All
I have an FM Tuner which has a two conductor (Live and Neutral) mains cable with a 'figure of eight' plug at the tuner end. Although the tuner circuit GND on the PCB is connected to the metal chassis, the chassis isn't connected to mains earth because there isn't one.
Three questions:
1. If there is no mains earth due to the two conductor mains cable, isn't there a danger the metal chassis could become live without a fuse blowing?
2. If the chassis isn't connected to mains earth how is it acting as a shield?
3. Can I just switch the two conductor (L + N) mains cable to a three conductor (L, N + E) one and connect mains earth to the chassis to address questions 1 and 2?
Any ideas?
I have an FM Tuner which has a two conductor (Live and Neutral) mains cable with a 'figure of eight' plug at the tuner end. Although the tuner circuit GND on the PCB is connected to the metal chassis, the chassis isn't connected to mains earth because there isn't one.
Three questions:
1. If there is no mains earth due to the two conductor mains cable, isn't there a danger the metal chassis could become live without a fuse blowing?
2. If the chassis isn't connected to mains earth how is it acting as a shield?
3. Can I just switch the two conductor (L + N) mains cable to a three conductor (L, N + E) one and connect mains earth to the chassis to address questions 1 and 2?
Any ideas?
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1. No danger in normal use because the unit will comply with double insulated standard. This also assumes that anything you connect it too (amplifier etc) also comply with safety regulations.
If you consider "non normal use" then yes, connecting the chassis to the live side of the mains would make the unit live without blowing the fuse... but how could that happen?
As mentioned, any partnering equipment must also conform fully to safety standards.
So any DIY amp etc must meet all relevant safety requirements.
2. The tuner will meet it's specs without a ground connected... why would it need the chassis or case to act as a shield? Remember ground and zero volt lines are all arbitrary references. In practical use it probably is grounded via other connections, the audio leads, maybe even the aerial but these can not be considered in any way an "earth" connection as far as safety goes.
3. You could fit a 3 core lead but that [might] cause ground loop issues when it's all connected up.
If you consider "non normal use" then yes, connecting the chassis to the live side of the mains would make the unit live without blowing the fuse... but how could that happen?
As mentioned, any partnering equipment must also conform fully to safety standards.
So any DIY amp etc must meet all relevant safety requirements.
2. The tuner will meet it's specs without a ground connected... why would it need the chassis or case to act as a shield? Remember ground and zero volt lines are all arbitrary references. In practical use it probably is grounded via other connections, the audio leads, maybe even the aerial but these can not be considered in any way an "earth" connection as far as safety goes.
3. You could fit a 3 core lead but that [might] cause ground loop issues when it's all connected up.
I must disagree with
3.) changing the cable.
A double insulated appliance must never be modified to bring it to ClassI
Similarly a ClassI appliance must always have a permanent Safety Earth connected to PE back at the distribution board. Until you unplug it !
2.) A shield operates by ensuring all the internal components are shielded from external fields, usually by connecting the shield to the circuit ground for the range of frequencies that one requires the shield to be effective.
A small cap from Audio Ground to metal chassis would effectively shield the internal circuits from RF interference.
1.) Does the appliance have the concentric squares symbol indicating double insulated? Use it, do not modify it !
3.) changing the cable.
A double insulated appliance must never be modified to bring it to ClassI
Similarly a ClassI appliance must always have a permanent Safety Earth connected to PE back at the distribution board. Until you unplug it !
2.) A shield operates by ensuring all the internal components are shielded from external fields, usually by connecting the shield to the circuit ground for the range of frequencies that one requires the shield to be effective.
A small cap from Audio Ground to metal chassis would effectively shield the internal circuits from RF interference.
1.) Does the appliance have the concentric squares symbol indicating double insulated? Use it, do not modify it !
Thanks for the replies. Pardon my ignorance but a few more queries if you could:
1. Mooly and AndrewT - regarding the metal case acting as a shield, where's the RFI intercepted by the case actually going to if it isn't connected to mains earth? As a I understand it, shields act by reflection and absorption but doesn't the latter require some kind of sink?
2. Does double insulated equipment mean that you can't add commercial metal can mains filters that have L, N & E tabs with caps connected between L and E and N and E inside the can?
3. AndrewT - I do use it - every day - and have improved the sound quite a bit by doing the usual component mods, adding extra regulators and re-routing lengthy pcb traces etc. but I'd like to understand why a double insulated appliance must never be modified to bring it to Class I? What damage could switching to a 3 core mains cable and connecting the chassis to mains earth actually do if the tuner is double insulated ?
1. Mooly and AndrewT - regarding the metal case acting as a shield, where's the RFI intercepted by the case actually going to if it isn't connected to mains earth? As a I understand it, shields act by reflection and absorption but doesn't the latter require some kind of sink?
2. Does double insulated equipment mean that you can't add commercial metal can mains filters that have L, N & E tabs with caps connected between L and E and N and E inside the can?
3. AndrewT - I do use it - every day - and have improved the sound quite a bit by doing the usual component mods, adding extra regulators and re-routing lengthy pcb traces etc. but I'd like to understand why a double insulated appliance must never be modified to bring it to Class I? What damage could switching to a 3 core mains cable and connecting the chassis to mains earth actually do if the tuner is double insulated ?
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I don't think that is how a shield works.
The shield radiates from all it's surfaces to all the interior components.
no modifications to double insulated appliances, not even swapping out the standard IEC socket for a filtered version.
I don't know what damage it might or might not do.
That's the Engineer in me. I want to know before I decide.
You could ask your household insurance company the same leading questions.
I can guess they would be rubbing their hands. Give you no answer and wait until you need to claim, then invalidate your insurance for unauthorised modifications.
A metal case can act as an RF shield without being grounded, provided it is not breached in any way i.e. all cables in and out have RF barriers. Then it acts as a Faraday screen, so external electric fields cannot get inside and external magnetic fields are cancelled by currents induced on the outside. If, as is likely, the external cables do not have RF barriers then the metal case can help RF get in and out. I guess the reason for using a metal case is that Class I and Class II versions can share most components, so saving costs for a worldwide item.
I would have thought that you could modify it to Class I, but you would have to do it properly with all external conductors grounded (not just the chassis).
I would have thought that you could modify it to Class I, but you would have to do it properly with all external conductors grounded (not just the chassis).
The shielding effect doesn't need the "shield" to be connected to earth. A metal plate in free space blocks (attenuates) RF signals. A mumetal shield will reduce induced 50 hz interference.
Of course you can use a filter as you describe... what you can't do is modify the equipment. So you have to connect the tuner using it's supplied lead to the 3 pin filtered outlet... result, the ground connection is not in place from the filter to the tuner.
Speaking practically... if the case is grounded it will not be a safety issue.
What is of concern is that the work done to make the case grounded may render the equpment dangerous in other ways... you might fit a screw that is to near a live connection etc.
Also the "chassis" may or may not be at what you refer to as ground. The primary side of a PSU in a SMPS (switch mode power supply) is at half mains potential regardless of the polarity of the live and neutral.
It comes down to the fact that the equipment is designed to exact safety standards, anything you do by taking the top off renders that invalid.
UK site
EMC Information Centre - The EMC Journal (Free in the UK)
http://www.compliance-club.com/
US site
EMC Journal portal | EMCJUSA
http://emcjusa.com/
The latest issue #91 of the EMC journal has an excellent article by Keith Armstrong; "Maxwell's Equations, Quantum Electrodynamics, and good installation practices for SI, PI and EMC".
Covers a lot of stuff on shielding, plus other articles on the site regarding the problems of shielding electronics these days.
Have Fun
EMC Information Centre - The EMC Journal (Free in the UK)
http://www.compliance-club.com/
US site
EMC Journal portal | EMCJUSA
http://emcjusa.com/
The latest issue #91 of the EMC journal has an excellent article by Keith Armstrong; "Maxwell's Equations, Quantum Electrodynamics, and good installation practices for SI, PI and EMC".
Covers a lot of stuff on shielding, plus other articles on the site regarding the problems of shielding electronics these days.
Have Fun
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I don't get this Class 2 business.
If a live wire comes loose inside a metal cased class 2 appliance like my tuner then the case will be live and will happily remain so because it's not connected to mains earth and so no large current will flow to earth, blowing the fuse in the live line. If you touch the case the current could instead flow to earth via you, killing you before the fuse has a chance to blow.
A class 1 appliance is earthed so if the case becomes accidentally live the live line fuse will blow, disconnecting the chassis from live, and you're safe. The only way you could be harmed is if you were unlucky enough to be touching the case at the moment the live wire came loose.
Class 2 appliances sound extremely dangerous to me. What's the point of Class 2 appliances since they will always be plugged into an earthed mains socket and so could easily be earthed using a 3 core mains cable?
Am I missing something?
If a live wire comes loose inside a metal cased class 2 appliance like my tuner then the case will be live and will happily remain so because it's not connected to mains earth and so no large current will flow to earth, blowing the fuse in the live line. If you touch the case the current could instead flow to earth via you, killing you before the fuse has a chance to blow.
A class 1 appliance is earthed so if the case becomes accidentally live the live line fuse will blow, disconnecting the chassis from live, and you're safe. The only way you could be harmed is if you were unlucky enough to be touching the case at the moment the live wire came loose.
Class 2 appliances sound extremely dangerous to me. What's the point of Class 2 appliances since they will always be plugged into an earthed mains socket and so could easily be earthed using a 3 core mains cable?
Am I missing something?
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Yes
That the scenario you mention as an example can not occur in practice. You are imagining what might happen... if you look at the actual design you will see it can not happen without intervention from an outside source... such as you
Mooly
What if a poor solder joint becomes brittle, the tuner is moved for cleaning and the live wire flicks up and touches the case? Who's to say that can't happen even if it's unlikely.
My point is... what's the point of Class 2? It just seems an unnecessary risk when all you have to do is use a 3 core cable and earth the case. Adds bugger all to the cost. Surely the electrical regs should be about minimising risk.
What if a poor solder joint becomes brittle, the tuner is moved for cleaning and the live wire flicks up and touches the case? Who's to say that can't happen even if it's unlikely.
My point is... what's the point of Class 2? It just seems an unnecessary risk when all you have to do is use a 3 core cable and earth the case. Adds bugger all to the cost. Surely the electrical regs should be about minimising risk.
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I was surprised when I saw a metal case on a Class 2 item (Philips DVD player). It was radiating RFI (even when switched 'off') and touching the case made this worse - I could not listen to LW radio in the same room. I swapped it for a Sony, which is much better.
I think the reason for Class 2 is that some places do not have a good earth, and Class 2 is safer than ungrounded Class 1. It also means you can sell the same junk almost anywhere in the world.
For Class 2 you have to show that two separate insulation barriers have to be breached before the user is in any danger. For Class 1 it is just one breach and one grounding failure, so I don't know which is more likely on a worldwide basis. Here in the UK Class 1 is safer, as our electricity installations are usually done properly.
I think the reason for Class 2 is that some places do not have a good earth, and Class 2 is safer than ungrounded Class 1. It also means you can sell the same junk almost anywhere in the world.
For Class 2 you have to show that two separate insulation barriers have to be breached before the user is in any danger. For Class 1 it is just one breach and one grounding failure, so I don't know which is more likely on a worldwide basis. Here in the UK Class 1 is safer, as our electricity installations are usually done properly.
ClassII (Double Insulated) appliances are designed and built and tested and guaranteed to an onerous set of regulations.
The "Double Insulated" is the clue. This effectively requires two separate faults to happen at the same time for the danger you describe to become a serious risk.
If your ClassII equipment is bought in the UK, it should have passed all the required tests for safe use. Use it, don't modify it.
The "Double Insulated" is the clue. This effectively requires two separate faults to happen at the same time for the danger you describe to become a serious risk.
If your ClassII equipment is bought in the UK, it should have passed all the required tests for safe use. Use it, don't modify it.
and yet informed commentators around the world suggest that the UK regulations are over protective.
Just read some of the Forum's comments to get a feel for how lax some Countries' regulations are and what their opinion is of UK regs, plug tops, fuses, RCCBs, MCBs, PE, LIVE & NEUTRAL switching. The list is too long for me to remember what should be included.
From a personal point as a familiy we have encountered two Class 1 failures at home. Both vacuum cleaning devices. One was years ago, my grandad commented that "the hoover sparks" when it touches the cooker. Cause... a broken ground lead. The metal hoover case was "live" with induced currents from the motor.
Second was a Hoover Dustette that caused severe interference to radio. Cause, a fractured earth lead.
On a Class 2 double insulated appliance... again a vacuum, this one suffered a break in one of the two cores resulting in intermitent operation. Imagine if that had been the earth core in a Class 1 appliance.
You mention a lot of "what ifs". You can say this about many things but it's a fact that double insulated gear that genuinely complies with the standards is super safe.
If your live wire did fracture and flirt up you would probably find it was also restrained mechanically and also not long enough to touch anything.
Ultimately it is up to you what you do with your own gear, just remember other people may use use it as well.
Second was a Hoover Dustette that caused severe interference to radio. Cause, a fractured earth lead.
On a Class 2 double insulated appliance... again a vacuum, this one suffered a break in one of the two cores resulting in intermitent operation. Imagine if that had been the earth core in a Class 1 appliance.
You mention a lot of "what ifs". You can say this about many things but it's a fact that double insulated gear that genuinely complies with the standards is super safe.
If your live wire did fracture and flirt up you would probably find it was also restrained mechanically and also not long enough to touch anything.
Ultimately it is up to you what you do with your own gear, just remember other people may use use it as well.
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