Bonding 0V to ground in an enclosure with mixed supplies

Short answer: bond each supply's DC common to ground at one star point on the mounting plate. The 24 V and 5 V systems then share one earthed reference, which is what PELV means. Drives bolted to a grounded plate are an advantage, not a problem: the plate settles the filter-capacitor float on their cases and shields your logic from their switching. Run one green/yellow conductor from each supply's negative output to a single stud on the plate; 14 AWG [2.5 mm²] covers a 10 A output. Bond the plate to the wall on dedicated studs. Done this way, the worked example below holds the 5 V common within 0.75 mV of earth. A long thin lead drifts 32 mV and moves every logic threshold with it.

What bonding the common to earth does

An industrial DIN-rail supply ships floating: neither output terminal references earth. One bond from the negative output to the grounded plate turns it into a PELV system — protective extra-low voltage, bonded to earth, as IEC 60364 defines the term. Before the bond the circuit is SELV, separated; after it, PELV. The limits are 50 V AC or 120 V DC touch voltage in dry locations, and 25 V AC or 60 V DC where water raises the shock risk. A bonded 24 V or 5 V common sits far below both, so every DC terminal stays touchable even after a fault to the plate. In a washdown cell that settles the question on its own.

The bond also gives sensor returns, encoder shields and both commons a single potential, instead of letting them chase each other around the panel. What the bond is not: a fault path for the mains side. NEC 250.4(A)(5) says it directly — the earth is not part of the effective ground-fault current path. The AC equipment grounding conductor still runs from each supply's line terminals to the panel's ground bar, and the grounding and bonding guide covers that side in full. The DC bond rides on top of it as a reference.

Drives on a grounded plate: the fix, not the fault

Metal-cased stepper drives carry EMC filter capacitors from their power terminals to the case. Leave the case floating and those caps pull the DC side to roughly half the mains: about 115 V AC from a 230 V supply, measured case to earth, through roughly a megohm. It is enough to feel on your hand, and it is a classic cause of encoders that miscount on a bench that otherwise measures fine. Bolt the drives to the bonded plate and the case sits at earth, the caps leak to a defined point. The plate then doubles as a shield against the drives' switching edges.

Two details make the mount itself a bond. A toothed washer under each drive's mounting screw bites through the stainless oxide film into the plate, and where any surface is painted, NEC 250.12 requires the coating to come off the contact area. Into the panel, the bond travels on dedicated studs: hinge and loose screws are not a current path, which is why the door bonding jumper exists and why the grounding screw thread and lug fit matter at the stud.

What you need

• Green/yellow insulated copper: one 12 in [300 mm] 14 AWG [2.5 mm²] strap per supply, plus 10 AWG [6 mm²] for the plate-to-wall bond. Insulated grounding conductors of 6 AWG and smaller are green or green with yellow stripes under NEC 250.119.
• Crimped ring terminals (no solder) and toothed washers.
• One M6 bonding stud on the plate, within 12 in [300 mm] of the supplies.
• A clean landing on the enclosure wall or the ground bar for the plate bond.
• A four-wire milliohm meter, or a DMM with a relative lead-zero function.
• The box: a 16 x 12 x 6 in 304 stainless wall-mount enclosure takes a drives-plus-breakout layout with room for the straps, and the same box in 316L suits brine and chloride washdown. Both are manufacturer-rated NEMA 4X / IP66, not UL listed. The wall-mount range runs from 6 x 4 x 4 in up to 60 x 36 x 16 in.

Steps

1. Lock the panel out and prove it dead before you land anything.
2. Open each supply's manual and find the earthing line for the output. Isolated, floating outputs — the standard on industrial DIN-rail supplies — may be bonded at either terminal; bond the negative and leave +V floating.
3. Fit the star stud on the plate, close to the supplies, with a toothed washer behind each ring terminal.
4. Bond the plate to the wall first: 10 AWG [6 mm²] green/yellow on a dedicated stud at each end. NEC 250.96(A) calls for metal parts to be bonded so they can carry the fault current likely to be imposed on them; that strap is what turns the plate from a floating island into an equipotential surface.
5. Run each supply's DC common to the star on its own conductor, sized from that supply's output fuse the way NEC Table 250.122 sizes an equipment grounding conductor. 15 A or less takes 14 AWG [2.5 mm²]; 20 A takes 12 AWG [4 mm²]; 30 A takes 10 AWG [6 mm²]. The bonding conductor sizing article walks the same logic for the AC side.
6. Keep every strap under 12 in [300 mm]; the worked example shows what a longer, thinner lead costs.
7. Land shields and analog returns at the same star, with each cable shield grounded at one end only, so a shield is never the second path in a ring. The analog ground offset article shows what a stray second reference does to a signal.
8. Run +V and the common to each drive as a twisted pair. The star bond is a reference, not a return; load current belongs on the pair.
9. When the drives' plate and the PC live in different cabinets, bond the two plates with 16 mm² [6 AWG] copper. One automation vendor's panel documentation sets that minimum for equipotential bonds between two cabinets, and it keeps equalizing current out of the signal cable.
10. Tag the stud 0V STAR — PELV BOND so the next rebuild lands on the same point.

Worked example: 24 V drives plus a 5 V breakout board

Take the 16 x 12 x 6 in box from the list, inside 15.88 x 11.88 x 5.88 in. A 24 V DC 10 A supply feeds three stepper drives set to 2 A per phase, about 6 A together. A 5 V DC 3 A supply runs the breakout board and encoder at 0.3 A. Both manuals allow an earthed output, and the star stud sits 12 in [0.3 m] from the supplies.

Offset on the 5 V board: 0.3 A × 2.5 mΩ = 0.75 mV from common to earth. Copper resistance rises about 0.39 % per K, so at 75 °C [167 °F] the strap reads 2.5 × 1.21 = 3.0 mΩ and the offset 0.9 mV — nothing against a 5 V rail.

Fault on the 24 V side: a +V-to-plate fault drives up to 10 A through the strap, a drop of 10 A × 2.5 mΩ = 25 mV. The NEC ampacity table puts 14 AWG copper at 20 A continuous at 75 °C, so the fuse clears long before heat matters.

Contrast, and the reason for step 6: daisy-chain the board's common to a PC chassis on 6.6 ft [2 m] of 22 AWG, which standard wire tables put at 53 mΩ/m — 106 mΩ total. The same 0.3 A now shifts the common by 0.3 A × 106 mΩ = 32 mV, 42 times worse. Let that thin lead carry a drive's 6 A return and it shifts 6 A × 106 mΩ = 636 mV, 13 % of the rail, wobbling every threshold on the board.

Whole path, common to wall: 2.5 + 0.7 plus about 0.3 mΩ of stud contact ≈ 3.5 mΩ. That clears the 5 mΩ check below with room to spare.

Checks that prove it worked

With the drives running, measure from the DC common terminal to the enclosure wall: a four-wire meter should read under 5 mΩ — the example lands at about 3.5 mΩ. A standard DMM bottoms out near 0.1 Ω, so it proves the bond exists and nothing finer; zero the leads with the relative function first. Then the float check: on a floating build, case to earth reads about 115 V on a 230 V supply; bonded, it reads millivolts. Last, jog each axis at full speed for 10 minutes and watch the encoder count on the board. A steady count with no random faults means the reference is holding.

Mistakes that cost you the quiet panel

• A second earth point. Two DC bonds plus a shield landed at both ends make a ring, and induced current circulates in it. One star per cabinet; shields at one end.
• Bonding through paint or through the hinge. NEC 250.12 takes coatings off contact surfaces, and a hinge earns a bonding jumper rather than trust.
• Letting the bond carry load current. The 636 mV figure above is what a thin lead as a return costs; pair +V with the common to every drive.
• Bonding a supply that is not isolated. Where the manual's earthing line is missing, the output can share a reference with the mains, and the bond becomes a short through the PE; one line in the manual settles it.
• Long thin straps. Each 12 in of 22 AWG adds about 16 mΩ; stay under 12 in and at 14 AWG or larger.

The decision: bond every isolated output's common to one star stud on the grounded plate — 14 AWG [2.5 mm²] for outputs to 15 A, 12 AWG [4 mm²] to 20 A, 10 AWG [6 mm²] to 30 A — take the plate to the wall on 10 AWG [6 mm²], and run 16 mm² [6 AWG] between cabinets. Leave floating only the devices whose instructions demand it, and everything else on the plate shares one reference that sits within millivolts of earth.

Enclosures in this article

Questions