Power Supply Common and Earth - Page 1
Forum section: electronics / beginners

author: banedon
author_rank: Regular Contributor
timestamp: March 06, 2015, 08:32:29 pm
content: Hi guysI think I've gotten this right, but just wanted to check with someone who knows more (that'll be most of everyone here!).I'm trying to get the difference between 'common' and 'earth' right in my mind. Here's how I think of them:Common (GND): This is 0V rail from the power supply, is local to the power supply and not "earth". Ideally, this should be isolated from Earth. Represented by a black connector on most bench top power supplies.Earth: This is GND as well, but actually is connected to earth. Represented by a green connector on most bench top power supplies.Also, I've attached a basic power supply circuit which shows how they are connected (as I understand things).

author: Cloud
author_rank: Regular Contributor
timestamp: March 06, 2015, 08:55:15 pm
content: Commom pin can be on a diffrent potencial as the earth.

author: JoeAtl30319
author_rank: Contributor
timestamp: March 07, 2015, 12:40:17 am
content: It sounds like you have it pretty straight. I'm not sure why common would ideally be isolated from earth though, I guess it really depends on the application. In a home electrical system, neutral is tied directly to earth ground in the circuit breaker box (at least in the USA).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2015, 02:52:36 am
content: Ground is "protective earth". Its purpose is so that, if a conductor (especially a live mains conductor) should come loose inside the equipment (or outside of -- even for the wiring in the walls, in the case of metallic conduit or metal studs), it is extremely likely to touch metallic ground well before anyone can get a finger near enough to receive a shock.Shunting the AC line therefore serves to blow the fuse in the equipment, de-energizing it and making it safe (both against shocks and starting fires -- hopefully).Internal circuits don't need to be grounded, but it can help.Example: a desktop CPU is built inside a grounded enclosure. The SELV DC outputs (+12, +5, etc.) are grounded. Why? If a conductor comes loose and contacts the case, instead of potentially applying -12V (or whatever) to everything else in the circuit, it shorts out the power supply. (Which is typically designed to shut down safely in this event, but could be a fused type instead, for the same operation as the mains case.) The voltage is not a hazard (SELV is under 50V), but the available power could be (>20A through a single wire, or a few strands of one, could cause an internal fire I suppose).One case that is required: draining ESD. Suppose someone walks up to the equipment, it's a cold, dry winter's day, and there is carpet in the office. Zap. Where does the static charge go? It generally has to be sunk into a safety-grounded circuit, sooner or later. Otherwise dangerous voltages could accumulate on human-touchable surfaces, or internal breakdown could occur. There are different methods, such as direct grounding, capacitive grounding (Ethernet connectors are a good example -- a 1nF 1kV capacitor provides sufficient rating and capacity to absorb the charge in the spark, without having to DC-ground the connection), or some combination of R, C, D and other components (often, an R||C is used to provide RF grounding and DC leakage, without inviting low frequency or DC ground loop problems; or a diode, TVS, MOV or other limiting device is used to clamp large excursions while drawing even less DC current).I don't remember exactly what all the agencies say about it (NEC, NFPA, UL, IEC... plus myriad state and local agencies around the world!), but many of these strategies are preferred over none at all.Further, there may be technical reasons why you can't use one of these methods. Test equipment, specifically intended for isolated use at high voltages, obviously can't use any of these (at least, nothing worse than a small capacitor and high value resistor). I don't know if there's an official way to handle that, if you need to label it as such, or what. Medical equipment cannot tolerate any leakage, so they can't use an R||C or C; instead, such equipment must tolerate the full 8kV (or more) of ESD, directly!Tim

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 07, 2015, 03:15:35 am
content: Quote from: banedon on March 06, 2015, 08:32:29 pmYes, those are all generally true statements.QuoteThere is no way you can make this statement without looking at the bigger picture (not revealed in your question)Shielding and Grounding are a "gray art" at best. It is very difficult to make a general statement like that when there are so many variations in specific applications.I am afraid you are seeing only the tip of the iceberg when it comes to shielding and grounding.If you want to ask specific questions about some particular project, many people here will be able to help you work through the details and begin to learn how to analyze the circuit and entire system it operates within.

author: jeroen79
author_rank: Frequent Contributor
timestamp: March 07, 2015, 03:46:57 am
content: Quote from: Richard Crowley on March 07, 2015, 03:15:35 amIf it depends on the bigger picture whether 'common' and 'earth' should be connected or isolated then wouldn't it be wise if that choice is not made by the power supply but by the circuit that uses it?(at least in the case of a 'general purpose' power supply)

author: banedon
author_rank: Regular Contributor
timestamp: March 07, 2015, 10:08:36 am
content: Thanks for the explanations, guys.This was a general question as I have a little understanding of digital electronics now (nowhere near professional, though), but am hazy to unknowledgable when it comes to AC. I thought I'd start with how power works, checking out how I *think* things work.