I was looking at a set of electrical prints and I saw something that caught my eye. I saw a DC power supply that was shown with a ground jumper to earth ground.

This system has VFDs and I wonder if this is the best ground practice for this case noise is going to ground and could get back on the DC side since it's bonded. I personally have never grounded my DC common to earth ground. What does everyone else do?

I usually always bond the DC power supply 0V (or -) terminal to ground. I've come across too many field devices that are grounded internally, so I head off any problems by bonding everything at the control panel.

Some are grounded internally anyway

Usually use switchmodes 24VDC/24VDC or 240VAC/24VDC and have never grouded the secondary 0V. I do ground the case though - essential in case there is a big bang.

I rarely ground the 0VDC because you loose your galvanic insulation from 240 to 24V. The earthing of 24VDC (either + or -) comes from the electronics and automtive guys. In electronic schematics the - is typically grounded and cars put either the + or the - to the frame to reduce wiring. If I put the 0VDC to ground I do this using a frame/signal ground (separated from the power cable earthing)

As a lot depends on the surrounding machinery and wiring, there is no black and white rule here.

The sometimes wrongly called protective "ground" (instead of "earth") is an additional source of confusion. This is really a safety net which should always be there and electrically have nothing to do with any machinery function. However as it surrounds all electrical equipment it is capacitively coupled to everything and so can be part of noise currents like the high frequency one generated by inverters. An additional problem is when the plant neutral and earth are used as one, typically in old factories.

So, ideally one should not ground any isolated power supply potentials to maintain isolation and protection BUT by connecting to the earth one may divert noise currents from affecting susceptible devices.

The most handy tool when investigating problems like this is a 100 - 1000 Ohm resistor. By connecting it between a supply potential and earth one can decide if it helps or not. The 100-1000 Ohms are relatively low for noise currents to flow and high enough to prevent destructive ground loops flowing via an earthing connection elsewhere in the plant such as from grounded equipment shields.

Sometime ago such a resistor saved the day and a few red faces in a plastics factory where static was a big problem.

interesting idea... I'll keep it in mind...

I think it's a bad idea. Let's consider why...

Regardless of the power supply design, the front end may or may not have a transformer. The next stage will be a half or full wave rectifier (diode bridge). At this point, the output rectified AC is referenced to the neutral of the incoming power. Transformers are frequently not used with switching power supplies because they mostly add weight, cost, and heat. They are much more popular in linear power supplies because it's one of the few ways that you can get closer to a theoretical power efficiency of 25% (vs. 100% in theory for switching supplies). The transformer is almost always the #1 contributor of heat in a linear power supply.

The next stage is the power regulation circuitry. Regardless of whether it's a charge pump (switching) or linear design, the resulting power on the DC side is still referenced to neutral.

Unless the secondary side of the AC transformer isn't tied to anything (and it is invariably tied to the "ground" or sometimes "neutral", just as with your distribution system) in a linear power supply, this means that "neutral" on the AC side is directly coupled to the negative output on the DC side.

Most plants have common grounds, not distributed, because of ground loop concerns. So they tied all grounds back to a common point at the transformer and feed, as per Code. So by tying the DC negative terminal to the AC ground, you are creating a potential for ground loops and violating Code. Tying it to the neutral is redundant but won't hurt anything.

Is the 25% number a typo? they've gotta be more efficient than that. That would mean that a 100W linear power supply would be creating 300W in heat?!

There are several statements that I don't agree with.



only true if that front end was without transformer (or if transforer was an auto-transformer for example)



that is correct for non-isolated devices and most simple DC/DC converters but I would have to see yet industrial general purpose power supply to be non-isolated. See picture below, there is bunch of switching power supplies (some with open frame for clear view). I've pointed arrows at the transformers.




weight of transformer in switching power supply is tiny... so is the cost and the dissipated heat.



i have no idea what is this supposed to be...



this is true for basic reglator (DC/DC converter) but this is not how commercial power supplies work.
they use transformer instead of coil. transformer is small because they run at frequency much higher
than line 50/60Hz so the core is not soft iron anymore, it's ferrite of course. (see green arrows)

This is how basic stages are arranged:

linear power supply:
1. transformer (very large due low frequency such as 50-60Hz)
2. low voltage rectifier + filter (very large capacitors)
3. regulator

switching power supply:
1. high voltage rectifier + filter (medium size capacitors - low capacity but but for high voltage)
2. switching circuit (driver, oscillator and regulator, freq. some 1000 times higher than line f)
3. transformer (very compact due high frequency)
4. low voltage rectifier + filter (small capacitors due high frequency)

here is an example of switching power supply (i didn't find one for 24V but they are all very similar).
note that it also uses transformer and that AC and DC sides are isolated...

Also 100% efficiency of an switching power supply is a myth.
Standard switching power supplies have a power factor of about 60% due to the fact that they use a rectifier at the input. The better ones have a power factor correction and this takes it up to 85-95%. This PF is due to the fact that higher order harmonics are important even while the cos phi stays close to 1 (only first order harmonic) In high power applications like inverters they go over to 12/18 pulse rectifiers and DC reactors to improve that power factor.

If you take Input-output insulation you get somthing like 100Mohm resistance and 3KVAC dielectric strength (example from S8VS from Omron)

I don't do it because 90 % of my machines that I build are mig welding apps. They tend to burn the sensor wires and often give false inputs if they short to ground.

Those of you building UL 508A panels may be interested to read section 16.1A which requires grounding of power supply output under certain conditions. Most especially those ps with 3ph 480v inputs. This change was effective Mar. 1, 2007.

I don;t have this standard as I don;t export or doing any work for the states



But are they referring to power supply tranformers AC rather then say a low voltage 24VDC output switch mode power supply. Just curious..

My guess (I am completely unfamiliar with UL standards) is that this is to prevent primary-to-secondary fault current not flowing to protective earth but escaping outside the supply enclosure.

On this part of the world, a double insulation transformer with physically separate (ie each winding on its own section, separated by a barrier) primary and secondary is usually specified for such a case. It is also a requirement in consumer 230 VAC devices such as small battery chargers and supplies. Depending on "duty", the specs require a minimum insulation strength and primary to secondary "creepage" (words in "" per IEC terminology).

Wombat...

The spec covers "power transformer, control transformer, or power supply" which I and my local inspector interpret as covering a 24vdc power supply. Text in the code doesn't distinguish between ac and dc. I'd post the entire text but for copyright.

If possible, I always ground the common (-) of my 24VDC general purpose power supply. I do this for safety circuits. If the cable gets damaged there is a better chance that the +24VDC and blow a fuse or trip the safety circuit. The other reason is for noise imunity. If you have large motors, VFDs or Servos a lot of electrical noise can be induced into long cables expecially related to light curtains. These transients can reduce your 24VDC preventing relays and some solid state devices from operating. In this case you may need to ground your negative in multiple locations.

Giz

sounds like tipical wrong setup. in properly designed circuit ground faults don't produce false input, uncotrolled motion etc.