Grounding old equipment

[tronde]

I seemed to have started, or helped start, an argument.

My quarrel was with the "I never stop being puzzled about electronics people and their fear for mains voltage." It's like saying you can have a lackadaisical attitude to mains.

I fear with comments like that people googling and finding this forum as a newbie should be comfortable playing with it. "She'll be alright". That's not an example we should set.

Since this obviously is aimed at me:

No, I don't have that attitude to mains. I do really respect mains, but I do not fear it as so many other on this forum seems to do because I understand what can be dangerous, and what is not. I have formal education and work traing to say so, and I am allowed to work on this kind of equipment according to Norwegan law which actually has specific requirements for skills.

It is a huge difference between 250 volts and kilovolts. When people talk about electrocution because this stereo lacks grounding, they need to be told the truth, namely that 250 volt is a completely different beast than anything in the kilovolt-domain. There is a very good reason why the European low voltage directive ends at 1000V ac, and that a normal electrician is not allowed touch wiring above that.

When people keep on talking about mains as something extremely dangerous regardless of the setting, they cause more harm than good to beginners. Any person above 8-10 years old, and living in a country with mains will know that mains can be dangerous. If we treat it with common sense it is safe.

The fact that any properly designed and built USB charger with no more than 4mm clearance and 5mm creepage is perfectly safe according to both IEC and all European safety agencies should ring a lot of bells. But, unfortunately, people controlled by fear does not get it. They keep on with scaremongering that will hide the real danger electricity can cause.

Many people seems to believe that low voltage is safe.
The European regulations are based on that no current above 30mA shall pass through a person's heart, because 30mA will not cause severe harm to a normal person. Based on statistics, they found that a normal person will have a skin resistance that will give no more than 30mA if exposed to 50V ac. This is not the same as anything below 50V ac is safe. This is the reason why some types of environment will require 24V or even 12V SELV equipment. When in a bath tube for instance, anything above 12V is considered dangerous.

Even if you don't get a cardiac arrest by 30mA, a current below that can cause severe harm, given the rigt conditions.

Some years ago, the most important cause for death among Norwegan electricians was not electrocution. It was the combination of a tall ladder, a light shock and an unfriendly meeting with the concrete floor...


If you care that much about beginners googling around, you should not discuss anything related to mains. You will always find someone that will do silly things. If you find it difficult that I write something that is true, because somebody can misunderstand it, we are heading in a completely wrong direction. The only way we can interact safely with electricity, is when we act based on understanding.

Maybe you should add a couple of lines to your signature telling beginners to stay away from mains until they have got enough understanding tho handle it in a safe way?

[b_force]

I seemed to have started, or helped start, an argument.

My quarrel was with the "I never stop being puzzled about electronics people and their fear for mains voltage." It's like saying you can have a lackadaisical attitude to mains.

I fear with comments like that people googling and finding this forum as a newbie should be comfortable playing with it. "She'll be alright". That's not an example we should set.

Since this obviously is aimed at me:

No, I don't have that attitude to mains. I do really respect mains, but I do not fear it as so many other on this forum seems to do because I understand what can be dangerous, and what is not. I have formal education and work traing to say so, and I am allowed to work on this kind of equipment according to Norwegan law which actually has specific requirements for skills.

It is a huge difference between 250 volts and kilovolts. When people talk about electrocution because this stereo lacks grounding, they need to be told the truth, namely that 250 volt is a completely different beast than anything in the kilovolt-domain. There is a very good reason why the European low voltage directive ends at 1000V ac, and that a normal electrician is not allowed touch wiring above that.

When people keep on talking about mains as something extremely dangerous regardless of the setting, they cause more harm than good to beginners. Any person above 8-10 years old, and living in a country with mains will know that mains can be dangerous. If we treat it with common sense it is safe.

The fact that any properly designed and built USB charger with no more than 4mm clearance and 5mm creepage is perfectly safe according to both IEC and all European safety agencies should ring a lot of bells. But, unfortunately, people controlled by fear does not get it. They keep on with scaremongering that will hide the real danger electricity can cause.

Many people seems to believe that low voltage is safe.
The European regulations are based on that no current above 30mA shall pass through a person's heart, because 30mA will not cause severe harm to a normal person. Based on statistics, they found that a normal person will have a skin resistance that will give no more than 30mA if exposed to 50V ac. This is not the same as anything below 50V ac is safe. This is the reason why some types of environment will require 24V or even 12V SELV equipment. When in a bath tube for instance, anything above 12V is considered dangerous.

Even if you don't get a cardiac arrest by 30mA, a current below that can cause severe harm, given the rigt conditions.

Some years ago, the most important cause for death among Norwegan electricians was not electrocution. It was the combination of a tall ladder, a light shock and an unfriendly meeting with the concrete floor...


If you care that much about beginners googling around, you should not discuss anything related to mains. You will always find someone that will do silly things. If you find it difficult that I write something that is true, because somebody can misunderstand it, we are heading in a completely wrong direction. The only way we can interact safely with electricity, is when we act based on understanding.

Maybe you should add a couple of lines to your signature telling beginners to stay away from mains until they have got enough understanding tho handle it in a safe way?

Thank you

[Specmaster]

Personally I would go ahead and ground the chassis anyway and then worry about and hum loops later if and when any are produced. I suspect that its going to be unlikely as most audio I have seen normally only take one end of a screen cable to ground to minimise any risk of hum being induced. Its easier to fix hum loops then fix a person who has received a fatal shock thats for sure.. just ground it and be done with it. Unless its a real piece of hi-fi quality audio, any slight hum is unlikely to be heard anyway as 50hz is pretty low frequency and some of the so called audio gear would struggle to reproduce that level of sound anyway.

[Gyro]

A couple of corrections guys...

The ONLY risks of live wire getting contact with the chassis is when the wires will break off or when the chassis is heavily damaged (dented).
In the last case, the user isn't allowed to use the product anymore (and shouldn't)

There are several ways that the chassis can become live in Class II consumer gear, you mentioned two of them. There is also failure of fixings, inappropriate choice of board materials. Some may visibly evident, some may require vigorous shaking, some may have no outward signs. That's where defensive design, proper testing and certification come in. In addition I would always take a sample unit into the car park and drop it from head height a few times as a 'belt and braces' sure it will break but will it break safely. Slavishly clearance figures doesn't do it.

The European regulations are based on that no current above 30mA shall pass through a person's heart, because 30mA will not cause severe harm to a normal person.

That's Body, not heart. It takes way less than 30mA directly through the heart to cause fibrillation. The 30mA figure, assuming limb to limb is designed to ensure prevent the heart, shunted by other body tissues, won't go into fibrilation.

[tronde]

You don't need to fix any person.

You really must stop these claims. They are WRONG. Please tell us why you must fix a person with this stereo, but not with a legal USB charger designed without any earth or several layers of insulation. Please tell us.  You must obviously know someting neither the IEC nor any of the European safety agencies are aware of.

The best you can hope for with the charger is 4mm clearance and 5mm creepage. It does not kill you. Why do you think it is different with this stereo? It was designed to be safe as it is, and it is still safe that way. If the stereo is marked as "double insulated" as it would have been in Europe, it is illegal to add any PE, regardless of your feeling for "safe" or "unsafe".

[Zero999]

Personally I would go ahead and ground the chassis anyway and then worry about and hum loops later if and when any are produced.

Whilst that probably wouldn't do any harm, I see little point in connecting the chassis to earth. Do it if it will make you feel better but if you're really worried about safety, there are other things you can do which are much more effective, such as:

• Adding a cable tie to secure the power cord.
• Covering the connection from the mains to the PCB with heat shrink or acid free silicone (don't use the stuff which emits acetic acid when curing, it can cause corrosion and make it worse)
• Putting some flame retardant insulating material, such as kapton tape on the case below the PCB.

I suspect that its going to be unlikely as most audio I have seen normally only take one end of a screen cable to ground to minimise any risk of hum being induced. Its easier to fix hum loops then fix a person who has received a fatal shock thats for sure.. just ground it and be done with it. Unless its a real piece of hi-fi quality audio, any slight hum is unlikely to be heard anyway as 50hz is pretty low frequency and some of the so called audio gear would struggle to reproduce that level of sound anyway.

It isn't 50Hz which causes most of the mains hum because, as you've correctly state, 50Hz isn't very audible, but the harmonics which can be quite loud.

[tronde]

The European regulations are based on that no current above 30mA shall pass through a person's heart, because 30mA will not cause severe harm to a normal person.

That's Body, not heart. It takes way less than 30mA directly through the heart to cause fibrillation. The 30mA figure, assuming limb to limb is designed to ensure prevent the heart, shunted by other body tissues, won't go into fibrilation.

Does this change the reason for the choice of 50V? No?
Does it make voltages below 50V safe? No?

[Specmaster]

The European regulations are based on that no current above 30mA shall pass through a person's heart, because 30mA will not cause severe harm to a normal person.

That's Body, not heart. It takes way less than 30mA directly through the heart to cause fibrillation. The 30mA figure, assuming limb to limb is designed to ensure prevent the heart, shunted by other body tissues, won't go into fibrilation.

Does this change the reason for the choice of 50V? No?
Does it make voltages below 50V safe? No?

You're right, it depends on where the shock is received and also on the person state of health as well as the actual conditions in how the shock was received, could be standing in deep flood water and unplugging the equipment. A shock in conditions like that could well be fatal.  I believe I'm right, but not positive about this but I think a current of less than 10ma flowing across the heart is enough.

[Gyro]

You don't need to fix any person.

You really must stop these claims. They are WRONG. Please tell us why you must fix a person with this stereo, but not with a legal USB charger designed without any earth or several layers of insulation. Please tell us.  You must obviously know someting neither the IEC nor any of the European safety agencies are aware of.

The best you can hope for with the charger is 4mm clearance and 5mm creepage. It does not kill you. Why do you think it is different with this stereo? It was designed to be safe as it is, and it is still safe that way. If the stereo is marked as "double insulated" as it would have been in Europe, it is illegal to add any PE, regardless of your feeling for "safe" or "unsafe".

I don't know where you've dragged the USB charger up from, we were talking about metal cased class II consumer equipment. Metal cased Class II is a lot more difficult to get right than a USB charger.

I think one of the reasons that we're talking as cross purposes is that I'm talking from the equipment designer's perspective. I've already said that the OP's unit looks safe [EDIT: from what I can see within the limits of the photo!], after a basic check of the condition of the end of the mains lead.

My point is that, when designing equipment of this type, there are so many other things to be taken into account than meeting a basic creepage and clearance spec. There are many external factors such as handling shocks, aging, the old incontinent old cat finding a warm place to sleep! There are so many thing to consider to produce a consistently safe product...

- PCB material strenght and brittleness under shock - I've seen badly designed equipment where all four corners of the SMPS PCB have snapped off!
- Lengths of wiring assemblies - not just the mains lead if it becomes disconnected, it's also the length of internal secondary side wiring looms - can they become disconnected and touch components on the primary side of the power supply?
- Can fixings become loose?
- .... It's a long list

Maybe I am guilty of trying to insert some education into this thread. What I am saying is that creepage and clearance are the very start of the design process, not an assurance that, if met, then everything will be fine on every unit. That's just not acceptable for a volume product. After all, a designer has to be able to sleep at night!

Even when meeting all appropriate regulations I still have reservations about what they permit. For instance allowing ventilation holes to be slots. They will quite happlly pass the EU 3mm finger probe test but will still allow a small child to post metal paperclips into the enclosure and bridge the safety clearances. Such possibilitiess can be a worry in the early hours despite your best efforts.

Just one other thing, it can't be illegal to ground a Class II product. Unless it has zero external interfaces, it must be able to tolerate any / all of their shields being grounded and through them, the case itself. It is illegal to modify the case in any way however (To clarify, that includes fitting a ground lead connection under a fixing screw).

[John B]

Well this was a bit of a hornet's nest. To clarify a few things:

-Its a Sony HST V202R for anyone curious.

- There is no insulation under the mains PCB, probably just a shadow from the camera flash. The posts that the mains wiring connect to go straight through the board and are exposed underneath the board.

- The big transformer in the photo is bolted to the sheet metal base, the same thing that the mains PCB is bolted to.

- The photo might be a bit confusing. I have to lay the unit on its side because the whole thing is actually quite flimsy.

- This unit will probably operate as a floating system. It has an external vinyl record player which is powered through this unit, and also has passive speakers. The only potential grounding point is if its connected to a grounded CD player through the RCA jacks, or if the headphone jack is utilised. Then you have to rely on the piddly earth shield of an audio cable as the grounding cable.

This thing is probably over 20 years old. That could mean many things depending on your perspective. It could mean that it's proven to be safe, or it could mean that the chances of something NOT going wrong are diminishing over time. I think of it like my car: its over 20 years old, but I have done lots of modernisation with areas such as the suspension and brakes. Old stuff can benefit from a modern perspective.

As an aside, I am repairing cracked solder joints due to mechanical fatigue over time. So the flexible nature of the chassis is now manifesting itself in at least some mechanical failures.

At the very least I will explain my concern to the person who owns the unit. They are a friend and so I can talk to them easily. Would you not point out your concern?

[Specmaster]

Looking at the spec for this, any hum loops will be heard seeing as freq response goes down down to 17Hz so they could be quite deep in their sound. If you have the speakers to hand, I'd be inclined personally to ground the chassis on a 3 core lead for safety sake and then test the unit and deal with any hum pick-up while you have the unit. It is far better to do that now then wait until such time as your friend adds a newer and grounded CD player for instance and then discovers hum loops and then you have to go through it again.

It is also far safer especially as you pointed out, the headphone jack is connected to the chassis anyway and I'd personally rather have earth potential near my head than line potential for very obvious reasons. 

[floobydust]

John B, as you can see there's a lot of speculation, emotion and opinion regarding safety on older gear.

Why don't you do a hi-pot test, this is standard procedure on something like this. Unless we all want to speculate about the mains transformer construction too 

I don't see the product being dangerous, as much as you are scaring the owner into throwing it in the garbage.
It was designed with the method of protection as reinforced/double-insulated (floating) which is commonplace in AV equipment, power tools, hair dryers, fans, lamps etc.
The mains PCB is not supposed to move around, it's not a portable product. Sony sheet metal is flimsier when the cover is off.
I do not like the mains cord, SKX cable not used in North America yet. I think the tug test is tougher here.

[Electro Detective]

yep, a poor man's hi-pot test (if conducted properly without trashing DUT) using a basic insulation/megger/megohm tester at 250v and 500v 

soon decides the 'safety' condition of the device

Warning: "Don't try this at home" if not familiar with an insulation tester,
you may zap components and get bitten by it !   

[Cubdriver]

Well this was a bit of a hornet's nest. To clarify a few things:

-Its a Sony HST V202R for anyone curious.

- There is no insulation under the mains PCB, probably just a shadow from the camera flash. The posts that the mains wiring connect to go straight through the board and are exposed underneath the board.

- The big transformer in the photo is bolted to the sheet metal base, the same thing that the mains PCB is bolted to.

- The photo might be a bit confusing. I have to lay the unit on its side because the whole thing is actually quite flimsy.

- This unit will probably operate as a floating system. It has an external vinyl record player which is powered through this unit, and also has passive speakers. The only potential grounding point is if its connected to a grounded CD player through the RCA jacks, or if the headphone jack is utilised. Then you have to rely on the piddly earth shield of an audio cable as the grounding cable.

This thing is probably over 20 years old. That could mean many things depending on your perspective. It could mean that it's proven to be safe, or it could mean that the chances of something NOT going wrong are diminishing over time. I think of it like my car: its over 20 years old, but I have done lots of modernisation with areas such as the suspension and brakes. Old stuff can benefit from a modern perspective.

As an aside, I am repairing cracked solder joints due to mechanical fatigue over time. So the flexible nature of the chassis is now manifesting itself in at least some mechanical failures.

At the very least I will explain my concern to the person who owns the unit. They are a friend and so I can talk to them easily. Would you not point out your concern?

One question is where are the cracked solder joints you're repairing?  Are they in the line side of the power supply circuit, or elsewhere?  If in the AC section, I might be at least somewhat concerned, but that would depend on their location and nature.  If somewhere other than the power supply, then I see much less of an issue.

The fact that it's around 20 years old and is a consumer product likely built in the tens of thousands (along with additional tens of thousands of other, similar models of similar construction) leads me to believe that it's likely quite safe - if it were not, by now there would have been some sort of a ruckus raised about people being electrocuted by them, and lawsuits would have flown.

If it makes you more comfortable, then put a piece of fish paper on the chassis under the board, and/or put a dollop of electronics grade RTV on the exposed pins, as others have suggested.  If the cabinet flexes enough for the board clearance to be an issue, I suspect that it will be the result of having taken heavy physical damage, and at that point anyone foolish enough to plug it in almost deserves to get lit up - at some point one must exercise a tiny bit of common sense and be smart enough to realize that if the piece of gear looks like it was run over by a bulldozer that they should NOT plug it in and attempt to turn it on.

I agree with what was posted earlier that people need to be less freaked out about AC mains.  Please note that this is not a digital condition where "less freaked out" = "completely care free, let's down a fifth of Jack Daniel's finest Tennessee Whiskey and go work on the breaker panel".  Think linear - by less freaked out, I mean not getting nervous because the device built by the thousands that has worked for 10 or 20 or 30 or more years doesn't meet today's safety specs.  If, for instance, 6mm of clearance was the spec when it was built, and 6mm has been fine for 20 years, it's unlikely to begin arcing over tomorrow because the new spec now says 8mm.  AC Mains must be respected, and worked on with care, but if due care is taken they are not terribly dangerous to work on.  The key is to read up and learn about the dangers, and act accordingly. 

-Pat

[Gyro]

Well this was a bit of a hornet's nest. To clarify a few things:

-Its a Sony HST V202R for anyone curious.

- There is no insulation under the mains PCB, probably just a shadow from the camera flash. The posts that the mains wiring connect to go straight through the board and are exposed underneath the board.

- The big transformer in the photo is bolted to the sheet metal base, the same thing that the mains PCB is bolted to.

- The photo might be a bit confusing. I have to lay the unit on its side because the whole thing is actually quite flimsy.

- This unit will probably operate as a floating system. It has an external vinyl record player which is powered through this unit, and also has passive speakers. The only potential grounding point is if its connected to a grounded CD player through the RCA jacks, or if the headphone jack is utilised. Then you have to rely on the piddly earth shield of an audio cable as the grounding cable.

This thing is probably over 20 years old. That could mean many things depending on your perspective. It could mean that it's proven to be safe, or it could mean that the chances of something NOT going wrong are diminishing over time. I think of it like my car: its over 20 years old, but I have done lots of modernisation with areas such as the suspension and brakes. Old stuff can benefit from a modern perspective.

As an aside, I am repairing cracked solder joints due to mechanical fatigue over time. So the flexible nature of the chassis is now manifesting itself in at least some mechanical failures.

At the very least I will explain my concern to the person who owns the unit. They are a friend and so I can talk to them easily. Would you not point out your concern?

Ah, good. Back to practicalities.

I have the same question as Pat - if the joints you're having to rework are on that power board(?) then you might want to add a bit of belt and braces sheet insulation, otherwise, I wouldn't bother. I've already mentioned checking the mains lead.

Looking at the construction, you have bits screwed to the base and the rear panel - Sony were obviously aware of the issue of differential movement - you can see that by the length of the wire jumpers between the base PCB, the main transformer PCB and the transformer (creating a production nightmare for themselves in the process  ).  As long as those aren't the joints that you're having to repair then I wouldn't be too concerned.

There's no inherent reason why a 20 year old bit of kit would be any less safe than a modern one - it might actually be safer in some respects due to the nice chunky linear supply and Japanese design / components. SRBP boards don't age that well, track adhesion isn't that great and they become more brittlewith age but there are no obvious cracks (the main board might be a different story of course).

Probably the best advice I can give is complete your repairs as quickly as possible and get the lid screwed back on. Those cabinets rely heavily on the lid to provide rigidity. The longer you have the lid off, the more joint failures you're likely to have to fix.

Chris

[Specmaster]

It was designed with the method of protection as reinforced/double-insulated (floating) which is commonplace in AV equipment, power tools, hair dryers, fans, lamps etc.

Isn't double insulated only applicable when the equipment is constructed out of a non-conducting material, such as fans, hair dryers power drills etc are, find an industrial fan for instance and they will be made out of metal and fitted with a 3 core cable, 3 pin plug so that the exposed metal can earthed or grounded so as to protect the user from a shock in event of problem occurring which allows the metal to become live. Yes it shouldn't do in a well designed product but there is always that chance.  Granted, the current standards were not in force when it was made, but there was a reason why modern standards were introduced so if the old standard was perfectly safe, then why did the authorities feel the need to write new standards?

[Vtile]

There is always the insurance, politics and lawyer lobbying fear factors.

[Gyro]

It was designed with the method of protection as reinforced/double-insulated (floating) which is commonplace in AV equipment, power tools, hair dryers, fans, lamps etc.

Isn't double insulated only applicable when the equipment is constructed out of a non-conducting material, such as fans, hair dryers power drills etc are, find an industrial fan for instance and they will be made out of metal and fitted with a 3 core cable, 3 pin plug so that the exposed metal can earthed or grounded so as to protect the user from a shock in event of problem occurring which allows the metal to become live. Yes it shouldn't do in a well designed product but there is always that chance.  Granted, the current standards were not in force when it was made, but there was a reason why modern standards were introduced so if the old standard was perfectly safe, then why did the authorities feel the need to write new standards?

Nope, it's fully applicable to (appropriately designed of course) metal cased equipment too. Specifically, in the case of consumer A/V equipment the requirements are covered under EN60065 (it also covers things like fire risk). The latest version is EN60065:2014. Before that 2011 and, I think 2002.

Note that things like USB chargers get an easier ride by coming under the IT equipment safety standard EN60950. Jammy gits.

It's the nature of standards to evolve with time, otherwise what would the standards committees have to lunch on. <cynical emoticon missing>. Real life experience and changes in technology (eg widespread SMPSs) do result improved standards, that remain relevant over time, however.


P.S. Those are the European specs, there are equivalent specs for other parts of the world. The British Standards spec was harmonized into the European spec as BS-EN60065.

[tronde]

I don't know where you've dragged the USB charger up from, we were talking about metal cased class II consumer equipment. Metal cased Class II is a lot more difficult to get right than a USB charger.

The reason is becuase most people can relate to them.

When they are considered as safe with 4mm clearance and 5mm creepage it does not make any sense to talk about electrocution because the stereo is in an ungrounded metal case.

It does not matter whether the enclosure is plastic as in the charger or metal as in the stereo. The imortant thing is if the product has any "exposed parts" that can come in contact with live voltage. The size of the exposed part does not matter. The charger will usually be in a plastic enclosure, but the USB contact and the 5V out is "exposed", and it is only separated from the danger by 4 and 5mm.

People tend to misunderstand the term "double insulated". It does not necessarily mean "two layers of wrapping". It does mean that the insulation shall provide the same level of protection as two layers. In the standards they list clearance and creepage distances. Those listed as "reinforced" is meant to give the same level of safety as two layers of insulation material.




You are of course correct when you say that we should opt for more than the bare minimum when we design something. You suggest 8mm. That is in fact the creepage distance normally used in medical equipment, so it will give a very high level of insulation.
You are also right about mechanical stress. They test for mechanical stability if you do a certification test, so it is obviously important.




My intention with the first post was, and is, that people must get a realistic view on electrical safety and danger.

250V is the normal upper limit for domestic mains. It is of course dangerous if you touch bare wires, but it is not like higher voltages that will easily flash over several milimeters as many seems to believe  the mains will do too.

You say in a later post that the USB charger is subject to a lighter standard. Not necessarily. You can choose to certify and test some types of"multi media equipment" to the same standard.

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.


Edit: Some of the typos.

[Specmaster]

It was designed with the method of protection as reinforced/double-insulated (floating) which is commonplace in AV equipment, power tools, hair dryers, fans, lamps etc.

Isn't double insulated only applicable when the equipment is constructed out of a non-conducting material, such as fans, hair dryers power drills etc are, find an industrial fan for instance and they will be made out of metal and fitted with a 3 core cable, 3 pin plug so that the exposed metal can earthed or grounded so as to protect the user from a shock in event of problem occurring which allows the metal to become live. Yes it shouldn't do in a well designed product but there is always that chance.  Granted, the current standards were not in force when it was made, but there was a reason why modern standards were introduced so if the old standard was perfectly safe, then why did the authorities feel the need to write new standards?

Nope, it's fully applicable to (appropriately designed of course) metal cased equipment too. Specifically, in the case of consumer A/V equipment the requirements are covered under EN60065 (it also covers things like fire risk). The latest version is EN60065:2014. Before that 2011 and, I think 2002.

Note that things like USB chargers get an easier ride by coming under the IT equipment safety standard EN60950. Jammy gits.

It's the nature of standards to evolve with time, otherwise what would the standards committees have to lunch on. <cynical emoticon missing>. Real life experience and changes in technology (eg widespread SMPSs) do result improved standards, that remain relevant over time, however.


P.S. Those are the European specs, there are equivalent specs for other parts of the world. The British Standards spec was harmonized into the European spec as BS-EN60065.

Oh, so are you telling me then that when I buy up old test equipment like signal generators, Heathkit Valve Voltmeters and the like that were originally wired up with 2 core cable and I rewire them with 3 core and 13A plug suitably fused that I shouldn't do that but leave them as they were?

[ebastler]

Oh, so are you telling me then that when I buy up old test equipment like signal generators, Heathkit Valve Voltmeters and the like that were originally wired up with 2 core cable and I rewire them with 3 core and 13A plug suitably fused that I shouldn't do that but leave them as they were?

No, actually Gyro was telling you that "double insulated" is a category that is also applicable to devices with metal enclosures. He corrected your false conjecture, and gave specific references.

How about acknowledging that you were mistaken, and thanking Gyro for the correction? Why do you need to react with an exaggeration and (presumably intentional) misinterpretation of his further comments?

[Gyro]

Oh, so are you telling me then that when I buy up old test equipment like signal generators, Heathkit Valve Voltmeters and the like that were originally wired up with 2 core cable and I rewire them with 3 core and 13A plug suitably fused that I shouldn't do that but leave them as they were?

I'm not telling you anything of the sort. 

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.

[Gyro]

I don't know where you've dragged the USB charger up from, we were talking about metal cased class II consumer equipment. Metal cased Class II is a lot more difficult to get right than a USB charger.

The reason is becuase most people can relate to them.

When they are considered as safe with 4mm clearance and 5mm creepage it does not make any sense to talk about electrocution because the stereo is in an ungrounded metal case.

It does not matter whether the enclosure is plastic as in the charger or metal as in the stereo. The imortant thing is if the product has any "exposed parts" that can come in contact with live voltage. The size of the exposed part does not matter. The charger will usually be in a plastic enclosure, but the USB contact and the 5V out is "exposed", and it is only separated from the danger by 4 and 5mm.

People tend to misunderstand the term "double insulated". It does not necessarily mean "two layers of wrapping". It does mean that the insulation shall provide the same level of protection as two layers. In the standards they list clearance and creepage distances. Those listed as "reinforced" is meant to give the same level of safety as two layers of insulation material.




You are of course correct when you say that we should opt for more than the bare minimum when we design something. You suggest 8mm. That is in fact the creepage distance normally used in medical equipment, so it will give a very high level of insulation.
You are also right about mechanical stress. They test for mechanical stability if you do a certification test, so it is obviously important.




My intention with the first post was, and is, that people must get a realistic view on electrical safety and danger.

250V is the normal upper limit for domestic mains. It is of course dangerous if you touch bare wires, but it is not like higher voltages that will easily flash over several milimeters as many seems to believe  the mains will do too.

You say in a later post that the USB charger is subject to a lighter standard. Not necessarily. You can choose to certify and test some types of"multi media equipment" to the same standard.

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.


Edit: Some of the typos.

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.

I didn't want to further complicate things yesterday but you keep quoting your 4mm clearance 5mm creepage and all will be well with the world. You've clearly not designed high volume consumer (I have). You need to understand that different safety standards have different requirements, for instance the magic figure of 8mm rears it's head in EN60065 (in fact I think the 2014 edition increased the standoff requirement for a primary voltage PCB from 8mm above the chassis to 10mm without an intervening insulating layer, ie. air insulation only. I can't say for sure, I don't have a copy, it's thankfully after my time). The standards also clearly define what is meant by reinforced, double layer etc. in the context of that equipment type (yes there is overlap between standards.

Different safety standards take into consideration different usage and environmental conditions - for instance buildup of household dust, risk of foreign object introduction, the previously mentioned incontinent cat pee (happens a lot more often than you might think). The EN60950 IT equipment spec is quite lenient in comparison - it was born out of nice clean filtered computer rooms. It has since been stretched like a piece of elastic to cover Home PCs cheap, nasty, USB chargers (assuming that they have been manufactured to standard in the first place) - It has even been stretched to cover portable music players for heavens sake! I am just waiting for the day when some manufacturer manages to get his Smart TV certified as IT equipment rather than Consumer A/V, nothing would surprise me these days. Just to give a taste of recent EN60950 changes... http://ewh.ieee.org/r6/ocs/pses/EN60950.pdf  I can't find an equivalent off hand for EN60065.

If you want to see and compare the various standards then you have to buy them. They're not cheap either as they're intended for manufacturers... https://shop.bsigroup.com/ProductDetail/?pid=000000000030254244

I fully understand that you want to take the 'fear' out of electrical safety, and I agree with you. You, in turn, must understand that every clause in every safety standard is there for a good reason, it's not open to discussion. It must be complied with, regardless of any 'percieved' risk. If a death happens, then it is the design authority for the equipment who is responsible, he can't argue that 'mains isn't really as dangerous as people think'.

I really hope that this clarifies. 

[Zero999]

It was designed with the method of protection as reinforced/double-insulated (floating) which is commonplace in AV equipment, power tools, hair dryers, fans, lamps etc.

Isn't double insulated only applicable when the equipment is constructed out of a non-conducting material, such as fans, hair dryers power drills etc are, find an industrial fan for instance and they will be made out of metal and fitted with a 3 core cable, 3 pin plug so that the exposed metal can earthed or grounded so as to protect the user from a shock in event of problem occurring which allows the metal to become live. Yes it shouldn't do in a well designed product but there is always that chance.  Granted, the current standards were not in force when it was made, but there was a reason why modern standards were introduced so if the old standard was perfectly safe, then why did the authorities feel the need to write new standards?

Nope, it's fully applicable to (appropriately designed of course) metal cased equipment too. Specifically, in the case of consumer A/V equipment the requirements are covered under EN60065 (it also covers things like fire risk). The latest version is EN60065:2014. Before that 2011 and, I think 2002.

Note that things like USB chargers get an easier ride by coming under the IT equipment safety standard EN60950. Jammy gits.

It's the nature of standards to evolve with time, otherwise what would the standards committees have to lunch on. <cynical emoticon missing>. Real life experience and changes in technology (eg widespread SMPSs) do result improved standards, that remain relevant over time, however.


P.S. Those are the European specs, there are equivalent specs for other parts of the world. The British Standards spec was harmonized into the European spec as BS-EN60065.

Oh, so are you telling me then that when I buy up old test equipment like signal generators, Heathkit Valve Voltmeters and the like that were originally wired up with 2 core cable and I rewire them with 3 core and 13A plug suitably fused that I shouldn't do that but leave them as they were?

As far as modifying old equipment is concerned. I'd go on a case by case basis. In some cases using three core cable would make it more dangerous and wouldn't consider that, unless it's necessary.

[Specmaster]

Oh, so are you telling me then that when I buy up old test equipment like signal generators, Heathkit Valve Voltmeters and the like that were originally wired up with 2 core cable and I rewire them with 3 core and 13A plug suitably fused that I shouldn't do that but leave them as they were?

No, actually Gyro was telling you that "double insulated" is a category that is also applicable to devices with metal enclosures. He corrected your false conjecture, and gave specific references.

How about acknowledging that you were mistaken, and thanking Gyro for the correction? Why do you need to react with an exaggeration and (presumably intentional) misinterpretation of his further comments?

No, I was not misinterpreting his comments intentionally at all, merely asking the question because 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