An upstream circuit breaker and a downstream fuse are not selectively coordinated merely because the fuse has a smaller ampere rating. For a fault on the fused branch, the fuse should clear while the upstream breaker remains closed over the required overcurrent range. That result depends on the exact breaker frame, trip unit, settings and tolerance band; the exact fuse family, class, rating and temperature; the DC voltage and polarity arrangement; the prospective fault-current envelope at the branch; current-limiting behaviour; and manufacturer-tested or approved coordination evidence.
A time-current plot is essential in the overload and longer short-circuit regions, but it may not be sufficient in the current-limiting region. Curves can stop before the highest available fault current, breaker instantaneous response can overlap fuse total-clearing behaviour, and AC tables do not automatically apply to DC. Procurement should require a coordination study plus exact product tables or test evidence for the region that curves alone cannot resolve.
This guide covers an upstream circuit breaker feeding downstream fuses in low-voltage DC distribution. It builds on the DC fuse coordination, time-current and I²t guide, which explains fuse evidence more broadly, and the DC circuit-breaker trip curve and interrupting-rating guide. It does not set breaker trips, select a fuse, calculate an arc-flash boundary or replace a qualified protection study. No statement here claims an unverified SINAWATTS protection device, coordination table, certification, engineering service, test capability, stock, price, MOQ or lead time.
Direct answer: what should the RFQ require?
For every breaker-to-fuse pair and credible operating mode, require:
- one-line location IDs and confirmation that the breaker is supply-side and fuse is load-side;
- system nominal and maximum DC voltage, grounding arrangement and required pole connection;
- source topology, operating modes and prospective DC fault-current versus time at the downstream fuse and upstream breaker;
- cable and bus impedance included in the study, with tolerances and minimum/maximum cases;
- upstream breaker manufacturer, family, frame, rating plug/sensor, trip unit, firmware where relevant, pole arrangement and exact settings;
- downstream fuse manufacturer, series/class, catalog number, ampere rating, voltage rating, utilization category and holder;
- current ambient and enclosure temperatures or correction basis for both devices;
- breaker minimum/maximum trip bands and fuse minimum-melting/total-clearing curves on common axes;
- the selectivity limit for the exact pair at the specified DC voltage and polarity, or an explicit statement that only partial selectivity is demonstrated;
- current-limiting peak and pre-arcing/total I²t evidence where applicable;
- confirmation that each device can interrupt the prospective DC fault in its installed configuration;
- conductor/equipment protection checks and any upstream delay withstand requirements;
- controlled electronic-trip settings and commissioning records; and
- a signed exception schedule.
Do not accept “fully coordinated” without a current range. State whether selectivity is required through overload only, to a defined branch fault current, or to the maximum available fault at the downstream bus.
Define the protection objective before comparing curves
Selective coordination has a service-continuity objective: the device closest to the fault operates while upstream healthy branches remain energized. It is distinct from backup protection, series rating and device interrupting capacity.
Use four separate gates:
- Interrupting gate: Each device and assembly must safely interrupt or withstand the available DC fault under its applicable rating route.
- Conductor/equipment protection gate: Clearing energy and time must protect cables, busbars and connected equipment under the governing rules.
- Selectivity gate: The downstream fuse clears the defined downstream faults without tripping the upstream breaker.
- Settings/production gate: Installed part numbers, pole wiring and breaker settings match the approved study.
A tested series combination may establish a short-circuit rating while both devices open; that is not selective coordination. A breaker with adequate interrupting rating can still nuisance-trip with a branch fuse. A coordinated pair can still be unsuitable if one product lacks the required DC voltage or polarity rating.
ABB’s official selectivity page, checked on 2026-09-28, defines selectivity between series devices as the load-side device opening for an overcurrent while the supply-side breaker remains closed. ABB distinguishes partial selectivity, which is maintained only below a defined limit, from total selectivity within the applicable range. ABB selectivity portal.
Build a location-specific DC fault envelope
Do not use one bolted-fault number for every decision. DC sources can have time-varying contribution. Batteries can deliver high initial current affected by state of charge, temperature, internal resistance and interconnections. Capacitors can contribute a sharp early pulse. Converters can limit or stop current according to controls. Parallel strings and backfeed paths can change the direction and magnitude through each device.
The study input should show:
- source configuration and credible maximum/minimum states;
- battery chemistry, string count and impedance basis where applicable;
- converter or power-supply current-limit behaviour supported by manufacturer data;
- capacitor discharge contribution;
- conductor lengths, sizes, temperatures and connection resistance;
- fault location and type: pole-to-pole, pole-to-earth or another credible path;
- system grounding and midpoint arrangements;
- device terminal voltage during interruption;
- minimum fault current needed to confirm timely operation; and
- maximum fault current used for interrupting and selectivity limits.
Provide a current-versus-time envelope or the approved equivalent, not just an initial peak. The downstream fuse and upstream breaker may respond to different portions. If a simulation output is used, preserve model revision and source data.
Freeze the breaker configuration
A breaker family name is not enough. Record:
- frame and interrupting-rating variant;
- rated current and sensor/rating-plug value;
- thermal-magnetic or electronic trip unit;
- long-time pickup and delay;
- short-time pickup, delay and I²t on/off setting;
- instantaneous pickup and any override;
- ground-fault function/settings if part of coordination;
- DC pole count, series connection and polarity requirements;
- trip-unit power/current sensing limits;
- temperature correction/derating; and
- lock/seal or access control for settings.
IEC 60947-2:2024, checked on 2026-09-28, applies to circuit breakers up to 1,000 V AC or 1,500 V DC within its scope and supersedes the 2016 edition. It covers device characteristics and tests, but a claim to IEC 60947-2 does not by itself prove selectivity with a particular fuse. IEC 60947-2:2024 official page.
Use the exact product’s DC data. Pole series connections can be mandatory for DC interruption. Line/load polarity may be restricted. Changing one connection can affect interruption and trip behaviour. The DC breaker pole series-connection guide and polarity/bidirectional-use guide provide complementary controls.
Freeze the fuse and holder configuration
Record the complete fuse catalog number, utilization category, DC voltage, ampere rating, body/size, interrupting rating, manufacturer curve revision and holder. Similar physical sizes can carry different time-current behaviour. A fuse class or family cannot be represented by an arbitrary “typical” curve.
Use both boundaries:
- minimum-melting/pre-arcing curve: earliest expected start of element melting; and
- total-clearing curve: latest clearing including arcing under stated conditions.
For breaker-over-fuse selectivity, the downstream fuse’s total-clearing behaviour generally needs to be compared with the upstream breaker’s earliest/minimum trip behaviour in the curve region. Apply the device manufacturers’ method and tolerances. Do not compare average fuse melting to an average breaker line and call the gap a margin.
Ambient and preloading matter. A fuse carrying high continuous current in a hot enclosure can operate sooner. An upstream thermal element can also be warm. State whether published curves assume cold, preloaded or reference ambient conditions, and run the study under credible operating temperature.
The DC fuse ambient-temperature and enclosure guide covers this thermal boundary. A holder’s heat rise can also affect fuse response; use the exact holder and cable termination.
Plot common-axis curves correctly
Use a log-log time-current plot with both devices referred to the same actual current location. If transformer ratios or parallel paths exist, convert currents consistently. For this topology, the upstream breaker normally sees the branch fault current plus any other contribution through its location, while the downstream fuse sees the branch path. Mark the current range where the values are equal and where they differ.
The study plot should show:
- minimum and maximum breaker trip bands, including tolerances;
- fuse minimum-melting and total-clearing bands;
- long-time, short-time and instantaneous regions;
- breaker settings and temperature basis;
- fuse preloading/ambient basis;
- branch minimum and maximum fault currents;
- the transition to current limitation;
- equipment/cable damage curves where required;
- inrush or expected transient envelopes; and
- any manufacturer table limit that supersedes visual curve comparison.
ABB’s official selectivity-techniques page, checked on 2026-09-28, says time-current selectivity requires the load-side protection to trip faster and highlights the need to consider threshold/time tolerances and real currents. ABB selectivity techniques.
Do not stretch a curve beyond its published endpoint. If the breaker band or fuse curve stops before the maximum fault, use manufacturer current-limiting/selectivity tables or test evidence for the remaining region.
Separate overload-region coordination from high-fault coordination
In the overload and delayed short-circuit region, curve separation can demonstrate which device operates first. At high current, both devices may act in milliseconds. Breaker instantaneous mechanisms, magnetic tolerances, contact opening and the fuse’s current limitation interact. Static lines on a TCC may be insufficient.
Schneider Electric’s current 2026 Selectivity, Cascading and Coordination Guide, checked on 2026-09-28, covers named LV product combinations and lists DC product ranges including C60 for DC circuits and Compact NSX for Direct Current. Its existence illustrates the need to use current manufacturer combination data rather than derive every high-fault result from generic curves. The guide’s tables apply only to the listed products, settings, voltage and conditions. Schneider 2026 selectivity guide.
For an upstream breaker/downstream fuse pair, require one of these evidence routes for the high-fault region:
- a manufacturer table naming both exact devices and the applicable DC conditions;
- a tested combination report with a declared selectivity limit;
- a manufacturer-approved energy comparison using published breaker and fuse data; or
- a qualified study method accepted by the governing project authority.
State any gap honestly. “Selective through 5 kA; simultaneous/upstream operation not excluded above 5 kA” is useful. “Selective” without a limit is not.
Use let-through and I²t without reversing device roles
At high current, the downstream fuse can limit peak current and energy reaching the circuit. To show the upstream breaker remains closed, the responsible study may compare fuse total clearing or let-through with the breaker’s no-trip/operation thresholds using the manufacturer’s method.
ABB’s official Selectivity with fuses page, checked on 2026-09-28, treats three topologies separately, including upstream circuit breaker/downstream fuse. It says time-current characteristics are used in the overload region and discusses let-through energy and device limits in the short-circuit region. The page must be applied with ABB’s exact product data and definitions; it is not a universal arithmetic formula. ABB selectivity with fuses.
Keep these quantities distinct:
- fuse pre-arcing I²t;
- fuse total-clearing I²t;
- fuse peak let-through current;
- breaker instantaneous pickup/tolerance;
- breaker no-trip or energy threshold, if published;
- breaker opening/clearing energy; and
- equipment withstand energy.
Do not compare fuse pre-arcing I²t with breaker total clearing and assume selectivity. Do not compare AC let-through at one voltage with a DC application. Use the exact manufacturer method and units.
Reject AC-only tables for unproven DC use
Many widely available coordination tables are for 240/480/600 V AC building systems. Eaton’s current selective-coordination page, checked on 2026-09-28, publishes tested combinations of upstream Eaton circuit breakers and downstream Bussmann fuses for the stated applications and points users to exact tables. Its application material also says available fault current and device curves/tables must be considered. Those sources are valuable examples of manufacturer pair evidence, but an AC table is not DC evidence unless it expressly includes the required DC voltage and configuration. Eaton selective coordination page.
For every table, record:
- current type and frequency if AC;
- DC voltage if DC;
- system voltage and grounding;
- upstream and downstream orientation;
- exact catalog numbers and settings;
- conductor/test configuration if specified;
- selectivity limit and units;
- publication/revision; and
- footnotes.
If no DC table exists, ask the manufacturer for written engineering guidance or use a qualified study route. Do not relabel an AC result.
Check interrupting ratings independently
Selective coordination does not prove interruption. The fuse must have a DC voltage and interrupting rating adequate for the fault at its location. The breaker must have the applicable DC breaking rating for its pole connection, voltage and time constant where specified. The assembly must have an adequate short-circuit rating.
Use the DC fuse voltage and interrupting-capacity guide for fuse checks and the breaker guide linked earlier for circuit breakers. Do not rely on the downstream fuse to “protect” an inadequately rated upstream breaker unless a tested/approved combination rating explicitly covers the configuration—and remember that such a rating does not automatically provide selectivity.
Coordinate without defeating conductor and equipment protection
Delaying the upstream breaker can improve selectivity but increases the duration of fault stress if the downstream device fails or the fault lies between devices. Confirm busbar and cable short-time withstand, breaker short-time capability and arc-energy implications. Protection priorities can conflict.
The study should evaluate:
- fault on the fused load side;
- fault between breaker and fuse;
- fuse failure/opening abnormality;
- minimum fault current at the far end of the branch;
- overload below fuse operation;
- inrush or charging current;
- ground fault if its path differs; and
- maintenance/settings modes.
Do not raise instantaneous pickup above a value needed for conductor or equipment protection solely to gain coordination. Record the trade-off and responsible approval.
Use a bounded worked comparison
Consider a hypothetical DC panel with an electronic upstream breaker and one downstream branch fuse. Values are invented to show method, not to select products.
The maximum branch fault study gives 7 kA, while the minimum remote fault is 180 A. The plot shows the fuse total-clearing band to the left of the breaker minimum-trip band from 180 A to 1.5 kA. Above 1.5 kA, both published TCCs enter regions where curve separation cannot establish behaviour.
Bid A calls the pair “fully selective” because the fuse is 40 A and the breaker is 250 A. It provides no exact catalogs or high-fault evidence. The 6.25:1 ratio may be irrelevant for different device families and DC conditions; Bid A is open.
Bid B supplies an AC coordination table that names the same ampere ratings but a 480 V AC system. It is not evidence for this DC system. Bid B can use it only as a question for the manufacturer, not a release document.
Bid C supplies exact curves, settings and a current manufacturer table covering the specific breaker, fuse, DC voltage and pole arrangement up to 10 kA. It confirms the table orientation is upstream breaker/downstream fuse and that the actual 7 kA is below the selectivity limit. It also checks minimum-fault clearing and interruption. Bid C has the strongest documented coordination; final approval belongs to the qualified study owner.
Control electronic trip settings through commissioning
The approved study can fail in production if settings change. Put the settings schedule on the released drawing and commissioning form. Record device serial/ID, trip-unit revision, rating plug, each pickup/delay, I²t mode, instantaneous/override condition, ground function and access seal.
Use password/role control for digital trip units. Archive the downloaded settings file and human-readable report. Verify settings after firmware update, trip-unit replacement or breaker maintenance. A photographed dial can supplement but not replace a complete settings record.
Where the breaker has dual settings or maintenance mode, study each mode. A maintenance setting intended to reduce incident energy may intentionally reduce selectivity. Label and train accordingly.
First article and production evidence
The first enclosure should be inspected against the study. Confirm exact breaker, trip unit, fuse, holder, conductor and pole wiring. Verify breaker settings with the approved method. Check fuse labels without relying on package colour. Confirm source/load orientation and voltage polarity.
The release packet should include:
- approved one-line and fault-study revision;
- pair matrix and selectivity objective;
- exact curves/tables with revisions;
- settings report;
- interrupting/SCCR evidence;
- thermal/ambient assumptions;
- first-article photographs and inspection;
- commissioning test record; and
- change-control owner.
Production inspection should prevent substitution among visually compatible fuses. Use barcode or part-number verification where risk justifies it. Seal or password breaker settings and audit a sample.
RFQ evidence matrix
On small screens, swipe or scroll sideways to read every column. Keyboard users can focus the table and use the arrow keys.
| Decision field | Buyer requirement | Supplier return | Release evidence |
|---|---|---|---|
| Topology | Upstream breaker/downstream fuse | Exact one-line pair | Marked one-line |
| DC system | Voltage, grounding and pole arrangement | Covered configuration | Device data and drawings |
| Fault envelope | Minimum/maximum current versus time | Study values | Approved fault study |
| Breaker | Frame, trip and settings | Exact catalog/settings | Curve and settings report |
| Fuse | Series, rating, category and holder | Exact catalog | Min-melt/total-clear data |
| Curve region | Separation with tolerances | Demonstrated range | Common-axis TCC |
| High-fault region | Required selectivity limit | Total/partial limit | DC table/test/manufacturer method |
| Interruption | Device/assembly ratings | Adequate values | Certificates/data |
| Thermal condition | Ambient and preload | Corrections | Enclosure thermal evidence |
| Production | Part/settings control | Inspection plan | First article and commissioning |
Mark the result “not demonstrated” when evidence ends below available fault. Do not round a partial limit upward.
Change control
Reopen the coordination study after changes to source capacity, battery strings, converters, cable length/size, grounding, operating mode, breaker family/frame/trip unit/settings/firmware, fuse series/rating/holder, enclosure temperature, DC voltage, pole connection or available fault current. Also reopen after nuisance trips or unexplained simultaneous operation.
Preserve historical curves and table revisions with the as-built pair. A replacement fuse with the same ampere rating can have a different curve. A replacement breaker trip unit can change instantaneous tolerance.
Source boundaries checked on 2026-09-28
The IEC 60947-2:2024 page, ABB selectivity and fuse-coordination pages, Schneider 2026 selectivity guide, and Eaton selective-coordination page linked above were checked on 2026-09-28. Manufacturer tables apply only to named products, settings, orientation, voltage/current type and footnotes. AC evidence is not silently transferable to DC. IEC 60947-2 product compliance does not by itself establish pair selectivity. Project fault, protection and safety approval remain with qualified parties.
Send the one-line, DC voltage, fault envelope and exact breaker/fuse candidates for a structured RFQ. Include required selectivity range, cable data, ambient and settings-control requirements so bidders can return curves plus current manufacturer evidence. The actual supplier must confirm device availability, price, quantity and lead time.
Buyer FAQ
Does a larger upstream breaker guarantee selectivity with a smaller fuse?
No. Ampere-rating ratio alone does not establish trip-band separation or high-fault behaviour. Use exact curves and applicable manufacturer pair evidence.
Is a time-current plot enough for every fault current?
Not always. In current-limiting and instantaneous regions, manufacturer tables, let-through data or tested combination evidence may be required.
Can an AC breaker-to-fuse table be used for DC?
Only if the manufacturer explicitly states that the table covers the required DC voltage, pole arrangement and conditions. Otherwise it is not DC evidence.
What is partial selectivity?
Only the downstream fuse operates up to a declared fault-current limit. Above that limit, upstream operation cannot be excluded. Compare the limit with maximum available fault.
Which fuse curve should be compared with the breaker?
Use the manufacturer-approved method. Typically both minimum-melting and total-clearing boundaries matter, with the fuse’s latest clearing compared against the breaker’s earliest trip in the relevant region.
Does a coordinated pair automatically have adequate interrupting ratings?
No. Verify DC voltage, interrupting/breaking capacity, pole connection and assembly SCCR independently.
Can I delay the breaker to create more separation?
Only after checking cable, busbar and equipment withstand, faults between devices and other safety objectives. The qualified protection study must approve settings.
How do temperature and preload affect coordination?
Hot or heavily loaded fuses and thermal breakers can operate sooner. Use credible enclosure temperature and load conditions rather than reference curves blindly.
What should be locked after commissioning?
Control breaker settings, trip-unit files, fuse catalog numbers, holder, pole wiring and study revision. Use seals, passwords and inspection records as appropriate.
What event should trigger a new coordination study?
Any source, cable, device, setting, voltage, grounding or thermal change that affects fault current or operating curves should reopen the study.