High voltage wiring inside an enclosure: air clearance and insulation

Short answer: give a bare high voltage lead 1 in [25 mm] of air for every 10 kV to the nearest grounded metal. Hold 1 in even at mains voltage. Sleeve every joint in silicone tubing bedded in RTV; a 600 V tape cannot hold back 20 kV. Bond the enclosure wall to earth, because when insulation fails, the wall is where you want the arc. For a 20 kV laser supply that means the output lead runs 2 in [50 mm] clear of walls, DIN rail and closed door. The tube-terminal joint gets silicone tubing plus RTV over the glass stem. Every wall crossing sits on an insulating bushing as long as the gap it replaces.

How much air each kilovolt needs

Air is a good insulator until it is not. Between smooth, rounded electrodes in still, clean air it breaks down at about 3 kV/mm [76 kV/in]. A stripped wire end, a solder blob or a dust film cuts that figure several-fold. So you design at 10 kV per inch [25 mm per 10 kV], about one eighth of the uniform-field number, with a one-inch floor at any voltage.

The floor has a source: OSHA's rules for work near exposed energized parts set the minimum air insulation distance at 20 mm [1 in] from 301 V to 5 kV. The same inch therefore covers mains wiring and the first 10 kV alike.

Three things move the number up. Sharp points start corona well below the smooth-electrode figure, so round every joint. Thin air breaks down sooner: IEC 60664-1, the insulation-coordination standard, adds roughly 5 % clearance per 1,000 m [3,300 ft] above 2,000 m [6,600 ft]. The strike pulse counts too: a tube can need several kilovolts more to start than to run, so add one inch to the table value whenever the supply prints no strike voltage.

Clearance and creepage are different measurements. Clearance is the shortest line through air; creepage is the shortest path along an insulating surface. Both matter at the wall. IEC 60664-1 asks 8.0 mm of creepage for an 800 V DC circuit at pollution degree 2 on group IIIa plastics. A lead in the tens of kilovolts therefore wants a bushing whose surface path matches the air table: call it one inch of bushing length per 10 kV.

What you need

• Silicone tubing in two bores, one snug on the terminal pin and one snug on the glass stem; bring 3 mm and 6 mm [1/8 in and 1/4 in] sizes and fit the snug one.
• Neutral-cure RTV silicone; acetoxy grades release acetic acid as they cure, and that acid corrodes bare copper and nearby electronics.
• Smooth-jaw pliers and side cutters.
• Insulating standoffs, 1 in [25 mm] long, PTFE or ceramic.
• A bonded divider plate for the low voltage section.
• An insulating bushing or feedthrough gland for each wall crossing.
• A crimped-lug bonding conductor, sized from the overcurrent device ahead of the box with NEC Table 250.122.
• A steel rule, an ohmmeter, and the discharge resistor or discharge stick the supply manual names.

The tubing is stronger than it needs to be through its wall. Sleeve datasheets commonly quote about 20 kV/mm [500 V/mil] for silicone rubber, so a 2 mm wall holds roughly 40 kV. Joints fail along the surface or through trapped air, not through the wall, which is why the tubing must seat over clean glass with no air pocket underneath.

The job, step by step

1. Lock out and drain the supply. Unplug the machine and follow the discharge step in the supply manual before any HV cover comes off. Then measure from each output terminal to the grounded chassis and read zero before your hands go in. Multiplier capacitors hold a lethal charge for minutes after the plug comes out.
2. Draw the route before you strip a wire. Mark the HV path on the layout and write the table figure on it: 2 in [50 mm] for the rated lead, 3 in [75 mm] with the strike voltage unknown. Keep the same distance from 24 V signal wiring, and cross it at right angles where a crossing is unavoidable.
3. Wrap the terminal, and stop there. Strip about 1.5 in [38 mm] of the HV lead and make three or four tight turns around the terminal pin, pinched up with smooth pliers. Never solder on a glass-sealed terminal: the heat cracks the glass-to-metal seal, air leaks in, and a tube that held vacuum for years dies in an afternoon.
4. Sleeve the joint and pot it. Slide the tubing so it covers the pin and at least 0.5 in [13 mm] of the glass stem. Seal both ends with a neutral-cure RTV bead that lands on the wire insulation at one end and the glass at the other. Work the sleeve down so no air pocket sits under it; trapped air ionizes first.
5. Feed every wall crossing on insulation. Deburr the cutout, then pass the lead through an insulating bushing long enough that its surface path matches the air table from the first section. A bare conductor through a plain metal hole hands the arc a head start. Sizes and sealing are in the guide to enclosure cutouts, glands and windows.
6. Stand the bare points off grounded metal. Mount the HV terminal area on the standoffs so the nearest bare copper sits the full table distance from the back wall, the DIN rail and the closed door. Clip the lead so vibration cannot close the gap over time; it is the same habit as setting terminal clearance from an enclosure wall, scaled up for kilovolts. Where geometry leaves you short, an insulating pad between the live part and the wall buys margin.
7. Bond the wall to earth. NEC 250.96(A) requires metal parts serving as grounding conductors, enclosures included, to be bonded so they can carry any fault current likely to hit them. Land the crimped lug on bare stainless under a star washer; where a coating covers the contact point, NEC 250.12 calls for it removed. An ohmmeter across the finished joint should read well under 1 Ω from any wall point to the earth bar. The fault-path logic is the same one behind bonding around a failed transformer.
8. Cure, close, then test. RTV datasheets call a bead tack-free in about 1 h and fully cured after 24 h at room temperature, so power up only after the full cure. Run your first tests with every cover closed, at the lowest current that fires the tube.

Checks when the job is done

• Gap audit: steel rule from every bare point to the nearest grounded metal with the door closed. The HV lead reads its table figure or better, one inch more when you sized for the strike pulse.
• Bond reading: ohmmeter from a far corner of the wall to the earth bar reads well under 1 Ω. A whole-ohm reading means the lug landed on residue, so strip the point and redo it.
• Dark-room check: run the supply at working current with the lights off. Glow or hiss at a joint is corona, the geometry failing before the arc arrives, so fix it now.
• Week-one recheck: RTV shrinks slightly through its cure and the first heat cycles, so confirm both beads still seal and no lead has crept toward grounded metal.

Mistakes that cost the gap

Tape as primary insulation is the classic one. Standard PVC electrical tape is rated for 600 V service, and an arc tracks along its surface with ease, so tape belongs on the 24 V loom and never on the HV joint.

Sharp geometry hides in plain sight. A frayed braid end or a solder ball starts corona at a fraction of the smooth-bead voltage, and corona makes ozone, which attacks most rubbers. Silicone resists ozone, one more reason the sleeve is silicone.

Sealed openings carry the rating: our stainless boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed. Every added hole seals only as well as the gland set into it, so install the bushing each drawing calls out and seat it as its instructions show. On machines built around a glass tube, the grounded housing is often the designed arc target. When a joint or a tube fails, the supply arcs to bonded metal and the overcurrent device clears. Bond first, insulate second, and keep that path intact.

Size the box, then order it

Worked example, a 40 W engraver supply whose label reads 20 kV running and prints no strike figure: design at 30 kV. Clearance: 30 kV / (10 kV/in) = 3 in [75 mm] of air from every bare point to grounded metal. Depth stack: terminal block on a 1 in [25 mm] standoff, block plus sleeved lug 2 in [50 mm], clearance 3 in [75 mm], working margin 2 in [50 mm]. Minimum internal depth: 1 + 2 + 3 + 2 = 8 in [200 mm]. Width comes from a bonded divider plate: hold 3 in [75 mm] from the HV terminal to the plate, and the 24 V controls behind it need no separation, because a bonded plate is grounded metal. A body 16 in wide, 16 in high and 10 in deep [400 x 400 x 250 mm] carries that layout with the mains contactor to spare.

The guide to sizing an electrical enclosure walks the same arithmetic for ordinary panels. Decide it this way: mains and 24 V wiring share a box freely at the one-inch floor; anything above 10 kV gets its own corner with a bonded divider; and a laser supply gets its strike voltage in writing before you fix the depth. Where no strike figure exists, the 3 in [75 mm] figure covers this class of supply, and the depth stack above sizes the box.

Set those dimensions and the bushing cutouts in the 3D enclosure builder. You pick 304 or 316L stainless, the width, height and depth, and every opening, then approve the drawing; the box is delivered in 10 business days, anywhere in the world, and for custom work the count starts at your approval of the drawing. For a fully drawn job, the custom stainless cutouts page lists what to send with the drawing.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

Questions