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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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31st Jul 2016, 9:07 am | #41 | |
Octode
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Re: The deeper mysteries of 'ground'?
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http://www.isis.stfc.ac.uk/about/aboutisis.html
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31st Jul 2016, 9:08 am | #42 | |
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Re: The deeper mysteries of 'ground'?
Quote:
#2: Fault conditions. This applies equally to TN-S and TN-C-S supplies. In the event of a hard short between line and CPC within the installation, a potential divider is formed with the fault at its midpoint. Earthed parts electrically near the fault (which might be the entire installation and all connected appliances) will rise to the midpoint voltage while the OCPD operates - if the impedances are similar this might be in the order of 100-150V in 230V installations, or with a high-resistance earth electrode, nearer 230V. This is dangerously high above true earth, requiring extraneous conductive parts that could otherwise provide a shock path to be solidly bonded, so that if necessary these too rise to the midpoint voltage. A domestic wiring short can often pass a current of thousands of amps, which will divide amongst the service earth / rod / extraneous conductive parts. If a signal earth of low impedance is tied into this system, it could end up passing heavy current and possibly fusing open-circuit, disconnecting it from the equipotential and allowing a dangerous voltage to exist between it and the earth derived from the CPC. Likewise if it is present but not bonded, it will be near true earth while the CPC is elevated due to the fault current. In a nutshell, if a fault occurs on the shower circuit you could receive a fatal shock between the radio and the signal earth, while you should not receive such a shock between the radio and any other metalwork in the house. We have strayed deeply into the electrical safety aspects of earthing which I don't think were the OP's field of interest. There is great risk of confusion when discussing this across continents and eras, as terminology and conventions vary while seeking similar objectives. Perhaps we should stick to the electronic implications of grounding / earthing. |
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31st Jul 2016, 11:48 am | #43 |
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Re: The deeper mysteries of 'ground'?
I'm not sure the safety and radio aspects can be easily separated, Lucien. The moment anyone brings an earthed conductor into most homes nowadays, a whole pile of legislation falls on their head.
Keeping it outside and using an RF isolating transformer has its attractions... David
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31st Jul 2016, 7:55 pm | #44 |
Dekatron
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Re: The deeper mysteries of 'ground'?
Lucien, no worries - on the contrary, I actually wanted to see just whatever emerged...the safety side is highly convoluted, but also extremely relevant and interesting. I hope this runs in whatever direction it wants to!
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Al Last edited by Al (astral highway); 31st Jul 2016 at 8:14 pm. Reason: Quotation slip |
1st Aug 2016, 9:14 am | #45 |
Hexode
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Re: The deeper mysteries of 'ground'?
Taking the RF Isolating transformer concept one step further. The input and output terminals would have to be arranged at a distance apart so that you could not hold the input side whilst connecting up the antenna side. Easy to see how you would be holding one connector whilst tightening the other if the thing is in a small insulated box. True you could measure the potential difference prior to connecting/disconnecting but would you remember to do this every time!
Once connected disconnecting earth ties is an even greater safety challenge as you never know the potential difference until the tie is opened. An electrician locally lost his life doing just that when he lifted an earth tie strap on equipment that had an unidentified leakage path from high voltage. All low probability stuff but not totally impossible a bit like the National lottery! Pete |
1st Aug 2016, 10:09 am | #46 | ||
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Re: The deeper mysteries of 'ground'?
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Those protons are produced in an H- linac followed by a proton synchrotron. H- is injected because protons would be repelled by protons already circulating. Two electrons are stripped off the H- in the synchrotron to leave protons (H+), which then go to the target station to produce neutrons. |
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1st Aug 2016, 8:35 pm | #47 | |
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Re: The deeper mysteries of 'ground'?
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I can see how a poor set up on a vintage set could be exposed to some of that massive overvoltage, theoretically, depending on the timing of the two RCD trips.
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Al Last edited by Al (astral highway); 1st Aug 2016 at 8:59 pm. |
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1st Aug 2016, 9:48 pm | #48 |
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Re: The deeper mysteries of 'ground'?
Probably both RCDs were 30mA types with similar operating speeds so both popping at once isn't a surprise. What current they are rated to carry/break is a separate parameter.
The inductive transient should be purely in the line/neutral loop and shouldn't trip an RCD, but a fast, large transient can create imbalance currents in filter capacitors to the earth wiring, and the resultant imbalance in line and neutral currents triggers the RCD. We had all sorts of things damaged by transients in the lab at HP and it turned out to be someone with a floor polisher who looked for a bench left turned on, and plugged it into one of the easily reached sockets across the back of it. A customer in Italy had a terrible record of equipment failures... his power was on the same branch as a freight elevator motor. David
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1st Aug 2016, 10:15 pm | #49 | |
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Re: The deeper mysteries of 'ground'?
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2nd Aug 2016, 7:09 am | #50 | |
Nonode
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Re: The deeper mysteries of 'ground'?
Quote:
Doing the isolation in the power supply to the receiver rather than at the aerial end doesn’t eliminate the problem, either. The earth lead from the “remote” RF earth, if used with a domestic-type receiver, could well terminate in a wander plug, easily accessible in terms of touching by hand, and it could be at a different potential to any nearby earthed, metal-encased items. Again, the above-mentioned bonding would mitigate. This case does however show how difficult it is to address RF earthing without also running into safety and protective earthing issues. Cheers, |
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2nd Aug 2016, 7:44 am | #51 |
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Re: The deeper mysteries of 'ground'?
The usual calculation with suppressor capacitors is the current in them with 240v 50Hz applied. This gives a small leakage current. 31.8k reactance so 7.54mA leakage. So you can only have two or three items suppressed like this before the suppressor caps trip a normal 30mA RCD.
EMC legislation has put filters into an awful lot of products and it's easier to reach the 30mA leakage limit than the 30A limit imposed by the breaker on a ring. 0.1uF is rather large for a Y capacitor and is more common in the 'X' location from line to neutral. In a typical installation you have usually several mA of leak to start with, and so the remaining freeboard is a fraction of the 30mA trip point. When a transient occurs, the voltage can be appreciably more than 240v, and the frequency content will extend to higher frequencies than 240v. Together these conspire to trip RCDs. The Y capacitors are there to filter interference in the common mode, of line and neutral acting together against ground. To keep the size of Y capacitor needed down, a lot of care in needed inside SMPS to mimimise capacitance of fast moving nodes to ground. Some designs had Faraday (and I really mean Faraday) screens between power transistors and heatsinks so that the capacitive current was constrained to the line-neutral circuit. This meant less EMC current, so less Y capacitance was needed, so you could have a benchfull of gear on one RCD. David
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2nd Aug 2016, 8:43 am | #52 | |
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Re: The deeper mysteries of 'ground'?
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2nd Aug 2016, 11:18 am | #53 |
Pentode
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Re: The deeper mysteries of 'ground'?
Reading these post I now sort of understand why nearby lightning sometimes trips my RCD. Thanks to the posters..
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2nd Aug 2016, 1:38 pm | #54 | |
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Re: The deeper mysteries of 'ground'?
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These regs are convoluted and interesting and there for a reason and I take note of your point. There is, however, no scope for any danger to the operator of this set under a failure mode. The set is entirely cased in a wooden cabinet and electrical contact with the chassis can only be exposed if a knob falls off. It was originally designed and built with a safety earth to chassis, which is still intact.
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Al Last edited by Al (astral highway); 2nd Aug 2016 at 2:03 pm. |
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2nd Aug 2016, 2:06 pm | #55 |
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Re: The deeper mysteries of 'ground'?
Thank you David, very interesting backround and insights...
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2nd Aug 2016, 2:14 pm | #56 | |
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Re: The deeper mysteries of 'ground'?
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7mA is an abnormally high, potentially dangerous level of leakage current for a portable appliance, so I would recommend reducing it if possible. I often build portable equipment with high leakage current, due to perhaps 15 or 20 separate switch-mode PSUs built into one unit, but I never reach the 3.5mA limit. The highest I have measured from a single electronic product is around 1.5mA. E2A the tolerance on normal RCD tripping currents is one-sided. All must trip at rated IΔn, the lower (no-trip) limit is 0.5IΔn. Many will trip at 20-25mA, in theory you might get a trip at 15.1mA. Once you subtract the cable leakage (resistive and capacitive, the latter including noise where Xc is much lower) you might only have 10mA available on a circuit for all the appliances, or perhaps 8mA if the RCD protects a number of long circuits. Last edited by Lucien Nunes; 2nd Aug 2016 at 2:29 pm. |
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2nd Aug 2016, 5:52 pm | #57 |
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Re: The deeper mysteries of 'ground'?
I didn't realise that caps as big as 0.1uF were available in Y ranges. I'm sure I read somwhere that 1mA was the recommended maximum current leakage per item - even this is enough to give a significant tingle to someone who is caught unawares. Even Sony (who seem to like big leakage currents) rarely go above 0.5mA per item.
Much better to use smaller caps, combined with appropriate inductors. Or use a packaged mains filter, which someone has designed and written a datasheet for. |
6th Aug 2016, 3:17 pm | #58 |
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Re: The deeper mysteries of 'ground'?
Not sure about 'better...' Easier, sure, but arguably better to learn by trial as I am here. I find it illuminating how focussing on a solution can blinker one to wider issues - I was just completely focuses on eliminating hash effectively so didn't think about leakage currents for an instant, I'll admit!! I do have a small selection of such filters pulled from computer switched -mode power supplies, however!
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6th Aug 2016, 3:47 pm | #59 |
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Re: The deeper mysteries of 'ground'?
I had a Schaffner packaged filter/IEC receptacle go off with a bang in one of my sig gens which was in work for a job. A Rifa capacitor inside blew a hole in the side of the can and stank the building out. Even in sealed boxes the little b*gg*rs can still get you!
David
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6th Aug 2016, 8:36 pm | #60 | |
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Re: The deeper mysteries of 'ground'?
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Or am I being unreasonable and simply asking too much? Al. Last edited by Skywave; 6th Aug 2016 at 8:36 pm. Reason: Typo. |
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