Uimp
Peak prescribed impulse withstand under specified conditions. It does not clamp a surge or set continuous operating voltage.
Insulation coordination guide
Terminal Block Impulse Withstand Voltage Explained
Rated impulse withstand voltage, or Uimp, is the declared peak of a prescribed impulse that a terminal block can withstand under stated test conditions. It is not working voltage, surge protection, SCCR or a whole-panel lightning guarantee. Read the full condition row, not the kV number alone.
Uimp is transientIt does not authorize continuous operation at the kV value.
Conditions travel with itCategory, pollution degree, altitude and arrangement matter.
Clearance is not creepageAir and surface insulation paths answer different stresses.
An SPD is separateIt limits surge voltage and diverts surge current. A terminal only withstands its stated test.
Installed terminal blocks show the connection context; ratings and impulse performance come from exact product data. Photo: tony_duell / Wikimedia Commons, CC BY 2.0. Cropped and darkened for layout.
Direct answer
Uimp is a condition-bound insulation rating.
Uimp is the peak value of an impulse of prescribed form and polarity. The product must withstand it without failure under stated test conditions. Clearance is referenced to this value within the applicable insulation rules.
The rating belongs to the exact product and evaluated arrangement. Read it with the conductor-to-earth voltage, overvoltage category, pollution degree and altitude. Also check the product standard, terminal function, mounting and accessories. A look-alike or detached kV value is not proof of equivalence.
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One-line selection rule
Accept the terminal only when its documented Uimp, Ui where declared, and applicable nominal, rated or operational voltage data match the project. The environmental conditions and assembly evidence must also match.
Accept the terminal only when its documented Uimp, Ui where declared, and applicable nominal, rated or operational voltage data match the project. The environmental conditions and assembly evidence must also match.
Keep the fields separate
Eight ratings answer eight different questions
Capture the field name, value, unit, condition suffix and standard basis. Do not translate every value into a generic voltage or kA claim.
Prescribed impulse form
A waveform label is part of the evidence
Switching and lightning coupling can cause short transients. Some terminal test data cites a 1.2/50 microsecond impulse. The applicable standard sets the wave, limits, polarity sequence, test points and pass rules. Longer temporary overvoltage is a separate stress. Some fault conditions can cause this longer stress, which Uimp does not establish.
The graphic is a normalized teaching image. It is not the current standard text and it does not prove that a product passed. Do not build or apply an impulse source from this article.
Impulse testShort-duration insulation stress with a prescribed wave shape.
Power-frequency testDifferent waveform, duration, arrangement and acceptance purpose.
There is no reliable universal conversion formula.
Do not calculate Uimp from a fixed multiple of working voltage. First set the conductor-to-earth voltage and overvoltage category under the applicable rules. This gives the required Uimp. Then use Uimp, field shape and altitude to check clearance. Use working or insulation voltage, pollution degree and material group to check creepage. IEC 61000-4-5 is an EMC surge test, not an insulation-withstand test.
Insulation coordination
Evaluate air, surface, solid insulation and the real assembly
The visible terminal body is only one part of the insulation path.
Clearance
The shortest path through air. Required separation is tied strongly to expected impulse stress, field geometry and altitude.
Creepage
The shortest path along an insulating surface. Working voltage, pollution degree and material group or CTI are central inputs.
Solid insulation
The path through insulating material. Thickness, material, manufacture, ageing and the product test basis all matter.
Assembly separation
Adjacent terminals, rail, barriers, plugs, jumpers and function blocks can create the governing path.
Pollution degree describes the insulation micro-environment.
A room that looks clean does not prove the surface condition. Overvoltage category describes circuit location and expected transients. It is not a product quality grade. Use the deeper guides to pollution degree and terminal-block creepage and clearance for those separate decisions.
Altitude and field stress
A natural lightning photo is not the terminal waveform
Lightning can be one source of a transient. The stress at a terminal also depends on the network, coupling, protection design and gear location. A standard impulse is a controlled test. It is not a replay of a weather event.
Lower air density can reduce clearance withstand. IEC 60664-1 addresses normal use up to 2,000 m and provides guidance above that level. Check the manufacturer's baseline and the actual site elevation.
Do not infer OVCThe product's cabinet location alone does not set the category.
Do not ignore altitudeA sea-level result does not automatically transfer to a high site.
Read the full condition row
A copied kV value is incomplete procurement data
Treat the manufacturer data sheet as a matrix. Keep every condition attached to its value.
Capture the rating basis
- Exact part number and terminal function
- Uimp value, label, condition suffix and test basis
- Ui plus nominal or operational-voltage rows
- Overvoltage category and pollution degree
- Altitude or sea-level wording
- Product standard and target-market approval
Capture the construction
- Rail or mounting support
- End plates, barriers and partitions
- Jumpers, feed-ins and mixed potentials
- Plug and mating-half conditions
- Panel-wall or spacing requirements
- Current, conductor, temperature and fault-duty limits
Red flag: a distributor table removes the condition text.
Return to the original manufacturer document. Do not select the most favorable row unless the project actually meets that row's category, pollution, altitude, construction and approval basis.
Product examples
Examples teach field-reading, not universal values
Use a product page only for that exact product, revision and condition.
Separate voltage fields
Phoenix Contact ST 1,5 BK product data places nominal voltage, rated surge voltage, category and pollution information in the insulation data.
Lesson: copy the complete row, not only the kV entry.
Series and size matter
Phoenix Contact PT 16 N has its own condition-based data. A higher value is not a universal rule or quality rank.
Lesson: product family and terminal function stay attached.
IEC and UL data stay separate
WAGO 2002-1201 shows distinct approval columns. A matching kV or voltage number does not make the routes interchangeable.
Lesson: verify the destination market.
Physical form is not proof
Two power terminals can hide different insulation evidence
Case size, color and apparent spacing are not a data sheet. Similar bodies can hide other geometry, material, barriers or certification terms. Record the full part number before comparing Uimp, Ui, conductor, current or fault duty.
Panel feed-through and connector pages can list several build cases. Older pages may cite earlier IEC editions. They can show why condition rows vary. They do not prove conformity to a newer edition.
Same kVDoes not prove the same function, product type or arrangement.
Same footprintDoes not prove the same clearance, creepage or approval.
Keep IEC and UL certification routes intact.
IEC 60947-7-1:2025 and UL 1059 Sixth Edition are distinct product routes. UL 840 applies only when an end-product or certification route calls it up. It does not approve each terminal by default. UL 1059 component compliance also does not prove end-product fit.
Assembly boundary
A rated terminal does not rate the whole strip or panel
The released construction, accessories and potential relationships decide whether the component evidence applies.
| Assembly item | Why it matters | Evidence to request |
|---|---|---|
| Adjacent potentials | Mixed voltage levels can create the governing air or surface path. | Approved terminal plan and manufacturer arrangement guidance. |
| DIN rail or support | Terminal-to-rail insulation can be part of the verification. | Selected rail/support and mounting documentation. |
| End plates and covers | They may complete the housing or maintain a required separation. | Exact compatible accessory numbers. |
| Partitions and barriers | A missing or substituted separator can change spacing. | Required part, location and usage statement. |
| Jumpers and feed-ins | They change potential and current relationships and can limit ratings. | Accessory data and released strip configuration. |
| Fuse, disconnect, PE or electronic terminals | Internal geometry and applicable product evidence differ from a feed-through block. | Individual data for every functional terminal. |
Physical substitution is a design change.
Do not replace the rail, terminal function, barrier, jumper or mating half by appearance. Review temperature rise, current path, insulation coordination and fault duty. Also check the assembly's declared conformity or certification. Use the separate terminal-block SCCR guide for the U.S. panel method.
Laboratory evidence
Impulse equipment belongs in a controlled test program
Laboratory systems create dangerous high-voltage impulses with controlled energy, waveform and measurement chains. The pictured equipment serves high-voltage transmission-component work, not this terminal-block test.
Do not perform field impulse or hipot testing from a blog. Testing can damage insulation, electronic modules or connected equipment if the exact procedure, disconnections and acceptance criteria are not established by the responsible manufacturer and standard.
Product evidenceUse current data, reports, certificates and recognized conditions.
Project verificationUse the manufacturer-documented test plan and qualified laboratory or verifier.
SPD coordination
Withstand and protection are related, not interchangeable
A coordinated design can include both, but neither component replaces the other.
Terminal Uimp
- Describes prescribed impulse withstand
- Does not limit voltage
- Does not divert surge current
- Does not absorb surge energy
- Does not establish lightning protection
Surge protective device
- Limits surge voltage and diverts surge current
- Has its own MCOV, discharge and protection data
- Needs correct location and connection
- Needs fault-current and protective-device coordination
- Does not automatically validate lower-Uimp insulation
A nearby SPD is not a rating override.
Only a documented system design and applicable standard can set the residual stress and required insulation level. A high-Uimp terminal does not remove the need for surge protection. Read SPD Up, VPR and device-type data in the surge protective device range under their own standards.
Seven-step selection workflow
Start with the system, then freeze the exact component
This is a documentation and engineering sequence, not an installation or test procedure.
Name the rules
Identify the end-product, installation and target-market standards before choosing terminal ratings.
Define the voltage system
Record topology and conductor-to-earth voltage, not only a nominal line voltage.
Set the environment
Document overvoltage category, pollution degree, altitude, enclosure and micro-environment assumptions.
Map transient control
Record the SPD and protection architecture without using it as a generic Uimp override.
Freeze the assembly
Name the terminal, rail, end plates, barriers, jumpers, feeds, plugs and special functions.
Match every field
Compare Uimp, Ui, rated voltage, category, pollution, altitude, approval and accessory notes.
Release the proof
Issue the BOM, terminal plan, data sheets, approval record, verification plan and change-control rule.
This article does not provide a field impulse, dielectric-withstand or live-panel test procedure.
Do not inject test voltage into installed gear from this article. Only qualified persons may inspect, replace or test parts. Follow the maker's instructions and site procedure. Find and isolate every source. Control stored and backfed energy. Apply the required lockout/tagout controls. Use rated test gear to confirm that exposed parts are de-energized.
RFQ and substitution data
Ask the supplier to prove the offered condition
"6 kV terminal" is not a complete specification.
Project design basis
- End-product and installation standards
- Target market and approval route
- System topology and conductor-to-earth voltage
- Required Uimp, plus applicable Ui and/or nominal, rated or operational voltage fields
- Overvoltage category and pollution degree
- Altitude, enclosure and micro-environment
- SPD and transient-protection architecture
Offered assembly
- Exact terminal part, function and revision
- Original complete condition table
- Rail, end plates, barriers and partitions
- Jumpers, feed-ins, plugs and mating halves
- Conductor, current and temperature limits
- SCCR or other fault-duty evidence under its own rules
- Lifecycle, PCN, substitution and traceability process
Acceptance sentence
The supplier confirms that the exact terminal and listed accessories meet the project's insulation rules in the offered arrangement. The record must cite current manufacturer data and approval evidence. Use the full terminal-block RFQ checklist for the commercial handoff.
Frequently asked questions
Terminal-block Uimp FAQ
The answers define selection boundaries and must remain tied to the visible FAQ schema.
Is terminal block impulse withstand voltage the same as operating voltage?
No. Uimp is a transient insulation-withstand rating under specified impulse-test conditions. Operating or nominal voltage describes ordinary service. Both fields must suit the application, and the Uimp number must not be compared directly with continuous circuit voltage.
Is a higher Uimp always better?
Not automatically. The terminal must still match the required category, pollution degree, altitude, Ui or rated voltage, current and thermal duty, function, accessories and approval route. A higher kV value does not replace a complete application match.
How does Uimp differ from Ui?
Uimp concerns a prescribed short impulse. Ui is the voltage to which dielectric tests and creepage distances are referenced under the product rules. They address related insulation questions but are not substitutes.
Is rated surge voltage the same as Uimp?
Many terminal data sheets use rated surge voltage for a Uimp-style insulation field. Confirm the manufacturer's exact label, product standard, test context and condition suffix. Do not confuse it with an SPD voltage protection level.
Does high Uimp protect downstream electronics from lightning?
No. Uimp describes insulation withstand and does not clamp or divert surge energy. Downstream protection requires a designed architecture using suitable protective devices and equipment-level coordination.
Does a nearby SPD allow a lower-Uimp terminal?
Not by itself. A lower value is justified only when the documented system coordination and applicable standard establish the residual stress and required insulation level for that location.
Does Uimp apply unchanged at every altitude?
Do not assume so. Air-clearance withstand is affected by altitude. Check the manufacturer's declared baseline, the site elevation and the applicable correction or approval route.
Can rail, PCB, pluggable and panel-feed-through ratings be compared directly?
Only when the product type, function, conditions, standard basis and installed arrangement are comparable. A common kV value does not prove equivalent insulation behavior.
Why does one data sheet show several insulation rows?
Different categories, pollution degrees, material conditions, panel-wall cases or accessories can produce different documented ratings. Select only the row that matches the real project condition.
Does current rating prove adequate impulse insulation?
No. Current rating concerns the thermal contact path under defined conditions. Impulse insulation concerns voltage stress, clearance, creepage, material, environment and construction. Verify both independently.
Related SENTOP guides
Terminal-block voltage ratings Clearance and creepage selection Pollution degree for terminal blocks Terminal-block SCCR under UL 508A Terminal current rating versus actual load How to read terminal-block specificationsPrimary technical sources
- IEC 60664-1:2020+AMD1:2025 CSV - low-voltage insulation coordination.
- IEC 60947-1:2020 - low-voltage switchgear and controlgear general rules.
- IEC 60947-7-1:2025 - terminal blocks for copper conductors.
- IEC 61000-4-5:2014+AMD1:2017 CSV (amendment) - EMC surge immunity, not insulation withstand.
- IEC 61643-01:2024 and IEC 61643-11:2025 - SPD requirements.
- UL 840, Third Edition - North American insulation coordination.
- ANSI/UL 1059, Sixth Edition - terminal blocks.
- Weidmuller terminal-block electrical tests - manufacturer test context.
- Phoenix Contact electrical tests - clearance and insulation examples.
- UL 1449, Fifth Edition - active SPD standard, revised October 21, 2025.
- OSHA 1910.333 and OSHA 1910.147 - U.S. work-practice and energy-control scope.
Condition rows, not headline numbers
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Send the terminal plan, system voltage, category, pollution degree, altitude, SPD architecture, target market and exact offered parts.
