Grounded device case causes frequent ESD

[canol]

Hello,

We are manufacturing an electronics device which has a metal, grounded case. From time to time, when we touch this device ESD occurs. My questions are:

1 - Is this a common thing on devices with metal cases?

2 - How can I prevent it, or should I prevent it?

3- How is this prevented, for example, on laptops with aluminium cases? Are these cases not grounded?

[Thor-Arne]

You are discharging the builtup charge into the grounded case.
This charge is built up from something else, and should probably not matter for the device.

[mikerj]

Exactly, it's not the device which has a charge built up, it's the person touching it that has the charge.

Do you observe full ESD precautions in your production area, e.g. conductive floor, ground straps for shoes or conductive anti-static shoes, wrist straps etc?

[canol]

We are a very small software start up with no production facility or something similar. We have a pretty primitive device which we put together under my observation (I am electrical engineer, others are not). So when ESD happens they asks questions like "is this dangerous?" or "is this unhealthy for us?". I am not very experienced either, all I know is ESD can happen when the case is conductive and grounded. But I also would like to know if this is common with metal cased "consumer" devices. And also if this is unhealthy if it is happening once or twice a day to a person touching the device. Should I take precautions so that the people using this device are not harmed?

[Thor-Arne]

This is very common, happens with consumer devices regularly. There is no health issues with it.

Note that this can happen between anything, not only grounded devices. I.e. person-to-person, between a person or a plastic-device, and even between different devices.

Make sure you don't have any ESD issues during production or part handling and you should be fine.

[denelec]

Low relative humidity + carpet = ESD

Harmless for humans but can be deadly for unprotected electronics. 

I once fried my electronic alarm clock with an esd.  Repaired it with a brain transplant (Picaxe).

Check the relative humidity of your workplace. If it is too low (<30%) consider an humidifier.
In winter, dry air is ofter a problem.

[coppice]

We are manufacturing an electronics device which has a metal, grounded case. From time to time, when we touch this device ESD occurs.

If you have a grounded metal case, and people touch it, some of those people will be highly charged, and a discharge will occur. That's life. The question is, are problems occurring because of it? If the device is well designed it should function through the discharge with minimal disturbance. If it is poorly designed you might see poor behaviour, such as a blip in readings or a complete reset. You might even damage devices inside the box.

[Jeroen3]

3- How is this prevented, for example, on laptops with aluminium cases? Are these cases not grounded?

Laptops are usually not grounded, but floating using transformers.(see 2 pin mains plug)
That is what creates a "vibrating" sensation when you carefully drag you finger over the aluminum case. As do desktops that don't have earth connected.
It is something to be aware of, that when plugging in USB, a small current will flow shortly to get both grounds at the same potential. Sometimes a tiny current will always flow, depends on the isolation quality. This should be less then a mA, but can introduce noises and other issues.

Refer to Dave's "How not to blow up your oscilloscope" for some more details. Also, this topic has been discussed on the forum before.

[Monkeh]

3- How is this prevented, for example, on laptops with aluminium cases? Are these cases not grounded?

Laptops are usually not grounded, but floating using transformers.(see 2 pin mains plug)

No, they are ground referenced these days in any country where a ground can be expected to be available.

[timb]

3- How is this prevented, for example, on laptops with aluminium cases? Are these cases not grounded?

Laptops are usually not grounded, but floating using transformers.(see 2 pin mains plug)

No, they are ground referenced these days in any country where a ground can be expected to be available.

Not always true. Take any metal Apple laptop (original Titanium PowerBook to current Unibodies); the AC adapter can be plugged into the wall two ways: With the three-prong cord *or* with the two-prong wall adapter-adapter.

With the later, you get the vibrations when sliding your fingers over laptop's case, not so much with the former.


Sent from my Tablet

[Monkeh]

3- How is this prevented, for example, on laptops with aluminium cases? Are these cases not grounded?

Laptops are usually not grounded, but floating using transformers.(see 2 pin mains plug)

No, they are ground referenced these days in any country where a ground can be expected to be available.

Not always true. Take any metal Apple laptop (original Titanium PowerBook to current Unibodies); the AC adapter can be plugged into the wall two ways: With the three-prong cord *or* with the two-prong wall adapter-adapter.

With the later, you get the vibrations when sliding your fingers over laptop's case, not so much with the former.


Sent from my Tablet

That's because it's sold in a market where a reliable ground is not always available: The US. It's shit, and it shouldn't be allowed because the PSU is designed for a ground connection.

Virtually every switching power supply sold in the UK and most of the EU above 30W or so will have an earth contact.

[timb]

Uh, the adapter has a ground contact, it's just you have the option of connection said adapter to a three pin (grounded) cable *or* hooking a two-pin folding plug directly on, which makes it nice for travel! (To put it another way, you can convert an Apple AC adapter from a typical tabletop unit to a wall plug pack.)

And pretty much every outlet in the US is grounded, the exception being really, really old houses, but they've become the exception and not the rule in the last 20 years.


Sent from my Tablet

[T3sl4co1l]

What do you mean "ESD occurs"?

What do you mean by "grounded"?  Is the case grounded to the circuit within?  Does it have shielded connectors, and are they grounded to the case as well?

Tim

[IanB]

Hello,

We are manufacturing an electronics device which has a metal, grounded case. From time to time, when we touch this device ESD occurs. My questions are:

1 - Is this a common thing on devices with metal cases?

2 - How can I prevent it, or should I prevent it?

3- How is this prevented, for example, on laptops with aluminium cases? Are these cases not grounded?

When I travel in cars with seat covers made with some woven fabric, I get this thing where when I get out of the car and go to close the door, the door "bites" me with a snap and a static discharge. It really puts me on edge, and I have lately resorted to gingerly closing car doors by pushing against the non-conductive window glass to avoid the jolt.

I suggest you can prevent the problem in your device by installing glass windows in the metal case and having your workers make sure to touch only the glass and not the metal.

[Monkeh]

Uh, the adapter has a ground contact, it's just you have the option of connection said adapter to a three pin (grounded) cable *or* hooking a two-pin folding plug directly on

Which is a crock. It should not be allowed, the contact is there for a reason. Nobody else allows it.

[timb]

Uh, the adapter has a ground contact, it's just you have the option of connection said adapter to a three pin (grounded) cable *or* hooking a two-pin folding plug directly on

Which is a crock. It should not be allowed, the contact is there for a reason. Nobody else allows it.

Why? It goes through a transformer, so the laptop is floating. There's no high voltage on the output side and the adapter is made of plastic.


Sent from my Smartphone

[Monkeh]

Why? It goes through a transformer, so the laptop is floating.

If it has an earth contact I can fairly well assure you it'll be earth referenced.

There's no high voltage on the output side and the adapter is made of plastic.

And the laptop is made of metal, and has an unpleasant leakage current sourced by the filter capacitors.

These adapters are illegal in the rest of western civilization.

[Jeroen3]

These adapters are illegal in the rest of western civilization.

So an EMI filter is illegal? These have Y-caps to ground, and create your leakage. Would you rather have an unfiltered supply?
Or a huge 50 Hz transformer with your laptop?

I'm missing the point. If you have an isolated supply, it will have leakage. If is has a ground tab to make it referenced, bonus! It will still fit in an non-earth EU socket, so it must stay safe without mains-earth connection. If mains earth is required for safe operation, it won't have a standard plug.
Get an medical grade certified laptop if you don't want any leakage.

[Monkeh]

These adapters are illegal in the rest of western civilization.

So an EMI filter is illegal? These have Y-caps to ground, and create your leakage. Would you rather have an unfiltered supply?

No, an adapter to connect a device which requires an earth pin to a non-earthed outlet is illegal.

It will still fit in an non-earth EU socket, so it must stay safe without mains-earth connection. If mains earth is required for safe operation, it won't have a standard plug.

No, it will come with an appropriate 3 pin plug. Which is what you are meant to use. These are quite standard!

So far as I know, the US is the only market in which a device designed for a 3-pin supply comes with an adapter allowing you to connect it improperly.

[timb]

The device isn't designed for only a 3 pin plug though. That's the point you're missing.

Edit: I tell you what, I've got an old one here, I'll open it up and we'll solve this right now.


Sent from my Smartphone

[Monkeh]

The device isn't designed for only a 3 pin plug though. That's the point you're missing.

Edit: I tell you what, I've got an old one here, I'll open it up and we'll solve this right now.


Sent from my Smartphone

Or let's solve it by referring you to my original point: Laptop supplies are 3-pin and earth referenced pretty much everywhere where an earth contact can be expected. Which is why there is one power supply for the UK, EU, Australia, and most of the rest of the planet, and another (usually two pin, Apple are an exception and a notable one with their full metal shockboxes) for the US (and yet, it's still a universal input supply designed for 230V operation, because it's a laptop).

Could this be because the manufacturers don't expect US users to be able to provide an earth? Or do they just do it for fun?

[taylor12]

 Why would it even need to be earthed? They are a 15 volt supply in a plastic case? I did not get a copper earthing rod with my mac book for use when not connected to the 3 pin adapter they must have forgot to put it in the box! 

[Yansi]

Thats the american incomprehension of safety. In EU, safety rules are very strict and only a few, hardly chinese crap products aren't compliant to these safety rules.
When you need a unearthed device, it must comply to even stricter rules of double insulation, and so. Switchmode PSU is hard to get designed right for the class II rules, no manufacturer woud not do that for fun. The insulation of class II devices must withstand 4kV~ pri-to-sec for 60 seconds. Someone having link to english version of the standards for class II devices in EU?

Yes, I also have an old Compaq laptop computer here with only two wire unearthed psu. But these things are no longer made for EU region and I suspect, that the computer wasn't made for EU either, because these earthing safety rules for electrical engineering are there for quite some time. I suspect, the computer problably has been imported from outside EU. I have got that from a local junk store.

See the PELV:
http://en.wikipedia.org/w/index.php?title=Extra-low_voltage&redirect=no

[miguelvp]

In the US most appliances are light loads so that's why we do fine with lower voltage, also neutral is grounded somewhere (but don't go touching it)

But enjoy eating your steaks with butter knifes, because you know that will be next

Show me statistics that show the contrary and how unsafe the US system really is:
http://www.esfi.org/index.cfm/page/Injury-and-Fatality-Statistics/pid/12015
http://www.electricalsafetyfirst.org.uk/news-and-campaigns/policies-and-research/statistics/

Oh, btw take if from some UK EE guy:
http://boards.straightdope.com/sdmb/showpost.php?p=4535712&postcount=5

[denelec]

Grounded outlets became mandatory in North America somewhere in the '60s.
So a lot of homes built before that time mainly have ungrounded outlets.

I've seen a lot of ground pins ripped so the device could be plugged in an old outlet.  Very unsafe. 

I once got shocked using a rented rug cleaner with a ripped ground plug.