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| Grounding boxes in home wiring are totally safe for you? |
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| msuffidy:
I always assumed grounded metal boxes would save you because all the electrons would go down the wire. So I was watching a video about what is ground and it eventually got into the topic of using grounding boxes as safety for persons. It seems to me that it relies on the idea that a wire is supposed to have some degree of resistance as part of the proof. In the video, it sort of suggests the wire segment before the box is a resistor, thus the potential at the box is lower than mains. I am a bit worried about how treating the wire as a resistor has such drastic implications. I am a bit confused about how it actually works in reality, but suspect in that case the potential at the box is in fact lower than the mains potential. I drew this diagram trying to understand it without the idea the wire had resistance and came up with treating the box path as a parallel near 0 ohms resistor. Here is the video: |
| msuffidy:
The answer may be like. 1) The length of the wire from the station is much higher than the grounding side thus the potential is lower at the box 2) The amperage of the near zero resistance ground path may reach the amperage limits of the supply wire and deplete the usually accepted 2nd (human) path supply. 3) You may be briefly shocked but the simultaneous ground path will ensure the trip of the circuit breaker. Actually, theoretically in this scenario this would only happen if you were touching the box when something live hit the inside of the box, or a breaker failed to trip. In the case of the stuck breaker you would probably also be worried about an electrical fire in the wall. |
| Berni:
This is why bad connections on earth wires are very bad. Yes earth wires have a resistance to them like any other wire, so an earthed metal housing will still rise in voltage if live shorts to it. But due to the low resistances of wires and high voltages involved this will send a massive current down the earth wire(100 to 1000A). So within the next milliseconds the GFCI will trip and the fuse for that live wire will trip. This stops the current and the metal housing is at earth potential again. If you ware holding the metal box and something else (like a metal pipe) when such a hard short happened you would likely get a brief jolt, but you would not be dead. But if you did that with no earth connection to the box you would effectively holding a box at 110/220V and the current would flow trough your body until you let go of it. Hopefully you have a GFCI to trip and save you there and you might end up dead |
| msuffidy:
I have been shocked by this and that over the years, but not at the ozzy 240. Just our 120 in Canada. I finished a basement once and did some stuff with the breakers still on without turning them off and either got straight up shocked or like rotated a wall light and it made a big spark. Around 2003 when I was renting, I got a shock when touching some appliances and found out one ot the outlets had something backwards, neutral and hot and fixed it myself. I am going to try to be more careful in the future. |
| Siwastaja:
This "protective earthing for personal safety" actually has well established science and math behind it, but it's old, and in practice never worked too well. Let's start with the terminology, get rid of "ground" as it can mean anything and it's used arbitrarily. When it comes to safety, we have no room for such inaccuracies. Either you have protective earth (PE) or you don't. PE is sacred, mixing it up with anything else is a crime worth of capital punishment. This is a mistake you just can't do. Anyway, the old idea revolves around the short circuit current and fusing. No, it does not depend on wires having resistance, quite the opposite, it relies on the wires having small enough resistance so that the fuse blows in time. The short circuit current is actually measured in new installations as a part of acceptance, and if you are behind a long line, too small short circuit current prevents you having larger fuse sized. The idea is that if you have a short from L to PE, current will be so high that fuse blows in short enough time (tens of milliseconds) to prevent being exposed for the dangerous voltage for too long. Needless to say, low impedance of the PE is paramount. Which is also the Achilles heel of the system. Similarly, poor paths you can do nothing about (like the actual physical earth) pose an issue. But yes, in theory, if you have a properly earthed metal box, and live wire comes loose inside and touches the box, you don't need RCD to save you. If the live wire touches poorly, with a lot of resistance, the resistive divider causes the exposed voltage to be small, and it doesn't matter if it takes longer for the fuse to blow. OTOH, if the live wire gets a good contant, causing significant voltage on the box, then the short circuit current is also large, and fuse blows so fast that even if you are touching the box while this happens, you won't die. But as you can see, this all relies on assumptions about wiring impedances and everything being done exactly right. Now starting in 1990's, RCDs have all but saved this iffy paradigm: now any half-decent PE impedance will do, and live wire resistance also becomes irrelevant as you don't need hundreds of A of fault current to blow the fuse, tens of milliamps is ample to trig the RCD in equally short times which required 1000x more current before it! For this reason, if safety is of concern at all, I strongly recommend you upgrade old systems with RCDs, even if legislation requires them only for new circuits. |
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