| Electronics > Beginners |
| My first scope is arriving but my home (old) doesn't have mains grounding |
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| Electro Detective:
As stated earlier, ignoring the onboard TEST button for a moment, unless an RCD can be verified with an external tester to simulate a fault, and the RCD/GFCI performs reliably with consistency as per it's time/current specs (and not just a couple of 'lucky' trips :clap:) then OP Fergo is wasting money on a false sense of security buying into an RCD in a situation where it may/might/won't/ work, or all three together :horse: As it is, even with an RCD in place in a properly grounded system, if I'm playing 'risky business' with electronicals there is my own portable fast trip 10>15ma RCD, or RCD/MCB combo close by, so Murphy can stay on holidays and not bother me with cheap travel offers to hot places >:D It may be two RCDs to reset if a larger fault trips the main RCD too, but hey, still better value than post zap hospital therapy or zapping the relatives with a funeral bill FWIW, you only have to spend maybe one weekend to assemble all the generic bits needed get the safety thing right ONCE, to save you and your gear MANY MANY times. It's actually fun doing it, testing the lot just to see and know what the outcomes and limitations are, and realize after a few trips and sort of controlled BANGS! :-[ or pending snafus that it's not so predictable as you thought to make a system idiot proof. Cheap fast blow inline fuses, even no name knockoff/fakes? (pre-test a couple first!) rated just above the expected current draw, will save you LOTS of money on blunders that are guaranteed to come knocking. I've got an assortment from 100ma up to 10amps, and the best $20 investment I spent years ago. If it saves even one cheap multimeter or project or repair, you just got $20 back and the rest :-+ An isolation transformer (or three.. :palm:) with switchable modes of earth/ground isolation is a great tool, but not for those unfamiliar with those modes. Still, a proper wired 'medical' style isolation transformer is a good bet for 'some' isolation and interference/noise from the mains, even though the earth/ground and neutral output are bonded internally. I use those whenever I can as they will only pump out whatever power their limit is, and some expensive types will regulate and current limit on an overload or dead short. But all that stuff is sort of useless unless you've simple PPE eyewear/face shield, gloves and rubber soles, A bucket of sand, fire blanket and fire extinguisher wouldn't hurt either and won't break your pocket I keep all that stuff in one flaky carton box, which saves on looking around for the bits, and doubles as a traveller too Stay safe and best of luck to OP with the new scope :-+ |
| Mechatrommer:
--- Quote from: Electro Detective on August 10, 2019, 09:17:05 am ---RCD/GFCI needs an Earth/ground reference to work, --- End quote --- no... --- Quote from: Electro Detective on August 10, 2019, 09:17:05 am ---an imbalance between the Live 1 and Live 2 conductors (Active and Neutral) during a fault won't do anything after passing through the internal sensing coil --- End quote --- yes if you buy one hung low brand... --- Quote from: Electro Detective on August 14, 2019, 11:14:02 pm ---As stated earlier, ignoring the onboard TEST button for a moment, unless an RCD can be verified with an external tester to simulate a fault, and the RCD/GFCI performs reliably with consistency as per it's time/current specs (and not just a couple of 'lucky' trips :clap:) --- End quote --- have you encountered a properly brand working and installed RCD (2 wires LN in LN out), that wont trip during imbalance? regardless of ground line existence or not? afaik it will trip even if current lost in the air.. read under the "Purpose and operation" https://en.wikipedia.org/wiki/Residual-current_device --- Quote ---Operation does not require a fault current to return through the earth wire in the installation; the trip will operate just as well if the return path is through plumbing, contact with the ground or any other current path. Automatic disconnection and a measure of shock protection is therefore still provided even if the earth wiring of the installation is damaged or incomplete. --- End quote --- the only time you get a shock when current through you from live back to neutral, so no imbalance inside RCD. hence dont use both hand.. |
| forrestc:
--- Quote from: Mechatrommer on August 15, 2019, 01:21:51 am --- --- Quote from: Electro Detective on August 10, 2019, 09:17:05 am ---RCD/GFCI needs an Earth/ground reference to work, --- End quote --- no... --- End quote --- I'll second this. In the USA in particular it is perfectly acceptable to replace a 2 wire outlet with a 3 wire GFCI. The principle here is that a 3 wire GFCI provides more protection than a 2 wire outlet would provide. The only protection you lose in the US system by doing this is not providing a path for leakage currents/shorts to find their way to ground. With a good ground, a loose wire touching the grounded case will result in a trip of the circuit breaker or GFCI. Without the ground, the path will be potentially through a human to the ground, a path which will cause the GFCI to trip. 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. |
| Ian.M:
--- 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 --- There is a small error in your statement: A GFCI does *NOT* require a ground at its point of installation, but as I said before, it does require its supply to be ground referenced. Nearly all utility company mains supplies in the world are ground referenced. Exception: some parts of Norway and Finland, where due to low ground conductivity (typically due to massive igneous rock), older homes may be on an IT system (i.e. floating) supply. A GFCI wont work on a fully floating supply as it works by using a balanced current transformer to detect any imbalance between the current in the two (or more) live supply conductors to determine if there is a significant leakage current to ground, and, considering a two conductor GFCI directly fed from an isolating transformer secondary that is not connected to ground, unless the secondary itself is faulty, there is no way there can be an imbalance as the current in the wires to the two ends of the winding must be equal and opposite, no matter what fault is introduced on the load side of the GFCI, so it will never trip on any load fault. 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. 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. |
| Mechatrommer:
--- 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. |
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