Final test chamber for medium-voltage switchgear panels: screened room for withstand-voltage and partial-discharge tests
A screened, interlocked room at the end of a switchgear assembly line · 50 kV or 95 kV power-frequency withstand, partial-discharge background below 1 pC, and a door that fits a 2.5 m panel
Finished switchgear is wheeled into a screened room, its main circuit is connected to a high-voltage source through bushings in the roof or wall, the door is closed and interlocked, and the panels are taken up to their withstand voltage and then held at a lower voltage while partial discharges are measured. The room does two jobs at once: it keeps people out of the high-voltage zone, and its metal walls keep outside electrical noise from swamping discharge signals of a few picocoulombs. Its size, voltage and noise floor all follow from the panels it has to test.
Screened, interlocked test rooms (Faraday-cage enclosures) in which assembled medium-voltage switchgear panels receive their final power-frequency withstand and partial-discharge tests before shipment, including the high-voltage feed-through, door interlock and measurement position. Primary current injection, trip timing, contact resistance and functional test benches for breakers and distribution panels are covered on the Power distribution test bench page. General EMI shielded enclosures and EMC test chambers are covered on their own page; climatic and environmental test chambers, and type-test laboratories for short-circuit or internal-arc tests, are outside this page.
CNFX editorial research · Sources checked October 12, 2026 · 4 sources
How much voltage the chamber must hold: 50 kV for a 24 kV panel, 95 kV for a 40.5 kV breaker
The chamber's high-voltage source, bushings and clearances are sized from the highest rated short-duration power-frequency withstand voltage of the switchgear that will pass through it. Published ratings give the scale:
Editorial calculation: a chamber built for 24 kV panels needs a source that reaches at least 50 kV, or 65 kV if panels are also built to the GOST rating; adding 40.5 kV equipment almost doubles that to 95 kV (95 ÷ 50 = 1.9). Lightning-impulse withstand is a type-test value, so a final test chamber does not need an impulse generator unless the factory also runs type or special tests there.
For comparison, Siemens' own high-voltage test laboratory lists a power-frequency test system of 220 kV / 100 kVA at 50 Hz and lightning and switching impulse tests up to 500 kV / 6.25 kJ, with impulse wave shapes 1.2/50 µs and 250/2500 µs per IEC 60060-1. [3] A factory final test chamber covers only the first of these, at the voltage of the panels it ships.
Give the chamber builder the highest rated voltage and power-frequency withstand voltage of the panels to be tested, any national rating such as GOST, and the capacitance of the largest line-up, which sets the kVA of the test transformer.
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Why the walls are metal: keeping the partial-discharge background below a picocoulomb
Partial discharges are measured in picocoulombs of apparent charge, and the measurement only means something if the background noise is well below the acceptance level. Siemens describes its shielded laboratory as enabling partial-discharge tests with a ground noise level below 0.5 pC up to 100 kV, using two separately operated measurement systems. [3]
Marcel Fortin of Hydro-Québec Distribution, presenting to the IEEE Switchgear Committee, set out the order of work: check for ambient noise first and clean up the circuit (or change to a balanced circuit) if needed; connect the apparatus; calibrate the circuit, preferably at three levels; raise the voltage to a pre-stress level, then lower it to the test voltage, noting inception and extinction voltages. [4] He adds that the measuring system's pulse-train response should be better than 1/30 s to catch discharges that occur only once per cycle, such as corona-like discharges in vacuum or gas. [4]
The same presentation classifies the noise a tester must recognise: corona, bad contacts and noise from the mains. [4] Those are what a screened chamber is built against: the metal enclosure blocks radiated interference, and the earthing, filtered supply and corona-free high-voltage lead-in deal with the conducted and self-generated kinds. Editorial note: a shielding-effectiveness certificate for the enclosure does not by itself prove the partial-discharge background; ask for a measured background in pC with the test source energised and the door closed.
State the partial-discharge acceptance level the panels must meet, the background level the chamber must reach at the test voltage, and how that background is demonstrated at acceptance.
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Door, floor and roof bushings sized to a switchgear line-up
The chamber has to take whole panels or short line-ups on their own wheels or on a trolley. Siemens' NXAIR M 24 kV panel is 800 or 1,000 mm wide, 2,510 mm high and 1,600 mm deep, with busbars rated up to 2,500 A. [1]
Editorial calculation from those dimensions: a line-up of three 800 mm panels is 2,400 mm wide, and three 1,000 mm panels 3,000 mm; the door opening must clear 2,510 mm of panel height plus any transport trolley, and the internal depth must take 1,600 mm of panel plus working and clearance space behind and in front. High-voltage connections then come in from above, as in the roof bushings visible in the photograph, so the clearance from the bushings to the panel top and to the chamber walls has to be set for the chamber's maximum test voltage.
NXAIR M is described as operable only with the high-voltage door closed and locked, and offers cable testing without isolating the busbar. [1] Editorial note: the chamber's own door interlock follows the same principle one level up: the high-voltage source must not be energisable while the chamber door is open, and the door must not open until the panels are earthed.
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What to send a builder of a switchgear final test chamber
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Panels: [rated voltage; power-frequency withstand voltage; national ratings]
Size: [largest line-up width, height, depth, weight; trolley]
Test source: [maximum kV; kVA from line-up capacitance; frequency]
Partial discharge: [acceptance level in pC; required background; measuring system]
Screening: [shielding effectiveness; filtered supply; earthing]
Safety: [door interlock; earthing switch; warning lamps; emergency stop]
Acceptance: [measured background with source energised]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
What test voltage does a final test chamber for 24 kV switchgear need?
At least the panels' power-frequency withstand voltage: Siemens rates its 24 kV NXAIR M at 50 kV, or 65 kV under GOST. [1]
How low can partial-discharge background go in a shielded room?
Siemens states its shielded high-voltage laboratory reaches below 0.5 pC up to 100 kV. [3]
In what order is a partial-discharge test run?
Hydro-Québec's Marcel Fortin lists: check ambient noise, connect, calibrate at preferably three levels, raise to a pre-stress voltage, then lower to the test voltage and measure. [4]
How big must the chamber door be?
Large enough for the tallest panel and its trolley; Siemens' NXAIR M panel is 2,510 mm high and 1,600 mm deep. [1]
Sources and what still needs a direct check
Sources checked 2026-10-12. Panel ratings and dimensions come from Siemens' NXAIR M profile and ABB's VD4 36/40.5 kV manual; laboratory test capacity and partial-discharge noise level from Siemens' high-voltage testing laboratory page; the partial-discharge test sequence and noise types from a Hydro-Québec presentation to the IEEE Switchgear Committee.
