transformer in service, because both sources feed the fault. With a generator running, fault levels rise again. A responsible short-circuit study therefore checks the maximum fault level — all sources in service, the arrangement that produces the highest current — for equipment ratings, and the minimum fault level — the arrangement that produces the lowest current — for protection sensitivity, because a relay that must detect a small fault current in a lightly loaded arrangement is a very different problem from a breaker that must interrupt a huge one.

Running a short-circuit study in ETAP

Start from the same validated model you used for load flow, because a short-circuit study is only as good as the impedances behind it. Transformer impedance, cable impedance and source fault level drive the results directly, so re-check those values before running anything. Select the short-circuit study and the fault type you need: three-phase faults usually give the highest currents and are the standard check for equipment ratings, while single-line-to-ground faults matter for earth fault protection and for systems where the earth fault current is limited. Run the study for the buses you need, and review the results bus by bus rather than glancing at a single summary number.

For each bus, the study reports the fault current and, for switching devices, a comparison against the device rating, often flagged when the duty exceeds the rating. Treat every flag as a finding, not as a warning to dismiss. An over-duty breaker is a design problem that must be solved by changing the device, changing the arrangement, or adding impedance — for example by running transformers separately rather than in parallel — and the solution must be recorded and re-studied. A study that reports over-duty devices and is filed without action has not protected anyone.

Time-current curves: the language of protection

Protection coordination is studied with time-current curves. Each protective device has a characteristic curve that plots its operating time against the current flowing through it: at low multiples of its rating it may take seconds or minutes to operate, while at high fault currents it operates in a fraction of a second. When you plot two devices in series on the same axes — say a feeder breaker and the main incomer breaker — you can see at a glance whether they coordinate. For the fault currents that matter, the downstream device's curve should sit clearly to the left of, or below, the upstream device's curve, with a sensible time margin between them, so the downstream device clears the fault first.

Relays add settings to this picture: pickup current, time dial or time multiplier, and curve shape (such as standard inverse, very inverse or extremely inverse, and definite time). Fuses have their own melting and clearing curves, and the coordination between a fuse and a breaker, or between two fuses, is checked the same way — by comparing curves at the fault currents of interest. The skill is not in drawing the curves; the software does that. The skill is in choosing settings that coordinate across the whole range of credible fault currents while still protecting cables and equipment from overload and damage, and that is a judgement call that requires understanding the plant, not just the plot.

Checking selective coordination step by step

Work outward from the load. For each feeder, identify the protective device nearest the load and the next device upstream, and check their coordination across the fault current range at the load bus, from the minimum fault level up to the maximum. Then move one level up and repeat the check for the next pair, all the way back to the main incomer. Where curves overlap or the margin is too small, adjust settings: a relay time dial, a breaker trip unit setting, or in some cases a different device rating or curve shape. Re-plot after every change, because fixing one pair can break coordination with the next pair upstream.

Pay special attention to the edges of the range. Coordination that looks perfect at high fault currents can fail at low fault currents, where a downstream fuse and an upstream relay may both operate, or where a relay's pickup is set above the minimum fault current so it never sees the fault at all. Also check motor starting and transformer inrush: a protection setting that coordinates beautifully for faults but trips every time a large motor starts or a transformer is energised is not a working setting. The coordination study must be read alongside the motor starting and inrush checks, which is why these studies are done as a set rather than in isolation.

Common mistakes and how to avoid them

The most damaging mistake is studying the wrong arrangement: modelling both transformers in parallel when the plant normally runs them separately, or forgetting a generator that is normally running, and then issuing settings based on fault levels that do not match reality. The second is using default device data instead of the actual installed device curves. Two breakers of the same frame size from different manufacturers, or even different trip units in the same frame, can have very different curves, and coordination checked against the wrong curve is not coordination at all. Always load the manufacturer's curve for the exact device and trip unit installed, and record the device schedule the study was based on.

The third mistake is treating coordination as a one-time exercise. Every change to the plant — a new feeder, a larger transformer, a changed utility fault level, a replaced breaker — can change fault levels and break coordination that was previously correct. A good protection study is a living document, revisited whenever the single-line diagram changes, and re-issued with a revision note explaining what changed and why. If your site cannot produce the current protection study for a board, that board's coordination is effectively unknown, however good it was on the day it was first studied.

What to learn next

With load flow, short-circuit and coordination understood, the next studies in the set are arc flash and motor starting analysis, which use the same model and the same fault levels to answer safety and operability questions. For the design documents behind these studies, see our AutoCAD Electrical basics and schematics and panel layouts guides, and for the control side of the plant, our PLC basics and advanced ladder logic guides. Our AI tools for engineers page covers how modern tools can help with study reporting, while the engineering judgement in this guide remains firmly human work.

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