Electrical
Selective coordination of overcurrent devices field guide
Why only the device nearest the fault should open, how fuses and breakers get there, what the NEC mandates for critical power, and where it collides with the arc-flash study.
Direct answer
Selective coordination is the arrangement of overcurrent devices so only the device nearest a fault opens, leaving every upstream device closed and the rest of the system energized. It turns a system-wide blackout into a single tripped circuit. The NEC requires it for emergency, legally required standby, and critical operations power systems.
Key takeaways
- Selective coordination arranges overcurrent devices so only the device nearest a fault opens, leaving all upstream devices closed and the bus energized.
- The NEC requires selective coordination for emergency (Article 700), legally required standby (Article 701, often 701.32), and critical operations power systems (Article 708, often 708.54).
- True selectivity allows no time-current curve overlap anywhere up to the available fault current; curves touching below that level means not coordinated.
- For common low-peak current-limiting fuses, holding a minimum ampere ratio, often 2:1, gives selective coordination up to the fuse interrupting rating.
- No 0.1 second coordination rule exists for Article 700; the 0.1 second carveout belongs to health care essential systems under Article 517.
Selective coordination, and the outage it keeps small
Selective coordination is the arrangement of overcurrent protective devices, fuses and circuit breakers, so that when a fault happens only the device immediately upstream of that fault opens. Everything above it stays closed. The faulted circuit goes dark. The rest of the building keeps running.
The opposite is cascading. A fault on one branch trips the branch breaker, but the feeder breaker above it also sees the fault current and opens, and sometimes the main above that opens too. One short on one receptacle circuit takes out a whole panel, a whole floor, or the whole bus. That is a coordination failure, and on a normal commercial building it is an annoyance. On a hospital or a data center it is the event everyone is paid to prevent.
The word that matters in the Article 100 definition is localization. The NEC defines selective coordination as localizing an overcurrent condition to restrict the outage to the circuit or equipment affected, achieved by the selection and installation of the devices and their ratings or settings for the full range of available overcurrents. Read that last part twice. Not at one fault level. Across the full range, from a small overload up to the maximum available fault current at that point in the system.
Why selective coordination matters for critical power
The cost of a fault is not the fault. It is what trips with it. On a data center bus, a single PDU fault that opens the upstream main drops every cabinet on that bus, and the recovery is not a reset. It is racks rebooting, storage rebuilding, and an outage report to a customer who has a service level agreement with teeth. The fault was always going to take out one circuit. Bad coordination is what turned it into an outage that makes the news.
In a hospital the stakes are physical. The essential electrical system feeds operating rooms, life support, and the systems that keep a building habitable. A fault on one branch that trips a feeder can darken a wing that has patients on it. That is exactly why the code stopped treating coordination as good practice and started treating it as a requirement for these systems.
The whole argument for selective coordination is the difference between a localized outage and a cascading one. You will always have faults. Conductors fail, equipment fails, someone drops a wrench across a bus. The design question is whether the system contains the damage to the one circuit that faulted, or lets it climb the tree and take the bus with it.
Coordination study vs true selective coordination
These two phrases get used as if they mean the same thing, and they do not. A coordination study can minimize the overlap between device curves and still not be selectively coordinated. Selective coordination is the harder bar.
An older coordination study often aimed to reduce miscoordination, to make the devices trip in a reasonable order most of the time, and accepted some overlap at high fault currents as a practical compromise. That is coordination as an optimization. True selective coordination is absolute: there is no overlap of the time-current curves anywhere up to the maximum available fault current at the device. If the curves cross or touch at any current the system can actually deliver, the devices are not selectively coordinated, full stop.
This is the distinction inspectors and commissioning agents press on. A pretty set of curves that separate nicely up to 10 kA means nothing if the available fault current at that bus is 40 kA and the curves collide at 22 kA. The available fault current is the line that decides it. Selectivity has to hold all the way out to that number, not to wherever the curves happen to stop looking clean.
How do you read a time-current curve and a coordination study?
A coordination study lives on a time-current curve, the TCC, plotted on log-log paper. Current runs along the horizontal axis in amps, time runs up the vertical axis in seconds, both on logarithmic scales so the plot can show a tenth of a second next to thirty seconds and 100 A next to 100,000 A on the same sheet.
Each device draws a band, not a line. A fuse or breaker has a tolerance, so its curve is a shaded region: anywhere inside that band the device may operate at that current and time. To coordinate two devices in series, the downstream device's band has to sit entirely below and to the left of the upstream device's band, with clear air between them, all the way out to the available fault current. The downstream device clears faster at every current, so it always wins the race to open.
The vertical line that matters is the available fault current at that point in the system, taken from the short-circuit study. Selectivity is judged against that line. The study engineer plots the source, the main, the feeders, and the branch devices on one sheet per path, drops in the fault-current line, and checks that no two bands in series ever touch below it. Where they touch, the curves overlap, and the overlap is the coordination problem you have to design out.
Fuses and the 2:1 ratio rule
Current-limiting fuses are the easy path to selective coordination, and the reason is the way they clear a high fault. A current-limiting fuse opens so fast on a large fault that it cuts the current off before it reaches its peak, inside the first half cycle. Because the clearing is that fast, two fuses in series separate cleanly in the high-current region where breakers tend to collide.
Manufacturers reduce the whole thing to a ratio. For the common low-peak families, holding a minimum ampere ratio between the upstream and downstream fuse, often 2:1, gives selective coordination up to the fuse's interrupting rating, which on modern current-limiting fuses can be 200 kA or 300 kA. A 400 A downstream fuse under an 800 A upstream fuse is a 2:1 ratio and coordinates. The exact ratio depends on the fuse class and family, so you read it off the manufacturer's selectivity ratio guide, not from memory.
The catch with fuses is the single-phasing risk on three-phase motor loads and the need to stock and replace the right fuse, but for pure selectivity they are hard to beat. One fuse, one number off a table, coordinated to a very high fault level with no settings to get wrong and nothing to calibrate. That last part matters at commissioning, where breaker settings are exactly where the work goes sideways.
Circuit breakers: thermal-magnetic vs electronic trip units
Circuit breakers are where coordination gets interesting, because the breaker's trip behavior is what you are coordinating, and not all breakers give you the same control.
A thermal-magnetic breaker has two mechanisms. The thermal element handles overloads with an inverse time delay, slower for small overloads, faster for big ones. The magnetic element is the instantaneous trip, and it opens with no intentional delay once the current crosses its pickup. That instantaneous region is fixed or only coarsely adjustable on most molded-case breakers, and it is the part that wrecks coordination, covered in the next section.
An electronic trip unit gives you adjustable parameters, usually described by the letters LSIG. L is long-time, the overload pickup and delay. S is short-time, a pickup and an intentional delay that lets the breaker hold in for a set time before tripping on a fault. I is instantaneous, the no-delay trip, which on better trip units can be turned off entirely. G is ground-fault, its own pickup and delay. With short-time delay and the ability to defeat the instantaneous, an electronic trip breaker can be set to ride through a downstream device's clearing time, which is how you coordinate breakers in series. The price of that delay shows up in the arc-flash study, and that conflict has its own section.
Why does the instantaneous trip break coordination?
The instantaneous trip is the enemy of breaker coordination, and the mechanism is simple. Two breakers in series both see the same fault current. If that current lands in the instantaneous region of both breakers, both get the command to open with no intentional delay, and now it is a coin toss.
There is no time separation between them at that current, because instantaneous means no delay on either device. The downstream breaker might clear first and the upstream might never finish opening. Or the upstream breaker commits to the trip before the fault clears and opens too. You cannot predict which, and the unpredictable case is the upstream breaker tripping with the downstream one, which is a loss of selectivity. On the TCC this shows up as the two instantaneous bands overlapping below the available fault current. Where they overlap, you have no coordination.
This is why a normal thermal-magnetic breaker pair coordinates for overloads and low-level faults, where the time delays separate them, but loses coordination at high fault current where both go instantaneous. Many engineers accept that overlap on general, non-critical circuits, because the bolted fault that reaches the instantaneous region of both is usually a wiring error and rare. On a system that must be selectively coordinated by code, that acceptance is gone. You have to defeat the instantaneous on the upstream device and replace it with a short-time delay, or use the manufacturer's tested combinations, or use fuses.
How do you achieve selective coordination?
There are three working methods, and a real design usually mixes them by voltage level and by where the gear sits in the chain.
Fuse ratios, the simplest. Hold the manufacturer's selectivity ratio between fuses in series and you coordinate to the fuse interrupting rating off a table. Short-time delay on breakers, the workhorse for switchgear and large breakers. You set the upstream breaker with a short-time pickup and an intentional short-time delay so it holds in long enough for the downstream device to clear the fault first, then trips only if the fault is still there. The delay is what buys the time separation that the instantaneous trip refuses to give you. Zone-selective interlocking, the method that gets you the delay's coordination without the delay's full arc-flash penalty, covered next.
On any breaker you give a short-time delay to, you have to deal with the instantaneous override. Most trip units keep a fixed instantaneous override at a very high current, a level set near the breaker's withstand rating, that will fire regardless of the short-time setting to protect the breaker from a fault it cannot hold through. If the available fault current at that breaker exceeds the override threshold, the breaker goes instantaneous at that level no matter what your short-time delay says, and coordination can collapse right at the top of the range. Verify the override threshold against the available fault current. That is the check people skip, and it is exactly where a study that looks coordinated on paper falls apart at the real fault level.
What is zone-selective interlocking?
Zone-selective interlocking, ZSI, is a way to get the coordination of a short-time delay without paying its full time penalty on every fault. It uses a signal wire between the trip units of breakers in series.
Here is the logic. When a fault occurs downstream of the lower breaker, that breaker detects it and sends a restraint signal up the wire to the upstream breaker. The upstream breaker, restrained, holds on its short-time delay and lets the downstream breaker clear the fault. Normal coordination, the downstream device opens, the upstream stays in. But when the fault is between the two breakers, in the upstream breaker's own zone, the upstream breaker detects the fault and receives no restraint signal from below, because the downstream breaker sees nothing. With no restraint, the upstream breaker trips with no intentional delay and clears its own zone fast.
The benefit is that a fault in the upstream zone clears quickly instead of waiting out the full short-time delay, which both speeds up protection and lowers the arc-flash energy for that fault. ZSI applies to short-time and ground-fault protection on electronic trip units that support it, and it has to be wired and commissioned, the interlock signal verified end to end. An interlock wire that was never landed or never tested gives you the slow delay everywhere and none of the benefit, and nobody knows until the study is rerun or the fault happens.
Where is selective coordination required?
Selective coordination is required by the NEC for three system types, and required is the word, not recommended. Emergency systems under Article 700, legally required standby systems under Article 701, and critical operations power systems, COPS, under Article 708. For each, the system's overcurrent devices must be selectively coordinated with all supply-side overcurrent devices.
The section numbers have moved between code cycles, which trips up citations. The legally required standby requirement is commonly at 701.32 and the COPS requirement at 708.54. The emergency-system requirement has been renumbered across editions, appearing as 700.27, then 700.28, and 700.32 in different cycles, so confirm the exact section against the edition the jurisdiction has adopted before you put it on a submittal. The requirement itself is stable even when the number moves. Health care essential electrical systems carry their own coordination language under Article 517.
One myth needs killing. There is no general 0.1 second selective coordination rule in Article 700. The Article 100 definition requires coordination across the full range of available overcurrents and the full range of device opening times, not down to some fixed time like 0.1 second. The 0.1 second figure people quote traces to the health care essential-systems context under Article 517, where coordination between devices is not required for faults cleared in 0.1 second or less. Applying that 0.1 second carveout to an Article 700 emergency system is a common and wrong shortcut. For 700, 701, and 708 the bar is full-range selectivity to the available fault current.
Manufacturer selectivity tables and tested combinations
For circuit breakers, the manufacturer's selectivity tables are often what actually proves coordination at high fault current, and they can do something the TCC cannot. A pair of breakers whose instantaneous bands overlap on the curve can still be listed as selectively coordinated to a stated fault level, because the manufacturer tested that specific combination and demonstrated the downstream breaker clears first in the real, dynamic event the static curve does not capture.
These tables give a coordination value, an amp rating or a kA level, up to which a named upstream device coordinates with a named downstream device. The combination is specific. Change the breaker frame, the trip unit, or the manufacturer and the table no longer applies. You cannot mix a breaker from one maker under a breaker from another and read either company's table.
Use the tables the way the manufacturer intends. Confirm the available fault current at the location is at or below the table's coordination value for that exact pair, and confirm the trip unit settings the table assumes are the settings actually dialed into the gear. A table that proves coordination to 65 kA does you no good if the trip unit is set differently than the table assumed, or if the available fault current is higher than the listed value.
The arc-flash tradeoff: coordination vs incident energy
Selective coordination and arc-flash safety pull in opposite directions, and you cannot pretend otherwise. The short-time delay that buys coordination also keeps the upstream breaker closed longer during an arcing fault, and the longer the fault burns, the more incident energy lands on a worker standing in front of the gear. Incident energy scales with clearing time. A delay that holds the breaker in for an extra twenty cycles can multiply the arc-flash energy at that bus.
Put a number on it the way the studies do. A breaker that trips instantaneously might clear an arcing fault in five cycles, roughly 0.083 second. Take the instantaneous away to coordinate and the same breaker might take thirty cycles to clear on its short-time delay, several times longer, and the incident energy climbs with it. So the very setting that makes the system selectively coordinated can push a bus from a moderate arc-flash category into a worse one. This is the direct conflict with the arc-flash study, and it is why the two studies have to be done together, not in sequence by two people who never talk.
The resolution is a maintenance mode. An energy-reducing maintenance switch, ERMS, sometimes called ARMS or an arc reduction maintenance system, is a second set of trip settings, usually a lower instantaneous, that a worker switches on before opening the gear for work. It drops the clearing time and the incident energy for the duration of the task, then gets switched back to the coordinated settings. The system runs coordinated normally and runs safe during maintenance. The known failure is human: the switch left in maintenance mode after the work, which gives you fast tripping and lost coordination until someone notices. For the full arc-flash picture, the labels, the boundaries, and the PPE methods, see the arc-flash study and labels guide.
What the coordination study delivers
A coordination study is a document, and a useful one tells you three things. The TCC plots, one per coordination path, showing every device in series from the source to the load with the available fault current line drawn in. The settings table, listing every adjustable device and the long-time, short-time, instantaneous, and ground-fault settings the study calls for. And the recommendations, where the engineer flags any place selectivity could not be achieved and what it would take to fix it.
The study is usually run alongside the short-circuit study and the arc-flash study, because all three share the same system model. The short-circuit study gives the available fault current that bounds the coordination check. The coordination settings feed the arc-flash calculation. Done by one engineer in one model, they stay consistent. Done piecemeal, they drift, and you find the drift at commissioning or worse.
Read the recommendations section first. That is where the engineer is honest about where the system does not fully coordinate, which is normal at the very top of the fault range on all-breaker systems. What you want to know is whether any non-coordinated overlap falls below the available fault current on a system the code requires to be coordinated, because that is the gap an inspector will find.
Commissioning the settings to the study
The study is only real if the settings are actually in the gear, and the classic field finding is that they are not. Breakers and trip units ship from the factory on default settings, usually with the dials at or near maximum and the instantaneous on. A breaker installed and energized straight out of the carton is running on factory defaults, not on the study, and a system full of factory-default breakers is not coordinated no matter what the study says.
Commissioning the settings means going breaker by breaker, setting every adjustable parameter to the value in the study's settings table, and verifying it. On electronic trip units, verify the actual value on the display, do not trust the dial position, and do not trust that the last technician set it. Photograph the settings as left. Where ZSI is used, verify the interlock wiring and prove the restraint signal does what it should, because an unwired interlock looks fine until you test it.
This is the same discipline as the rest of power commissioning, and it belongs in the same record set. The data center commissioning and power QA guide covers how these checks fit the witnessed, signed commissioning sequence. The point here is narrow and blunt: a coordination study that nobody set into the breakers is a binder, not a coordinated system. Verify the settings match the study, in the gear, with eyes on the display, before turnover.
Ground-fault coordination
Ground-fault protection has its own coordination problem, and it is frequently missed because people coordinate the phase devices and forget the ground-fault function. The NEC requires ground-fault protection of equipment on certain large solidly grounded wye services, commonly at 480Y/277 V and 1000 A or more, and where you have ground-fault protection at more than one level, those levels have to coordinate too.
The trouble is that a main with ground-fault protection and a feeder without it do not coordinate on a ground fault. A ground fault on the feeder is seen by the main's ground-fault function, and with nothing downstream to clear it first, the main trips and drops everything. You get the cascading outage on a ground fault even though the phase devices were perfectly coordinated. The fix is to add ground-fault protection at the downstream level and coordinate the two with pickup and time-delay separation, or use ZSI on the ground-fault function so the downstream level restrains the main.
Check the ground-fault coordination as carefully as the phase coordination, especially on those large wye services where the code forces ground-fault protection on at least the main. A study that coordinates the phase overcurrent and ignores the ground-fault path is half a study.
Coordinating the data center power chain
A data center power chain is a long series string, and coordination has to hold from the utility down to the branch. Walk it from the top: the utility or service main, the generator paralleling and distribution, the upstream switchgear, the UPS input and output, the downstream distribution, the PDU or RPP, and finally the branch device at the rack. A fault at the rack should open the branch device and nothing above it.
The hard spots are predictable. The UPS is its own world, because in battery or inverter mode the available fault current is limited by the UPS electronics, sometimes to not much more than the rated current, which changes the coordination picture completely between utility mode and battery mode. You have to coordinate for both. The PDU breakers and the upstream feeder are a common overlap point at the high end of the fault range, and that is where the tested-combination tables or the short-time delays earn their keep. And the generator source has a lower available fault current than the utility, so a system that coordinates on utility power can lose coordination on generator power, where the fault current is too low to reach a downstream device's instantaneous and the upstream rides in on its delay.
Coordinate the chain for every source the bus can run on: utility, generator, and UPS battery mode. A study done only at the utility fault level misses the mode where the building actually rides through an outage, which is the mode that matters most in a facility built around riding through outages.
As-builts and re-study after changes
Coordination is a property of the system as it exists, not as it was designed, so it expires when the system changes. Add a transformer, swap a service, grow the available fault current, add a large motor, replace a breaker with a different frame, and the coordination study you commissioned may no longer be true.
The available fault current is the input most likely to move. The utility upgrades its transformer, the service fault current climbs, and a coordination that held to the old fault level can fall apart at the new one, because the overlap that used to sit above the available fault current is now below it. The same is true of arc flash and short-circuit ratings, which is why the three studies get reviewed together and on a cadence.
Keep the study as an as-built. When a breaker setting is changed in the field, the study and the settings table get updated with it, not left to disagree. When the system is modified, the study is rerun for the affected paths. A coordination study from the original build, never updated through years of changes, describes a building that no longer exists, and the gap between the binder and the gear is exactly what bites during the next fault.
What to document
The coordination record has to let the next engineer reproduce the result and the next technician confirm the settings. Tie the study to the trip settings actually dialed into the gear, or it falls apart the moment a fault or an inspector tests whether the breakers match the binder.
Capture each device by name and location, its type, the rating or the trip-unit settings it is set to, and the fault level it is coordinated up to, against the available fault current at that point. Record the source assumptions, the available fault current per bus, and which sources the coordination was checked for. When a setting is changed in the field, record who changed it, to what, and why, and update the settings table so the as-built study and the gear never disagree.
| Field to record | Why it matters |
|---|---|
| Device, type, and location | Identifies exactly what was coordinated |
| Rating or LSIG settings | Coordination is only real if these are set |
| Coordinated to (kA / amps) | States the fault level selectivity holds to |
| Available fault current at bus | The line selectivity is judged against |
| Sources checked (utility/gen/UPS) | Coordination can differ by source mode |
| ZSI / ground-fault interlock verified | Unwired interlocks fail silently |
| Settings as-left, who set them | Ties the gear to the study and a person |
Common mistakes
- Leaving the upstream breaker's instantaneous trip on, so two breakers overlap in the instantaneous region and both trip on a high fault.
- Energizing breakers on factory-default settings instead of setting them to the study's settings table.
- No short-time delay on the upstream device, so it cannot ride through the downstream device's clearing time.
- Checking selectivity only to where the curves look clean, not all the way to the available fault current.
- Ignoring the instantaneous override threshold, so the breaker goes instantaneous above it and coordination collapses at the top of the range.
- Coordinating only at the utility fault level and missing generator and UPS battery modes.
- Adding a short-time delay for coordination without checking what it does to the arc-flash incident energy.
- Coordinating the phase devices and forgetting the ground-fault function entirely.
Field checklist
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Standards and references
The NEC, NFPA 70, carries both the definition and the mandate. Article 100 defines selective coordination as localizing an overcurrent to the affected circuit across the full range of available overcurrents. The mandate sits in Article 700 for emergency systems, Article 701 for legally required standby systems, commonly at 701.32, and Article 708 for critical operations power systems, commonly at 708.54. The emergency-system section has been renumbered across cycles, so confirm it against the adopted edition. Ground-fault protection of equipment and its requirements come from Article 230 and 240. Article 517 governs health care essential electrical systems and is the home of the 0.1 second coordination language that does not belong to Article 700.
For the engineering method, IEEE 242, the Buff Book, is the reference on protection and coordination of industrial and commercial power systems. IEEE 1584 is the arc-flash calculation method that sits on the other side of the coordination-versus-incident-energy tradeoff, and NFPA 70E governs the electrical safety program that the arc-flash study feeds. Manufacturer selectivity tables and the fuse selectivity ratio guides are the listed, tested basis for coordination at high fault current, and they govern the specific device pairs they cover.
Cite the document that controls the point. The code mandates that these systems be coordinated and that a qualified engineer select and document the coordination. The project specification and the equipment listings fill in the rest, and the adopted code edition with local amendments controls the section numbers and the thresholds.
Units, terms, and conversions
Coordination work moves between the curve, the study, and the gear, and the same idea carries a few names across them.
Selective coordination is sometimes shortened to selectivity or full selectivity. The overcurrent protective device is the OCPD, fuse or breaker. The time-current curve is the TCC or the coordination curve. Available fault current is also called available short-circuit current or the bolted fault current, in amps or kA. Breaker trip settings go by LSIG: long-time, short-time, instantaneous, ground-fault. The maintenance mode goes by ERMS, ARMS, or arc-energy reduction maintenance setting depending on the manufacturer.
- Selective coordination
- Only the OCPD nearest the fault opens, leaving every upstream device closed, across the full range of available overcurrents
- OCPD
- Overcurrent protective device, a fuse or circuit breaker
- TCC
- Time-current curve, the log-log plot of device operating time versus current
- Available fault current
- The maximum short-circuit current at a point in the system, the level selectivity must hold to
- LSIG
- Long-time, short-time, instantaneous, and ground-fault adjustable settings on an electronic trip unit
- ZSI
- Zone-selective interlocking, a restraint signal between trip units that speeds clearing of in-zone faults
- ERMS / ARMS
- Energy-reducing maintenance switch, a second trip setting that lowers clearing time and arc-flash energy during work
FAQ
What is selective coordination?
Selective coordination is the arrangement of fuses and breakers so that only the device immediately upstream of a fault opens, while every device above it stays closed. A fault on one circuit darkens that circuit alone instead of cascading up and tripping a feeder or the main and dropping the whole bus.
Where is selective coordination required by the NEC?
The NEC requires selective coordination for emergency systems under Article 700, legally required standby systems under Article 701 (commonly 701.32), and critical operations power systems under Article 708 (commonly 708.54). Confirm the section numbers against the adopted edition, since the emergency-system section has been renumbered across code cycles.
Why does the instantaneous trip break selective coordination?
Two breakers in series both see the same fault. If that current reaches the instantaneous region of both, both trip with no intentional delay and there is no time separation, so the upstream breaker can open with the downstream one. That overlap on the curve is a loss of coordination at high fault current.
What is zone-selective interlocking?
Zone-selective interlocking, ZSI, runs a signal wire between trip units. A downstream breaker sends a restraint signal up on a fault below it, so the upstream breaker holds on its delay. A fault in the upstream zone sends no restraint, so the upstream breaker trips fast, clearing in-zone faults quickly with lower arc-flash energy.
What is the difference between a coordination study and true selective coordination?
A coordination study can just minimize curve overlap and accept some at high fault current. True selective coordination allows no overlap of the time-current curves anywhere up to the available fault current. If the curves touch below that fault level, the system is not selectively coordinated, regardless of how clean the lower range looks.
Does a short-time delay increase arc flash energy?
Yes. A short-time delay keeps the upstream breaker closed longer during an arcing fault, and incident energy scales with clearing time. Defeating an instantaneous trip to coordinate can multiply arc-flash energy at that bus. A maintenance switch (ERMS or ARMS) drops the clearing time during work to manage the conflict.
What is the 2:1 fuse ratio rule?
For common low-peak current-limiting fuse families, holding a minimum ampere ratio between the upstream and downstream fuse, often 2:1, gives selective coordination up to the fuse interrupting rating. An 800 A fuse over a 400 A fuse meets it. The exact ratio depends on the fuse class, so read the manufacturer's selectivity ratio guide.
Is there a 0.1 second selective coordination rule in the NEC?
Not for Article 700 emergency systems. The Article 100 definition requires coordination across the full range of available overcurrents and opening times, not down to a fixed time. The 0.1 second figure belongs to health care essential systems under Article 517, and applying it to Article 700 is a common mistake.
Fuses or circuit breakers for selective coordination?
Fuses coordinate easily at high fault current with a single ampere ratio off a table and nothing to calibrate. Breakers need adjustable electronic trip units, defeated instantaneous, short-time delay or tested combinations, and careful commissioning. Fuses are simpler for pure selectivity; breakers give settings, monitoring, and resettability. The design usually mixes both by location.
How do you verify selective coordination is actually installed?
Compare the gear to the study's settings table device by device. Read each electronic trip unit's actual value on the display, not the dial, since breakers ship on factory defaults. Verify the instantaneous override is above the available fault current, prove any ZSI interlock wiring, and record the settings as-left before turnover.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.