--- title: "Should a USB-C shell go straight to ground, or through an RC?" topic: "USB-C shell" answer: "Straight, unless an earthed box or cable shield touches it too. An 8 kV zap puts the same 30 A into the board’s ground either way. Through 1 MΩ beside 4.7 nF, the cap is left holding 249 V. Common 0805 caps are rated 50 V." url: "https://nearfieldnotes.com/notes/usb-c-shell/" site: "Near Field Notes" author: "Michael Walker" note: 2 family: "EMC" published: "2026-11-03" revised: "2026-11-03" revision: 1 video: "https://nearfieldnotes.com/videos/usb-c-shell/" --- Note 2 · [EMC](https://nearfieldnotes.com/families/emc/) · Rev 1 · [Watch the video · 0:09](https://nearfieldnotes.com/videos/usb-c-shell/) USB-C shell # Should a USB-C shell go straight to ground, or through an RC? **Short answer:** Straight, unless an earthed box or cable shield touches it too. An 8 kV zap puts the same 30 A into the board’s ground either way. Through 1 MΩ beside 4.7 nF, the cap is left holding 249 V. Common 0805 caps are rated 50 V. **Note 2’s opening figure: The zap, both ways.** From 1 to 300 ns ![Note 2’s opening figure: The zap, both ways](https://nearfieldnotes.com/notes/usb-c-shell/fig-opening.png) *What the figure shows:* An 8 kV contact zap into a USB-C shell, from the first nanosecond to 300 ns. The same current goes into the board's ground whichever way the shell is tied, 29.8 A at its first peak. Through 1 MΩ beside 4.7 nF, the shell climbs and stays up. Tied straight, it bumps for a few nanoseconds, then sits at 0 V. At 1 ns, 27.1 A into ground, 15 V through the RC and 3 V straight. At 10 ns, 14.4 A into ground, 37 V through the RC and 0 V straight. At 30 ns, 16.1 A into ground, 109 V through the RC and 0 V straight. At 60 ns, 8.1 A into ground, 184 V through the RC and 0 V straight. At 100 ns, 2.8 A into ground, 226 V through the RC and 0 V straight. At 300 ns, 0.0 A into ground, 249 V through the RC and 0 V straight. **Key** - Through the RC - Straight - A 50 V part’s rating The test gun is 150 pF charged to 8 kV, behind 330 Ω. The RC is 1 MΩ beside 4.7 nF. ## Two ways to tie a shell down *USB-C on Raspberry Pi’s CM5 IO board* A USB-C receptacle has a metal shell, soldered down at four tabs. Reach for the cable with a charged finger and the zap lands on that shell. Open boards tie it to ground one of two ways. Straight puts every tab on the ground net. An RC puts the tabs on a net of their own, tied to ground through 1 MΩ beside a 4.7 nF cap. **Fig. 2: The circuit.** ![Fig. 2: The circuit](https://nearfieldnotes.com/notes/usb-c-shell/fig-2.png) *What the figure shows:* Figure 2: the circuit, both ways. The test gun, 150 pF charged to 8 kV behind 330 Ω, drives 30 A into the shell. Tied straight, the shell goes to ground through its 4 tabs, about 1 nH each. Through an RC, it goes to ground through 1 MΩ beside a 4.7 nF capacitor. The gun is the one IEC 61000-4-2 sets out for test labs. Each tab with its vias is about 1 nH. **Fig. 2b: On the board.** ![Fig. 2b: On the board](https://nearfieldnotes.com/notes/usb-c-shell/fig-2b.png) *What the figure shows:* Figure 2b: the USB-C receptacle's corner of the CM5 IO board from above, as the field model built it. The shell, seen through, stands out 1 mm past the board edge. Four tabs at its sides go down into the ground copper, which fills the board round them. The tabs are the shell's only way into ground. The CM5 IO board’s USB-C power input, as the field model built it. The shell is seen through. Its four tabs are its only way into ground. **Fig. 2c: What the cap is left holding.** ![Fig. 2c: What the cap is left holding](https://nearfieldnotes.com/notes/usb-c-shell/fig-2c.png) *What the figure shows:* Figure 2c: the voltage left on the shell after one zap, at 4 and 8 kV. Straight leaves 0 V. Through 1 MΩ and a cap: 10 nF is left at 58 V after 4 kV and 117 V after 8 kV. 4.7 nF is left at 124 V after 4 kV and 249 V after 8 kV. 2.2 nF is left at 266 V after 4 kV and 532 V after 8 kV. 1 nF is left at 585 V after 4 kV and 1,169 V after 8 kV. A 50 V part's rating and a 1 kV part's are marked. One contact zap. Every RC has 1 MΩ beside the cap. USB-C shell ### 4.7 nF is left holding 249 V **Field marks.** The gun’s 150 pF at 8 kV holds 1.2 µC, and the zap delivers 1.17 µC of it. All of that lands in the cap, so the cap ends at that charge over its capacitance. **Look-alike.** A smaller cap is worse. 1 nF is left at 1,169 V. **Range.** At 4 kV, the usual EU test level, 4.7 nF still gets 124 V. The 1 MΩ drains it below 1 V in 23 ms. **Habitat.** Arduino’s Uno R4 fits 4.7 nF rated 1 kV beside its 1 MΩ. Raspberry Pi and MNT Reform tie their shells straight. ## Zap it yourself *5 ties, 1 to 8 kV* **Note 2’s explorer: USB-C shell.** ![Note 2’s explorer: USB-C shell](https://nearfieldnotes.com/notes/usb-c-shell/fig-explore.png) *What the figure shows:* The voltage on the shell for 300 ns after the zap, for the chosen tie and zap, with a 50 V part's rating and a 1 kV part's marked. Through an RC the cap charges and holds. Straight stays at 0 V. Every RC has 1 MΩ beside the cap. The circuit is linear, so each zap is the 8 kV run scaled. **Fig. 2d: Where the zap crosses the ground.** ![Fig. 2d: Where the zap crosses the ground](https://nearfieldnotes.com/notes/usb-c-shell/fig-2d.png) *What the figure shows:* Figure 2d: Raspberry Pi's CM5 IO board from above, its ground copper shaded where the zap's charge crossed it in the first 100 ns, after an 8 kV zap on its HDMI0 shell. With the power cable on the same edge, the zap runs along that edge to the power input, and 2.9 A goes through the CM5 module. With the power input on the far edge by the module, the zap runs round the board's edges to it, and 11.0 A goes through the CM5. An 8 kV zap on the HDMI0 shell, and the charge that crossed the ground in its first 100 ns. The CM5 module sits above the board on two connectors. USB-C shell ### With power on the far edge, 11 A goes through the CM5 **Field marks.** The zap leaves through whatever cable goes to ground. On the board as built, the power cable is on the zapped edge. The zap runs along that edge to it, and 2.9 A goes through the CM5. **Look-alike.** Move the power input to the far edge and the zap runs round the board to it. 11 A goes through the CM5. That’s the current that can reset a board. **Range.** Straight or RC, the zap spreads the same way. The field model ties every shell straight, and at 100 MHz 4.7 nF is 0.3 Ω. **Habitat.** Raspberry Pi’s CM5 IO board, zapped on its HDMI0 shell. Every cable connector sits on one edge. ## Which to use *Shell ties* **Fig. 2e: The bump while the zap is on.** ![Fig. 2e: The bump while the zap is on](https://nearfieldnotes.com/notes/usb-c-shell/fig-2e.png) *What the figure shows:* Figure 2e: tied straight, the peak voltage on the shell while the zap's current rises, for 1, 2, 4 and 8 paths to ground of about 1 nH each. 1 path: 37.5 V. 2 paths: 18.7 V. 4 paths, the 4 tabs: 9.4 V. 8 paths: 4.7 V. Straight, on the 8 kV zap. Each path is a tab with its vias, about 1 nH. The bump is over within a few nanoseconds. Key to shell ties ### How should this shell be tied down? 1. 1a Nothing earthed touches it Straight, every tab, 2e 2. 1b An earthed metal box, or a cable shield earthed at the far end, touches it Go to 2 3. 2a You can fit a cap rated 1 kV or more An RC, 2c 4. 2b Only a 50 V part fits Straight, 2c With an earthed box, the RC keeps mains hum off your ground. Leave footprints for both, so a fix at the test lab is a part swap. #### Conditions - **Zap:** IEC 61000-4-2 contact discharge, from the standard’s own formula. At 8 kV its first peak is 30 A, with a 0.8 ns rise. - **Gun:** 150 pF behind 330 Ω, run as a current source. That holds while the shell stays far below 8 kV, which it does here. - **Straight:** 4 paths to ground side by side, each a tab with its vias at about 1 nH - **RC:** 1 MΩ beside one cap with 0.8 nH and 0.05 Ω in series, like a 1206 high-voltage part - **Circuit:** ngspice, 0.02 ns steps to 400 ns - **Board:** Raspberry Pi’s CM5 IO board. Its two inner ground planes as one sheet of 1 mm cells, each shell a metal block tied down at its tabs, and the CM5’s ground 1.5 mm above on its 51 ground pins. - **Bench:** The gun’s rod on the HDMI0 shell, its source 80 mm out. The exit is a wire from the power input to a ground plane 60 mm under the board. - **Field:** openEMS, solved per ampere up to 1.2 GHz, then put through the 8 kV zap Simulated only. Nothing on this page has been measured on a board yet. #### Limits - The bench is smaller than the IEC’s 0.8 m table, and cables are wires. That changes how the outer loop rings, not which way the current runs on the board. - FR-4 is lossless and copper is perfect. - 1 nH per tab is an estimate. - The field model has no RC case. At ESD speeds the cap is a short, so the current spreads as it does straight. #### Files - esdsim/iec.py The IEC 61000-4-2 current - esdsim/bond.py The shell ties in ngspice - esdsim/model.py The CM5 IO board’s field model - esdsim/synth.py From per-ampere runs to the 8 kV zap #### Sources - [IEC 61000-4-2:2008, the ESD immunity test](https://webstore.iec.ch/en/publication/4189) - [Raspberry Pi’s CM5 IO board design files](https://pip.raspberrypi.com/categories/1098-design-files) - [Arduino Uno R4 Minima schematic](https://docs.arduino.cc/resources/schematics/ABX00080-schematics.pdf) What should the next note be? [Ask for one](https://nearfieldnotes.com/#ask-for-a-note) --- Source: https://nearfieldnotes.com/notes/usb-c-shell/ Cite as: Michael Walker, “USB-C shell”, Near Field Notes, Note 2, Rev 1, 3 Nov 2026. https://nearfieldnotes.com/notes/usb-c-shell/ Figures: free to reuse under CC BY 4.0 (https://nearfieldnotes.com/license/). Each one's image is linked under its name. Credit Michael Walker, Near Field Notes, and link the figure: this address plus #fig- and its number, like https://nearfieldnotes.com/notes/usb-c-shell/#fig-2.