When to connect ground to earth and when not

[Brainless]

One of the things that continues to delude me is when to connect a ground to the chassis of its metal/aluminum casing.

Many small projects use an adapter with two prongs and a power supply takes that 6-19DC voltage and regulates it to what is needed. Often 3.3v or/and 5v.
The pcb has a ground plane.
My feeling is that connecting that to the chassis would introduce noise and possibly even effect the circuit. Is that correct?

When a 3 prong plug is used, like with a power supply salvaged out of a PC, the casing of the power supply is connected to the ground wire of the mains.
When using a 5v and 3.3v from that power supply on a pcb would connecting from the pcb's ground to the chassis be necessary?

What are the 'rules' fro connecting to a chassis?

[Neilm]

By ground, do you mean the Earth (ground) provided by a wall outlet? This is provided for safety reasons for the mains power supply so if is provided it should be used. It is connected to the metal case on PC PSUs as the idea is if something goes wrong inside the supply (i.e a wire falls off) it prevents the case becoming hazardous so you will not get a shock off it - generally it blows a fuse first.

Power supplies that only have two prongs are (or should be) "double insulated" which means basically a user can not touch anything that could ever possibly be hazardous.

For general rules, there is no need to connect a circuit to Earth unless it is required for performance reasons.

[ampdoctor]

Good thumb rule is that if the end user can ever touch anything either directly or indirectly to something conductive, ground the chassis.

[AmmoJammo]

You've just reminded me...

I bought a cheap DVD player, and upon plugging in the RCA cable (touching the ground the rca cable) when I came into contact with the back of the DVD player case, I got a nice tingle....

Measured 200+ volts from the dvd player case, to mains earth...

Pulled it apart, and found that instead of connecting the capacitors across the mains, they'd connected them from active, to the case....
(If it was plugged into something else that was earthed, it wasn't an issue)

Took it back:
"Hey, this dvd player has a design fault, the case is at mains voltage, they probably shouldn't be sold anymore till its fixed"

"ahhh, so... do you want a refund?" 

[Zero999]

You've just reminded me...

I bought a cheap DVD player, and upon plugging in the RCA cable (touching the ground the rca cable) when I came into contact with the back of the DVD player case, I got a nice tingle....

Measured 200+ volts from the dvd player case, to mains earth...

Pulled it apart, and found that instead of connecting the capacitors across the mains, they'd connected them from active, to the case....
(If it was plugged into something else that was earthed, it wasn't an issue)

If the capacitors were appropriately rated, that design could be perfectly safe, since the leakage current would be limited to a safe level by their impedance.

[AmmoJammo]

Safe, yes, but who designs a piece of equipment that gives the user tingles!

[grumpydoc]

Safe, yes, but who designs a piece of equipment that gives the user tingles!

Rather a lot of people I'm afraid.

The culprit AIUI is a small cap that commonly gets connected between the low voltage side and the high voltage side of a SMPS - it provides a path to ground for high frequency signals to reduce EMI

I find I can feel the leakage through these caps as well.

[Brainless]

What if you use a metal case and an adapter like the small 5v USB power supplies.
Is connecting the ground to the case good or bad?

[moepower]

I don't think it is important if you're only dealing with 5V and your mains wiring is well away from all that.    If you have mains wiring in that metal case, then yes, you should ground the case for safety.

[vk6zgo]

Safe, yes, but who designs a piece of equipment that gives the user tingles!

Probably someone designing for use on a North American 240v system,where both sides are "Actives" & the chassis is at "virtual Earth".
In Australia,such a circuit puts the chassis halfway down a Capacitive voltage divider,so that it is at 120v w.r.t Neutral,& hence,Earth.

Even with an Earth connection,this can give you trouhle,as in that case,one cap has "near as dammit" zero volts across it,& the other has 240v.
If the designer has fitted caps which are adequate in the US,with a max of 120v across them,they may go "bang!"

[wraper]

Safe, yes, but who designs a piece of equipment that gives the user tingles!

Who? Answer is all. You will find Y rated capacitors connected from the mains to the chassis in 100% of devices which have SMPS. Exception is small phone chargers.

[AmmoJammo]

Safe, yes, but who designs a piece of equipment that gives the user tingles!

Who? Answer is all. You will find Y rated capacitors connected from the mains to the chassis in 100% of devices which have SMPS. Exception is small phone chargers.

From active to the chassis? so every single "double insulated" device has 200+ volts on it with respect to ground? 

[wraper]

Half of the mains voltage in most of tha cases. There are usually two Y rated capacitors from both mains inputs to the chasis. Somethimes when skimping costs use one capacitor after rectifier, from minus rail to the chasis. Or use both combined, 3 caps in total. That's why it is not so good idea to connect TV and DVD player, computer or whatever while devices are connected to the mains. You can blow inputs/outputs if signal contact connects before ground contact

[Zero999]

One way to get rid of the capacitor would be to put a shield between the primary and secondary of the SMPS transformer and connect it to the DC on the primary side. There would still be some leakage but it'd be less than a capacitor.

[Nerull]

Safe, yes, but who designs a piece of equipment that gives the user tingles!

Probably someone designing for use on a North American 240v system,where both sides are "Actives" & the chassis is at "virtual Earth".
In Australia,such a circuit puts the chassis halfway down a Capacitive voltage divider,so that it is at 120v w.r.t Neutral,& hence,Earth.

Even with an Earth connection,this can give you trouhle,as in that case,one cap has "near as dammit" zero volts across it,& the other has 240v.
If the designer has fitted caps which are adequate in the US,with a max of 120v across them,they may go "bang!"

US 240V plugs still provide an earth ground, in the vast majority of cases.

[vk6zgo]

Safe, yes, but who designs a piece of equipment that gives the user tingles!

Probably someone designing for use on a North American 240v system,where both sides are "Actives" & the chassis is at "virtual Earth".
In Australia,such a circuit puts the chassis halfway down a Capacitive voltage divider,so that it is at 120v w.r.t Neutral,& hence,Earth.

Even with an Earth connection,this can give you trouhle,as in that case,one cap has "near as dammit" zero volts across it,& the other has 240v.
If the designer has fitted caps which are adequate in the US,with a max of 120v across them,they may go "bang!"

US 240V plugs still provide an earth ground, in the vast majority of cases.

Indeed,they do,but not all power cords have an Earth connection--hence my comment about a "virtual Earth".
My other point about only one cap being  exposed to the full 240v if used with an Earth lead on the Australian system is also still valid.
With the US 240v system,the maximum voltage on each cap is 120v.

There are quite a few people in the USA using  240v outlets,in particular,"Hams" if they have "legal limit" linear amplifiers.
These devices.if run off 120v usually need to be "wound down',so as to not exceed the rated current draw of normal circuits.

[Zero999]

That would just possibly eliminate the need for the Y1 cap across the isolation barrier. You would still need the Y2's (L-G and N-G) for the EMI filter I think.

I think they also sometimes connect the output secondary side return to earth ground either directly or through a capacitor for safety from primary to secondary insulation failure. I vaguley recall reading an app-note about that.

What about class 2 appliances with no earth connection?

Normally there are 2 capacitors: secondary to primary (Y2) and an X2 capacitor from phase to neutral. Eliminate the Y2 capacitor between the secondary to primary and you can reduce the leakage current.

[SeanB]

If you remove the Y2 cap across the isolation barrier it will fail EMC tests and will become a deliberate radiator.