Grounding an RS485 shield inside a metal enclosure

Terminate the RS-485 shield to the enclosure wall where the cable enters, metal to metal, within about 25 mm [1 in] of the wall. Use a 360-degree braid clamp or a shield-terminating gland, and keep the PCB ground out of the path. The isolated transceiver on your board never touches the shield; it works with A, B and the reference wire, and the reference lands on the transceiver's isolated ground pin. Bond the enclosure itself to protective earth at its ground stud. On a ten-box chain with 5 m [16 ft] between neighbors, bond the shield direct to the wall at the master end only and fit a 10 nF capacitor at every other drop. That split does two jobs cleanly: the wall carries RF current away, the reference wire holds common mode in range, and shield current never crosses the board.

Where each conductor lands

The cable brings four things you have to place: the A and B pair, the reference wire, the shield braid, and a protective-earth bond for the wall itself. Each lands at a different point. Mixing two of them is what makes RS-485 grounding arguments run in circles.

The transceiver's isolation separates the bus from your logic, and the isolated 24 V supply on the board completes that split. Inside the supply, Y-capacitors tie the two grounds together for EMC; that is the path shield current has to stay away from. Land the braid on the PCB ground and every ampere the braid picks up walks across the board to those capacitors on its way to earth. Terminate the braid on the wall instead and it never reaches the board at all. The plastic housing around your PCB stays out of the decision, because the shield terminates before it, at the metal wall. Whether the PCB ground itself should also tie to the wall is a separate question; the guide to grounding and bonding works through how the wall, the stud and protective earth relate.

The reference wire does a separate job. Ten identical drops leave the bus floating unless something defines its potential, and the RS-485 standard (TIA/EIA-485) allows receivers -7 V to +12 V of common mode. Run the third conductor to every drop and land it on the isolated ground pin; that is what holds A and B in the window when the motors around the line switch. The shield and the reference share a cable and nothing else.

The mechanics of the 360-degree termination are the same for any fieldbus, and the guide to cutouts, glands and HMI windows covers the clamp and gland choices at the entry. For the system-level trade-offs between a single-point bond, both-end bonds and capacitive bypasses, the RS-485 shield bond selection guide walks each option and where it fits.

What you need

• A shield-terminating gland or a 360-degree braid clamp sized to the jacket. A Cat 5e cable measures about 6 mm [0.24 in] over the jacket and an M16 gland typically clamps 4 to 8 mm, so an M16 covers it; read the clamp range off the gland before you order the cutout.
• One 10 nF ceramic capacitor per drop, plus the direct-bond stud for the master cabinet; the worked example below fixes the value.
• Termination resistors matched to the cable: 120 Ω for classic RS-485 twisted pair, 100 Ω for a Cat 5e run.
• Crimp ring lugs and a ground stud. Our stainless walls are bare metal, so the contact needs no coating stripped; the rule NEC 250.12 states — remove paint, lacquer and enamel from contact surfaces at grounding and bonding connections — targets coated boxes.
• A PE conductor sized from the overcurrent device ahead of the panel, per NEC Table 250.122; the grounding and bonding guide covers the fault-path side.
• The entry hole. The cutout designer lays holes and glands on any face of a catalog or custom box and sends the layout with the quote; the box is delivered in 10 business days, counted from drawing approval for custom work.

If the drops terminate on shielded RJ45 plugs instead of a gland, plan the jack-side bond before you cut: the same guide's gland and entry spacing rules decide where the jack's shield land sits on the wall.

Terminating the shield at the wall

1. Kill the panel power and verify absence of voltage before you drill or open anything.
2. Put the entry low on the bottom wall so water that gets past the gland runs back out, and keep it clear of corners. The cutout designer's spacing check settles the edge clearance.
3. Strip the jacket to the gland's own strip dimensions and spread the braid 360 degrees under the clamp. The clamp is the termination: metal to metal on the stainless wall, with no pigtail wire standing in for it.
4. Tighten the gland to its maker's torque so the seal grips the jacket, and leave the cable slack so the clamp carries no side load.
5. Run A, B and the reference wire to the transceiver's isolated side, and land the reference on its isolated ground pin. The braid ends at the clamp.
6. Bond the enclosure's ground stud to protective earth. NEC 250.96(A) requires metal parts that act as equipment grounding conductors, enclosures included, to be bonded so they can carry the fault current likely to be imposed on them. That stud is also where your shield lives. If the stud sits on the removable mounting plate, bond the plate to the wall as well; the grounding and bonding guide covers that bond.
7. Fit the termination resistor at the two ends of the bus only, matched to the cable: 120 Ω on twisted pair, 100 Ω on Cat 5e.

The 10 nF bypass, worked for a 10-box bus 5 m apart

Your layout is ten identical enclosures daisy-chained with the master at one end and 5 m [16 ft] between neighbors. Nine gaps of 5 m give 45 m [148 ft] of cable between the first and last drop; the master's tail takes the run to about 50 m [164 ft]. The direct bond lives at the master alone. Every other box gets a capacitor between its clamp tab and the wall, picked from the 1 to 100 nF band shield-bonding practice uses.

The rule: a capacitor's impedance falls as frequency rises, Xc = 1/(2πfC). Pick the value that reads like a brick wall at mains frequency and like a plain wire at RF.

At 50 Hz, with C = 10 nF:

Xc = 1/(2π × 50 Hz × 10 nF) = 1/(3.14 × 10⁻⁶) ≈ 318 kΩ

At 1 MHz, the same capacitor:

Xc = 1/(2π × 1 MHz × 10 nF) = 1/(0.063) ≈ 16 Ω

So one 10 nF part shows 318 kΩ to the 50 Hz that drives ground loops and 16 Ω to the megahertz noise the motors and drives put into the air. Push the value up to 100 nF and the 50 Hz impedance falls to about 32 kΩ, low enough that loop current starts to matter across a 45 m span. Take it down to 1 nF and the 1 MHz impedance climbs to about 159 Ω, and the RF bond you bought the clamp for fades.

Decision: fit 10 nF ceramic at every drop except the master, keep the leads under 10 mm [0.4 in] so the RF path stays short, and stay inside 1 to 100 nF. Below that the RF bond weakens; above it the 50 Hz loop strengthens. 10 nF holds both ends of the trade, and the master alone gets the direct bond.

Checks when the box is back together

• Clamp to ground stud reads under 0.1 Ω on the meter; a good crimp sits at a few milliohms.
• A to B at one end of the wired bus reads about 60 Ω with two 120 Ω terminators fitted (two in parallel), or 50 Ω for two 100 Ω parts. A reading near 40 Ω means someone fitted a third terminator mid-run; an open reading means one is missing.
• Transceiver ground pin to the master's reference land reads near zero ohms; that is the common-mode path.
• The meter's capacitance range reads about 10 nF at each drop; on the ohms range the same part reads open, because DC cannot cross it.
• Every gland sits torqued on its jacket and the door gasket is clean, so the entry you cut leaves the rating intact.

Mistakes that cost the bus or the seal

• Braid floating inside the box. A plain grommet pass-through leaves the shield unterminated; it picks up noise and re-radiates it centimeters [under an inch] from your transceiver. Cut the entry for a shield gland instead; the guide to cutouts, glands and HMI windows lists what else goes wrong at entries.
• Shield landed on the PCB ground. The braid current then crosses the board and exits through the supply's Y-capacitors, and the offset shows up as data errors at the drops furthest from the master. Land it on the wall.
• A gland rated below the box. An accessory that seals to a lower IP drags the whole assembly down to its own figure. Fit glands with the same rating and install them to the maker's strip lengths and torque. Our stainless wall-mount and HMI enclosures are rated NEMA 4X / IP66 by the manufacturer; not UL listed — the assembly holds that rating only while every part in it does.
• A direct bond at every drop. Ten direct bonds on boxes tied to one protective earth close 50 Hz loops along the whole run; the 318 kΩ the 10 nF bypass shows is what keeps them open.
• Termination set at 120 Ω into 100 Ω cable, or fitted at every drop. Match the resistor to the cable and keep it at the two ends.
• Reference wire skipped because the transceivers are isolated. Isolation protects the board; the bus still needs the third conductor to hold A and B inside the standard's common-mode window.
• Cutting the entry after the quote. Send the hole layout with the drawing through the custom stainless enclosure page so the approval drawing shows the gland positions and edge clearances before the box is built.

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