Is safety ground different from battery ground? - Page 1
Forum section: electronics / beginners

author: geratheg
author_rank: Regular Contributor
timestamp: August 05, 2014, 05:56:02 am
content: Is Earth ground different from battery ground used in circuits?I'm curious about the ground application used for safety, such as the round plug in a 3 prong power plug.I learned that the ground is supposed to collect any excess electrons and could be connected all over the device, including its frame to protect the users.But since electrons flow from negative to positive, then would this ground be considered positive so that it can collect electrons. Or is ground neutral in this case meaning it will still collect excess electrons, but it's so big it would remain neutral.I always thought ground as a source of which electrons come from such as a battery's negative, not a source of which electrons go into.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 05, 2014, 06:51:57 am
content: Safety ground only exists/makes sense in the context of AC power distribution in your house. It is essentially a backup "neutral" conductor that normally carries no (or very little) current. It is designed such that if there is a wiring fault in a piece of equipment and the "hot" wire touches the case, that will be conducted back to the electrical panel through the safety ground, and cause the breaker to flip, rather than leave the case 'hot'. Remember that current always flows in closed loops/circuits. If you connect one terminal of a battery to safety earth, nothing happens because there is no loop until you connect the other terminal somewhere.As a side note, don't think about electrons. Despite the name they are basically irrelevant for electronics. What you need to understand are current and potential/voltage. Voltage is also relative. Absolute voltage has no physical meaning, all that matters is potential difference. A 9V battery is so called because the potential difference between the two terminals is 9V -- that isn't a statement about some absolute voltage at either terminal. And remember, current only flows in loops.

author: geratheg
author_rank: Regular Contributor
timestamp: August 05, 2014, 06:57:52 am
content: Ok thank you for the response.I think I confused myself because I thought that every device that is grounded means you won't get static shocks. Yeah I know.... lol.Then thought about my question and realized you can get static shocks even if it's grounded.But what you will be protected from is that electrical shock of current flowing through you regardless of which direction the current is flowing, thus electrons can flow either way and you will be protected if it is connected to ground since they are most likely to go through the path of lowest resistance.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 05, 2014, 02:02:29 pm
content: EEs have for many years been taught "Conventional Current Flow",& have a bad reaction to the very thought of "Electron Flow".That said,ejeffreyis right-----most of the time,electrons (or holes,or whatever) don't come into it!Inside active devices,like Vacuum Tubes,BJTs,& FETs charge carriers are important,& help in understanding of their operation,but most of the time,current can just be regarded as a thing in its own right,without delving intohowit works!For instance Ohms Law works with Voltage,Resistance & Current as "things".

author: JonnyBoats
author_rank: Regular Contributor
timestamp: August 05, 2014, 02:19:31 pm
content: QuoteForgetting about which direction things flow for a moment, it is important to understand that there can be several kinds of "ground" in electronics. For example a DC ground is often different from an RF ground on circuit boards.The key thing that differentiates a safety ground is that in normal operation no current should be flowing through the safety ground.

author: retrolefty
author_rank: Super Contributor
timestamp: August 05, 2014, 02:27:47 pm
content: QuoteA battery doesn't have a 'ground' terminal. It can be wired into a circuit to supply either +9vdc or -9vdc depending on which battery terminal is wired to the circuit 'common' potential point.

author: Wilksey
author_rank: Super Contributor
timestamp: August 05, 2014, 02:38:36 pm
content: 0V or -9V or whatever you want to call it is the return path for the current in the circuit, safety ground, I guess is the same as what I know it as protective earth is what I would typically use if I am putting equipment onto a plate or into a metal enclosure, it earths the chassis so if the live came off or in the event of a lightning strike the quickest way to ground is through the protective earth bond rather than through the circuit.I am not 100% I am talking about the same thing, but on the off chance I am, that is my understanding.