Grounding old equipment

[Gyro]

The difference is that metal cased Class II consumer equipment is designed to entirely different and much stricter standards in terms of primary to secondary insulation, pcb clearances, safety critical component specifications, clearances from chassis metalwork etc. compared to the stuff you mentioned.

[Specmaster]

I'm telling you what the current standards are for Consumer A/V equipment, the question you seemed to be asking. The OPs A/V equipment was built to a Class II double insulated standard.

None of the stuff you just mentioned is anything of the sort. Don't blame me if you've bought a bunch of old test equipment that needs making safe! 

P.S. Thank you ebastler.

I'm really struggling to see the difference here, just how would anyone be able to differentiate between an item then like "old test gear" that needs making safe and "A/V gear built to class II double insulated standard". Does it carry some sort of sticker that states it conforms, modern stuff has the square in a square symbol to indicate that it is double insulated.  In fact in the early days I often came across early valve radios and indeed TV chassis which were directly connected to the live supply, especially American equipment which had 2 pin plugs which could be plugged in either way round. In fact there are YouTube channels featuring repair shops where they come across this even today, 12voltvids is a typical channel that I watch and this cropped up lots of times on it.

[ebastler]

[...] from where I'm sitting, I could not see any difference between test gear in metal cases that was unearthed and audio gear in metal cases that was also unearthed. In both cases the user has to interact with the equipment and while everything is working as intended, no problems. The moment a problem does occur then the user could potentially be touching the full line voltage which here in the UK is 240v 

As already stated by Gyro, the difference is that the modern Class II equipment is double insulated. That means that, if the outer enclosure is metal, there actually is redundant insulation of all mains-carrying parts inside. This can take various shapes and forms -- extra large clearances, two layers of insulation around cables, etc..

Another way to put it, "a problem" (as in "a single problem") does not cause any danger in properly designed Class II equipment. The double insulation is one incarnation of the "single fault tolerance" paradigm, which is a guiding principle in many safety standards.

[Zero999]

There's also a big difference between audio/video and test equipment. They're used for different applications. Test equipment will be designed to more stringent standards than audio equipment.

[floobydust]

How many decades is "old"?

For this Sony, the safety standards changed little since the 1990's. Aside from protective insulation aging and cracking, the construction is not entirely unsafe.

Silicon Chip Magazine antique tube radio restorations: add a fuse and proper strain relief, ground the chassis, new mains cord. Antique equipment had weak or non-existent safety standards. Makes sense to bring it up to something that won't kill or burn the house down, like a lot of antique gear could do.

[ebastler]

There's also a big difference between audio/video and test equipment. They're used for different applications. Test equipment will be designed to more stringent standards than audio equipment.

Right. But the rationale is not that the life of a test engineer deserves more protection than the life of a consumer using an audio amplifier. Rather, for test equipment one can construe additional "single fault scenarios" that the user needs to be protected against.

For example, the mains power supply is not the only source of potentially dangerous voltages in a piece of test equipment. If you measure a high voltage with your bench DMM or oscilloscope, a faulty test lead or coax cable (which is not double insulated) or a user error may cause that voltage to reach your test equipment's GND connectors (and its metal enclosure if applicable). Hence, these need to be connected to protective earth for safety! Just protecting them from the test equipment's built-in power supply via double insulation is not enough to keep you safe.

[Gyro]

I'm telling you what the current standards are for Consumer A/V equipment, the question you seemed to be asking. The OPs A/V equipment was built to a Class II double insulated standard.

None of the stuff you just mentioned is anything of the sort. Don't blame me if you've bought a bunch of old test equipment that needs making safe! 

P.S. Thank you ebastler.

I'm really struggling to see the difference here, just how would anyone be able to differentiate between an item then like "old test gear" that needs making safe and "A/V gear build to class II double insulated standard". Does it carry some sort of sticker that states it conforms, modern stuff has the square in a square symbol to indicate that it is double insulated.  In fact in the early days I often came across early valve radios and indeed TV chassis which were directly connected to the live supply, especially American equipment which had 2 pin plugs which could be plugged in either way round. In fact there are YouTube channels featuring repair shops where they come across this even today, 12voltvids is a typical channel that I watch and this cropped up lots of times on it.

To answer the easy question first, yes, double insulated items are marked with the 'Square within a Square' symbol - not a sticker, it's a requirement that it is permanently marked, silk screen or whatever.

Early TVs were indeed often live chassis. If you think back though, their knobs and controls were plastic - even the ones with metal trim had plastic inners / shafts. Their aerial sockets weren't directly connected to the chassis - they included series RF capacitors in both the centre and shield paths (trust me). SCART and Headphone connectors didn't come in until after grounded chassis. Where TVs were the 'modern' pushthrough style (rather that recessed behind glass), the Implosion/fixing band around the screen was connected to chassis via a high voltage (~20Meg) resistor with parallel small value ceramic capacitor. That was just in case someone slipped something down the side. Those old TVs could be loosely regarded as 'reinforced' insulation in modern language, not bad, even if the back was fibreboard with coin slot fasteners. Cabinets were of course wood / plastic, not metal.
 
Test gear is a whole bag of worms, I remember a Japanese signal generator I had with a 2 core cable, metal case and definitely wasn't safe internally. A lot of cheap US origin too with dodgy looking transformer insulation (manufacturers more used to working with 110V?). It's difficult to lump them together. As ebastler said, treat each one as an individual case and act accordingly.

There's also a big difference between audio/video and test equipment. They're used for different applications. Test equipment will be designed to more stringent standards than audio equipment.

I think I have to disagree with you in safety terms. Consumer AV is manufactured to go in a domestic environment (kids) and in such high volumes that in safety terms there is no room for compromise. Test equipment components and operation will be obviously be, well, test equipment quality, there's no reason to argue that it is built to more stringent standards with regard to safety.

[Gyro]

So much for a quiet night watching/ listening (via the RCA connectors on my double insulated TV and my grounded chassis valve amp and Tannoys!) to the Proms! 


P.S.

How many decades is "old"?

For this Sony, the safety standards changed little since the 1990's. Aside from protective insulation aging and cracking, the construction is not entirely unsafe.

Young enough.

Agreed, particularly for a linear power supply model like that. No Y-cap spec changes to worry about etc.  An insurance company might actually argue that modifying it by adding extra insulation is an unauthorized change from its approved state and has potentially changed its flamability characteristics. But then, insurance companies will argue anything if it avoids paying out. 

[Specmaster]

Thank you for providing some illumination on the subject, it is a bit clearer now 

[Specmaster]

There's also a big difference between audio/video and test equipment. They're used for different applications. Test equipment will be designed to more stringent standards than audio equipment.

Right. But the rationale is not that the life of a test engineer deserves more protection than the life of a consumer using an audio amplifier. Rather, for test equipment one can construe additional "single fault scenarios" that the user needs to be protected against.

For example, the mains power supply is not the only source of potentially dangerous voltages in a piece of test equipment. If you measure a high voltage with your bench DMM or oscilloscope, a faulty test lead or coax cable (which is not double insulated) or a user error may cause that voltage to reach your test equipment's GND connectors (and its metal enclosure if applicable). Hence, these need to be connected to protective earth for safety! Just protecting them from the test equipment's built-in power supply via double insulation is not enough to keep you safe.

Thank you some enlightenment but what about in the instance where either the DUT or the test equipment is connected via an isolation transformer? Also in an example of say using an oscilloscope  to probe about in a DUT, where should the isolation transformer be, on the scope or the DUT?

[ebastler]

Thank you some enlightenment but what about in the instance where either the DUT or the test equipment is connected via an isolation transformer? Also in an example of say using an oscilloscope  to probe about in a DUT, where should the isolation transformer be, on the scope or the DUT?

In my understanding, the scope should always remain connected to PE. (So you can be sure that its metal parts are safe to touch.) Connect the DUT via an isolation transformer if you want extra protection while probing it; especially if it is one of those "chassis at high potential" contraptions.

[tronde]

I'm sorry tronde, we wasted a lot of OT time on this yesterday, I really don't want to get into it again but I'll try to summarise some points.

The reason I called you on USB chargers is because they are covered by a completely different safety standard to consumer A/V equipment that the OP was talking about - In Europe, EN60950 versus EN60065 as I indicated in my last post. It doesn't matter if people can relate to them or not, different equipment type, different safety standard.

Well, some multimedia equipment is allowed to be tested to 60950, so not so wrong as you indicate.





The origin of the discussion was the claim of being killed. I said, no killing here, and I used the charger as an example because it clearly shows that even small distances are safe in a domestic environment. Else it would have been illegal to sell and use.

The 4 and 5mm is more relevant to the discussion than "being electrocuted" because it shows clearly that you will not be killed by those distances in a domestic setting. This applies regardless of the product being a charger, a TV or a stereo. I have also always made clear that the environment is part of the danger. As I wrote in my previous post, the expected environment plays a big part. That is why some multimedia equipment is allowed to be tested to 60950, and some not. The danger does not necessarily change by type of product, but more often by how the product is used and in which environment.

It was you that complicated everything by talking about how a product could be improved by changing the design to allow for problem-free manufacturing of thousands of units. This is nice for the manufacturer, but it does not does not matter in this spesific discussion about what can be considered safe or not, so blame yourself on that.

No, I have not designed high-volume consumer stuff, but I have done a lot of layouts for high-volume light-equipment. All tested and certified by NEMKO without problem, so I am not that far-out, I think.



No need to elaborate more. I guess we agree one the most important; that the stereo in question does not appear to be unsafe by design as claimed by some.

[Specmaster]

Thank you some enlightenment but what about in the instance where either the DUT or the test equipment is connected via an isolation transformer? Also in an example of say using an oscilloscope  to probe about in a DUT, where should the isolation transformer be, on the scope or the DUT?

In my understanding, the scope should always remain connected to PE. (So you can be sure that its metal parts are safe to touch.) Connect the DUT via an isolation transformer if you want extra protection while probing it; especially if it is one of those "chassis at high potential" contraptions.

Well I like to obtain old scopes and try and rescue them becoming landfill so you would always have the scope being used as the test instrument connected to PE and isolation on the other under test, OK.

[tronde]

This thread has diverted into two different problems.

A:
Guy finds old equipent and wonder whether it is safe or not.

1:
Open enclosure, and find out if it looks mechanically sound and without any obvious signs of electrical faults.

2:
If sound, measure actual clearance and creepage from anything mains to exposed parts. If clearance > 4mm and creepage > 5mm and intended use is normal office or living room environment you will most likely have no reason to wet your pants.




B:
This is what Gyro mixed in to the discussion.

Guy gets a great idea for a product that will sell in huge numbers.

1:
Shell out a lot of money to IEC for all relevant standards.

2:
Spend a lot of time trying to figure out what is important.

3:
Implement what you think is important as best you can.

4:
Shell out a lot of money for a certification test.

5:
Repeat #3 and #4 and maybe #1 and #2 because you forgot something.




OR as we did

Find out what to sell

1:
Make an agreement with for instance NEMKO and pay something upfront.

2:
Tell them what you intend to make.

3:
Let them tell you what they would like to see.

4:
Implement what they asked for.

5:
Shell out a lot of money for a certification test.

6:
Be happy. If you didn't fail on EMC then, but that is a different case.

[Electro Detective]

LOL, you'd have to be rich or bonkers to stuff about modifying a formerly approved two core mains device to a 3 core question mark,

with the possibility of an UNRELATED to the mod mishap,

being sued to oblivion by some arrogant opportunistic pr!ck (no shortage of those today that must believe a well deserved visit to hospital on life support payback isn't going to happen sometime in the future)

and being allowed back on the street with only your jocks left, classed as an un-seizable asset

Leave the gadget as is, it worked then, it works now, move on   

[Vtile]

Early TVs were indeed often live chassis. If you think back though, their knobs and controls were plastic - even the ones with metal trim had plastic inners / shafts. Their aerial sockets weren't directly connected to the chassis - they included series RF capacitors in both the centre and shield paths (trust me). SCART and Headphone connectors didn't come in until after grounded chassis. Where TVs were the 'modern' pushthrough style (rather that recessed behind glass), the Implosion/fixing band around the screen was connected to chassis via a high voltage (~20Meg) resistor with parallel small value ceramic capacitor. That was just in case someone slipped something down the side. Those old TVs could be loosely regarded as 'reinforced' insulation in modern language, not bad, even if the back was fibreboard with coin slot fasteners. Cabinets were of course wood / plastic, not metal.
 
Test gear is a whole bag of worms, I remember a Japanese signal generator I had with a 2 core cable, metal case and definitely wasn't safe internally. A lot of cheap US origin too with dodgy looking transformer insulation (manufacturers more used to working with 110V?). It's difficult to lump them together. As ebastler said, treat each one as an individual case and act accordingly.

If I add a bit more air around the scenery..

One must remember that the distribution wiring scheme is also changed and so is building materials and techniques to some extend. Lets take the log cabin with wooden floor as a example. It does have one building material and it is a wood, which is a rather good insulator while its moisture content is typical what we typically call dry. As it is a log cabin it doesn't have running water or other metal stuctures. It does have one mains socket (110 or 240 V). Now the only socket is used to power the only electronic device in the cabin, which were old tuberadio with only 2 lead power cord. The chassis is all shiny bare metal (for sake of the story) and the other line of the cord is attached directly to the metal chassis. At the morning while mr.Mears were dry cleaning his living room with brucetwigs he removed the socket and didn't notice that he put it back at wrong way around. Now the chassis became to mains potential (voltage) and poor mr.Mears is toasted, isn't he?? Nope, because the archaic environment didn't have any path for the fault current to form. The wooden floor were insulator and all the other surfaces in the cabin except the radio chassis. So the user were still insulated (from ground at this case) and were like a bird sitting on the power line.

Since then the environment have drastically changed, with electronics top of each other and more conductive surfaces in houses (water pipes and so forth). This leads to environment that more often than not is always dangerous for the user if the person touch a mains potential.

This applies in some extend to test gear also. The routine to use of them might have been dramatically different than todays rather liberal "poke around culture" and so were often the environment where it were used.

Edit. some typos and idiomics.

[Gyro]

@Vtile:

If I add a bit more air around the scenery..

You have taken this thread way OT (again!) with your fanciful conjectures of why exposure to mains voltage isn't dangerous (is that 'at all' by now?  ). I'm not sure at this point whether you are deliberately trolling, or just have difficulty with estabished facts.

The OP was asking specific questions about an item of Class II Consumer A/V equipment. I have gone to some length providing relevant details, spec references etc. and have provided my own (for what it's worth) clear opinion on the specific item.

I don't care if Log cabins swept with spruce twigs, or any other domestic environment are envolved. Equipment is designed to strict an binding safety specifications where they have no latitude for fanciful stories.


EDIT: The following quote from tronde, not Vtile (My apologies to Vtile):

B:
This is what Gyro mixed in to the discussion.

Guy gets a great idea for a product that will sell in huge numbers.

1:
Shell out a lot of money to IEC for all relevant standards.

2:
Spend a lot of time trying to figure out what is important.

3:
Implement what you think is important as best you can.

4:
Shell out a lot of money for a certification test.

5:
Repeat #3 and #4 and maybe #1 and #2 because you forgot something.

Nobody (I certainly wouldn't, and I've designed several high volume consumer Class II  A/V products) would get an idea for such a Class II product and go ahead and develop it without purchasing the relevant safety spec - which is actually very cheap in comparison with other costs for "a product that will sell in huge numbers". Yes, of course he would need to spend considerable time designing to meet the full requirenments of that safety spec. Yes, of course you take advice if something is unclear to you, either from an internal specialist or an external authority. Run several prototypes to finalise the design this is needed regardless for a product of any complexity. Submit a 'golden sample' to an acredited test house, together with full supporting documentation, which includes things like copies of safety body approval certificates from the manufacturers of all safety critical components. Then, be prepared the cost of rework and re-test of any issues of non compliance that the test house identifies. Without all of those steps it would be completely unsafe - and illegal to take a Class II product to market.

... All of this is what real manufactuctures of Metal cased Class II eqiupment do, every day. It's not 'Amateurs night at the Circus' 

If you want to develop products by shortcut means, then put them in safety grounded enclosures or use an already approved external power brick.

You go ahead and add as much "air" to the thread as you want. As far as I am concerned this thread has run its useful course and I'm not wasting any more time on it. I hope the OP has gained the information he needs and maybe others may have gained some value from it.

Finally stop quoting your 4mm and 5mm mantra as the only safety metric, that advice is dangerous for other people! I am sure that NEMKO would take a very dim view of you doing it on a public forum!  It may be applicable to something that you have done, but the only figures that are relevant are the ones stated in the safety standards for that specific product category. It certainly isn't applicable to Metal cased Class II consumer!

I'm not going provide you care and feeding any more. I'm out of here. 

[Vtile]

@Vtile:

If I add a bit more air around the scenery..

You have taken this thread way OT (again!) with your fanciful conjectures of why exposure to mains voltage isn't dangerous (is that 'at all' by now?  ). I'm not sure at this point whether you are deliberately trolling, or just have difficulty with estabished facts.

The OP was asking specific questions about an item of Class II Consumer A/V equipment. I have gone to some length providing relevant details, spec references etc. and have provided my own (for what it's worth) clear opinion on the specific item.

I don't care if Log cabins swept with spruce twigs, or any other domestic environment are envolved. Equipment is designed to strict an binding safety specifications where they have no latitude for fanciful stories.

No, the OT were already antique radios and antique test equipments. Yes the exposure to mains voltage is relative as the voltage is a potential difference of two points. If you have only one point of contact there is no potential difference (except through non-galvanic paths) and no current flow and no energy to do the harm. Indeed I look this through electrical (power) engineering glasses not electronics engineering glasses.

PS. The 'fanciful story' and the next chapter after it hopefully gives some more understanding for random reader what is happening with the mains. It is not directed to you or not written as any sort of design instruction.

[ebastler]

No, the OT were already antique radios and antique test equipments.

Umm, no?! 

Look, the original post is still here for your convenience:
https://www.eevblog.com/forum/beginners/grounding-old-equipment/
And is is worded pretty clearly, and talks about one specific piece of stereo gear (only).

[hermit]

I had a customer that said every time she closed the refrigerator door it blew her house fuse.  Cause?  Her "electrician" had decided since neutral and ground were the same he would ground the socket by tying them together.  Problem?  He tied ground and hot.  There most not have been a good earth ground in this place because it didn't blow the fuse until she shut the door and it contacted the register duct.  This thing was right next to the sink where she did dishes.  Can you imagine if she decided to reach over and get a cold drink?

Again, you add something that wasn't there and you become a potential target.  Yes the fault would REALLY be with the socket.  You want to go to court and prove that?  You charge enough for that repair to cover the legal costs if they arise?  Some of my old stereo equipment came with a ground screw on the back.  Of course back them most house receptacles were 2 wire.  I lived in one place where they ran wire up through the old gas light pipe.  But I digress.  But then again, so has this thread.   

[Vtile]

No, the OT were already antique radios and antique test equipments.

Umm, no?! 

Look, the original post is still here for your convenience:
https://www.eevblog.com/forum/beginners/grounding-old-equipment/
And is is worded pretty clearly, and talks about one specific piece of stereo gear (only).

Well, I were indeed wrong about post #81 it is drifted to antique TVs and antique test instruments.  Not antique radios and test gear. Good catch. Anything else?     

It is good that Gyro and some others have bring the specific information what current standards have to say on the subject, since those aren't accessible to general public due the huge $$$ of cost related to get a representive set of publications (while given some of the properties of the law) and the ever increasing update cycle (ripping off).

[ebastler]

Well, I were indeed wrong about post #81 it is drifted to antique TVs and antique test instruments.  Not antique radios and test gear. Good catch. Anything else?     

I think there is a difference between (a) repeatedly trying to reel the thread back in, by pointing out why references to old test equipment etc. are not relevant to the OP's question, due to differences in the safety requirements and standards, and (b) telling anecdotes about log cabins.

Now excuse me while I look for the icon depicting "beating a pop-corn eating smart-alec"...

[Vtile]

It definedly is not my fault if it is too much to ask to understand the anecdote, which by the way is pretty close to the typical imaginary excercises (like the story hermit shared) used in introduction level electrician training by many teachers.

[tronde]

Finally stop quoting your 4mm and 5mm mantra as the only safety metric,

Gyro, you must learn to READ.

I have never claimed 4 and 5mm as the only safety metric. I have always mentioned that the environment must be considered.
Already in my first post I wrote:

"This is a stereo. It's meant for a normal domestic environment, i.e. no excessive humidity or dust. It is not ment to be mechanically abused as power tools are."

I have also written:

"The intended use and environment plays a big role in the design and certification process. Equipment meant to be used in a normal office / living room environment is subject to lighter regulations than something meant for use in the kitchen or outdoors or as toys for instance."


But you prefer to be blind for this. Only you can tell why.





Electrical safety has nothing to do with words written on a paper.
Legal safety has something to do with words on paper.

No product will become safe because of words, and no product will become unsafe because we call the product something else.

What makes a product safe depends on the environment and physics. 4 and 5mm will provide the clearances necessary for normal mains voltages in a normal office or living room environment, regardless of what is written on any paper or what we prefer to call the product.

 The reason the standards increases this distances for certain products is not because we call the product someting else than computer or fax or monitor. The reason they demand more clearance for some products is because they know that those products probably will be exposed to a more demanding environment, for instance people acting stupid. They increase the safety margin. Not necessarily because the product itself needs it, but because of the expected environment. They don't make a separate standard for every product imaginable. They try to group them according to expected use and environment.

Your computer monitor with 4/5mm distances is considered perfectly safe when you use it as a monitor for your computer in your living room. This monitor will not become unsafe just because you hang it on the wall in the same living room and uses it as a TV together with a TV dongle. According to words written on paper, it is now a TV and should be tested as a TV to be conform with a more demanding standard. Because of this it is legally unsafe. It is still electrically safe because it is still used in the environment it was designed for. If you move it to the kitchen or outdoors, you can find it to be both electricall and legally unsafe because the environment is too different from what it was designed for regardless of being used as a TV or a monitor.


But - I don't think you ever will get it.
It seems like it is impossible for you to see the difference between how to find out if an existing product can be considered as safe to use, and the design process needed to create a new product that will have to follow all legal requirement in today's legislation.

[Zero999]

The standards are overkill in many respects, because they've been developed to cater for the worst case scenario and ageing. 4mm clearance and 5mm creepage are around an order of magnitude higher than what's necessary to insulate something at mains voltages. You could design a PCB with 0.5mm creapage between the mains and tracks exposed parts and it wouldn't shock anyone 99% of the time. It wouldn't pass the safety standards but it would still have more than adequate insulation for 230VAC.

So why don't the standards just say 0.4mm clearance and 0.5mm creepage? Why 4mm and 5mm? Because the mains can have high voltage transients on it, which will easily arc across a 0.5mm gap and have enough energy to hurt someone. Even if the transient itself isn't enough to kill, the conductive plasma will connect the metal case to the live parts, causing mains current to flow to the user. To address this the standards take into account the expected peak voltage of the transients, how often they occur and the potential for injury when the insulation is breached. This is why the minimum distances are greater for an industrial, than a domestic environment and are greater reinforced insulation than single insulation, which is greater than functional insulation and that's dependant on whether it's before or after a fuse/circuit breaker.