Mains earth - Page 1
Forum section: electronics / beginners

author: Hansemann
author_rank: Newbie
timestamp: February 18, 2015, 05:23:47 am
content: Hi,I was wondering about getting a oscilloscope, but one thing may be a problem of using it for me and that is that the room i have available to work with it does not have mains earthed sockets. Is this something i should worry about when using the oscilloscope. Another thing is will it introduce readerrors on the scope etc.?Thanks in advance for help!

author: electr_peter
author_rank: Unknown
timestamp: February 18, 2015, 05:26:49 pm
content: Quote from: Hansemann on February 18, 2015, 05:23:47 amDo you do some work with inadequate equipment? If so, don't work without ground and demand better conditions. If it is for hobby stuff, install proper ground in the room and then operate the scope.I would suggest at very least adding GFCI/RCD protected socket for a scope.In any case, I suggest you do not work with ungrounded oscilloscope, because you seem to not understand and appreciate potential dangers.Oscilloscope is supposed to be connected to mains earth because 1) oscilloscope itself is powered from mains and 2) DUT may be mains referenced. It is mostly a matter of safety. Scope power supply is negatively affected as well.Signal integrity wise, scope will pick up more mains hum and could potentially damage some equipment on connection (because of floating voltage).See some references about scope floating from ground:http://www.tek.com/document/technical-brief/floating-oscilloscope-measurements-and-operator-protectionhttp://www.testequity.com/documents/pdf/floating-measurements-fundamentals.pdf

author: Paul Price
author_rank: Super Contributor
timestamp: February 18, 2015, 10:02:53 pm
content: Grounds can as dangerous as no grounds.https://www.eevblog.com/forum/beginners/oscilloscope-on-2-wire-house-electric/msg607622/#msg607622

author: lowimpedance
author_rank: Super Contributor
timestamp: February 18, 2015, 10:35:32 pm
content: Also most scopes actually have a note on them or in the documentation telling you not to operate without a mains earth. Portable battery powered scopes excepted of course.So a battery powered type is all you should use until you have a proper mains earth installed.

author: cosmicray
author_rank: Frequent Contributor
timestamp: February 19, 2015, 04:08:22 am
content: Quote from: lowimpedance on February 18, 2015, 10:35:32 pmSo what would be the recommended method if you wanted to operate a scope off of a generator or inverter, drive a spike in the ground and connect it to that ?

author: Hansemann
author_rank: Newbie
timestamp: February 19, 2015, 05:30:37 am
content: Ok, am a relatively rusty automation technician since i changed trade like 20y ago. I dont really understand alot of the tech stuff i read in a another tread relating to this subject of if i should use ground or not, but here is the thing.I have just bought used this analog LEADER LBO 524 Oscilloscope and when i measure every bit of metal on the chassis including the groundpart of the BNC connectors against the earthwire on the mainslead on the scope, there is infinite resistance. No conn whatsoever. I opened it up and everything looks untouched. So looking at this video below i dont understand whats going on. My scope is a 230V one and its delivered and bought in Norway where i live. Is it correct what i measure? Also it looks like that there is danger in using mains earthed instruments too according to this video.

author: mswhin63
author_rank: Frequent Contributor
timestamp: February 19, 2015, 06:19:01 am
content: Quote from: Hansemann on February 19, 2015, 05:30:37 amAs mentioned in the video at the beginning most (not all) scope are connected to ground. In the dark ages there was a lot of power systems that required testing using isolated power. Some scopes, very few have connected earths but would be in todays terms quite dangerous. The other , is that the seller has disconnected the earth from the scope so they could carry out their tests they wanted. It is not a recommended practice. I would go through you scope around the mains section very closely and check to see if "power cord is missing and earth" or some other small non obvious earth disconnection has occurred.When I was in my apprenticeship my boss showed me to modify a soldering iron so that I could continue to work on live equipment while servicing. I don't practice this anymore, instead switch off the equipment.The most common method and safer option is the use of an isolation transformer.

author: Richard Head
author_rank: Frequent Contributor
timestamp: February 19, 2015, 06:20:47 am
content: So what would be the recommended method if you wanted to operate a scope off of a generator or inverter, drive a spike in the ground and connect it to that ?You don't have to drive a spike into the ground. Just ensure that the chassis of the generator is connected to the earth connection of the house. It's not so much about earthing as it is about equi-potentialisation.

author: cosmicray
author_rank: Frequent Contributor
timestamp: February 19, 2015, 07:38:26 pm
content: Quote from: Richard Head on February 19, 2015, 06:20:47 amI guess my question was incomplete. How would you ground the generator if it is operating in a remote location (no house or building mains handy) ? Generators usually have a lug on the front for grounding to something.

author: electr_peter
author_rank: Unknown
timestamp: February 19, 2015, 07:51:37 pm
content: In general (but check with local electric installation laws and requirements), generator should be connected to house/building earth point and additional earthing should not be used. If generator is standalone, local grounding should be implemented if generator is providing power to more than one device.Adding generator can be tricky in terms of proper connections - check all the circuit schematics, measure cables and switches. The problem can arises when two possible sources of mains power are present and switches do not completely isolate them.

author: Richard Head
author_rank: Frequent Contributor
timestamp: February 20, 2015, 05:29:54 am
content: CosmicrayIf you are using the generator outdoors in the bush then an earth spike is probably your best option. Ensuring all metal cased appliances are earthed to the generator chassis through the earth wire. If the appliance is double insulated then obviously no earth is required. There's also no reason an GFCI/RCD/earth leakage unit cannot be used on a generator output for added protection.