Circuit earth safety question

[that_guy]

Hi,

I have a single PCB project powered by the mains. An isolating transformer transforms the mains down to 6VAC where I rectify and regulate it to power the low digital side of the board. There are 4 PCB mounting holes for screws in each corner of the board. The incoming earth wire is connected to the metal chassis and the mounting screws are also connected to the chassis.

My question is, should I connect the low-side ground to the chassis (and hence earth)? Currently that side of the board is floating and given Dave's video warning of having a mains-earth referenced project I've become concerned about whether I should do that. However, the low side will have a connection to the external world through a case-mounted button and I'm concerned that if the mains should somehow find its way on to the low side then the lowest impedence path to earth would be through the operator pressing the button.

[madires]

Since you already grounded the metal chassis to earth/PE the operator is protected from electrical shock assuming the wall socket is wired correctly. If there's any benefit in connecting the circuit's ground to the chassis (PE), like less noise or EMI, simply do it. For example, the output ground of every ATX PSU is connected to earth/PE, as the PSU's chassis. But you have to be aware that most scopes are grounded too, i.e. if you connect the probe's ground clip to +Vcc you would create a short circuit.

[dom0]

Simple enough.

Appliance class I: Ground all secondaries
Appliance class II: Don't ground anything (requires double/reinforced insulation), if you have PE in the device, treat it like a phase or neutral.

You want class I, ground everything.

[that_guy]

dom0, madires: very clearly put both of you. Thank you for your time.

[rdl]

Appliance class I: Ground all secondaries

You mean earth ground? Transformer secondaries can be grounded? I have some HP power supplies and I don't think the secondary is earthed. Would the output still be floating?

[dom0]

Commercial lab supplies usually have floating outputs, but they do have of course the necessary insulation in the  transformer to do that. Many of them also have additional circuitry that isn't floating but grounded, e.g. supplies with Ethernet or GPIB ports.

Grounded = earthed, says my dictionary. To clarify: I mean a low-resistance path to PE.

[rdl]

I don't think I have ever seen a grounded secondary on a low voltage step down transformer (120v to less than 50v for example) when a 3 conductor connection with chassis ground is used. Are these types of transformers always double insulated or is grounding the secondary just considered not necessary if the primary is below a certain voltage and a 3 wire grounded connection is used?

[Richard Crowley]

I don't think I have ever seen a grounded secondary on a low voltage step down transformer (120v to less than 50v for example) when a 3 conductor connection with chassis ground is used. Are these types of transformers always double insulated or is grounding the secondary just considered not necessary if the primary is below a certain voltage and a 3 wire grounded connection is used?

I have never seen such a thing before, either.  The secondary winding and the circuit after that are isolated and do not require grounding/earthing to be "safe".  Indeed most bench power supplies have isolated outputs, and a separate "PE" green-wire safety ground terminal so that the user can connect one side of the power supply to PE where that is indicated.

[AxleD]

Thanks, very helpful.

[dom0]

I don't think I have ever seen a grounded secondary on a low voltage step down transformer (120v to less than 50v for example) when a 3 conductor connection with chassis ground is used. Are these types of transformers always double insulated or is grounding the secondary just considered not necessary if the primary is below a certain voltage and a 3 wire grounded connection is used?

This is the usual arrangement for class I appliances. Of course this is only necessary if you can touch some part of the circuitry, it it is all isolated no one cares. However, most of the time, one can touch at least circuit ground. Consider for example a class I audio appliance (e.g. amplifier), it probably has some cinch connectors, so you can touch circuit ground. Class I mandates that all conductive parts you can touch must be connected to PE, so you need to connect circuit ground to PE, grounding the secondary of the mains transformer. Now consider a class I alarm clock (dumb example, but it illustrates the point), you have a nice metal case and some LEDs behind plastic and some plastic pushbuttons. In this case you don't need to earth circuit ground, since you can't touch any conductive part of the circuit.

Yes, most smaller (<100 VA, or so) mains transformers you can buy at distributors are suitable for class II. This is usually noted in the data sheet by something along the lines of "prepared for class II" or "double/reinforced insulation".

[dom0]

I don't think I have ever seen a grounded secondary on a low voltage step down transformer (120v to less than 50v for example) when a 3 conductor connection with chassis ground is used. Are these types of transformers always double insulated or is grounding the secondary just considered not necessary if the primary is below a certain voltage and a 3 wire grounded connection is used?

I have never seen such a thing before, either.  The secondary winding and the circuit after that are isolated and do not require grounding/earthing to be "safe".  Indeed most bench power supplies have isolated outputs, and a separate "PE" green-wire safety ground terminal so that the user can connect one side of the power supply to PE where that is indicated.

I suggest you take a ohm-meter and connect it between circuit ground and the PE prong of various appliances, e.g. oscilloscope, audio amplifier, function generator, analyzers, computer, printer, ...

Indeed most bench power supplies have isolated outputs, and a separate "PE" green-wire safety ground terminal so that the user can connect one side of the power supply to PE where that is indicated.

And if you look at them you will find the required insulation to do that. Bench supplies are effectively mixed class I/II appliances (yes, you can do that). EN/VDE says about that "Geräte, welche teilweise nach Schutzklasse II, jedoch auch teilweise nach Schutzklasse I gebaut sind, werden als Schutzklasse I eingestuft." ("Appliances, which are partly construced as appliance class II, partly as class I, are rated as class I appliances"). Which is why you'll find the class I symbol on your bench supply. Also relevant here is the EN 61010-1, which describes the safety mechanisms. As most bench supplies, as mentioned, are partly class I you can also use a shield winding between primary and floating secondary to avoid double/reinforced insulation. Most supplies will have that winding anyway to cut down on coupling capacitance.

btw I don't think OP's question was about a bench supply

[Zero999]

I don't think I have ever seen a grounded secondary on a low voltage step down transformer (120v to less than 50v for example) when a 3 conductor connection with chassis ground is used. Are these types of transformers always double insulated or is grounding the secondary just considered not necessary if the primary is below a certain voltage and a 3 wire grounded connection is used?

It depends on the construction of the transformer. If there's only basic insulation between the primary and secondary, then the secondary needs to be earthed to ensure protection against electric shock. Nearly all modern transformers will have double insulation between the primary and secondary so the secondary doesn't need to be earthed.

Quite often the secondary of the PSU is earthed on industrial and IT equipment, either for EMC purposes or to prevent it from floating at dangerous voltages.

[rdl]

Yeah, I spent the morning tracking down data sheets for all the transformers I have bought in the last 10 years and most of them are good to use without needing to ground the secondaries. The majority of what I have at the moment are Tamura and some Avel Lindberg toroidals. I have some old ones from Radio Shack and a couple of surplus units that the jury is still out on. Proper ratings are something I'll definitely be checking for in the future.

[Richard Crowley]

I suggest you take a ohm-meter and connect it between circuit ground and the PE prong of various appliances, e.g. oscilloscope, audio amplifier, function generator, analyzers, computer, printer, ...

Thanks, but I regularly design, build, repair and refurbish mains-powered equipment. I don't need to measure anything, I know how it is designed and constructed.

And if you look at them you will find the required insulation to do that. Bench supplies are effectively mixed class I/II appliances (yes, you can do that). EN/VDE says about that "Geräte, welche teilweise nach Schutzklasse II, jedoch auch teilweise nach Schutzklasse I gebaut sind, werden als Schutzklasse I eingestuft." ("Appliances, which are partly construced as appliance class II, partly as class I, are rated as class I appliances"). Which is why you'll find the class I symbol on your bench supply. Also relevant here is the EN 61010-1, which describes the safety mechanisms. As most bench supplies, as mentioned, are partly class I you can also use a shield winding between primary and floating secondary to avoid double/reinforced insulation. Most supplies will have that winding anyway to cut down on coupling capacitance.

I think those requirements are much more intricately defined and enforced in your part of the planet. There isn't the same obsession in other regions.

btw I don't think OP's question was about a bench supply

Yeah, that is the problem. We don't know what the OP's project is, and he was asking generic questions that have no generic answers.  People generally have a very simplistic notion of shielding and grounding that doesn't fit very well in the Real World.

[rdl]

For some reason this all begins to remind me of lead-free solder. Oh the horror...