Electronics > Beginners

My first scope is arriving but my home (old) doesn't have mains grounding

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Electro Detective:
Believe what you will, flog the math and have faith,
but if an RCD/GFCI does not respond reliably and repeatedly to an external simulated fault test, and at 180 degrees,
good luck.

Then there's fault tripping and SMPS noise issues to contend with.

If the power system in OPs case is suss, playing with non isolated input oscilloscope and other single ended test gear,

he/she needs to be 100% sure a failsafe system in place works 

i.e. you rely on an RCD/GFCI to bail you out, it works for a while (apparently... :-//)  and one day it either fails to work, or was never going to work, 
and or the system earthing develops issues over time, or some electricity maintenance friday helper twat that works for beer disconnects it,
and the RCD faither may end up in the ground, with the 'unfortunate event' passed off as 'misfortune' or 'bad luck' 

An RCD is still better than nothing,
but nothing with extreme caution is good too, especially when one solo RCD won't work when you need it to

FWIW, even the best RCD will just sit there and look on  :popcorn: while the owner cooks on a live chassis with no working earth ground



Mechatrommer:
how about having an RCD, and be extremely careful? well i can think one safest foolproof way, be in a cave away from electricity. my wife is wiser to stay away from all this shit. remember, all this taboo is already there since the OP dwell that apartment, regardless if he buy a scope or not. unearthed refrigerator, unearthed tv, unearthed hair drier unearthed water heater unearthed everything, or the unreliable RCD thereof.

Jeroen3:

--- Quote from: Mechatrommer on August 15, 2019, 09:15:22 am ---
--- Quote from: Ian.M on August 15, 2019, 06:42:22 am ---A GFCI's built-in test button introduces an imbalance by connecting a resistor diagonally across the GFCI terminals, from one conductor on the supply side to the opposite one on the load side, so the test resistor current only flows in one conductor passing through the GFCI.   That will trip just that GFCI even if its fed from a floating supply that its otherwise incapable of tripping on.

--- End quote ---
but if one touch either wire downstream of the floating supply, the current will leak to wherever the person is standing to, as long as it is not returned to the other line, it will create current imbalance in the GFCI.

--- End quote ---
No significant current will flow aside from the capacitive leakage in the transformer.


--- Quote from: Ian.M on August 15, 2019, 06:42:22 am ---...
If the test button introduced a resistor to ground, it could potentially trip all GFCIs upstream of it and which one(s) actually tripped would depend on their relative speed and trip time vs current curves.

--- End quote ---
Considering some RCD's run amps of current trough their test resistor when their rating is only 30mA, yes, that would be a problem.

ArthurDent:
There is only one correct solution to solving this problem and that is to use a proper ground (earth) from the building’s A.C. power source that meets the current local code. Where this doesn’t appear to be immediately possible, anything done will be a kludge to one degree or another. What I would consider doing is constructing your own 3 wire A.C. supply completely isolated from the building’s mains supply and require no changes or additional connections to the building’s A.C. supply.  Solar would work but a complete solar system with storage and conversion is totally out of the question and impractical.

Here is a possible solution for you to discuss and critique that is a little different than some of the other suggestions above. What I think might work, if you’re just going to power your own little lab, is to buy an isolation transformer with enough capacity to power everything you could connect to the mains supply and then wire the secondary as you would a properly grounded 3 wire A.C. line from the power company. At the transformer secondary one lead would be the high side line and the other secondary lead, right at the transformer, would be the low and also have a ground or PE wire which would be a non-current carrying conductor, just like a properly grounded 3 wire A.C. supply. These wires would be treated just like a regular 3 wire A.C. line with a correctly rated breaker, GFCI/RCD, and outlets on your bench. Because there will be some capacitance coupling you might want to consider three about 330K ohm resistors in series from the core and the shield between the primary and secondary windings to the low primary lead to drain any small leakage - see drawing.

This should work o.k. especially if your lab is located where there are no earth referenced conductors coming in from the outside like phone lines, tv cable, or water pipes. The idea is to create what amounts to a space station or clean room type environment that is galvanically isolated from the outside so there is no possible accidental connection to outside earth yet you still have a local 3 wire system for your bench that will allow safety circuits to be used normally.

Fergo:

--- Quote from: forrestc on August 15, 2019, 05:21:32 am ---I don't know what the OP's national electrical system looks like, so I can't be 100% sure this is the same in their country.   But I can attest that a GFCI absolutely does not require a ground to work.

--- End quote ---

Here's is a schematic of a distribution box in accordance to our code (or as the author said so...), which is mandatory since 1997. My building is from the 70s, so it's nothing like this:


In this case, it's a 2-phase system (220V). The red device is an SPD. The big one in the middle is an RCD/GFCI. The other ones are regular breakers (2-phase input).

This is an English speaking forum, but if eventually someone is interested, here is our current building code for low voltage installations:
http://universidadeniltonlins.com.br/wp-content/uploads/2019/04/NBR-5410.pdf


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