General > General Technical Chat
Grounding boxes in home wiring are totally safe for you?
KaneTW:
Oh, I'm not against RCDs in general. We have 30mA RCDs on every subcircuit, and they did in fact detect a medium-impedance short to ground (Riso on the order of 10kohm).
Just arguing that in some applications an RCD is more trouble than it's worth. They're limited, and nothing else should be on that subcircuit, but still.
Jester:
A small note on ground resistance. Years ago I was part of a team that were developing a 240V power meter for utilities that was to be installed at the pole mounted transformer. We designed the meter with a suitable HRC fuse. The company was sold to investors and they were looking to cut manufacturing costs and suggested we replace the fuse with a narrow trace on the PCB. Their “engineer” trained who knows where explained to us that if a fault occurred the trace would simply melt and everything would be fine. We disagreed. In the end the investors agreed to run a test. Fortunately we had access to a high voltage, high current testing facility and they agreed to run the test. We placed a wire wrap style wire just down circuit from the narrow trace. The testing facility records these test on video. The device went off a bit like a hand grenade. We repeated the test with the HRC fuse and and it was a non event.
Their “engineer” did not seem to grasp the concept that the considerable fault current available at the transformer terminals with very little resistance ( no long line from the transformer to the load) makes a rather significant difference in the actual fault current. That’s why for example a 1A HRC fuses has 200kA breaking capability.
Their “engineer” also seemed to miss the point that yes the trace will melt, however the arc will not be quenched and all this will occur VERY quickly as in explosive quickly.
Siwastaja:
--- Quote from: Jester on January 25, 2022, 02:11:40 pm ---A small note on ground resistance. Years ago I was part of a team that were developing a 240V power meter for utilities that was to be installed at the pole mounted transformer. We designed the meter with a suitable HRC fuse. The company was sold to investors and they were looking to cut manufacturing costs and suggested we replace the fuse with a narrow trace on the PCB. Their “engineer” trained who knows where explained to us that if a fault occurred the trace would simply melt and everything would be fine. We disagreed. In the end the investors agreed to run a test. Fortunately we had access to a high voltage, high current testing facility and they agreed to run the test. We placed a wire wrap style wire just down circuit from the narrow trace. The testing facility records these test on video. The device went off a bit like a hand grenade. We repeated the test with the HRC fuse and and it was a non event.
--- End quote ---
I have seen disastrous management like this. When it's a con "engineer", it's even worse. That's exactly why we have regulations and standards - you don't need to waste time demonstrating that illegal design is dangerous |O. That's why it's illegal, end of story. Even if the end result of your story was good, a lot of time was wasted when all that was needed is to management believe in their own experts (the actual engineers) saying this is against the standards and dangerous.
Berni:
Yeah this story is the reason why we have standards and laws that force you to follow them.
There are times where engineers explain the proper way of doing something, but the management sees the whole thing as just unnecessary cost. Particularly if there are people in management that know a little bit of the field, as that gives them confidence to push some of the stupid ideas, yet not know enough of the field to realize it is a bad idea.
Siwastaja:
Also why protective laws for whistleblowers exist. Not here of course, but in many countries which value things like that.
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