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Is this product grounding situation legal and EMC/CE effective ?

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mikestefoy

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I need some help on deciding whether the following situation is legal and safe, and EMC compliant.

I have just joined a company and am concerned about the grounding/earthing of 2 products, one products is mains operated, the other is mains/battery.

The product is housed in a plastic case.
The inside is sprayed with a conductive metal coating.
There is a metal ON/Off button (which pins go to a micro, not power), where the metal housing of the button is connected to the sprayed conductive coating.
The PCB 0volts is bonded by several wires to the sprayed metal coating.

There is an external switch mode power supply supplying 0volts and +19Volts, so the product is Low Voltage .
The power brick is a commercial well known brand (XP), and so will be compliant with the necessary EMC and safety regs.
The power brick has a 2 pin mini DIN connector plug with 0V and +19V pins, and a outer metal housing which is electrically connected through its shield to Earth on a 3 pin 240Vac L-N-E supply. the 2 power pins are not connected to the Earth shielding.

The mini DIN socket on the product has the two pins 0V and +19V wired into the electronics as you would expect. As mentioned, the 0V is connected to the "chassis" of the product.

My problem is that the mini DIN terminals for the outer metal housing ( thus earthed via the power brick) IS NOT CONNECTED TO ANYTHING.

thus the whole thing is floating.

There are USB sockets, so the product can be connected to desktop/laptop PC's. The outer metal of the USB is connected to this "0V chassis", and so if USB is plugged in a connection is made to "Earth" via the desktop PC.


The product has passed all EU EMC emission and susceptibility and ESD zapping tests.

I would have expected to see the mini DIN outer metal housing (EARTH) to be connected to the sprayed metal coating, the 0V not be connected directly to the chassis, but through a 10nf/1KV capacitor.

any thoughts ?
 
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HTA

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Since it is only +12V there is no safety issue. It is also isolated from the mains via the power brick and this avoid ground loops when connected to the USB with a Laptop/PC. Maybe for shielding it could be useful to connect it to the shield.

Enjoy your designe work!
 

mikestefoy

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I fully understand that there are no safety issues.

My concern is on two counts.

1. ESD/Static - when a human charged up to20KV from walking on a nylon carpet touches the ON button, where does the charge go ?

2. from an EMC perspective, wont the whole chassis be a radiating antennae for the electronic noise generated, and with no sink to ground, wont it radiate ?
 

FvM

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An enclosed chassis doesn't radiate (unless you manage to inject differential surface currents). Cables running out from the chassis may radiate if not filtered suffciently against the chassis ground or provided with a large common impedance. Grounding the chassis does not necessarily reduce radiated or conducted interferences, the effect depends on the frequency range of interest and the common mode impedance of potential interfering cables.

Regarding ESD strength, we can expect that the external power supply has Y-capacitors to protective earth can withstand usual ESD.
 
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