Switch mode power supply

[lyledunn]

Why are switch mode power supplies responsible for current in the earth conductor as a function of thier normal operation? Does anyone know of a code that specifies a maximum protective conductor current?

 

[dpc]

Switching power supplies draw current non-linearly and generate harmonics on the supply system. Some of these harmonics (esp 3rd harmonics) are carried on neutral (grounded) conductor.

I'm not sure if you are referring to the neutral conductor or the grounding conductor. The harmonics should flow in the neutral conductor, not the ground conductor. However, if there is a parallel path or some problem in the neutral conductor, then current can flow in the grounding conductor.

 

[electricuwe]

To dpc:
If you are talking about a single phase power supply not only the 3rd harmonic current flows on the neutral conductor but als the fundamental current (where else ?)
The current on the ground conductor is usually caused by RFI-Capacitors included in the power supply.

The 3rd harmoncic currents drawn by switch mode power supplies can be responsible for a voltage on the ground conductor (refered to a real ground) depending on the location where ground and neutral are connected.

 

[dpc]

To electricuwe

Yes, I was referring to three-phase systems. Obviously the neutral must carry all the phase current in a single phase system.

But the question was about the "earth conductor" which I take to mean a grounding conductor separate from the neutral. In either case, there should be very little current flowing in the grounding conductor if the neutral conductor is there.

There could be some RF voltage I suppose, but very little current would flow due to that, I should think.

 

[electricuwe]

To dpc:

The switchmode power supplies usually include small capacitors from each phase or neutral conductor to ground to create a short circuit for the RFI-Voltages generated by the supply. As a side effect these small capacitors carry a few mA of fundamental current.

 

[margaret695]

Just wondering is the earth conductor for the switching power supply connected via the ground prong thingy on the plug which is of course plugged into a receptacle, or are they refering to an equipment ground like sometimes is used in radio station control rooms, or sound studios.

Equipment grounds often go from the various pieces of equipment to a ground bar in a "star" manner that is of course grounded.

Also is this receptacle an isolated grounding type (orange) that has a separate ground wire back to an isolated ground bar in some panel, or is the receptacle grounded via the conduit for the wiring serving the receptacle. If the equipment is uses a 20A 120V 1ph receptacle with a live, neutral and isolated ground then in by my understanding of the Canadian Electrical code, the breaker, live and neutral should match the amperage of the receptacle (taking voltage drop into consideration for the wiring sizes) and the ground should be sized as per code to match the breaker, or overcurrent device.... which in Canada is Table 16 of the electrical code.

If the ground is for the equipment and is run to a ground bar in the room, then there are other considerations I believe, which I don't remember off the top of my head.

Also, by experiance PC computers, which are stuffed with switching power supplies and other weird things that end up with current on the neutral should be circuited so that there are 4 maximum computers on a 20A 120V 1ph with 20A live and neutral, unless they have large screens and hard drives.... in that case 2, or 3 maximum. Oh but also, the neutral can be over sized if more PCs are to be connected to a circuit.

 

[lyledunn]

To Margaret 695

I refer to equipment like computers and business machines of various types. The UK wiring regulations BS7671 requires special precautions to be taken when the total protective conductor ( not the neutral) current exceeds 10ma. There is no limit on the amount of current in that conductor and no limit on the number of machines connected to a circuit other than by normal load constraints.
You mention Canada, are there restrictions on the number of PCs to a circuit because of protective conductor current?

Thanks

Lyledunn

 

[margaret695]

Lyle,

Yes, in Canada PC computers have 15A 120V plugs that are served by 15A 120V receptacles a North American type configuration of live, neutral and ground that are connected to 15A single pole breakers in 120/208V 3ph 4w or sometimes 120/240V 1ph 3w electrical panels.

As per the name plate readings of the majority of office PCs, they usually draw about 300 Watts, or 2.5A at 120V.

In Canada the breakers are sized to protect the receptacles ... hence 15A receptacles are protected by 15A breakers, etc. Note in the US they use 20A 115V receptacles and 20A breakers for most equipment.

So for a standard office layout, I usually connect the receptacles for the PCs at 4 work stations together in parallel and then to the breaker, as breakers for this type of load can be loaded to 80%.

It used to be that PC manufacturers recommended that isolated grounding type receptacles be used for PCs, but they don't seem to require that as much any more, because they claim that there is better protection within the computers now. Note, I still circuit computers with IG receptacles, because the handy orange colour makes instructions to clients a tad simpler.... especially the don't plug your fan into any orange receptacles whatever.

Isolated grounding type receptacles have a separate ground wire for each circuit that runs from the ground slot in the receptacle back to an isolated ground bar in the electrical panel. As per the Canadian Electrical code the wiring for a 15A breaker can be no. 14, but no. 12 minimum is standard for larger projects and no. 10 wiring ends up being used for long runs due to voltage drop.

Any ways for the circuit loading... I have heard some people ..... not engineering type people.... claim that you can connect 6, or 8 PCs to a 15A breaker, but they seem to think that this is acceptable becauses PCs rarely work at their name plate readings... but I have spoken to systems analysts who say that it is for the best to go with the name plate readings.

Also, even if the name plate readings are less the 300W, the switching power suplies and stuff in the PCs will often place current on the neutral, so my understanding of good practice is 4 PCs per circuit, or an over sized neutral.

Margaret

 

[lyledunn]

Margaret695,
Interesting. In the Uk we often use our bespoke 30 amp ring main to serve a number of computers. Unlike Canada, the breaker (32amp MCB) protects the fixed wiring and the fuse in our square pin 13 amp plug top protects the flex connection to the computer. Since our final circuits are at 240volts single phase, we have more flexibility with the amount of load that can be connected to the circuit and less problems with voplt drop.
My original question was more to do with the amount of current in the earth conductor or to give it its proper name, circuit protective conductor, due to SMPS and their associated filtering devices. Does Canada restrict this protective conductor current or insist on special provisions?