Selective coordination of breakers and fuses

A system is selectively coordinated when, for any overload or fault, only the protective device closest to the problem opens, and everything upstream stays on. A fault on one branch circuit should trip that branch breaker, not the panel's main breaker or the building's main. Good coordination limits an outage to the smallest possible part of the building and makes faults faster to find.

What goes wrong without it

Every breaker and fuse between the source and a fault sees the same fault current. If the upstream device is faster or set lower than the downstream one, it may open first or at the same time. The result is that a short circuit in one receptacle takes out a whole floor, or a fault on one feeder shuts down the entire building. In a hospital, a data centre or on an emergency power system, that can be serious.

Reading time-current curves

Coordination is checked using time-current curves. Each breaker or fuse has a curve showing how long it takes to open at a given current. Current runs along the horizontal axis and time up the vertical axis, both on log scales. A device trips slowly on a small overload and quickly on a large fault.

For two devices in series to be coordinated, the downstream device's curve must sit to the left of and below the upstream device's curve across the full range of current the downstream device could see, up to the available fault current at its location. Where the curves overlap or cross, both devices could open.

The instantaneous region

The hardest part to coordinate is at high fault currents, where breakers trip instantaneously. Two breakers with instantaneous trips that both see the same high current may both open, because neither has any intentional delay. Engineers handle this in several ways:

• Short-time delay. Upstream breakers with adjustable electronic trip units can be set to wait briefly, giving the downstream device time to clear.
• Manufacturer selectivity tables. Breaker manufacturers publish tested combinations that coordinate up to a stated fault current, which can show more selectivity than the curves alone suggest.
• Fuse ratios. Fuses of the same class and manufacturer generally coordinate when the upstream fuse is a set multiple of the downstream fuse rating, as published in the manufacturer's selectivity ratio tables.
• Zone-selective interlocking. Breakers communicate so that the one nearest the fault trips without delay and tells the ones upstream to wait.

Where coordination is required

Codes and standards call for selective coordination in some systems where an unnecessary outage would put lives at risk, most commonly emergency power systems and certain life safety loads. The Canadian Electrical Code and CSA C282, which covers emergency electrical power supply for buildings, set out requirements in this area, and health care facilities have their own standards. Requirements and editions change and are adopted differently by province, so the engineer of record should confirm which apply to your building. For other systems, coordination is a design choice that the owner should make knowingly.

For background on these systems, see emergency power vs standby power.

The trade-off with arc flash

Coordination often works by making upstream devices wait. But the longer a device takes to clear a fault, the more energy an arc flash releases. A well-coordinated system can have high incident energy at the main switchboard. Designers manage this tension with:

• Zone-selective interlocking, which keeps upstream delays only where needed
• Maintenance switches that temporarily reduce trip settings while people work on equipment
• Current-limiting fuses or breakers where they suit the system
• Differential protection on large switchgear

This is why a coordination study and an arc flash study are usually done together using the same model.

How a coordination study is done

1. Collect the data. Every breaker and fuse in the paths being studied: manufacturer, type, frame and trip rating, trip unit and present settings. Transformer and cable data are needed for the fault current calculation.
2. Calculate fault current. The study needs the available fault current at each bus, since coordination only has to hold up to that level. See available fault current explained.
3. Plot the curves. For each path from source to load, the engineer plots the devices in series and looks for overlaps, along with transformer inrush, motor starting and cable damage curves that the settings must respect.
4. Adjust and test options. Settings on adjustable devices are changed to separate the curves where possible. Where they cannot be separated, the engineer checks selectivity tables or considers different devices.
5. Check arc flash. Proposed settings are run through the arc flash calculation so that coordination is not bought at an unacceptable incident energy.
6. Issue settings and verify. The final settings table goes to the electrical contractor or testing firm, who applies the settings and records that they were applied.

A typical case

An office tower's tenants complain that a tripped breaker on one floor sometimes shuts down the whole floor's power and occasionally the elevators. A coordination study finds that the floor panels' main breakers have instantaneous trips that overlap with the branch breakers, and that the riser feeder breaker in the main switchboard was set at the factory defaults. The study recommends new settings for the adjustable trip units, which costs little, and identifies two panels where the breaker combination cannot coordinate at the available fault current and replacement would be needed to achieve it. The owner chooses to change the settings now and plan the replacements with the next renovation.

What to keep on file

• The coordination study, with time-current curves for each path studied
• A settings table for every adjustable breaker and relay
• Records showing the settings were applied and verified in the field
• Fuse types and ratings, so replacements match the design
• Notes on any accepted lack of coordination and the reason

How ehZee can help

Our short-circuit and coordination studies build a model of your system, check coordination for the paths that matter, recommend settings, and identify where equipment changes would be needed. We review the results with you so the trade-offs, including arc flash, are clear.

Common questions