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| Grounding boxes in home wiring are totally safe for you? |
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| Siwastaja:
Do remember that even a 200mA RCD is a huge improvement over no RCD at all. It is not officially for protecting from shock, but if you compare it to only relying on short circuit currents to protect from shock, it's still an improvement. |
| jonpaul:
Hello: A few points: Commercial and industrial equipment is tested for resistance from chassis to mains cable earth at plug, with some hy-pot and grounds resistance testers. UL, TUV, VDE, ETL etc all require the resistance testing at high currents eg 25 A. Depending on the regulatory environment and equipment rating the acceptable resistance varies. In power distribution and transmission, every tower, pole , protector and transformer has an earch ground via rods or a grid. A major risk it dry soli so the resistance increases. IEEE and ERPI research proves correlation between lightning damage and ground circuit resistance. In RF engineering I learned long ago the proverb: "There is NO such thing as a ground" Bon chance Jon |
| KaneTW:
Yeah, you'd have to use a 300mA Type-B RCD. But does this provide an actual increase in safety in a properly designed system? It's not enough to protect a person from shock, so it's only useful as fire protection. For non-critical use cases I don't think it's worth it (and the VDE agrees), as it'd require an extremely low probability PE failure to trigger the RCD but not the circuit breaker. |
| Berni:
--- Quote from: KaneTW on January 25, 2022, 10:43:42 am ---Yeah, you'd have to use a 300mA Type-B RCD. But does this provide an actual increase in safety in a properly designed system? It's not enough to protect a person from shock, so it's only useful as fire protection. For non-critical use cases I don't think it's worth it (and the VDE agrees), as it'd require an extremely low probability PE failure to trigger the RCD but not the circuit breaker. --- End quote --- Here it is standard to have a low sensitivity 300mA RCD for the entire house and a separate high sensitivity RCD for the bathroom. You can have issues with false trips if you try to use a high sensitivity RCD for the whole house because the leakage currents can add up. The 300mA RCDs are quite high when it comes to shock indeed, but can still save you in situations where you grab something live while having a good contact with something else metal. Even the high sensitivity RCDs will give you a hefty shock before the switch has time to react and break the power. But this does not make them useless. The 300mA RCDs do trip quite easily when mains stuff gets wet. Like an outdoor fixture getting water inside it, someone pouring liquid on a extension cord etc.. it also does not need a good low resistance earth path to trip a breaker. Without a RCD something developing a high resistance path to earth will generally burn more and more until the path gets carbonised enough to let a serious current flow, often setting things on fire as the carbon path starts to glow. If you are lucky the flame might form a arc to live in order to make enough current flow to trip the breaker quickly enough before everything is on fire. So it is very good practice to have a RCD in some form. |
| Siwastaja:
Here it's 30mA for the whole house. (Well, nothing prevents you from using multiple, with smaller chance for false positives, but guess how many electricians do that, these things cost like 30€! Professional work being expensive does not mean it shows in the results. ) Refrigerators and freezers can be legally wired to separate outlets without RCDs. If not, false positives would be really awkward. The American way of multiple 6mA circuits offers even better safety, although that being said, 30mA is pretty good at preventing death. 300mA - not so much, and it's not meant for that, but if you have to choose between taking the shock without RCD, or with a 300mA RCD, the choice is obvious! |
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