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Old 31st Jul 2016, 9:07 am   #41
evingar
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Default Re: The deeper mysteries of 'ground'?

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Originally Posted by RogerEvans View Post
The Rutherford Lab near Harwell used to have a proton synchrotron
Still does - It's called ISIS

http://www.isis.stfc.ac.uk/about/aboutisis.html
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Old 31st Jul 2016, 9:08 am   #42
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the appliance fed from the isolating transformer may not be earthed. For example there must be an appropriate level of insulation between earth and any earth socket/terminal in the isolated system. If this condition is not satisfied, then it won’t meet the requirements of an isolated system,
This is true of an isolated supply system and would apply where an isolating transformer is built into the electrical installation. However I think this proposal is for a portable transformer plugged into a normal earthed outlet, which effectively makes it part of the appliance. I have only a smattering of knowledge of AS / NZS 3000 but I don't think it applies in this case. Casual single-earthing of an appliance fed from an isolating transformer meeting the requirements for separation of circuits will be no more dangerous than not earthing it.

#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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Old 31st Jul 2016, 11:48 am   #43
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Default 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...

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Old 31st Jul 2016, 7:55 pm   #44
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Default Re: The deeper mysteries of 'ground'?

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We have strayed deeply into the electrical safety aspects of earthing which I don't think were the OP's field of interest...
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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Old 1st Aug 2016, 9:14 am   #45
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Default 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!

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Old 1st Aug 2016, 10:09 am   #46
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Default Re: The deeper mysteries of 'ground'?

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Originally Posted by evingar View Post
Quote:
Originally Posted by RogerEvans View Post
The Rutherford Lab near Harwell used to have a proton synchrotron
Still does - It's called ISIS

http://www.isis.stfc.ac.uk/about/aboutisis.html
It is not strictly ISIS, which is a neutron and muon machine. But it produces neutrons by a process called spallation. Basically a proton beam is smashed into a target. The target is mainly tungsten http://www.isis.stfc.ac.uk/learning/...rget12345.html ), and a beam of neutrons are knocked out.

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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Old 1st Aug 2016, 8:35 pm   #47
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Default Re: The deeper mysteries of 'ground'?

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It must also be bonded back to a point upstream of any RCD in the supply, with no other connection to Neutral downstream thereof, so every milliamp flowing out of the RCD can be accounted for as it flows back into the RCD.
Interesting! I accidentally pulled the plug on the RG16 Serenade the other day before switching the set off. The local RCD tripped (inductive spike from the deck motor?) but so did the much larger main supply RCD. At the time, the fridge/ freezer motor was also running, as well as a dishwasher and washing machine. Big old back EMF from all these inductive loads, presumably, as the larger RCD tripped!!! I live in a new build (3 year old) apartment block, if that's relevant.

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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Old 1st Aug 2016, 9:48 pm   #48
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Default 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.

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Old 1st Aug 2016, 10:15 pm   #49
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Default Re: The deeper mysteries of 'ground'?

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...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.
In my case, do you mean the 0.1uf Y2 caps that I used as a noise-suppression mod, one phase to safety Earth and one Neutral to Earth??!!
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Old 2nd Aug 2016, 7:09 am   #50
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Default Re: The deeper mysteries of 'ground'?

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Originally Posted by G4_Pete View Post
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!
I think that that is an argument in favour of bonding the “remote” RF earth stake back to the main house earthing point, in order to minimize the hazard arising from different earth potentials.

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.


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Old 2nd Aug 2016, 7:44 am   #51
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Default 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.

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Old 2nd Aug 2016, 8:43 am   #52
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Default Re: The deeper mysteries of 'ground'?

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the 0.1uf Y2 caps that I used as a noise-suppression mod, one phase to safety Earth
0.1 or 0.01μF? As David points out the leakage from 0.1μF at 230V is over 7mA which is excessive for a portable appliance connected via a 13A plug - the normal maximum permissible is 3.5mA. Above this, regulations mandate a high-integrity protective condutor - you would be obliged to wire it directly into the wall or use an industrial plug and socket with its own dedicated circuit to reduce the risk of shock if continuity was lost. It would automatically fail a PAT.
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Old 2nd Aug 2016, 11:18 am   #53
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Default 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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Old 2nd Aug 2016, 1:38 pm   #54
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Default Re: The deeper mysteries of 'ground'?

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0.1 or 0.01μF? As David points out the leakage from 0.1μF at 230V is over 7mA
0.1uf, Lucien. I tried lower values line to neutral and also lower values common mode. 0.1uf common mode is the effective configuration and sufficiently above threshold to eliminate the awful hash across the entire FM range without.

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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Old 2nd Aug 2016, 2:06 pm   #55
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Default Re: The deeper mysteries of 'ground'?

Thank you David, very interesting backround and insights...
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Old 2nd Aug 2016, 2:14 pm   #56
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There is, however, no scope for any danger to the operator of this set under a failure mode.
There is more to it than this (and it is dangerous to assume that there can never be any danger!) When it is connected to a final circuit in an installation, the CPC is common to other pieces of equipment. In the event of a CPC failure, a dangerous shock current may be taken from the exposed metalwork of other items of equipment downstream of the fault. In addition to the 3.5mA limit per portable appliance, there is also a regulation that any fixed wiring circuit likely to be required to pass 10mA or more of leakage current must have a high-integrity (usually duplicated) CPC. Since a few mA would be anticipated from other appliances and cable stray capacitance, this would push you over the limit to requiring special wiring in your house!

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.

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Old 2nd Aug 2016, 5:52 pm   #57
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Default 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.
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Old 6th Aug 2016, 3:17 pm   #58
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Much better to...use a packaged mains filter, which someone has designed and written a datasheet for.
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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Old 6th Aug 2016, 3:47 pm   #59
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Default 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!

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Old 6th Aug 2016, 8:36 pm   #60
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Question Re: The deeper mysteries of 'ground'?

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I had a Schaffner packaged filter/IEC receptacle go off with a bang . . . .
Ditto here - and on more than one occasion, too. They don't seem to be as reliable as I would expect. Any ideas as to why they fail in the manner they do - or why they fail at all? Usual trouble with leakage in the capacitors? If so, couldn't the manufacturers have fitted high-reliability ones, considering that they are integrated in a complete package?

Or am I being unreasonable and simply asking too much?

Al.

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