I'm dealing with an electrical equipment which should be compliant to the CE low voltage directive (LVD), following the EN60950 standard.
The equipment is connected to the mains (220V) through an external power supply which provides DC power at 12V. The equipment has a metal casing which is not connected to ground.
Is it a requirement to use a Class II double insulated power supply. Or would a standard Class I power supply be enough?
Electronic – Power supply CE compliancy requirements
cecompliancepower supplyregulationssafety
Related Solutions
Below is a typical schematic of an AC/DC power supply EMI filter.
You can see that the X-capacitors (between line and neutral) plus the leakage inductance of the common-mode inductor give the differential-noise rejection, and the CM choke inductance combined with the Y-capacitors give the common-mode noise rejection.
I would also not be surprised if the output return is directly connected to earth.
Should I be doing the above step as the chassis is effectively not grounded?
Chassis ground is to protect against faults (shorts from power inside of the box to itself) and ESD. With low voltage DC it is not always necessary to protect against faults, with AC mains in a product it is and the mains voltages must be fused.
If there is a fault, it is probably best for the fault current to return through the chassis so grounding the chassis to the ground of the supply is probably the best way to go. In the past I've done this at the input with a DC jack negative terminal connected right to the chassis.
One question I have is what will happen to your DC supply if ESD hits it, it should have certifications that it has been tested for ESD immunity. Again the best pathway is also ground.
If its connected and there is ESD on the metal chasis, wouldn't the circuit get damaged as its connected to the Circuit GND at multiple points via mounting holes?
Electricity always returns to the source via the lowest impedance pathway, so if you shunt the current away from the PCB through the chassis and down the cable that is the best you can do. If you do need to star the ground on the PCB, try and do it as close to the power inlet as possible.
You don't need to connect all of the mounting holes to the PCB ground (you can leave a 10's of mils gap around the mounting hole) just the one by the inlet could be connected to PCB ground (if its a DC jack mounted right to the board). If the PCB has a pigtail, then star the negative terminal on the chassis.
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Best Answer
In the event of a failure with a Class I your metal enclosure could potentially be dangerous to touch. So you have to ground it. The class I requires to be connected to a properly grounded outlet.
The class II doesn't need the grounded outlet and do not require your enclosure to be grounded.