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Topic: Earthing a metal case (Read 1 time) previous topic - next topic

GilchristT

Sorry for what's probably a total newb question but...

I've a project that's going to have to be installed in a metal (Aluminium) enclosure. The project is USB bus powered.

I'm assuming it would be best practice to earth the enclosure even though it's only low voltage? If so, should I use the shield of the USB cable or the ground?

The ground would seem to be the obvious choice and I've read several times that the USB spec recommends (or demands, depending on where you read it) that the shield is only ever connected to ground at the host end.

However I thought I'd better ask rather than guess  :smiley-red:

Grumpy_Mike

If you connect the case to the shield it will not matter as long as you don't connect the case to anything else. However there is no safety advantage in grounding the metal case and I would leave it unconnected to anything.

GilchristT

Thanks Mike

That would certainly keep things simpler (I tried soldering onto the Al, gave up, did some reading, realised I should never have started  :) )

I just assumed that the case would need grounded for safety reasons. Speaking as a (obviously) ignorant end-user, I'm used to our facilities people coming around and doing electrical safety testing on our laptops every year or so and assumed it was something like this would be needed. Does this sort of testing only apply to something with a power supply or am I barking up the wrong tree completely?

jackrae

If you are connected to a laptop (whether on battery or mains adapter) the USB shielding is floating from "true" electrical earth so there is no need to earth the casing, other than via the USB cable shield as Mike says.  However if you are using a mains powered workstation then the casing of that machine and possibly the USB shield will be connected to "true" electrical earth.  If you then also connect your metal enclosure by a separate earth bond to the mains ground you could possibly create an earth circulating current into the USB shield which may induce noise into the USB data. So better to simply use the USB shield as a single earth link as per the laptop system.

Grumpy_Mike

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However if you are using a mains powered workstation then the casing of that machine and possibly the USB shield will be connected to "true" electrical earth.

That is only true for some classes of equipment:-
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In general, there are a two classifications of portable appliance used on the normal mains supply, Class I and Class II.  Class I appliances usually have metal casing and must be connected to an earthed supply.  Class II (also known as double insulated) have two layers of insulation to prevent a fault causing an electric shock.  Class II appliances are marked with the "Double Square" symbol and do not need earthing.


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Does this sort of testing only apply to something with a power supply

Yes it is called PAT testing.
http://www.pat-testing.info/

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I tried soldering onto the Al,

It can be done but you need special solder, it is very expensive at about £35 per meter.

GilchristT


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