UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

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.

Closed Thread
 
Thread Tools
Old 1st Jan 2019, 12:37 pm   #21
tony brady
Pentode
 
Join Date: Mar 2014
Location: Peacehaven, East Sussex, UK.
Posts: 174
Default Re: "Elevated ground" Good idea?

I design audio in the day job to CE and CQC ( China) regulations. I would like to be very clear on this point NEVER remove the mains earth from the chassis of a class 1 unit and replace by anti parallel diodes or combinations of a resistor and capacitor. It is perfectly acceptable to lift the AUDIO ground from the chassis and use diodes or say a 10 ohm fusible in parallel with 100nF while maintaining the earth bond to the chassis.

for CE we test the earth bond at 25A for 5 seconds at less than 100 milli ohms. For CQC it is 25A at less than 100 milli ohms for 60 seconds. this includes the lead from the test gear to the unit under test
tony brady is offline  
Old 1st Jan 2019, 12:46 pm   #22
GrimJosef
Nonode
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 2,672
Default Re: "Elevated ground" Good idea?

25A at less than 100 milliohms corresponds to less than 2.5V between chassis and plug pin. The PB3510-E3/45 diode pack would pass that test. It might even pass it with two diodes in series.

EDIT: I don't think the 10 ohm resistor should be fusible should it ? We want the earthing circuitry to blow the protective fuse, not the other way round.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline  
Old 1st Jan 2019, 12:49 pm   #23
tony brady
Pentode
 
Join Date: Mar 2014
Location: Peacehaven, East Sussex, UK.
Posts: 174
Default Re: "Elevated ground" Good idea?

but would a testing house accept it?

cheers
Tony
tony brady is offline  
Old 1st Jan 2019, 12:53 pm   #24
GrimJosef
Nonode
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 2,672
Default Re: "Elevated ground" Good idea?

I suspect that only a testing house could answer that.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline  
Old 1st Jan 2019, 12:59 pm   #25
tony brady
Pentode
 
Join Date: Mar 2014
Location: Peacehaven, East Sussex, UK.
Posts: 174
Default Re: "Elevated ground" Good idea?

yes I suspect that would be the case but it's not something I would be doing when it's so much easier to lift the audio or design to class 2

cheers
Tony
tony brady is offline  
Old 1st Jan 2019, 1:11 pm   #26
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 12,625
Default Re: "Elevated ground" Good idea?

The testing we did had to cove CE mark, UL, CSA etc etc. It included visual inspection of metallic earth connection to chassis. Screws not permitted, rivet was accepted. Sticking a diode/diodes in the path would have lost approvals for some markets.

Biggest ground impedance problem was softened springs in the IEC cable ends.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 1st Jan 2019, 2:29 pm   #27
mhennessy
Dekatron
 
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 3,591
Default Re: "Elevated ground" Good idea?

AFAIK, there are no safety requirement for the signal earth to be bonded to the chassis, and something like the outer shell of a phono socket, for example, would not be expected to carry the 25A of an earth continuity test.

If the phono sockets were expected to provide a low-impedance, high-current connection to mains earth, then presumably, so must the negative speaker output terminals? I wonder if those who make bridged amps know about this

Quote:
The Golden Rule is: When testing Earth Continuity, if you do get a fail initially, do not immediately fail the appliance. Make sure you have a good connection and try PAT testing again. If necessary try a different Earth point as not all metal parts might be connected to earth.
From: http://www.patinfo.co.uk/pat-testing...uity-test.html


A connection between signal earth and chassis is desirable for EMC reasons, hence the ~100nF capacitor(s).

A "weak" DC connection from a 10 to 100 ohm resistor is obviously sensible so that things are broadly in the right ballpark while not causing earth loops. And if you do that, it's customary to add the diodes (or bridge) that's being discussed here. But these diodes are not for PAT testing or electrical safety reasons, and it's not reasonable or necessary to rate them at 25A+. Rather, they are simply to provide a bit of protection to the 10-100 ohm resistor. They obviously remain o/c in normal use. 1N4001s are generous. I've never heard of failures.

If you wanted to use a wirewound resistor, or indeed an NTC instead of a resistor (the sort used for limiting mains inrush currents), then the diodes aren't needed.

Argus, differential (or pseudo-differential) analogue video inputs are very common. I'd go so far as to say that they were pretty much the norm in pro gear
mhennessy is online now  
Old 1st Jan 2019, 2:38 pm   #28
llama
Octode
 
llama's Avatar
 
Join Date: Jun 2006
Location: St Osyth, Nr Clacton, Essex, UK.
Posts: 1,150
Default Re: "Elevated ground" Good idea?

Well that's something to think about - and act on!
I'd though the only issue with what I'd done was the slight tingle between signal earth (to projector) and signal earth of the lounge equipment.
I think I have some braid-breaker chokes from video telecoms equipment in my stash. No-one has mentioned such things so I may just find one and drop it into circuit, reconnect the mains earth and check for hum bars.
I'd like to see a video opto-isolator circuit or even one to buy at an OK price (say 20).
Graham
__________________
Half my stuff is junk - trouble is, I don't know which half!
llama is offline  
Old 1st Jan 2019, 4:15 pm   #29
GrimJosef
Nonode
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 2,672
Default Re: "Elevated ground" Good idea?

Quote:
Originally Posted by mhennessy View Post
AFAIK, there are no safety requirement for the signal earth to be bonded to the chassis, and something like the outer shell of a phono socket, for example, would not be expected to carry the 25A of an earth continuity test ...
Doesn't it seem a bit unlikely that there are formal requirements for us to be protected from electric shcok when we touch the chassis but these requirements don't apply when we touch a plug-and-socket connector (my rather ordinary Neutrik phono plugs all have metal shells connected to the phono outer) ? The plugs are designed to be touched, in fact gripped quite hard.

I'm not an equipment manufacturer so I haven't taken out a second mortgage to buy a copy of EN 60065:2014 or its replacement EN 62368. But I do have a copy of its predecessor EN60065:2002. That says, in Section 8.5, and I quote

CLASS I apparatus shall be provided with a protective earthing terminal or contact to which the protective earthing contacts of socket-outlets, if any, and ACCESSIBLE conductive parts shall be reliably connected.

There are some exclusions relating to any accessible parts which are Class II insulated or which are protected e.g. by an intermediate screen, but I don't think either of these apply to phono socket outers. So it's pretty clear that phono socket outers must be reliably connected to the protective earthing terminal (commonly the centre pin on an IEC inlet). I would argue in court that a pair of 35A (cw)/350A (pulsed) diodes in parallel constitute a reliable connection. I'd be more nervous about relying on a single metal film resistor in the event of the mains driving a hundred amps or so through it.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline  
Old 1st Jan 2019, 5:09 pm   #30
Lucien Nunes
Octode
 
Lucien Nunes's Avatar
 
Join Date: Apr 2008
Location: London, UK.
Posts: 1,756
Default Re: "Elevated ground" Good idea?

The possibility of providing a low-integrity functional-earth path to signal common, in class I equipment, relies on the insulation between the live functional parts and the DC rails being more substantial that that to the case. This occurs quite often; for example a device with lots of line voltage AC wiring, switching and connectivity might be designed as class I, while all the ELV side of things that is common to the signal grounds is fed from split-bobbin transformers. Provided the transformers are rated for the purpose and electrical separation is maintained between ELV and LV around the unit, there's no reason why exposed conductive parts such as phono sockets need a safety earth, and the barrier network can be safely used.

I have used anti-parallel diodes in the past and conducted some tests on their fault-current withstand capability. A 35A bridge will survive some fairly serious knocks, I don't have figures to hand, but I was confident in their ability to take out a 5A BS1362 fuse at pretty much any value of supply loop impedance. One advantage of a semiconductor in this position, as opposed to a resistor, is that they often fail S/C which sacrificially improves safety. One of the applications was to work around a faulty design on a piece of equipment that had digital ground earthed in two places, and analogue ground both earthed and connected to digital ground. Noisy ground current from the digital side of things tended to divert via the analog ground and appear all over the system, requiring the diodes to force it back down the digital PSU -Ve leads.

Anti-parallel diodes are often connected in shoreline supplies to boats, to interrupt the circuit for low DC voltages caused by electrolysis. Marinas use TT earthing (i.e. the earth available from a socket is obtained locally from a rod) to ensure it is equipotential with the surroundings and unaffected by drop along the supply CNE if it is TN-C-S for example. But electrolytic potentials between adjacent boats and/or extraneous parts of the installation can set up circulating DC currents, sacrificing the anodes on one boat (or the boat itself) while preserving another boat or steel conduits etc. Diodes are the conventional sure for this.

FWIW I have a commercial rackmount zone mixer apart in front of me at the moment. It is class I in general construction, but the transformer provides isolation sufficient to treat the signal circuitry as though it were class II. Between signal common and chassis there are two anti-parallel 1N4001s, and 10R || 100nF. This works very well and solves a lot of potential problems. Although all of its I/O is balanced, it is necessary to assume that some unbalanced equipment might be connected remotely and therefore that signal ground might be part of an audio circuit somewhere.
__________________
Three anodes good, six anodes better!
Lucien Nunes is offline  
Old 1st Jan 2019, 5:12 pm   #31
mark2collection
Hexode
 
mark2collection's Avatar
 
Join Date: Jul 2010
Location: Royal Berkshire, UK.
Posts: 325
Default Re: "Elevated ground" Good idea?

It's a difficult one.

With my daft 'homebrew' projects, all metalwork, chassis or cabinet parts including fancy metal volume knobs, are always connected to the mains Primary Earth. That way, any fault on the mains Line or Neutral connections are swiftly dealt with.

I usually have a single earth line from the mains P.E to transformer 0V, then going on through to the input stage of say, the amplifier, with the rest of the circuit taken to ground via this single 'buss bar(?)' much like the symmetry of a Leak Stereo 20.

Maybe I've been lucky and not had to make a change to modern 'manufactured' equipment though have made safety changes to vintage radios, grounding otherwise 'floating' chassis, but then, these sort of sets do not meet any of today's safety requirements and although a 'modification' it's surely for the best, in this case?

My first valve power amplifier caught me out, proudly making the wiring loom, installing & setting it up with 'star' earthing ......... Oh my giddy aunt, baaad idea! I had of course fitted the loom after the sockets, and everything else afterwards, happy with my 'laced-up' loom I powered the amp up! With nothing connected it was full of earth loops and buzzed like a busy bee. Had to rethink & start all over again.

Interesting ideas and views thus far, not sure how I'd face court if one of my projects bit someone, though they'd have to have it open in the first place to do that, a whole different kettle ... unless an unguarded valve was damaged whilst running. Hmmm.

Mark
__________________
Slowly turning the 'to-do', into 'ta-dah'

Last edited by mark2collection; 1st Jan 2019 at 5:18 pm.
mark2collection is offline  
Old 1st Jan 2019, 5:23 pm   #32
mhennessy
Dekatron
 
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 3,591
Default Re: "Elevated ground" Good idea?

Quote:
Originally Posted by GrimJosef View Post
Quote:
Originally Posted by mhennessy View Post
AFAIK, there are no safety requirement for the signal earth to be bonded to the chassis, and something like the outer shell of a phono socket, for example, would not be expected to carry the 25A of an earth continuity test ...
Doesn't it seem a bit unlikely that there are formal requirements for us to be protected from electric shcok when we touch the chassis but these requirements don't apply when we touch a plug-and-socket connector
Not really. That's the difference between "functional earth" and "protective earth".

And you can side-step the question entirely by going Class 2, which is what the overwhelming majority of hi-fi equipment is.

IEC 62368-1:2018 is less than 300. Money well spent, especially for anyone who modifies equipment for their customers
mhennessy is online now  
Old 1st Jan 2019, 5:30 pm   #33
mark2collection
Hexode
 
mark2collection's Avatar
 
Join Date: Jul 2010
Location: Royal Berkshire, UK.
Posts: 325
Default Re: "Elevated ground" Good idea?

On the C&G course for PAT'ing, the trainer said 'never use external signal connections for testing the Primary Earthbond'.

Knowing a lot of gear has its signal ground tied to Primary Earth, we quizzed him as to why.

In short, it doesn't form part of the safety system, and under 'normal' operating conditions, you will not be touching, for example, the phono sockets, besides, they (signal grounds) are not designed to carry 25 amps.

Mark
__________________
Slowly turning the 'to-do', into 'ta-dah'
mark2collection is offline  
Old 1st Jan 2019, 5:41 pm   #34
TonyDuell
Dekatron
 
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 3,356
Default Re: "Elevated ground" Good idea?

I once had to rebuild a BBC microcomputer that had been PAT tested with the PAT earth lead connected to the outer of the video BNC socket. Suffice it to say there were melted traces on the PCB and numerous blown ICs...
TonyDuell is offline  
Old 1st Jan 2019, 6:45 pm   #35
GrimJosef
Nonode
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 2,672
Default Re: "Elevated ground" Good idea?

Quote:
Originally Posted by mhennessy View Post
Quote:
Originally Posted by GrimJosef View Post
Doesn't it seem a bit unlikely that there are formal requirements for us to be protected from electric shcok when we touch the chassis but these requirements don't apply when we touch a plug-and-socket connector
Not really. That's the difference between "functional earth" and "protective earth".
If people are now building bits of kit where the external sockets, and therefore the metal plug shells, are not protectively earthed then I think my wife might prefer me to spend some money on insulating gloves. I quite often grab hold of plugs with my bare hands. Somehow I thought that that was what they (the plugs) were for and that I should be safe doing it.

As an aside, although I've worked with big electrical kit for 40+ years the nearest I've ever come to seeing someone killed by electricity was when a colleague pulled a small metal plug out of an ordinary but faulty piece of bench equipment (an oven). The shock knocked him unconscious and threw him across the lab. He was very lucky to get away with that.

Quote:
And you can side-step the question entirely by going Class 2, which is what the overwhelming majority of hi-fi equipment is.
I specialise in valve stuff (strictly speaking valve stuff which has gone wrong). I can't remember ever seeing a piece with the Class II symbol on it.

Quote:
IEC 62368-1:2018 is less than 300. Money well spent, especially for anyone who modifies equipment for their customers
I'll see what the library system can do for me . EN 60065:2002 told me clearly and unambiguously that accessible parts (not some accessible parts) had to be reliably connected to the protective earthing terminal. I'm surprised to hear that 62368-1:2018 has rolled back on that, but if you assure me that it has then I absolutely believe you. I'd still struggle to recommend it to my customers as a risk worth taking though. Especially when eliminating the risk involves just a 4 bridge rectifier.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline  
Old 1st Jan 2019, 11:00 pm   #36
Argus25
Nonode
 
Argus25's Avatar
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,446
Default Re: "Elevated ground" Good idea?

Quote:
Originally Posted by mhennessy View Post
Argus, differential (or pseudo-differential) analogue video inputs are very common. I'd go so far as to say that they were pretty much the norm in pro gear
Interesting, because back in the 1980's I worked on a lot of pro video monitors, JVC, & Sony, also cameras with genlock inputs, and never saw a differential video input on any of them, or on the camera control units. Maybe it came into fashion later? Also, on the ones I saw at least it was rare to see the body of the BNC sockets insulated, but even when they were there, the wiring to the outer shell ended up connected to the signal earth on the pcb's, which was at mains earth anyway.

Last edited by Argus25; 1st Jan 2019 at 11:06 pm.
Argus25 is online now  
Old 1st Jan 2019, 11:19 pm   #37
Argus25
Nonode
 
Argus25's Avatar
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,446
Default Re: "Elevated ground" Good idea?

One issue is now there is a propensity for plastic enclosures for everything. If inside that enclosure there is mains power, then every metal object projecting to the outer surface, screw heads, connector shells etc, a metal base, the transformer stack (if there is one) must be earthed if the screws project to the outer surface. A loose wire inside connected to phase could touch any of those fittings, a failure in the transformer insulation etc could take the stack high and its mounting screws. This is why , for home made mains apparatus at least, its simply better to use a solid metal enclosure with a good earth bond, but its more expensive.
Argus25 is online now  
Old 2nd Jan 2019, 8:35 am   #38
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 4,671
Default Re: "Elevated ground" Good idea?

Can we clarify whether we'd need a 35A rated bridge or paralled diodes in the ground lift? If for example we have an amp with a total DC peak current of 1A, with a star ground, the RCA's and speaker negative terminal the only grounded metalwork on the outside of the amp cabinet, which is made of a non conductive material, would we still need a 35A rated diode/BR in parallel with a 10r resistor and 100n cap? Would the cap need to be Y rated?

The amp I'm building at present throws up some interesting questions/problems. The PSU is mounted on an internal 2mm ali sheet (not accessable from outside the amp box), this is earthed to the IEC socket as per good practice. The PSU ground's go to a star ground on the same ali sheet, IE, HT ground and ground for a low DC PSU. All supplies etc are connected to a female McMurdo red range 16 way chassis connect, the ground of which goes to the star point. On top of the wooden box sits the metal amp chassis with the male McMurdo connect attached. Ground sits on a insulated bus bar which is connected at one point to the metal chassis.

So the chassis above does not have an earth wire as proscribed, it's earthed via the connector which is very highly unlikely to fail. Ground is connected via the big locating pin which is the first/last thing to connect. Problem?

Another potential problem is one side of a tfmr secondary (12v AC) is connected to ground on the inner PSU "plate". This is because the 12v winding supplies a very small +15v 0v -15v split rail PSU off a sec winding with no center tap. Would a BR/R/C ground lift as discussed cause issue's?

This thread has also thrown up the question of liability when working on gear, perhaps best split off into another thread?

Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is online now  
Old 2nd Jan 2019, 10:24 am   #39
GrimJosef
Nonode
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 2,672
Default Re: "Elevated ground" Good idea?

There are two reasons why I use a bridge as an earth lifter. The first is that it's a handy package containing four really chunky diodes for not much money. The second is that it offers me versatility since there are two connection options: either i) having two paralleled diodes in each direction which doubles the fault current capability and gives a safety net if one diode should fail or ii) two series diodes in each direction which raises the isolation voltage from 0.7V to 1.4V. This might be ueful in very noisy environments. I start by connecting the + and - terminals together. For option i) I also connect the two ~ (AC) terminals together. Then I take wires from the ~ terminal and the +/- terminal, one to safety earth and one to signal ground. For option ii) I leave the two ~ terminals unconnected to each other and take one wire from each ~ terminal.

The role of the capacitor, as I understand it, is not a safety one. It can help with noise from high-frequency transients on the ground wiring and also with interference from external radio frequency sources. Since it's not safety-critical it doesn't need to be either class X or class Y.

I'm less clear about why people add a resistor. I suppose if the signal ground would otherwise be completely isolated from earth then a static voltage of up to 0.7VDC might build up on it, and the resistor would prevent that. But to be honest it looks like a hangover from the days when the resistor was the only component in the earth lifter. The downside of having it is that it does allow a bit of the ground loop current to carry on flowing and generating a bit of the hum that we're trying to eliminate.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline  
Old 2nd Jan 2019, 9:58 pm   #40
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 4,671
Default Re: "Elevated ground" Good idea?

Thanks GJ.

Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is online now  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 2:51 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.