what's your box look like ? plastic ? metal sputtered ? metal ?
if metal sputtered or metal you want to tie that metal to the usb shield ( Not the system ground ! . you want any energy picked up by the case to blasted into the shield of the usb cable.
the drawing below gives the principle.
The red outline is the metal box containing circuitry. The red box is connected ONLY to the metal shield of the USB connector ( or RS232 or other connector ). inside the USB cable ( if it is a properly onstructed one and not a one-hung lo) there should be NO electrical connection between the cable GND abd the cable shield. the cable shield is a foil warapped around the 4 conductors (vbus,gnd,d+ and d-) and connected only to the plug shield.
if an ESD strike occurs on your device ( the metal box around it ) the current (thick dashet pink line) will flow only through this shield , through the connector sield, through the host casing and into the ground ( provided your computer is grounded. ) none of this current flows in your system signals or system ground. so no harm. if it were to flow through system ground it could create a vlotage delat ( any stub of wire has inductance and resistance. a fast transient will create a large standing voltage across the cable. and all of a sudden the voltage between vbus and cable ground would be superimposed on the voltage across cable gnd created by the zap. so you do not want to send the energy in your ground return. you need that shield return
There is a bit of parasitic capacitive coupling (thin dotted lines) in the cable. part of the zap energy may (will...) be coupled into the signal lines and create a voltage delta there. that delta can be large enough to still kill sensitive circuitry. Enter the esd protection diode. any coupled energy ( blue thick dotted line ) will be shunted by the protection circuitry. so it cannot enter the real circuitry. i fogot to draw the protection diode in the host. but there it will be shunted as well.
so, no matter how you zap this thing the bulk energy is deviated to earth , the capacitively couple smaller energy is absorbed by the protection.
to make the the loop between cable gnd and system ground equipotential you use the 4k7 parallel with the cap. since the transient is a very fast signal it will be shorted by the small capacitor. ( for a fast transient that cap is a virtual short. ) residual standing voltage across the cap is then discharged through the 4k7. so the cap is the real energy deflector. the resistor is simply there to discharge the cap after the event. ( you don't want the energy shot into the cap bouncing back from the cap back into your system. )
that is the ESD protection principle.
if you do the PCB layout you should ideally have a ring around the entire pcb ( a trace about 20 mils wide that goes around the entire circumference of the board and is connected to the all connector shields and to the screw holes that will mount the board to the chassis. this trace should be free of soldermask. for doublesided boards : run trace on top and bottom and stitch via's every cm or so.
if you use a plastic case with vent holes or slits : the esd strike may go through the holes and be attracted by the low impedance path. that nice low impedant shield ring you put down , that has no soldermask, is a damn good attractor. you bet it will strike there .. doing no harm. you may need to study where the vent holes are and provide extra strips of copper attached to that shield ring. or you can mount a piece of copper tape or bent metal electrically connected to that ground ring.
i posted a picture on this forum long ago ( several months ) that showed a pcb layout with exactly such a construction. can't remember what topic.
What asgard above suggested ( the ferrite bead in VBUS ) only protects your system power rail against a coupled in discarge into vbus. but you typically have so much capacitance between vbus and cable gnd tha that energy is absorbed. the ferrite bead is an EMC solver , not an ESD solver .
any noise generated by your system will either radiate ( and be captured by the metal box and grounded ) or be distributed conductive.
that's where ferrite beads and common mode chokes com into play.
a ferrite bead in vbus stops any crap you generate from sneaking out via the connector onto the cable... and stops any crap picked up along the way from sneaking into your system. just make sure to shunt the emc energy to cable ground ( NOT the shield ! ) by placing a small cap ( 47pf) between vbus and cable gnd , before the ferrite bead.
for the data lines you place a common mode choke. this leaves the differential nature of the signal intact but kills off any crap pikced up bye the data lines stone cold. the data lines are a twisted pair so whatever they pick up is an in-phase signal. so common mode choke is the answer. TDK has really nice ones for that purpose. common mode chokes also stop small esd pulses.