A short-circuit study works out how much current would flow if a fault happened at each point in your electrical system, and checks that every breaker, fuse and panel can safely withstand and interrupt it. A coordination study then sets the protective devices so that the one closest to a fault trips first, leaving the rest of the building running. Together they are the foundation of a safe, reliable distribution system and the starting point for an arc flash study.
These studies are often skipped until something goes wrong: a fault in one office trips the main breaker and blacks out the whole building, or an inspector or insurer notices panels rated well below the fault current the utility can deliver. Both problems are invisible on a normal day and expensive on a bad one.
What we look at
- The utility's available fault current and the impedance of the service transformer.
- Every significant bus in the system: switchgear, distribution panels, transformers, motor control centres and panelboards.
- Interrupting and short-circuit current ratings of breakers, fuses, panels and equipment, including series-rated combinations.
- Cable and conductor lengths and sizes, which limit fault current as you move downstream.
- Motor contribution, generators and other sources that add current during a fault.
- Protective device types, trip units and their present settings.
- Ground fault protection and how it coordinates with the phase protection.
Why the closest device should trip first
When a fault occurs, current flows from every source through every device between the source and the fault. All of those devices see it. Coordination means choosing devices and settings so that the branch breaker clears the fault before the feeder or main breaker reacts. The study does this by plotting each device's time-current curve and checking the gaps between them.
Full coordination is not always possible with the equipment already installed, especially at high fault currents where breakers react almost instantly. The study shows where coordination is achieved, where it is not, and what it would take to improve it. Codes and standards can require selective coordination for certain emergency and life safety systems; we check what applies to your building and edition of the code.
How it works
- Brief. We start by asking why: new equipment, a utility change, nuisance trips, an arc flash study, an insurer's question or a planned service upgrade.
- Data collection. We gather the utility fault data and walk the system to record nameplates, breaker and fuse types, trip settings, cable sizes and approximate lengths. This is usually the biggest part of the work in an existing building.
- Modelling. We build a model of the system in study software and calculate fault currents at each bus, then compare them with equipment ratings.
- Coordination. We plot time-current curves for each path from the service to the loads and propose settings that improve selectivity without leaving equipment unprotected.
- Report and settings. We deliver the findings, the recommended settings and the list of equipment that is under-rated. Any change to settings or equipment is made by qualified electrical workers, and new equipment is installed by licensed electrical contractors under permit.
What to have ready
- Single-line diagrams and panel schedules, even if they are out of date.
- Utility contact or account information so fault data can be requested.
- Previous studies, settings sheets and maintenance test reports.
- Access to electrical rooms and a qualified person to open equipment where nameplates or trip units are behind covers.
What you get
A report that lists the available fault current at each piece of equipment, compares it with the equipment's rating and flags anything under-rated. Time-current coordination plots for the main paths, with notes on where coordination is achieved and where it is not. A settings table that a qualified technician can use to adjust adjustable trip units. An updated single-line diagram reflecting what was found on site. The model itself is kept so it can be updated when the system changes, and it feeds directly into an arc flash study.
A typical case
A manufacturing plant has had two plant-wide outages in a year, each caused by a fault on a single machine that tripped the main breaker. The study finds that the main and feeder breakers have adjustable trip units still at their factory settings, which overlap with the branch devices. New settings separate the curves on most paths. The same study shows that one older panel near the service is rated below the available fault current, so it is replaced. The arc flash labels are then produced from the corrected model.
Two paths cannot be fully coordinated with the existing breakers because the fault currents are too high for the devices to tell a nearby fault from a distant one. The report lays out the choice for those paths: accept partial coordination, or replace the feeder breakers with a type that can be set more selectively. The owner can then weigh the cost against the cost of another outage.
Who it's for
- Facility managers dealing with nuisance trips or wide outages from small faults.
- Owners preparing for an arc flash study or a service upgrade.
- Buildings where the utility has changed the service transformer or available fault current.
- Industrial sites adding large motors, drives or generators.
For background, read available fault current explained, selective coordination of breakers and fuses, ground fault protection and what happens in an arc flash study.
Common questions
How often should the study be updated?
Whenever the system changes in a way that affects fault current or protection, such as a new transformer, a utility change or new major equipment. Because arc flash studies are built on it, many owners review it on the same cycle as their arc flash study.
Does the work require a shutdown?
Data collection is mostly visual. Reading nameplates and trip settings behind covers may need equipment opened by a qualified worker following your electrical safety procedures, and sometimes a short planned shutdown is the safest way to do it.
What happens if equipment is under-rated?
The report explains the options, which may include replacing the equipment, adding current-limiting devices or changing the system. Until it is corrected, the equipment should be treated as a hazard and not relied on to interrupt a fault safely.