PCB inside metal chassis. Is chassis ground required?

Question

I have a project that has a PCB housed in a metal enclosure. The PCB is powered by a USB-C connector. Power input is 20 V DC @ 4 A max. Power is sourced from an isolated AC/DC wall wart adapter. The PCB contains a Wi-Fi module, so I have put a cutout window in the enclosure, which will be replaced with plastic.

My current intention is to leave the PCB isolated from the chassis by using nylon stand-offs and make a plastic housing shell for the USB-C connector cutout.

Considering the following:

• chassis is not used for electrical purposes elsewhere
• safety
• common mode noise
• EMC
• static build up on chassis
• effect on Wi-Fi antenna
• the end device will go EMC testing
• consumer use product (non-portable)

Do I need to tie chassis to GND of the circuit? Are there any other things to consider before deciding?

Answer 1

chassis is not used for electrical purposes elsewhere

Not relevant

safety

Not applicable - only relevant when dealing with safety grounds.

common mode noise
emc

The general rule of thumb when it comes to EMC is to ground everything to chassis when possible. This helps with emissions and susceptibility both.

static build up on chassis

This is a bigger concern. Just like above, the general rule of thumb for ESD is to ground down all exposed metal surfaces to prevent arcs/sparking across air gaps between metal parts. ESD testing will involve contact discharges against the enclosure and anything of metal attached to it like metal connectors.

affect on wifi antenna

Nearby metal surfaces always have impact on an antenna, whether you ground them or not. Sometimes they improve the signal, sometimes they worsen it. In general you'll want to avoid metal in parallel with the antenna and the ideal placement is at a 90° angle away from the metal.

The antenna will unlikely benefit from this enclosure, but it will for sure not benefit if you don't ground it. Standard practice is to connect the antenna ground/shield to chassis.

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Overall you have nothing to lose but potentially a number of things to gain from grounding the chassis. The standard way would be to attach a cable lug to a screw or similar in the chassis, then connect it to where the supply ground enters the enclosure or where it enters the PCB.

Answer 2

I would at the very least wire bypass capacitor(s) to it, if not hard-ground it.

The adapter will require a REINFORCED type isolation rating, or if the panel/circuit can be earthed, BASIC or better.

The power connector shield shall be tied to PCB ground (plane), so that DM emissions are confined to the cable/PS.

By similar reasoning, the wires to the LEDs shall be filtered, locally, on the ground plane, near the connectors, since they will be more exposed as potential antennae.

I assume you'll have some SMPS on there for WiFi power and running the LEDs, which will need good layout, and adequate filtering to keep noise off connectors. Using the chassis as part of the ground plane design helps with this. Again, it doesn't need to be galvanically / DC grounded, but low RF impedance is helpful.

Answer 3

Depends upon jurisdiction:

If you have a USB powered device in a metal chassis, where the metal is accessible to the user then for class II safety approval the electronics must be double insulated and your PCB and metal chassis MUST NOT be connected.

You could:

• Connect an approved ground to the original chassis, such as the earth pin of a three pin mains cable and plug.
• Double insulate the equipment with a plastic enclosure outside of the original metal chassis. In which case the PCB and metal chassis could be connected for EMC shielding.
• Double insulate the equipment with a ungrounded metal enclosure outside a plastic enclosure, outside of the original metal chassis. Which is standard practice when the device is intended to look pretty and metallic.

Edit:

A mains to USB power brick will be accepted as one layer of the double insulation for safety approval. But depending upon the use of the equipment and how prevalent "web bought" USB power bricks are in your market you may actually want two layers of insulation inside the device.