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Withdrawable construction allows a functional unit to be isolated and removed using the manufacturer procedure. Whether the remaining board can stay in service depends on the assembly design, interlocks and approved maintenance rules; removal under load is not permitted.
This page covers what withdrawable construction actually changes, which circuits justify it, and what has to be specified before the board is built.
What withdrawable construction changes
The mechanism is simple and the consequences are not. A withdrawable unit has three positions, and the interlocks are what make it safe.
- Connected. The unit is in service, with both power and secondary circuits engaged.
- Test. The power contacts are separated but the control circuits are live, so the protection and the indication can be checked with the circuit isolated.
- Isolated. Both power and secondary are separated, with a visible gap, and the unit can be withdrawn.
The interlock that matters is the one that prevents withdrawal or insertion under load. It is mechanical, it is usually backed by an electrical interlock, and it is the item to verify at commissioning rather than assume.
Specifications
Configuration reference
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| Parameter | Typical value | What it depends on |
|---|---|---|
| Rated voltage | Up to 690 V AC | Plant voltage and local standard |
| Main busbar current | Up to 6 300 A | Load profile and diversity factor |
| Short-time withstand | Up to 65 kA for 1 second | Prospective fault level at the connection point |
| Withdrawable units | Modular units from 8E/4 upwards, and withdrawable ACB | Which circuits must be swappable |
| Unit positions | Connected, test, and isolated | Commissioning and maintenance procedure |
| Interlocks | Mechanical and electrical; no withdrawal under load | Safety procedure and local code |
| Form of separation | Form 3b or 4b to IEC 61439-2 | How much must stay live during maintenance |
| Secondary circuits | Plug-in connectors for control and instrumentation | Whether units must swap without rewiring |
| Enclosure rating | IP31 to IP54 indoor; NEMA 3R outdoor | Room conditions or outdoor siting |
| Standards | Confirm applicable assembly standards and required listing | Selected assembly type and destination-market requirements |
| Internal arc | Confirm applicable internal-arc test evidence and conditions | Selected assembly type and project requirements |
| Finish | Epoxy powder coat or galvanised | Environment and design life |
Figures are selection references. Confirm the final ratings, dimensions, available certifications and warranty against the current datasheet and written proposal for the selected model or system.
What arrives in a typical order
- The panel assembled with the withdrawable units fitted, wired and routine-tested.
- Routine test report to IEC 61439-2 or the applicable standard for that panel.
- Single-line and wiring diagrams, with the unit schedule and positions marked.
- Interlock description and the operating procedure for withdraw and insert.
- Installation, terminations and maintenance documentation.
Where the project needs it, the transformer, the upstream switchgear and the sub-distribution boards are quoted on the same order so the fault rating and the discrimination are settled once.
Where a withdrawable panel usually goes
- Industrial incomers and large feeders, where a fault has to be cleared in hours rather than in a shutdown window.
- Continuous process plant, where stopping the line costs more than the panel does.
- Data centres and hospitals, where an approved design may allow an isolated unit to be serviced while other sections remain available.
- Sites with a spare-unit strategy, where a failed unit is swapped and repaired off site.
Sizing notes before you order
- Decide withdrawable per circuit, not for the whole board. Incomers and large feeders justify it; small feeders rarely do, and mixing is normal and cheaper.
- Specify the unit sizes from the protection device ratings. Modular units come in steps, and the next size up takes more space in a board that is already sized.
- Confirm the interlocks and the shutters. A withdrawable unit without shutters leaves live parts exposed when the unit is out.
- Plan the spare strategy. A withdrawable panel only saves time if a spare unit exists, and the spare has to be the same rating and the same secondary wiring.
Documentation to confirm for your order
- Routine test report to IEC 61439-2 or the applicable standard.
- Single-line and wiring diagrams with the unit schedule and the three positions marked.
- Interlock description and the withdraw and insert procedure.
- Dimensional drawing, termination details and the spare unit list.
- Installation and maintenance documentation, and warranty terms.
Questions buyers ask
When is withdrawable worth the premium?
Where an outage costs more than the panel does: industrial incomers, continuous process, data centres, hospitals. On small feeders that are rarely touched, fixed construction costs less and takes less depth.
What are the three positions for?
Connected is in service. Test isolates the power contacts but leaves control live, so protection can be checked without energising the circuit. Isolated separates both and gives a visible gap, which is when the unit can be withdrawn.
Can a unit be withdrawn under load?
No, and the interlock exists to make that impossible. It is mechanical, usually backed by an electrical interlock, and it is one of the items to verify at commissioning rather than assume.
Do we need a spare unit?
Only if the outage cost justifies holding one. A withdrawable panel saves time only when a spare of the same rating and secondary wiring is available, so the spare strategy belongs in the first order rather than after a failure.
Which standard applies?
Confirm the standard and any required listing for the actual assembly type and market. The quotation should identify the applicable construction, test evidence and any specified internal-arc performance; standard names alone do not establish certification.
What to send us
Send the single-line diagram, which circuits need to be swappable, the prospective fault level if you have it, and your target date. We confirm the unit schedule, the form of separation and the build date in writing.
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