Safety tips for using mains power - Page 1
Forum section: electronics / projects

author: shadewind
author_rank: Frequent Contributor
timestamp: February 25, 2011, 12:10:16 am
content: I'm working on a project to install some lighting in guitar amplifier head (that might sound weird because it will look neatand I don't want have an extra power plug just for the lighting. I thought about using the power supply from the amplifier but I don't want to mess with that and possibly (probably) introduce noise and interference to the amplifier output. So I was thinking I could add my own power supply to this and learn something in the process.So my thinking was using a transformer from 230V to 12V AC, rectify, filter and then use a switching regulator to give me a stable 5V output. Maybe not the best solution, I don't know, but that's not the point really. I believe in learning by experimenting so we'll see.But since I'll be experimenting with mains power, safety is very important and something I take very seriously. Do you have any tips on working with mains powered projects and power supplies? Safety precautions, tips, tricks?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 25, 2011, 12:23:53 am
content: Why don't you just buy an off-the-shelf PSU? Will be easier and cheaper than building something.

author: shadewind
author_rank: Frequent Contributor
timestamp: February 25, 2011, 12:28:05 am
content: Simply because then I will have learned nothing. But sure, I might end up doing that in the end.

author: Psi
author_rank: Super Contributor
timestamp: February 25, 2011, 01:17:53 am
content: - Use a RCD- Keep one hand in your pocket when live 230v parts are exposed- Its a good idea to have another person nearby who can see/hear you and give CPR if its necessary.- Don't think of it just as a 'circuit' when it's live, think of it as a loaded gun pointing at you.That being said, while 230VAC can definitely kill, most people on here will have had multiple shocks from mains voltage and are still here.This next tip can be used to protect a circuit from faults while you design and test it.- You can put an ordinary 40-200W mains voltage lightbulb in series with the 230v to your circuit, that way you form a current limit. Normally the current will still be enough to kill but it will stop the components from exploding instantly if you get something wrong, instead the lightbulb will illuminate. It will protect your components from damage and could save you from an eye injury. You need to pick the right wattage lightbulb so it doesnt illuminate with normal operating current.This trick doesn't work so well for some switchmode power supplies, as they want to draw a huge current at the start and common wattage lightbulbs don't allow enough current for the switchmode to start up. A really high wattage lightbulb that would alllow enough current for the switchmode to start wouldnt protect it much when the current is much lower during normal operation.

author: Unknown
author_rank: Unknown
timestamp: February 25, 2011, 01:27:02 am
content: Here is an idea, find a wall transformer that outputs 12V AC, then rectify, filter and regulate if needed. There are many such wall adaptors and this solution will allow you to learn without being exposed to mains voltage.Here is what I mean, I am sure you can find them locally.http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0458923

author: Jimmy
author_rank: Regular Contributor
timestamp: February 25, 2011, 09:31:13 am
content: OpIf I get your Idea correctly you want to install this system into you amp box with only pulling 230v from the plugIf this is what you want to do open up your amp to see how much space you have behind the pannel to work with.You can use an old PSU from a computer if you want it will be realy cheap. You wont need the fan for cooling if you are just going to run a couple of leds and a micro controller off of them. You would just cut an old psu power lead and wire it into the internal of the amp so you get power to the psu.Then Brige Green and Ground to turn on the psu just like the computer would.I would spend more time on the actual lighting than the supply you could use a Arduino to control some rgb led's that can do cool lighting effects

author: shadewind
author_rank: Frequent Contributor
timestamp: February 25, 2011, 11:49:56 am
content: Exactly, I want to draw 230V directly from the plug (preferrably, I want to include the power switch into my own circuit so that the lighting turns off with the power). A PC PSU is too big to fit in the case. Preferrably, I want to hide it behind one of the large transformers in the amp so that my own power supply is not visible.The lighting I've got pretty much figured out. I have an ATMega8A-PU, a couple of Cree RGB LEDs which I'm gonna drive with PWM and some transistors.

author: Lance
author_rank: Frequent Contributor
timestamp: February 25, 2011, 05:37:36 pm
content: Quote from: Psi on February 25, 2011, 01:17:53 amOh sure. I haven't been hit by 230 though. I got a shock from 120 V. My hand felt a little funny, but it wasn't that bad.

author: BrickBoiler
author_rank: Contributor
timestamp: February 25, 2011, 06:04:26 pm
content: Probably the best tip I can give you is to never lose your attitude. Safety is important and so is learning, even if that means doing things the hard way.The second best tip I know for line voltage is to use alligator clips. De-energize what you are working on, verify with a good non-contact-voltage tester, attach your alligator clips, and then power it up. Your hands never go near exposed live conductors.

author: Zero999
author_rank: Super Contributor
timestamp: February 25, 2011, 09:07:16 pm
content: I did write a summary of basic tips for safely designing mains projects but didn't complete it and can't post it at the moment as it's on another computer. I'll post it some other time.All I can say is follow the following basic guidelines:Don't, whenever possible use a pre-made wall plug power supply.If you have no choice then do the following, use some common sense and you should be fine:Use a power transformer with built in thermal protection or a fuse.Fit as lower current rating fuse as possible in series with the live conductor.Keep the mains as far away from the secondary side and user as possible, which means having at least 5mm spacing between 230VAC and extra low voltage on the PCB and 7mm for rectified mains (325VDC).1Use a non-conducting plastic enclosure whenever possible.If you must use a metallic enclosure then connect it to protective earth/ground.Ensure there's at least 2.5mm of creapage between the mains conductors and the protective earth/ground conductor on PCBs or metalwork, 3.5mm for rectified mains.2Keep hot surfaces away from flammable materials and ensure the enclosure doesn't get hot enough to burn the user.1Minimum requirements for double insulation.2Minimum requirements for basic insulation.DisclaimerIf you don't understand the above then don't build a mains powered circuit.This is only a basic guide, seek advice from the relevant bodies for getting your project manufactured.And don't blame me if you kill/injure yourself or anyone else.

author: shadewind
author_rank: Frequent Contributor
timestamp: February 26, 2011, 12:01:04 am
content: I was first thinking that I could do this on a perfboard but I'm starting to realize that it's probably not a very good idea. I'm probably going to need a proper PCB which is going to be too expensive or too much work for this project (yes, really). I don't know if I could mount the transformer on something else maybe since it does have what seems to be mounting holes through the magnetic core. That's about the only thing that is really connected to the mains. Hmm... I have my doubts.The thing is, it would be much less of a hassle if there was only one single power cable to the amplifier. On the other hand, it might not be such a good idea to mount a power supply directly behind another large transformer becuase of magnetic flux leakage and whatnot.

author: Unknown
author_rank: Unknown
timestamp: February 26, 2011, 12:09:54 am
content: Quote from: Hero999 on February 25, 2011, 09:07:16 pmDo.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 26, 2011, 02:13:04 am
content: Quote from: Alex on February 26, 2011, 12:09:54 ami also had similar answer as you Alex. but when some discussion arise on lightning strike, it made me think twice, as i usually made my own simple ac-dc adapter using transformer. even though well isolated, but the spike could be transfered to secondary side damaging anythingwithoutproper protection circuit. so next time i build my adapter, i will study safety and protection circuit more.as for main safety. i know long time ago how electrocution felt like, but just few weeks back while installing my home lightings, i've been struck by mains several times, guess i never learn, even though knowing the safety, sometime i missed it. so thinking, maybe the best way to handle mains is to remember carefully everytime to switch off the main supply before handling it (the circuit).

author: Jimmy
author_rank: Regular Contributor
timestamp: February 28, 2011, 12:43:25 pm
content: QuoteI would never even concider using alligator clips on one of my projects it is way too dangerous.NO psu then thats the easy route gone. What you can do is buy a 240v to 24v transformer as they are usually common and cheap. On the primary side put a fuse and a 300v varistor after fuse and to n of the primary side that will take care of lightning strikes then you can just use a small brige wave rectifier and a voltage regulator.You can keep your component count low and just buy a pcb prototyping board "like a breadboard" and hack it together as you are only making one.IF you decide to go ahead with it just rember if it looks dogey then there is potential for a large risk to your health. Buy a fuse holder dont make one use proper insulated wire. Put your swich on the secondary side of the transformer. Send us photo's if you are not sure on something

author: Simon
author_rank: Unknown
timestamp: March 03, 2011, 08:38:03 pm
content: the mains can kill but being sensible will keep you safe. Do always have an RCD in operation, my lifes been saved by one before.Yes the best way to think of the mains and live parts is: a loaded gun. Respect it and you will be ok and make double sure of whatever you connect to it.

author: Unknown
author_rank: Unknown
timestamp: March 03, 2011, 10:45:27 pm
content: I'll make a couple of comment and suggestions here.Using an RCD can be considered an essential for any form of smoke testing. And given the environment you'll likely use a guitar amp I'd consider it an essential piece of kit for gigs (do they still call them gigs) etc. There are lots of two bit experts when it comes to music amplification and lots of interconnected equipment and poor sources of supply. It was common (and deadly) practice throughout the 60's 70's and 80's to cut the earth leads of valve amps to minimise hum loops. And there is still plenty of equipment in that state circulating.Wall Warts are not always the best solution either given the rough environments they are likely to be used in. Drop/kick them 20 times and the bent pins loose internals and cracked cases become a safety hazard and a reliability PITA.Given your current demand for LEDS is relatively small you may be able to use existing Amplifier power wiring to source a mains supply, but be certain of the ratings of any switches power filters etc, these will probably only be good for a few amps total and will likely be carrying near their maximum rating already.Double heatshrink every mains connection you makeIf soldering mains connections ensure you have a big enough iron for the likely large terminals encounteredHeatshrink those newly soldered joints toouse grommets and mechanical protection everywhere a cable passes through a panel and anywhere cables may otherwise chafe or rub over time.Earth the frame of any transformer you use and use shake-proof washers anywhere a screw pr post is used for the connection.Use a transformer with an electrostatic shield and an associated mains if at all possible if a transformer breaks down it will pop the fuse before contact with any ELVMake your earth wire the longest at any termination so that equipment will remain earthed should Any cable be pulled out due to excessive forceMake sure every thing you connect to the mains is rated for mains duty. That's right 600V rated insulation for 240V mains. No insulation cut from the lids of old shirt boxes, ice cream tubs, etc, please. It may stop a flash but will not stand up to any burnout or meltdown.Check and double check your work and have at least another set of (skilled) eyes check your work for you before power up. Its easy even for old hands to miss their own silly mistakes which are usually obvious immediately to another observer.

author: Chasm
author_rank: Regular Contributor
timestamp: March 04, 2011, 12:12:04 am
content: Quote from: Uncle Vernon on March 03, 2011, 10:45:27 pmThat one is especially true when it comes to fixing power and extension cords. If something gets ripped apart the earth connection should always be the last to go.Do not forget the pull relief either! It has that name for a reason.

author: Unknown
author_rank: Unknown
timestamp: March 04, 2011, 12:39:59 am
content: Quote from: Chasm on March 04, 2011, 12:12:04 amRound these parts we call those cord clamps to avoid any confusion with the Friday night pastime of lonely lads.

author: Chasm
author_rank: Regular Contributor
timestamp: March 04, 2011, 12:53:24 am
content: Well... Strain relief was to close to stain relief.

author: Unknown
author_rank: Unknown
timestamp: March 04, 2011, 12:56:38 am
content: touche

author: Zero999
author_rank: Super Contributor
timestamp: March 10, 2011, 06:38:40 pm
content: Quote from: Uncle Vernon on March 03, 2011, 10:45:27 pmAll good advice but I have to take issue with that point.That's not applicable for toroidal transformers and is unnecessary for transformers with mains and ELV windings wound on separate bobbins.In most cases the transformer's metal frame will be double insulated but if it's not, by all means earth it. Any modern transformer should have double insulation between the primary and secondary, otherwise the secondary should be earthed.

author: Unknown
author_rank: Unknown
timestamp: March 10, 2011, 08:52:29 pm
content: Quote from: Hero999 on March 10, 2011, 06:38:40 pmI'd argue the effectiveness of double insulation in an unattended overload and subsequent meltdown situation, melt through onesEven for a toroidal transformer ensuring the mounting bolt is earthed offers some protection despite it being layers of insulation away.For fixed wiring AS3000 requires the secondary to be earthed in all situations. Transformers with an electrostatic screen (dumb description) have a metallic winding or foil between the primary and the secondary and are the only approved way of floating the secondary.Plug in appliances escape this level of prudence so it's more often than not a tiny wrap of finest quality Taiwanese insulation is all there is between normality and possible electrocution. This also applies to many SMPS with a 2 wire cord, how many 2 metre drops can a notebook power brick sustain whilst maintaining the integrity of it's mains insulation.Your points are valid, transformer construction does need to be taken into account when considering any safety and earthing measures necessary.

author: Zero999
author_rank: Super Contributor
timestamp: March 10, 2011, 09:28:22 pm
content: Quote from: Uncle Vernon on March 10, 2011, 08:52:29 pmIf you apply that logic, none of your mains equipment should be considered safe, regardless of any UL or TUV approvals.QuoteI don't see how that offers any protection at all. If the insulation between the primary and secondary fails, the metallic blot would provide no protection at all.QuoteWe're not talking about a fixed installation but a portable appliance. I despute that claim otherwise IT (floating mains) installations wouldn't be allowed in places like hospitals. If I remember rightly, a floating secondary is allowed but it's advisable to fit an insulation monitoring device to issue a warning in case of a fault. 12V halogen lighting is powered from a transformer with an unearthed secondary. It's known as SELV, Separated (meaning not earthed), Extra Low Voltage (under 50VAC).And here in the UK it's mandatory for all plug sockets in the bathroom to be powered by an isolation transformer with a floating secondary.QuoteNot true, all that's required is the secondary be double insulated from the primary, BS EN 60742. If it isn't you need to earth the secondary.QuoteIf the reinforced insulation can be broken that easily, it shouldn't be approved.I'd think damaging the case would expose the user to live parts before shorting the secondary to the primary. All the insulation between primary and secondary is inside the device sealed in a plastic box which would have to be badly damaged to cause it to fail.

author: Chasm
author_rank: Regular Contributor
timestamp: March 10, 2011, 10:22:39 pm
content: SELV, aka Class 3The important part about SELV is the insulation voltage. (For 230V primary voltage 2.5kV minimum.)Important: This rule applies to all parts of the circuit.A typical pitfall in automation system would be to use SELV to get around some rules regarding the user interface for machinery and then having the need to switch something bigger on a non SELV circuit. Simple, use a suitable relay, but: the relay has be certified for the above mentioned 2.5KV insulation voltage, or usually more since it usually will be a 230/400V net. The same also applies for all the simple stuff like switches if a panel does not exclusively consist of SELV circuits.As you may have noticed, this drives up the cost of parts real quick.Or, if the need for such parts and other applicable rules is not noticed, creates a /real/ liability problem once an accident happens...That is the definition in Germany, on a quick check thereseemto be different definitions what exactly SELV is in other nations.

author: Unknown
author_rank: Unknown
timestamp: March 10, 2011, 10:26:54 pm
content: Quote from: Hero999 on March 10, 2011, 09:28:22 pmNone? A lot of mains double insulated equipment is not safe. Just one example. A double insulated drill with a metallic chuck and gearbox can be (and is) approved. In normal use an operator will have one hand on the metal part of the drill, yes he is double insulated from the drills cord but he is far more at risk if drilling through a live cable than if he was using a tool with a fully earthed frame.Quote from: Hero999Of course not, hell it's much safer to have any metal adjacent a mains winding floating from ground? </sarcasm>Quote from: Hero999 on March 10, 2011, 09:28:22 pmJust a portable appliance, projects are only portable appliances?The same physics apply regardless of the source of supply. Or does a mains plug offer some magic control of of all that passes through it?Not questioning your memory but our rules which have a basis from an old copy of our rules are quite specific in terms of earthing a secondary. Nothing short of an engineers sign off and type approval allows anything different. The latter of course unfortunately can be purchased for a fee for almost any piece of imported rubbish.Unearthed halogen lighting falls into the same categories, type approved appliance or fixed wiring out of reach. Check the sections on earthing of exposed luminaires there is no specific exclusion for ELV lamps fed from mains transformers. The fact they generally don't get done doesn't imply any assumed safety margin.Quote from: Hero999 on March 10, 2011, 09:28:22 pmThat's both dumb and expensive. There are many much more effective ways of reducing the risk of electrocution in damp situations.QuoteAh blind faith and regulations. Of course we all know that double insulation can never break down and BS-whatever will keep use safe from harm. Double insulation is not a miracle cure.QuoteAgreed! This doesn't however make all that garbage out there in the marketplace, or any of those low cost parts shop transformers any more safe. I've seen live mains flashed across abused power bricks more times than I'd like to remember and seen more than the odd transformer meltdown/burnout.We are primarily talking home built DIY projects here. The home constructor does not have the testing resources necessary to test and rate double insulation. It's just common sense to soundly earth anything metallic part or chassis of any project with even the remotest possibility of being touched or coming in contact with a loose wire or burnt winding.QuoteI could show you numerous examples where this has not been the case. Badly damaged yes, but often with internal damage hidden by a battered but still intact outer casing. You've never seen plastic PCB pillars snap and come adrift within appliances?

author: Unknown
author_rank: Unknown
timestamp: March 10, 2011, 10:42:01 pm
content: Quote from: Chasm on March 10, 2011, 10:22:39 pmExactly and the mechanical rigidity stability of that same insulation.QuoteA gold star to Chasm for pointing out a much overlooked part of double insulated design.QuoteThe definition doesn't change much, the real worry is in interpretation and allowable exclusions. In Australia whilst fixed wiring rules apply almost anything goes for appliance with the onus falling back on the importer or holder of the C-Tick number. A C-tick here is not an indication of testing or compliance, it's simply a bureaucratic indication that there is someone to blame when things go pear shaped,which may help recover funeral expenses.

author: Zero999
author_rank: Super Contributor
timestamp: March 10, 2011, 11:23:37 pm
content: Quote from: Uncle Vernon on March 10, 2011, 10:26:54 pmWhat nonsense? If you use that logic, a cordless drill should be earthed as the operator could easily drill through a live cable in a wall.Earthing a double insulated mains drill won't do much to protect against the user drilling though the cord because it could sever the earth conductor first - it's pot luck.If you disassemble a double insulated mains drill, you'll also find that lots of the gears used are nylon.QuoteNo, it's a very good method of protecting against shock, much better than an RCD which relies on moving parts and can easily stop working. Isolation transformers used in bathrooms are also normally low power (<50VA) so inexpensive. They are rugged, reliable and have to be double insulated and thermally protected.QuoteNeither is earth bonding which can easily fail as well as increasing the risk of shock under the wrong circumstances. Connecting a tool's case to earth reduces the impedance of the user to earth and therefore increases the risk of a lethal shock if they come into contact with an external live part.Earthing the secondary side of a transformer can also cause ground loops and all sorts of bad things if it's done in consumer electronics as two different 0V rails become bonded together when more than one appliance is interfaced with another. If you're designing something like an audio amplifier or lab power supply, earthing the secondary is a very bad idea.[quoteThe home constructor does not have the testing resources necessary to test and rate double insulation.There is no need for any testing, proper design, construction using quality components and a thorough visual inspection is all that's required to ensure a design is safe to be connected to the mains.For example, if you have a PCB with mains and SELV on it, providing you're using a good double insulated transformer, given a clearance of 7mm betreen mains an SELV, checked the PCB to ensure there's no copper islands or blotches to reduce the clearance and covered the tracks with a conformal coating, you can be certain that the PCB will pass and tests so there's no point in performing them.Testing is only used to prove to ignorant people it's safe and to cover the backs of managers. Over reliance on testing is also very bad practise: so what it passes a flash test? That doesn't prove it's mechanical strength, just it can take 2.5kV under the conditions present when the test was performed.

author: Unknown
author_rank: Unknown
timestamp: March 11, 2011, 12:13:45 am
content: Quote from: Hero999 on March 10, 2011, 11:23:37 pmNo what ignorance on your behalf!!!!QuoteYou probably should. Not so easy apply or enforce given any wiring regs won't apply for a non mains powered device.QuoteYour scenario would only be applicable if you were drilling through the appliance own mains cord. Applying your pot luck principle it is likely there is no earth conductor within the hidden cable. If a drill bit damages an earthed cable it will likely flash across all damaged conductors anyway. A solid earth bond on a tool will ensure that the tool tip and first point if inadvertent mains contact is always earthed. That's a far far better path for most electrons than through the operator.Quotecough. cheap rubbish tools. cough.QuoteI know which one I'd rather rely on if an appliance was inadvertantly immered or splashed. Isolation transformers remove the risk of shock to earth they do not reduce shock risk entirely. Hell in the right situation an isolation transformer could fry you without ever tripping an upstream RCD.QuoteNeither is earth bonding which can easily fail as well as increasing the risk of shock under the wrong circumstances. Connecting a tool's case to earth reduces the impedance of the user to earth and therefore increases the risk of a lethal shock if they come into contact with an external live part.And the impedance from earth to tool case ? A properly earthed case will be the safest unless multiple faults exist. In your scenario a faulty or poor earth could be dangerous just as faulty double insulation can be.What DIY contructor has access to the materials and testing necessary for effective double insulation? A sound chassis earth is essential in project construction!QuoteBut nowhere near the bad idea it is to put audio performance ahead of commonsense safety. There are tens of thousands of guitar amplifiers about with their chassis earth removed by tinkeres making them into absolute death traps. There are better ways of eliminating ground loops and shield current than chassis isolation.QuoteA lot of if's and provisos there for any recommendation to beginners. Fail on any one and a mains related fail will be dangerous and/or spectacular.QuoteCan you believe you just typed that? Hell yeah lets all throw away our scopes and multimeters and just work off the data sheets. You cannot be serious.QuoteThat makes much more sense than your previous statement. Any test is only as good as the way it is performed. 2.5Kv insulation may not be 2.5kv insulation after tiime and mechanical stress take their toll.QuoteOne would hope that testing extended to parameters such as mechanical rigidity.Newbie advice:Test Everything, including the way you do your testing!Trust no-one or no corporation test it yourself and test it again!Soundly earth your chassis! And anything near mains suppply!

author: Unknown
author_rank: Unknown
timestamp: March 11, 2011, 01:30:18 am
content: Right. You are very passionate about your mains safety.Although the wiring of the house is usually done by an electrician, there are many designers out there that could use some design advice for mains-connected products. Say you have a design and you want to include a power supply and box it up real nice ready for certifications.Why not list the major standards covering device classes, insulation and internal wiring requirements, earthing, EMI filtering requirements, certifications, PAT testing and anything else needed in your opinion to bridge the gap between a 5V design and a wall socket? How much track clearance does one need for a mains transformer on a PCB? Where can one go and find how to connect a typical product to mains?It is a challenging task, can you bring all important references together from your experience? You will be helping designers reduce the risk of electrocution too. It would be great if this is posted here or maybe a new thread.Hero could focus on UK and Uncle Vernon on Australia (?). It would be great if we can have a reference for Europe too.Lets see what you can do.

author: Unknown
author_rank: Unknown
timestamp: March 11, 2011, 09:22:24 am
content: Quote from: Alex on March 11, 2011, 01:30:18 amTo see me in action you'd probably think I was quite cavalier. I am passionate about pointing out that the nonsense we suffer under the guise of OH&S has nothing to do with safety. I am also a great believer that lurking within all of us is a modicum of common sense. What I do myself is quite different to what I'll offer as advice to newbies.QuoteCuriously much of that wiring, in recent years, looks like it has been done by blindfolded Gibbons with access to power tools.But I digress.QuoteYour right it is a challenge! It is also a very good idea. i wouldn't have the time to do such a challenge any real justice, but I'd certainly be up for participation in a collaborative effort. Perhaps a wiki would be the best medium?QuoteInternational input would be the way to go. Somewhere in my deep dark past I was an electrician. (technically I still am) so I am fortunate enough to have skills in a number of camps. I've see some oversees workmanship both in and out of Australia.I work in automation and see a lot of cross trade arrogance and a lot of poor quality "know it all" workmanship. Techs can be just as ignorant as knuckle dragging SparksNone of it is rocket science but there are lots of considerations with any product particularly those to be interconnected to other systems.If there is there enough participatory interest out there I'll be up for some contribution/

author: shadewind
author_rank: Frequent Contributor
timestamp: March 11, 2011, 03:36:32 pm
content: I'm not sure if I'll end up going the mains powered road (we'll see...) but regardless, a thread or similar about this sounds like a really great idea.

author: Zero999
author_rank: Super Contributor
timestamp: March 11, 2011, 05:29:24 pm
content: Quote from: Uncle Vernon on March 11, 2011, 12:13:45 amThat's what I'm talking about. You're holding an earthed drill and come into contact with a live part elsewhere and you'll receive a much more severe shock than you would if the drill were double insulated.QuoteIt's impossible for the regulations to protect you against everything. The standards the drill has to satisfy only cover the drill, not user error or its interaction with a buried wire. In the UK the wiring regulations mandate that all wiring under a certain depth shall be protected with an RCD.QuoteI'm not talking about using an isolated chassis, although that is a possibility and is perfectly safe if designed correctly. I'm talking about a floating 0V rail on the DC side of the supply. We're also not talking about modifying an existing appliance but designing one safely in the first place.QuoteHmm let me see.The razor falls into the bath.Isolation transformer: a short circuit occurs causing a breaker to to blow. No current will flow through you because the source is not bonded to earth. If the breaker fails to trip, the thermal fuse inside the transformer will fail.RCD: 30mA will flow until the RCD trips which may give you a small shock. If it doesn't trip soon enough, you're dead.QuoteYou're right, isolation transformer don't protect against a line to line shock and neither do RCDs. In bathrooms the shock risk is minimised by only allowing one piece of equipment to be connected to the isolation transformer at once and not having an earth connection on the socket which means only class 2 appliances can be connected to it. The socket is also different to those used for other mains appliances so it's not possible to connect an extension lead to power more devices.QuoteThat's not what I meant. I mean it's not always necessary to test and that just doing so doesn't prove anything.QuoteThat's exactly what I meant by my previous statement making it pretty obvious I didn't really mean all testing is pointless.

author: Unknown
author_rank: Unknown
timestamp: March 11, 2011, 09:07:22 pm
content: Quote from: Hero999 on March 11, 2011, 05:29:24 pmAn obscure scenario indeed. Now concentrate hard ! Your drilling through a wall to place a fixing to hang your favorite EEVblog photo and you inadvertently strike a hidden cable. What part strikes that hidden live conductor first? The drills metallic tip or a stray body part?I will concede that it is not a good idea to hold an earthed drill while sticking your tongue in a working toaster, but I'm sure even you would consider this an event far more unlikely than to inadvertently drill through a live cable.QuoteIt's impossible for the regulations to protect you against everything. The standards the drill has to satisfy only cover the drill, not user error or its interaction with a buried wire. In the UK the wiring regulations mandate that all wiring under a certain depth shall be protected with an RCD.Then those regulations are garbage. If everything was always done correctly and within correct operating procedure, then we would hardly need regulations. Regulations are very much about how systems and equipment will cope with misuse and failure! Sure you cannot cover every situation but to not cater for the common and more obvious scenarios is negligent.QuoteWhich is why recommending a solid earth to any chassis or mains related component is a very good idea. It's the safest course for the beginner or intermediate constructor.QuoteShort Circuit? Which short circuit is this? A bath full of tepid water will more often than not well short of a short circuit capable of tripping a circuit breaker upstream and rated for protecting from overload of the wiring of an entire circuit. (A circuit breaker is a mechanical device, by the way operating as an interruption device in a similar manner to an RCD )Quoteat my age I'm right most of the time.QuoteI'm sure children will select only class II objects to poke/force into the socket.QuoteAnd that's the advice you offer when beginners or intermediate constructors are asking about mains safety of their future creations? Dr Kevorkian would be proud!QuoteNo testing is ever pointless, the laws of "diminishing returns" and "missing the bleeding obvious" however still apply. My advice to earth chassis and metallic parts and to where possible have your work checked by another set of eyes still stands.

author: Zero999
author_rank: Super Contributor
timestamp: March 11, 2011, 10:34:52 pm
content: Quote from: Uncle Vernon on March 11, 2011, 09:07:22 pmThat's your problem you think you're always right when you're sometimes wrong. There's no point in continuing the discussion any further. If it makes you feel better, you can think I've given up because I don't have any decent counterarguments but I know better.I'm actually pretty pissed off, not with you but more with myself from having not learned not to waste my time replying.

author: Unknown
author_rank: Unknown
timestamp: March 11, 2011, 11:03:34 pm
content: Quote from: Hero999 on March 11, 2011, 10:34:52 pmAlways? how do you get always from most of the time?QuoteNah I'd far prefer to see constructive and well considered counter argument. I've never been a fan of defeatism.QuoteHow is replying a waste of time? It's only a waste of time if discussion is approached as some form of conquest or point scoring.Vigorous defense of a well considered point of view will always be healthy, as is testing any argument or point of view against rational criticism.Good engineering is considering why a regulation or practice is in place and also questioning whether alternatives could offer a better solution. There is a place for conservatism just as there is a place for innovation. Conservatism helps temper the consequences of ill considered innovation.

author: Franki
author_rank: Regular Contributor
timestamp: March 20, 2011, 01:29:51 am
content: One general thing, though not related to electric safety measures, I would recommend to people designing audio amps of all sorts and their respective power supplies, is to both have a good reference amplifier and good reference adjustable, stable, low-ripple, symmetric power supply besides having a good function generator and measuring devices, eg. oscilloscope.For example, I use an old stabilized, over-temperature/current protected, adjustable linear symmetric 2x30V/2x2A power supply with builtin V/A displays from Ratho(difficult to get one, many other brands are fine too).As scope, I currently only have an old Hameg 203, this is not a really good one, but still does the job.

author: Zero999
author_rank: Super Contributor
timestamp: March 20, 2011, 09:55:32 am
content: Why do you need a stabilised power supply for an audio amplifier? If the ripple rejection is good (it should be) you don't need a regulated power supply for an audio amplifier.

author: Franki
author_rank: Regular Contributor
timestamp: March 23, 2011, 11:50:07 pm
content: Quote from: Hero999 on March 20, 2011, 09:55:32 amYou don't need one, and many power amplifiers don't have their power supply stabilized. But In my personal experience, experimenting with a stabilized power supply is better to avoid the output stage from distorting the signal by even small voltage drops by high current demands especially at low signal frequencies, even if you use independent power supplies for the input and output stages.Of course you could be using an overpowered power supply and use exceedingly high caps, but that's neither sensible nor economical.

author: Zero999
author_rank: Super Contributor
timestamp: March 24, 2011, 07:19:26 pm
content: Quote from: Franki on March 23, 2011, 11:50:07 pmI completely agree with you on using a stabilised PSU for experimenting but wouldn't recommend it for production for cost reasons. If the voltage drops are causing problems then it means either the capacitors are too small or the transformer voltage is too low.QuoteA stabilised power supply with the same rating as its unregulated counterpart needs a larger transformer (higher output voltage transformer with the same current rating) and monster heat-sinks to cool the regulators which is less economical. You could opt for an SMPS but that can create noise problems and will need to be rated to the peak current so is not a good idea for high fidelity audio.Take the following example:You need an amplifier to put 15W RMS into an 8 Ohm load.The peak voltage at the output stage is:V = sqrt(2*PR) = sqrt(2*15*8) = sqrt(240) = 15.5VWe'll assume 2V is lost in the output stage, plus 1.2V in the rectifier and a couple of volts for ripple, taking the peak AC voltage to 15.5+2+1.2+2 = 20.7V. For an unregulated PSU we could use 20.7/sqrt(2) = 14.64VAC, nearest transformer is 15V and put a 10,000µF capacitor across each rail.But for a stabilised PSU we need a larger transformer. The output voltage to the output stage needs to be >17.5VDC so it's wise to use an 18V regulator. Here's the calculation for the transformer voltage:The dropout voltage for a typical linear regulator is 3V, then there's 1.2V for the rectifier again so the minimum ripple valley needs to be 18+3+1.2 = 22.2V so a 15V transformer won't do. We could use an 18VAC transformer (20% larger) so the maximum acceptable ripple is 18*sqrt(2) = 25.5 - 22.2 = 3.26V.The capacitor size could be reduced to C(uF) = (10* I(mA)/ Vripple = (10*2000)/3.26 = 6135uF, nearest E6 value 6,800uF but that's not much of a saving considering the higher voltage rating, then there's the cost of the heat sinks and additional regulators to consider