Drive heatsink grounding and isolation on an enclosure plate

Short answer: bond the mounting plate to the AC earth terminal through bare metal, tie the drive's DC common to that plate at one point, and isolate the heatsink tab with a greased mica kit only when the drive datasheet calls for it. The kit costs roughly 0.4-1 °C/W between tab and plate, so at the assumed 20 W of drive loss in the worked example below it adds 8-20 °C that has to come from somewhere else. Mounted metal-to-metal, the same drive pours its heat straight into a bonded plate that stays within a few degrees of the air inside the box. Here is how to tell which construction your drive uses, how to make each bond so it still conducts under fault current, and what isolation does to the temperature budget on a real stainless box.

What the datasheet says about the tab

A stepper drive sheds its transistor losses through a metal tab or through its own finned case. That metal sits in one of two places electrically, and the datasheet tells you which. It either states that the case is isolated from the power circuit, or it shows the tab sharing a terminal with the DC bus or a motor output. Many small boards isolate only their control inputs with optocouplers; that isolation sits on the signal side and says nothing about the power tab.

A live tab bolted straight to an earthed plate puts the DC bus to earth through its own mounting screws. An isolated case mounted through an insulating kit wastes 8-20 °C for nothing at that loss. The pad-and-washer mechanics match isolating a transistor heatsink inside an enclosure; the drive-level decision is the table above, and the temperature bill is the worked example below.

Bond the plate to the AC earth

The plate is a metal part that can carry fault current, so it bonds to the equipment grounding conductor. NEC 250.96(A) requires metal parts intended to serve as equipment grounding conductors, including raceways, cables, equipment and enclosures, to be bonded together so they can safely conduct any fault current likely to be imposed on them. Run that bond from an earth stud on the plate to the enclosure's earth terminal: crimped ring lug, conductor sized from the rating of the overcurrent device ahead of the cabinet, the way NEC Table 250.122 sizes it.

Coatings break bonds. NEC 250.12 requires paint, lacquer and enamel removed from threads and contact surfaces at grounding and bonding connections, and anodize on an aluminium plate insulates the same way. Strip a bare-metal ring about 3/4 in (20 mm) across at the stud and let an internal-tooth star washer bite through it. A lug sitting on anodize can read fine on a bench meter and still be the joint that opens under fault current. For the plate detail see grounding a mounting plate inside a metal enclosure, for the enclosure side see bonding the ground bus to the enclosure with the supplied screw, and enclosure grounding screw threads and lug fit covers the thread and lug fit.

The earth itself carries none of that fault current. NEC 250.4(A)(5) treats the earth as no part of the effective ground-fault current path, and a ground rod driven beside the cabinet is an auxiliary electrode under NEC 250.54: it may be connected to the equipment grounding conductor, but it is never a substitute for it. The bond that matters is the conductor from plate to earth terminal.

Tie the DC common to the plate at one point

AC earth and DC common share the plate, and they should share it deliberately. Land one bond from the supply's 0 V terminal to the earthed plate: one ring lug, one cabinet, one point. That bond fixes the DC reference at earth and gives the chopped motor current a short local loop to return over. The pattern that causes trouble is the accidental second bond: a motor cable shield landed at both ends, or signal commons chained drive to drive and earthed again at the far end. Either one turns the plate into a loop the chopping outputs energise all day. Land shields on the plate at the entry gland and stop there. For the full bond-or-isolate treatment see AC and DC grounds in one enclosure: bond or isolate; servo drive and computer wiring separation in one enclosure covers keeping drive wiring clear of the signal side.

What you need

• Mica or silicone-impregnated insulating pads, one per live tab; kit datasheets quote roughly 0.4-1 °C/W
• Insulating shoulder washers for the tab screws
• Heat-conducting grease for both faces of each pad
• Crimped ring lugs, a crimp tool and internal-tooth star washers
• A digital multimeter with resistance ranges and an IR thermometer
• The plate itself, ordered with the earth stud and drive pilot holes in the drawing: 304 stainless for indoor washdown and most outdoor sites, 316L where salt air, brine or chlorides wash the box, built to your cutout drawing and delivered in 10 business days, counted from your approval of the drawing

Fitting the bond and the insulator, step by step

1. Lock out and tag the supply feeding the enclosure, then confirm the drive's bus capacitors are discharged before any meter work.
2. Off the plate, meter the tab (or case) to every power terminal in both polarities. A low reading either way means a live tab: fit the insulating kit. Open both ways supports a metal-to-metal mount.
3. Strip the plate to bare metal at the earth stud, a ring about 3/4 in (20 mm) across.
4. Bolt the earth lug on with a star washer under its head and run the grounding conductor from it to the enclosure's earth terminal at the size NEC Table 250.122 gives for the overcurrent device ahead.
5. For a live tab, grease both faces of the pad, fit the shoulder washers, and clamp the drive at the torque the kit datasheet states: over-torquing cracks mica, under-torquing raises the joint temperature. Metal screws with shoulder washers hold clamp pressure; nylon screws drop it.
6. For an isolated case, bolt straight down with a star washer under each screw so every fastener bites through coating to bare metal.
7. Bond the supply's 0 V to the earth stud once. Shields land on the plate at the entry gland and nowhere else.
8. Mount the drives with fins vertical; datasheets for this drive class recommend vertical mounting to maximise heatsink area.
9. Meter the finished assembly before power-up: plate to earth terminal a fraction of an ohm, and tab to plate open on the highest resistance range where a kit is fitted.

What isolation costs on a 24 x 20 x 8 in box at 40 °C ambient

• Drive loss: 20 W, a worked-example assumption for a 7 A axis driving a NEMA 42 motor; replace it with the dissipation figure your drive's datasheet states
• Insulator: 0.4 °C/W greased mica to 1 °C/W dry silicone pad, the range kit datasheets quote
• Ambient: 40 °C (104 °F), the usual design ambient for drives and electronics
• Box: 24 x 20 x 8 in (H x W x D) stainless wall-mount from the stainless range that runs from 6 x 4 x 4 in to 60 x 36 x 16 in, rated NEMA 4X / IP66 by the manufacturer; not UL listed

Walls that see air: front 24 × 20 = 480 in², two sides 2 × (24 × 8) = 384 in², top and bottom 2 × (20 × 8) = 320 in². The back sits on the wall and is left out, so A = 480 + 384 + 320 = 1,184 in² = 0.764 m².

Two rises stack between tab and outside air. The sealed stainless walls lift the interior above ambient by ΔT = P ÷ (k × A), with k = 3.7 W/m²K for stainless; the insulator then lifts the tab above the plate by P × R. The wall model is the one estimate enclosure thermal resistance from heat load works through.

• Wall rise: 20 W ÷ (3.7 W/m²K × 0.764 m²) = 20 ÷ 2.83 = 7 °C, so the plate sits at 40 + 7 = 47 °C
• Metal to metal: the tab rides with the plate at about 47 °C, 13 °C under the limit
• Greased mica: P × 0.4 °C/W = 8 °C, tab at 47 + 8 = 55 °C, 5 °C under the limit
• Dry silicone pad: P × 1 °C/W = 20 °C, tab at 47 + 20 = 67 °C, 7 °C over the limit

The limit is the working temperature below 60 °C (140 °F) that datasheets for this class of industrial stepper drive state.

So the decision: isolate only when the datasheet calls for it, and when it does, grease the kit, because the grease alone is worth 12 °C on this tab. Anything still over 60 °C (140 °F) at full current needs more fin area behind the drive, the path heat sink thermal resistance in sealed enclosures sizes, or a bigger box. Every extra 10 W of loss adds about 3.5 °C to this box's interior; a second identical drive takes the plate to 54 °C and its greased tab to 62 °C, which is the point to step up a size.

Checks before you close the door

A tab that reads over the limit at a 40 °C ambient is telling you one of three things: the pad went in dry, the clamp torque is light, or the box is a size small. Fix the first two at the bench and the third at the ordering stage.

Mistakes that cost the bond and the margin

• A lug on anodize or powder coat. It meters fine on the bench and is the joint that opens under fault current; strip the ring and keep the star washer.
• Counting on the drive's mounting screws as the earth path. They are mechanical fasteners; the earth bond is the dedicated stud and lug.
• A dry silicone pad left in after the bench test. At the assumed loss it holds 20 °C instead of 8 °C: the difference between 55 °C and 67 °C on this tab.
• Shields bonded at both ends, or a second 0 V-to-earth point, either of which builds the loop the chopping outputs energise.
• Nylon screws in place of shoulder washers, which drop clamp pressure and raise the joint temperature.

Pick by construction and the rest follows: isolated case, bolt it down metal-to-metal on the bonded plate; live tab, greased kit and shoulder washers with the plate still bonded; nothing stated, treat the tab as live until your meter says otherwise. Either way the plate is earthed and the DC common lands on it once.

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