Should the power supply float or be grounded?

[imsmooth]

I have read pros and cons for both, but would like to hear what people have to say. I can build a simple power supply for a project that supplies 15v that is isolated from earth ground by using the output from a step-down transformer. I can also put one together that shares earth ground.

Which do people use for home projects and which do people use if its supplies power to some device used commercially or out in the field.

 

[Baluncore]

You need to prevent insulation breakdown, between the primary and secondary windings of the transformer. That suggests a common ground.

If a common ground is a problem, use a metal-oxide varistor, or a gas discharge suppressor, between the two ground systems. That will protect the transformer and you from static discharge.

 

[DaveE]

Circuits are normally grounded somehow, somewhere. As @Baluncore said, avoiding HV from leakage currents is one common reason. But multiple grounds can be a problem, if they aren't actually at the same potential. This is one reason people like isolated PSs. Then they can choose their own ground configuration.

In practice it can get complicated and depends on your performance requirements and the environment. Often things will work anyway, even if it's not perfect. To really answer this we would need much more information.

 

[Babadag]

I agree with Baluncore.
In U.S.A the law it is NEC
NEC 2020
250.20 Alternating-Current Systems to Be Grounded
(A) Alternating-Current Systems of Less Than 50 Volts.
Alternating-current systems of less than 50 volts shall be grounded
under any of the following conditions:
(1) Where supplied by transformers, if the transformer
supply system exceeds 150 volts to ground
supply system is ungrounded
(2) Where supplied by transformers, if the transformer
supply system is ungrounded
(3) Where installed outside as overhead conductors

 

[Babadag]

BS 7671 is more complicate .See for instance BS7671/2008

 

[imsmooth]

Thanks for the answers so far. My question about power supplies was just a general question, but here is a more specific one.

Let's suppose I was going to produce a commercial induction heater. (I'm not if anyone really wants to know). In it the coil and tank capacitor are not grounded - they are floating. The tank is coupled by a transformer to the inverter system.

The casing of the unit would be grounded. Would I have to have an earth ground to the LC tank? If I did then someone would complete the circuit by touching the work coil and make a path to ground. If it's isolated then there is no chance for shock unless the person grabs two ends with both hands, which would be strange thing for someone to do.

 

[Baluncore]

The casing of the unit would be grounded. Would I have to have an earth ground to the LC tank? If I did then someone would complete the circuit by touching the work coil and make a path to ground. If it's isolated then there is no chance for shock unless the person grabs two ends with both hands, which would be strange thing for someone to do.

To comply with the rules, and to be safe, you must ground the secondary somehow.
You must protect the transformer insulation from static leakage or short circuits.

 

[DaveE]

There are ways to meet the safety standards with a floating secondary. This would be similar to an AC powered "double insulated" drill with a two prong plug, or your isolated power supply, for example. But, it's a pain in the #$$ and costs money. It probably wouldn't actually be done compared to grounding it. I won't go into all of the methods/requirements here, there are a few different implementation choices. Ask if you want more info/ references.

 

[Babadag]

If the supply voltage of the transformer - before inverter- it is more than 150 V with respect to the ground, or if the transformer secondary is not grounded, you have to ground the tank. However, I agree with Baluncore , in any case, it is better to ground the tank.

 

[TonyStewart]

The choice of power supply floating or grounded depends entirely on the design specs, which includes requirements for safety and EMI (ingress and egress)

PC's with low voltage have sensitivity and susceptibility to EMI and many SMPS regulators in PSU and on MOBO around CPU must be earth bonded and well filtered for CM noise suppression.

Many peripherals with wall-warts are not earth-bonded and use isolated supplies may rely on earth bond connections to the host. (e.g. video)

Isolated ungrounded SMPS are notorious for RF leakage issues to secondary. e.g.USB peripherals with floating power and PC towers with earth bonded may fail to communicate on USB while laptops without charger pass.