A fuse holder marked “1000 V DC” can still be unsuitable after installation. A mounting screw, grounded wall, neighboring busbar, wire strand, dust layer, condensation path or high-altitude air gap can reduce the insulation margin that existed in the component test. The voltage label is therefore not a substitute for an installed creepage and clearance review.
Clearance is the shortest distance through air between conductive parts. Creepage distance is the shortest path along an insulating surface. They respond to different stresses and inputs. Clearance is strongly related to impulse voltage, electric field, air and altitude. Creepage is strongly related to working voltage, insulation material surface behavior and the local pollution/condensation environment. A holder needs both.
This guide turns those concepts into an RFQ evidence package. It does not calculate one universal spacing, replace the applicable product or installation standard, or claim that a component rating automatically applies to an enclosure. It makes no unverified claim about a SINAWATTS fuse holder, polymer, voltage, pollution degree, certification, test capability, stock, price, MOQ, lead time or project performance.
Direct answer: what must the bidder return?
Require an insulation-coordination schedule for the exact installed fuse holder:
- system nominal, maximum continuous and transient DC voltage, including charging and fault states;
- circuit relationship to earth/chassis and voltage between every relevant conductive pair;
- applicable equipment/product standard, edition and insulation-coordination method;
- overvoltage category, rated impulse withstand voltage and protective measures used to establish it;
- pollution degree for the local micro-environment, with enclosure and condensation assumptions;
- altitude range and any correction required above the base conditions;
- insulation function: functional, basic, supplementary, reinforced or another category defined by the governing standard;
- insulating material group or comparative tracking evidence where required;
- minimum creepage and clearance requirements plus actual dimensions on controlled drawings;
- measurement method around ribs, slots, fasteners, fuse clips, covers and mounting surfaces;
- component certification/conditions of acceptability and exact model scope;
- production dimensional control, material traceability and dielectric/impulse evidence; and
- enclosure-level first-article inspection and change-control triggers.
A certificate stating a voltage and current is supporting evidence, not a complete installed schedule. Ask what conditions, enclosure, spacing and intended use make the rating valid.
Use current insulation-coordination sources
IEC’s official page for IEC 60664-1:2020, checked on 2026-09-28, describes insulation coordination for equipment up to 1000 V AC or 1500 V DC connected to low-voltage supply systems, up to 30 kHz. It states that the standard provides requirements for determining clearances, creepage distances and solid-insulation criteria, applies directly up to 2000 m and provides guidance above that altitude. The page also identifies the consolidated IEC 60664-1:2020+AMD1:2025 edition. IEC 60664-1 official record.
This is a basic safety publication, not a declaration that every fuse holder should be dimensioned from its tables without the relevant product standard. Start with the product/equipment standard and use insulation coordination as that standard directs. Record edition because amendments can change the calculation path.
UL Solutions’ official fuse and fuseholder services page identifies UL 4248 for U.S. fuseholders and lists different holder/fuse categories, including photovoltaic, Class T and other classes. This shows why “UL fuse holder” is incomplete; the exact part of the series and product category matter. UL Solutions fuse and fuseholder services.
For an IEC-based product, identify the relevant IEC 60269 or other fuse/holder standard and its system-specific part. For a North American product, identify Listing versus Recognition and any end-product conditions. Never infer cross-system acceptance from physical fuse fit.
Freeze the electrical stress between each pair
Do not write only “system voltage 48 V” or “1000 V DC.” Create a potential map for:
- line-to-line or positive-to-negative across the open fuse;
- each live pole to grounded enclosure/chassis;
- input to output during fuse opening;
- live parts to accessible cover or mounting hardware;
- separate circuits in the same holder or adjacent holders;
- indicator/alarm terminals to the protected power circuit; and
- service states with the fuse removed.
Maximum continuous voltage may occur during charging rather than at battery nominal voltage. PV strings can reach their highest open-circuit voltage in cold conditions. Inductive circuits can produce transients during interruption. Define whether the upstream surge protective device, fuse and enclosure architecture limit impulse stress and how that conclusion is documented.
The DC fuse voltage and interrupting-capacity guide addresses whether a fuse can safely clear the prospective fault at voltage. This article addresses insulation around the holder before, during and after that event. Passing one review does not pass the other.
Define clearance from impulse, altitude and geometry
Clearance follows the shortest path through air. The relevant impulse withstand, field conditions, altitude and governing standard determine the required distance. Measure from the nearest conductive points, not from nominal centerlines. Include screw heads, clip edges, strands, washers and metal mounting surfaces.
Above 2000 m, lower air density can reduce dielectric withstand, so an altitude correction may apply. State the maximum installation altitude in the RFQ even when the current project is near sea level if the product is intended for a global platform. Do not apply an altitude factor to creepage automatically; follow the governing method.
Potential clearance reductions include:
- an overlong mounting screw entering the holder cavity;
- a lug rotated toward a grounded panel;
- conductor whiskers outside a cage clamp;
- a fuse indicator board added near live clips;
- a cover rib with conductive contamination bridging the gap;
- a metal label or shield added after certification; and
- deformation at high temperature.
Specify conductor preparation and stripping so no strand violates the measured boundary. Inspect the worst permitted wire and terminal size. A drawing with an ideal straight conductor is not enough when the field cable can rotate.
Define creepage from working voltage, pollution and material
Creepage follows the surface of solid insulation. It depends on working voltage and the local micro-environment, plus material tracking behavior where the governing standard uses a material group. Surface ribs can lengthen the path only when their geometry qualifies under the measurement rules. A cosmetic groove may not count.
IEC 60112:2025’s official record explains that its test evaluates insulating material behavior and that results must be used with overvoltage, creepage-distance and pollution-degree considerations under IEC 60664-1. It also cautions that its test result is not directly a safe creepage distance for an apparatus. This is the correct evidence boundary: CTI/material-group data is an input, not an end-product spacing approval. IEC 60112:2025 official record.
Require the exact polymer grade, color and any recycled-content rule where material tracking performance is relied upon. Pigment, filler and process changes can matter. A generic “PA66” callout is insufficient when certification or spacing depends on a qualified grade.
Do not confuse bulk dielectric strength with surface tracking performance. A material can resist a short laboratory voltage through its thickness yet perform differently along a contaminated surface. Require the property named by the governing standard.
Assign pollution degree to the micro-environment
Pollution degree is not just an outdoor/indoor label. It describes contamination at the insulation surface. An outer enclosure can create a cleaner micro-environment, but condensation, conductive dust, salt, carbon residue or service opening can change it.
Eaton’s official low-voltage switchgear application guide summarizes the IEC/EN pollution-degree concepts: degree 1 is no or only dry non-conductive pollution; degree 2 normally has non-conductive pollution with occasional temporary conductivity from condensation; degree 3 includes conductive pollution or dry pollution that becomes conductive due to condensation; degree 4 is permanently conductive pollution. The guide emphasizes the equipment micro-environment and enclosure influence. Use the applicable standard text for design values, but this manufacturer guide is useful evidence for RFQ definitions. Eaton low-voltage switchgear application guide.
Document sources of pollution:
On small screens, swipe or scroll sideways to read every column. Keyboard users can focus the table and use the arrow keys.
| Source | Possible surface effect | Control to document |
|---|---|---|
| Condensation | Temporary conductive film | Thermal/humidity design, vent or heater |
| Salt mist | Conductive residue and corrosion | Sealed enclosure, coating, maintenance |
| Carbon/brush dust | Conductive deposits | Separation and cleaning interval |
| Metal swarf | Direct conductive bridge | Manufacturing/service contamination control |
| Battery electrolyte | Conductive/corrosive path | Compartment separation and spill control |
| Road dust/water | Wet contaminated film | Automotive enclosure and seal verification |
| Insects/debris | Bridging and moisture retention | Screen/seal and service inspection |
Do not lower pollution degree merely because the holder has a cover. Show how the installed enclosure maintains the assumed micro-environment throughout life and service.
Separate component cover, touch protection and insulation coordination
A fuse-holder cover may reduce accidental contact and debris ingress, but it does not automatically create the required pollution degree or enclosure IP rating. Determine whether the cover is part of the evaluated construction, whether it remains closed in normal operation and what happens when the fuse is removed.
The fuse-holder footprint and cover guide addresses cover fit and access for MIDI/MEGA arrangements. The fuse-holder thermal derating guide covers heat rise. This article adds spacing and surface-insulation evidence. A thicker cover can improve touch protection while trapping heat; both gates must pass.
Define live access during maintenance. Pulling a fuse can expose both source and load contacts, and the load side can remain energized from another source. Labels, barriers and isolation procedures are separate from creepage but essential to the installed risk control.
Check conditions of acceptability and enclosure spacing
Recognized components can require evaluation in the end product. Obtain the certification file information allowed for use, conditions of acceptability, required enclosure, maximum ratings, wire range, mounting and spacing. Match suffixes exactly.
Eaton’s official fuse-holder selection paper states that voltage ratings involve dielectric strength, creepage and clearance and warns against assuming a published holder voltage can be field installed as such, particularly in metal enclosures where application conditions may be unknown. That is a strong procurement boundary: require installed evaluation rather than treating the catalog rating as self-executing. Eaton “How to select a fuse holder”.
Measure from the holder’s live parts to the enclosure at tolerance extremes. Include mounting-slot movement, panel flatness, hardware length and cover deformation. If standoffs establish clearance, freeze their material, height and compression. If a PCB carries the holder, include copper, solder fillets, contamination and conformal-coating boundaries.
Do not infer spacing from voltage label alone
Two holders with the same rated voltage can rely on different product standards, impulse categories, enclosure conditions or certification scopes. One can be a bare PCB component intended for controlled equipment; another can be an enclosed field-wired assembly. Their labels do not mean their installed creepage and clearance are interchangeable.
Littelfuse’s official 304 PICO fuse data sheet, revised in 2025 and checked on 2026-09-28, expressly states that the fuse must be mounted so creepage and clearance distances are not impaired. This statement is for that fuse, not a holder design rule, but it reinforces that end mounting can invalidate component-level assumptions. Littelfuse 304 PICO fuse data sheet.
Ask the bidder to state whether dimensions are minimum after molding tolerance, flash, warpage, assembly and required mechanical loads. Nominal CAD distance is not the acceptance value.
Evaluate heating, fuse opening and contamination as coupled stresses
Spacing drawings are commonly made at room temperature with a new holder. The installed insulation also experiences fuse heat, terminal heat, mechanical clamping and possible arc products. The RFQ should state the maximum permitted fuse power dissipation, holder ambient, conductor temperature and enclosure thermal state used for dimensional and dielectric evidence. Use the DC fuse ambient-temperature guide to coordinate current derating with this insulation review.
Ask whether the holder body, cover or barrier can soften, warp or creep at the controlling temperature. A few millimeters of movement can reduce air clearance, bring a cover closer to a live clip or open a surface gap that traps contamination. Verify dimensions after the applicable thermal conditioning and terminal mechanical loads when the product standard or project plan requires it.
Fuse interruption can deposit soot, metal vapor or fragments inside a holder or enclosure. Do not assume an electrically opened fuse leaves the insulation system fit for continued service. Define whether the holder is replaceable after a specified fault, what visual/insulation checks are required and how prospective current and voltage affect that decision. Carbonized tracks are potentially conductive and must not be treated as ordinary dust.
Indicators create another coupled path. A blown-fuse light or remote-alarm circuit can bridge across the fuse by design. Require its maximum leakage/current, component voltage ratings, creepage and clearance to the power clips, isolation to low-voltage communications, and behavior with the fuse removed. A holder that passes without the optional indicator is not automatically approved with it installed.
Finally, include maintenance contamination. Tools, finger oils, fuse grease, cleaning residue and dropped hardware can change the micro-environment. State permitted cleaners and compounds, require power isolation before service, and verify that the cover and barriers are restored. These operational controls support the assumed pollution degree; they do not replace adequate design spacing.
Use a bounded spacing example
Assume a fictional DC assembly has 900 V maximum continuous line-to-line voltage, an assigned impulse withstand from the applicable equipment standard, pollution degree 2 inside a controlled enclosure and installation altitude up to 3500 m. The buyer cannot select a spacing from 900 V alone.
The engineer first uses the governing standard to determine required clearance from impulse and altitude. Creepage is then determined from working voltage, insulation function, pollution degree and material group. The actual holder/enclosure drawing must meet both independent results. If actual clearance is 7.0 mm and creepage is 12.0 mm, comparing both with one “required spacing” number is wrong.
Now assume a metal mounting screw is changed from 10 to 14 mm and protrudes 3 mm farther toward a live clip. That change can reduce clearance while leaving nominal creepage on the plastic unchanged. A later condensation path can affect creepage even if clearance remains. The fictional example gives no design value; it demonstrates why different changes trigger different checks.
Inspect and measure the first article
Prepare a marked-up drawing that identifies every critical pair and the path used. Then inspect the installed first article with production parts. Record:
- fuse holder, fuse, cover and accessories;
- polymer grade/color and lot evidence where required;
- conductor sizes, strip lengths and terminal positions;
- mounting hardware, standoffs and torque;
- shortest clearances through air;
- shortest creepage paths along surfaces;
- slots, ribs and recesses treated under the measurement rules;
- enclosure wall and adjacent component distances;
- cover open, closed and fuse-removed service states;
- altitude/pollution/enclosure assumptions;
- dielectric or impulse test evidence required by the product plan; and
- photographs with scale and path labels.
Use gauges and optical methods appropriate to the tolerances. Do not damage insulation with a metal gauge on an energized assembly. Measurements should be made de-energized and under the approved safety procedure.
Environmental conditioning can be necessary where condensation or contamination affects the insulation system. Use the applicable test sequence and acceptance values; do not spray an arbitrary salt solution and call it pollution-degree certification.
Compare supplier evidence
On small screens, swipe or scroll sideways to read every column. Keyboard users can focus the table and use the arrow keys.
| Decision field | Acceptable return | Hold point |
|---|---|---|
| Electrical stress | Pair-by-pair voltage and impulse map | One nominal voltage |
| Standard | Applicable product/equipment edition identified | IEC 60664 cited alone without route |
| Pollution | Micro-environment and enclosure basis | “Indoor = PD2” assertion |
| Altitude | Maximum site and correction documented | Sea-level test used globally |
| Material | Exact qualified grade/group evidence | Generic plastic family |
| Dimensions | Minimum actual paths with tolerances | Nominal CAD centerlines |
| Component scope | Exact model and conditions matched | Family certificate |
| Enclosure | Hardware and adjacent metal included | Holder reviewed in isolation |
| First article | Marked paths and test/inspection record | Catalog page only |
| Change control | Material, geometry, hardware and environment triggers | Silent “equivalent” substitutions |
Control production and contamination
Critical dimensions need mold/process capability and inspection. Control flash, short shots, warpage, broken ribs, insert position and mounting features. Link resin lots to the qualified material. Define whether regrind is permitted. A material substitution can invalidate tracking and certification evidence.
Assembly controls should prevent stray strands, solder balls, clipped wire ends, metal chips and loose hardware. Cleaning chemistry must not leave conductive residue or attack the polymer. If conformal coating is credited, define material, thickness, coverage, process and inspection under the applicable standard; a casual spray does not automatically reduce creepage.
Service instructions should specify fuse isolation, correct replacement, cover restoration and cleaning interval. Do not use compressed air that drives conductive dust deeper unless the equipment procedure allows it. Replace carbonized or tracked insulation; cleaning a visible arc path may not restore dielectric strength.
Repeat the installed review after changing fuse type/size, holder, indicator, wire, terminal, mounting screw, standoff, cover, enclosure, altitude range, ventilation, coating or pollution control. A thermal change that warps plastic can also become a spacing change.
Send a complete insulation-coordination RFQ
Attach the one-line, maximum voltage/transient basis, earth relationship, destination, altitude, pollution environment, enclosure drawing, wire/terminal schedule and critical-pair matrix. Ask for exact certification scope, drawings, material evidence, conditions of acceptability and deviation response. Request commercial terms for the exact part only.
Use the fuse-holder indicator guide where an indicator adds another circuit, and the SINAWATTS knowledge center for fuse coordination and enclosure checks. When the package is complete, send the project-specific RFQ with the installed critical paths highlighted.
Buyer FAQ
What is the difference between creepage and clearance?
Clearance is the shortest path through air. Creepage is the shortest path along an insulating surface. They use different design inputs and both must pass.
Does a 1000 V DC marking prove adequate installed spacing?
No. The rating has a product-standard and construction scope. Enclosure metal, wiring, mounting hardware, altitude, pollution and conditions of acceptability can change the installed result.
Is pollution degree the same as an IP rating?
No. IP describes ingress protection under its own test. Pollution degree describes the insulation micro-environment, including whether contamination becomes conductive. Enclosure ingress performance can support, but does not equal, the pollution-degree decision.
Does altitude affect creepage and clearance equally?
No. Reduced air density primarily affects clearance/withstand through air. Apply the governing standard’s altitude method and do not automatically multiply creepage by the same factor.
Can a plastic rib always be counted in creepage distance?
No. The rib or groove must meet the governing measurement rules. Decorative texture or a small recess may not count. Show the measurement path on the drawing.
Does CTI provide a safe creepage distance directly?
No. It supports material classification under the relevant method. Working voltage, pollution degree, insulation function and product standard still determine creepage.
Can conformal coating reduce required creepage?
Only when the applicable standard permits it and the coating system, process, coverage and tests meet that route. Uncontrolled coating is not a spacing credit.
What must be checked with the fuse removed?
Check exposed live parts, source/load backfeed, cover/barrier state, clearances to tools and enclosure, and the maintenance isolation procedure. Service state can differ materially from normal state.
Why include mounting screws in the spacing drawing?
They are conductive parts whose length, washer stack and slot position can reduce clearance or create a surface path. A hardware substitution can invalidate the measured minimum.
When must creepage and clearance be reviewed again?
After changes to voltage/transient basis, altitude, pollution control, polymer, molding, holder/fuse/indicator, wire termination, mounting hardware, coating, cover or enclosure. Repeat affected dimensional and dielectric evidence before release.