Power circuit grounding

[cyberblak]

Hi,

We designed a SCR driver for variable speed AC motor (split-phase AC fan motors). This board includes some control and thus uses a 24V power supply. The ground of this supply is linked to the ground plane of the control circuitry.

My question is : should I link this ground to the Earth conductor of the cable that brings the 120V/240V to the board, should I leave the supply floating in relation to the Earth, should I attach it with a capacitor, or any other way?

What is good practice in such a case?

Thanks,

cyberblak

 

[IanP]

My views on this subject can be summarized by the following quote:

Grounding


In 3-phase, 480V applications, large inverters, drives, and motors can introduce significant distortion and interference on the power lines. Using a single grounding system can, and will, introduce unwanted problems. Depending on the application, system integrators choose between a single ground point that ties all AC and DC voltages together, or separate grounds for the AC and DC voltages. In creating a common ground, engineers typically connect the incoming AC voltage ground with the negative leg of the DC voltage (see GND in Fig. 2, on page 33).

Of course, this common ground compromises the isolation between the AC and regulated DC and ultimately negates the requirement of an isolating transformer. Because a power supply that uses transformer isolation achieves single (and even double) isolation, the common-ground approach comes further into question.

By using the right circuit layout and components that feature touch-proof connections, system designers can create insulating boundaries between different voltages. These boundaries maintain the integrity of the power supply and all of the devices connected to its DC voltage output. In short, it's possible to design a safe system with true isolation between AC and DC and two separate grounding systems.

All DC power supplies offer a plus (+) and minus (-) output connection that is isolated from the AC input. With the DC circuit, a common plus or minus connection is possible using touch-proof, DIN-rail terminal blocks that allow voltage isolation from the DIN-rail itself. By keeping different voltages physically and visually separated, the need to keep a common ground is no longer important. The physical distance between the various voltages negates the possibility of creating a dangerous situation, unless it's intentional.

Because 24VDC would normally power analog 4mA to 20mA loops, or 24VDC relays, contactors, and proximity sensors, it's important to make sure that all metal casings and shields are tied to ground (unless otherwise specified). However, the AC ground should in no way be connected to either the positive or negative connection of the 24VDC power supply.

Regards,
IanP

 

[cyberblak]

Ok... Could you precise what you call the AC ground? Our AC inputs includes three conductors: the line (usally black), the neutral (usually white) and the earth (green or bare conductor). Of course I can't connect the neutral to the DC- terminal, if that's what you mean, but should I connect the DC- (or the ground reference of my circuit, if you prefer) to the earth conductor, that is itself connected to the enclosure?

Thanks,

cyberblak

 

[IanP]

By AC ground I mean the earth wire (yellow/green or green).
I wouldn't connect earth to 0Vdc. Have a look at the drawing below ..
The electronics is not referenced to the case/earth.
In some cases, specially in audio/video equipment, you can find a 1MΩ - 4.7MΩ resistor connected between 0Vdc and chassis..
Regards,
IanP