|Elliott Sound Products||Electrocution|
Electrocution & How To Avoid It
(... and what to do if the worst happens)
Rod Elliott (ESP)
|Please Note - I am not a Safety Expert, I am not a Doctor|
|I have no accreditation as an electrical safety expert, and this article is based on common sense, personal experience and basic research. The material is for your information only - it contains sensible advice, but the information here is not to be considered as wholly reliable or complete. For complete safety information, please consult appropriate professionals in electrical hazard reduction, victim rescue and CPR techniques.|
Electrocute - To kill or be killed by electricity (This is the proper definition, but many people use the term to mean electric shock)
There is much confusion about electricity and its ability to send you to your ancestors, and while much of the information is sensible, it is not always easy to find, and usually doesn't cover your area of interest - especially if that includes audio (or electronics in general).
An electric shock may be variously referred to by survivors as being zapped, bitten ("That f'ing microphone just bit me!"), "copping a belt" (probably uniquely Australian), electroplated (that's one I use), or simply by a string of expletives. However it's expressed, the experience is never pleasant, and almost always signals your body to release adrenaline - our bodies react as if we are under attack - which is true enough. All installations, whether fixed or mobile, should be properly connected, all equipment with an earth pin must be connected to an earthed outlet, and safe wiring practices must be used.
This article is not about wiring practices, because there are so many variations worldwide that it is impossible to cover everything. If you are involved with setting up, building or repairing audio or lighting equipment, make sure that you know and follow the regulations that apply where you live. Failure to do so can result in death or serious injury, possibly followed by expensive litigation. If the worst does happen though, the material here should be useful - plus you will hopefully learn something new.
Terminology: Unfortunately, different terms are used in many countries for the same thing. Active, phase, hot, line, live - they all mean the live mains wire, but this is not always clear - especially to those who may not know the terminology. I can make no guarantees that I have covered all possibilities, since there are many foreign (to me) languages that will use different terms again. The table below covers those that I know of - that there are others is certain.
|Wire Name (Oz)||Wire Colour ¹||Also known as ...|
|Active||Brown, Red, Black||live, line, phase, hot, plus, positive (these last two are wrong, but I have heard them used)|
|Neutral||Blue, Black, White||cold, common, grounded conductor (US), minus, negative (as above for the last two)|
|Earth||Green/Yellow, Green||ground, protective earth, earth ground, safety earth, grounding conductor (US)|
Note 1 - Be careful with wire colours. The standards are gradually changing worldwide to the Brown, Blue, Green/Yellow scheme, but a great deal of older equipment will use one of the old standards - and it might not be one ever used in your country! Make sure that you treat all incoming mains wires that are not connected directly to the chassis as hostile.
While most terms are reasonably easy to get right, take great care with the US terms grounded and grounding conductor. They are not the same thing. The neutral lead is earthed (grounded) in almost all installations - this is done at the switchboard of every connected premises. Many people have claimed that the mains would be "safer" if neither conductor were earthed, but this is simply not the case. Anyone, anywhere in your street or neighbourhood could have an undetected earth fault that shorted one conductor to earth. The system is converted by one failure (which could be undetected for weeks or months) to the way we have it now, but no-one knows ! Where everyone would assume that both wires were "safe", only one would meet this expectation. The other would be live, with the full potential from conductor to earth. See below for more information.
Standards: The European standards (and those of many other countries) can be particularly confusing, and some of the information is either marginally wrong or is incorrectly used with respect to audio and audio-video systems. As an example, I have included section 7.16.2 from TLC-Direct in the UK. This applies mainly to fixed installations, but is included primarily for reference. Fixed installation SELV circuits are not intended to be handled, and they are required to be insulated against accidental contact.
7.16.2 - Separated extra-low voltage (SELV)
The safety of this system stems from its low voltage level, which should never exceed 50 V AC or 120 V DC, and is too low to cause enough current to flow to provide a lethal electric shock. The reason for the difference between AC and DC levels is shown in (Figs 3.9 & 3.10).
It is not intended that people should make contact with conductors at this voltage; where live parts are not insulated or otherwise protected, they must be fed at the lower voltage level of 25 V AC or 60 V ripple-free DC although insulation may sometimes he necessary, for example to prevent short-circuits on high power batteries. To qualify as a separated extra-low voltage (SELV) system, an installation must comply with conditions which include:
- it must be impossible for the extra-low voltage source to come into contact with a low voltage system. It can be obtained from a safety isolating transformer, a suitable motor generator set, a battery, or an electronic power supply unit which is protected against the appearance of low voltage at its terminals.
- there must be no connection whatever between the live parts of the SELV system and earth or the protective system of low voltage circuits. The danger here is that the earthed metalwork of another system may rise to a high potential under fault conditions and be imported into the SELV system.
- there must be physical separation from the conductors of other systems, the segregation being the same as that required for circuits of different types (see 6.6)
- plugs and sockets must not be interchangeable with those of other systems; this requirement will prevent a SELV device being accidentally connected to a low voltage system.
- plugs and sockets must not have a protective connection (earth pin). This will prevent the mixing of SELV and FELV (Funnctional Extra-Low Voltage) devices. Where the Electricity at Work Regulations 1989 apply, sockets must have an earth connection, so in this case appliances must be double insulated to class II so that they are fed by a two-core connection and no earth is required.
- luminaire support couplers with earthing provision must not be used
I dispute the claim that 50V AC or 120V DC is "too low to cause enough current to flow to cause a lethal electric shock" - IMHO this is bollocks. While it is impossible to cover every possibility, statements like that make people complacent (self-satisfied and unconcerned), and complacent people and electricity do not mix.
Note the comment that contact with SELV is not intended, and that lower voltages apply where uninsulated terminals are accessible for contact. This would include plug-pack (wall-wart) transformers with a standard DC connector, loudspeaker terminals (both on amplifiers and speaker boxes). Based on these recommendations, any power amplifier (or loudspeaker) capable of greater than 75W (approx.) should have insulated terminals designed to prevent contact by the user.
I don't have a full copy of the latest A/NZ standards, and unfortunately this information is almost impossible to find on-line (for any country - not just Australia). One is expected to purchase the standards (this applies almost everywhere), all are copyright, and none will allow any re-publication without a hefty fee (assuming that such re-publication is allowed at all - usually is not permitted). So much for keeping people informed about electrical safety matters, and making sure that hobbyists and DIY people have access to essential safety information. Ultimately, it is left to individuals like me to provide this data, with no ability to legally disclose the relevant sections of the rules. One section of the Australian/NZ standard to which I have access covers double insulation requirements. Since laboratory testing is mandatory to ensure that all double insulation requirements are met, this is an impractical approach for DIY.
After much description, the standard states ...
It will be apparent that double-insulated appliances built according to these principles must not be earthed.
One wonders how it will be apparent (and to whom) when the item is fitted with a bunch of connectors at the rear. Putting a string of words together is all well and good, but the above is almost impossible to maintain. There is an inevitable mix of earthed and unearthed A/V equipment in almost all households, and these will be interconnected. Quite clearly, this violates the basic principle of double insulation for A/V equipment, and I'd be interested to meet the lunatic who included this rule. I have never seen (or heard about) any "new" sub-ruling within the standards that allows such interconnection, so can only conclude that it is technically illegal to connect my DVD player to the hi-fi. Likewise, it is technically illegal to connect my TV set to an antenna (outdoor antennas must (by law) be earthed as they are a definite safety hazard otherwise). Pity the poor person who tries to maintain the rules (that no-one will tell him about) and not earth double insulated appliances. He's not allowed to connect the TV to an outside antenna, can't connect the CD or DVD player to an earthed amplifier, so is forced to watch and listen to ... what, exactly? I suppose that one can listen to the sound through the TV speakers, but they usually sound like a goat pooping on a tin roof .
While most of the rules make some kind of sense, there are countless pieces of A/V equipment that are double insulated (suitably marked, and no earth pin on the mains plug), and these will almost always be connected to another piece of equipment that is earthed (grounded).
This defeats the SELV and/or double insulation isolation requirements, and while it will not usually cause a problem there is definitely a risk involved - mainly to the sanity of the regulations. It is not at all uncommon that regulations are at odds with reality (just look at the RoHS directive), but the SELV requirements only consider new equipment, and do nothing to address the vast amount of older gear that is still in use. One risk that does exist is if the earthed amplifier (for example) has its earth connection removed - either due to a fault or deliberately. If this amplifier subsequently develops a fault that makes the chassis live, then all connected equipment becomes live too - including all the double insulated gear that is supposedly safe.
To make matters even worse, it seems that it is now alright to use a type Y2 capacitor between the internals and the non-earthed metal case of double insulated equipment. Why? Because they have to pass electromagnetic interference tests, and will fail if the case is floating. The metalwork of such equipment can give you a tingle because of the capacitor, but this is perfectly acceptable according to the regulations. This (IMHO) is unadulterated madness! No mains powered appliance should ever give you a tingle. An Australian electronics magazine recently published a project to prevent the tingles, however it's use is probably (technically) illegal. How insane have things become when it is theoretically illegal to prevent your DVD player from giving you a tiny zap whenever you touch the metalwork?
It is also assumed that Y2 capacitors will never fail as a short, and that no-one will ever use a counterfeit Y2 capacitor (for example, no-one will ever fraudulently re-label X-class caps as Y2 to make a quick buck). Counterfeiters have never been known for their moral fibre, so at some stage there's every likelihood that fake Y2 caps will surface. The biggest problem is that no-one will even know about it until someone is killed or injured as a result, and that could be many years after manufacture. Not one regulatory body seems to have thought about this probability, and any attempts to convince government bodies is sure to fall on very deaf ears indeed. I have tried, and got exactly nowhere.
Note that I have already seen switchmode plug-pack /wall wart /etc. supplies where the manufacturer (from guess where) decided that a Y2 cap was too much hassle, so used a very ordinary 1kV ceramic cap instead (these are far cheaper than Y2 caps as you would expect). These supplies have fraudulent CE markings, and would not pass electrical safety tests in any first-world country. They are readily available from any number of sellers operating on-line auction accounts.
Expect this trend to continue, and expect that unknowing (or unscrupulous) sellers will import equipment and sell it without regard to mandatory electrical safety or electromagnetic interference tests that may apply in your country. I've already caused one to be shut down because almost every product he sold required safety tests or other mandatory certification, none of which was done. This is rife on a well-known on-line auction site, where international sellers (and also some locals in many different countries) are selling goods that require approvals, but have none. It's common to see the CE logo on everything that comes from the East, but most will not have a single test report to prove that the goods actually meet regulations.
I could recount many other tales of sheer stupidity that have come about as a direct result of the application of ill conceived standards and the blind following of "the rules" by the electrical safety test laboratories and regulatory bodies. To do so will not achieve anything though, so I shall refrain.
Suffice to say that it is almost certainly safer overall if all hi-fi, TV and other home theatre equipment is earthed, so a fault in one cannot cause anything else to become live. This isn't going to happen though - more and more equipment will be double insulated (or at least claim to be) as time passes.
For electricity, in a word ... this is bullshit! There is no immunity and no single reason that one person survives where another dies. Someone who has had countless electric shocks over many years might not panic and may therefore be able to apply reason and disconnect before dying, but don't count on it. While panic certainly helps people die, it's the electric current that kills them, not the surprise! People who have worked with electrical systems all their lives are killed regularly.
It is generally considered that a current of around 50mA is deadly. This is true if that current passes through the chest cavity, and the likely outcome is a nasty condition known as ventricular fibrillation. If this occurs, the heart muscles are all working, but are out of synchronisation with each other - no blood is pumped and the host dies (about 4-6 minutes before irreversible brain damage to carbon based lifeforms such as humans). If your heart is in fibrillation, it is hard to stop - hospitals have machines called defibrillators that are designed to provide such a powerful shock that the heart stops. Once the heart is stopped, it may re-start by itself, or a smaller controlled shock may be needed. A stopped heart may be able to be re-started by external heart massage (CPR - cardiopulmonary resuscitation) and assisted breathing is almost always needed in such situations. If the victim's heart is fibrillating, you probably won't know this is happening, but there will be no detectable pulse. CPR should be started immediately. A comment on one website is worthy of repeating ...
Good CPR is better than bad CPR, but even bad CPR is infinitely better than no CPR at all.
It used to be considered essential to check for a pulse before administering chest compressions (the 'cardio' part of cardiopulmonary resuscitation). If the victim has a heartbeat, inappropriate application of chest compression may cause damage (and may even conceivably stop a weakened heart). This notwithstanding, the latest CPR recommendations suggest that there is less chance of damage than death through delayed action, as explained by a reader (who is a physician, advanced cardiac and advanced pediatric life support instructor, and practices emergency and family medicine) ...
The old recommendations (checking for a heartbeat) are superseded because of the difficulty faced by lay people trying to find a pulse. The delay can cause far more damage than "inappropriate" chest compressions.
I strongly suggest that you do a course in CPR - you may need it one day yourself, and the more people who know how to perform CPR properly, the better. A useful fact sheet is available from The American Heart Association that explains the new techniques, and why the recommendations have been changed. From the AHA ...
2005 (New): After delivering the first 2 rescue breaths, the lay rescuer should immediately begin cycles of 30 chest compressions and 2 rescue breaths. The lay rescuer should continue compressions and rescue breaths until an AED arrives, the victim begins to move, or professional responders take over.
Why: In 2000 the AHA stopped recommending that lay rescuers check for a pulse because data showed that lay rescuers could not do so reliably within 10 seconds. Lay rescuers were instructed to look for signs of circulation. There is no evidence that lay rescuers can accurately assess signs of circulation, however, and this step delays chest compressions. Lay rescuers should not check for signs of circulation and should not interrupt chest compressions to recheck for signs of circulation.
(Above section updated 11 Feb 2007 to include latest recommendations).
Should the current be less than 50mA (or is present for a very short time), you will probably only enrich your vocabulary with a few suitable phrases and get on with what you were doing (after a short break to allow the adrenaline to dissipate - highly recommended!). A current above 50mA may stop your heart completely - while it may re-start by itself, I wouldn't count on it.
An electric shock across one hand will not kill you. For example, if two fingers of one hand are in contact with the active and neutral conductors but there is no circuit to earth, it will hurt, it may burn you, but death is highly unlikely except by a secondary effect (falling, heart attack (cardiac arrest), etc.).
An electric shock that passes through your legs will throw you against the rear wall if your knees are bent! The muscles that straighten your legs are much stronger than those that bend them, so an electric current will cause your legs to straighten violently, possibly causing serious injury or even death. This is not uncommon, so always wear insulated shoes when working with electrical appliances, amplifiers, etc.
Telephone Circuits: You can get a tingle from a telephone circuit, which is at -48V with respect to earth when the line is not in use. Ring voltage is about 90V RMS - that will give a good tingle, but I don't know of a case where it's killed anyone, because it's both current limited, and has 'cadence' - i.e. it is pulsed with a repeating sequence that varies from one country to the next. This is done deliberately. Not only is a continuous ring really annoying, but it less safe than a pulsed waveform. The pulsed waveform gives you a chance to let go of a wire with ring current present, because it stops after a short time (typically a couple of seconds maximum) before starting again.
Amplifiers: A conventional power amp usually has zero volts at the output when there is no signal. There is no AC or DC (maybe a few millivolts at most). In the case of even a very large PA rig with no signal, it is usually safe, but some Class-D (PWM) power amps will have a continuous DC voltage even with no signal. This could range from about 30V up to maybe 90V or so. The same signal exists on both leads, but if you contact either lead and earth you may get a tingle. Even at full power, it is unlikely that an amplifier will kill you if you get yourself across the speaker leads. This is not to say that it won't though, and safe working practices would suggest that you keep yourself away from such temptations.
Batteries, Power Supplies: Any power source capable of supplying more than 30mA has the potential (sorry ), to kill you - regardless of voltage. While electrocution is highly unlikely from a 1.5V alkaline cell, 12V may be more than sufficient with the right combination of unfortunate circumstances. One of the nastiest electric shocks I have ever received was from a 12V car battery (and there have been a few in the 40-odd years that I've been messing with electrical stuff). I had a small cut on one hand, and a strand of wire stabbed me in the other. This provides a low resistance path because of direct contact with the bloodstream, and I have never forgotten the experience. Definitely not recommended. This was a freak accident, in that the circumstances needed are not common - it has never happened since, and that was over 40 years ago.
There are many tales (and many of them are at least based on fact) of people sustaining horrific burns as a result of tools dropped across telephone exchange bus bars. The phone system uses 48V DC, and the current available in a large exchange (central office) may be hundreds of amps. Quite literally, tools will simply vaporise if they contact the positive and negative bus bars. If you are nearby, you can be badly burnt from the arc and flying molten metal. Most modern exchanges to not use exposed bus bars, and the current requirements are generally lower now because of electronic switching, rather than relatively power hungry electro-mechanical switching systems of old. Still, a bank of massive cells or batteries (almost always lead-acid) supplying 48V is a force to be reckoned with.
Mains: Of the world's mains supply voltages, that used in Australia, New Zealand, Europe, the UK and South Africa is sometimes claimed to be the worst. 220 - 240V (nominal) delivers a lethal shock quite readily, since the combination of voltage and typical skin resistance is just about ideal. 220V may seem marginally safer, but there's very little difference in reality. Even 120V is quite capable of causing a lethal shock, but it is generally considered to be reasonably "safe" - especially when compared to 220-240V. There are still a lot of people killed by 110V supplies though, so complacency is not an option. Higher voltages (for example 415V is used for three phase systems in Australia) can deliver a mighty wallop (personal experience again!). I have heard said that in some respects higher voltages may be "safer" - the shock will either throw you across the room (and away from the source), stop your heart or both. A stopped heart is marginally better than fibrillation - at least there is a chance that it can be re-started relatively easily. Don't count on this though - it isn't much more than a passing thought on my part, and high voltages are most certainly not safe.
Burns: Many cases of electrocution are accompanied by burns. These can be very nasty, and if an appreciable arc is drawn, it has the same temperature as an electric arc welder. This can not only cause extreme burns, but can also cause eye damage because of the intense ultra-violet emitted by an electric arc. DC is generally worse, especially where massive current is available. A car battery can supply 300A quite easily, and this can create a very intense arc. Metal watch bands or rings can easily be melted into your flesh by the heat generated if they form part of a short circuit across a high current source such as a car battery. DC arcs much more readily than AC because there is no momentary break in the supply and no polarity reversal as with AC.
Survival: There are many tales of people surviving electric shocks that should have killed them several times over. One that I know of (for a fact, but it wasn't me this time) happened a long time ago, in an electrical sub-station adjoining a water pumping station. One of the 1,100V 3-phase pump motors caused the mains fuses to blow, so the maintenance chap went to the sub-station to replace them. Normally, only two fuses fail in a 3 phase system, and that's what he took with him. In this case, all three had failed, so the ladder (timber of course) was removed from the switchgear as required and he went to get another fuse. Unfortunately, someone else was working in the sub-station at the time, and moved the ladder! (You can guess what's coming.) As expected, our man forgot to verify that he had the right switching point, and replaced the ladder against a live switch. The arc almost destroyed his elbow, and burnt a hole about 8mm diameter through 6mm thick steel angle. He survived (and apparently got most of his arm movement back), but the hole remained as a warning to any who might start to think they were invincible.
In short, almost any electrical supply capable of supplying over 50mA can kill. Most don't - you feel a tingle or even perhaps a proper wallop that sends you reeling, but that's it. At low voltages, you probably won't feel anything at all. It is wise to remember that even "safe" voltages can be dangerous - if you are up high, on a roof or perhaps a flown PA speaker system, even a mild shock may cause you to lose balance. Deceleration trauma from a reasonable height is definitely life threatening.
Many tales from survivors tell of how they grabbed a tool, wire, light fitting, etc., received a major electric shock, and couldn't let go. The reason for this is very simple - the muscles that close your hand and fingers are much stronger than those that open them. Muscles are triggered by minute electrical impulses in the body, and the external electrical current is many times greater than those we generate. The 'closing' muscles will almost certainly win. As a result, you will genuinely be stuck - unable to let go. Panic is the mind's instant reaction to something like that, but if you panic, you can't think. If you can't think you will probably die.
It's really easy to say "Don't Panic" (the technique seemed to work well enough in the Hitch-Hikers' Guide to the Galaxy). It's not quite so easy in real life, so I suggest that you follow every possible precaution to prevent the shock that causes the panic that causes the death. If you do find yourself in that situation, the best option is usually to try a different means of getting rid of the current source. I once saved myself by smashing an electric drill on the ground. It was desperately trying to kill me, but when I smashed it, that caused a major short circuit internally that blew the main fuse - and caused some embarrassment for the workshop manager, because the mains socket I used wasn't earthed! - I didn't know this at the time. I was lucky, and have had to use similar techniques on several occasions - yes, I've been zapped many times from all manner of things. To some extent, it comes with the territory - if you are building and fixing mains powered things for long enough, an electric shock is almost inevitable. We all get a bit complacent when it's something we do every day.
With anything that you suspect, never touch it with your finger tips - if it's live, you may grab it and be unable to let go. If no test equipment is available, use the back of your hand. Because the skin is softer, you can feel quite low voltages this way, but if it's potentially lethal, your hand will pull away from the faulty appliance. You may find that you can detect as little as 1mA quite reliably by using the back of your hand - this is considered to be about the minimum we can feel, although some people will be more or less sensitive.
If someone else is stuck, never simply rush to their aid. If their body is live, you may find yourself stuck too - it may bring to mind comical scenes of hundreds of helpers all jiggling about wildly, but it's not funny. The first thing you must attempt is to remove the source of power. If this cannot be done for any reason, you may try to pull the person clear by holding onto dry clothing or by using a dry stick or similar (nothing metal or wet for obvious reasons) to pry the person from danger, or to pry the danger from the person. Don't be too concerned about using force and causing minor injury - no-one ever died of a broken finger (probably not strictly true, but you know what I mean ).
There are innumerable possibilities, and it is obviously impossible to try to explain a method for each case. If you are the rescue party, then your first responsibility is to yourself - you can't rescue anyone when you are dead. Attempting a foolhardy rescue may mean that not only does your rescue mission fail and the person dies, but you die too. This is not a good outcome, and is best avoided.
There are quite a few websites that discuss electrocution, what to do and what not to do. The vast majority will give good advice, and although it might be a bit over-cautious in some cases, it is better to be safe (and alive) than sorry (and dead).
If there is any doubt about the victim's condition whatsoever, call for an ambulance. Elderly people in particular may suffer cardiac arrest (heart attack) or even a stroke as the result of the often violent electric shock. In many cases, the electric shock itself may not kill the victim, but can easily be a trigger for some other life-threatening condition.
Of all the possible sources of electric current, most do have the capacity to kill you, either directly or by causing a fall, heart attack, etc. There are many that are considered safe, but that doesn't mean that you should be complacent. Electrocution from low voltage sources (< 32V AC or about 48V DC) is extremely uncommon - I couldn't find any references to a death from such sources in my searches. This doesn't mean they can't kill you, and sensible precautions are still needed.
A list of do and don't items is always difficult, because one must generalise. However the following may be helpful ...
This is not a complete list, nor is it intended to be the last word on electrocution from any source. The purpose of this article is to give the reader a few basics, and to encourage further study on the topic. There are over 3 million sites on the Net that discuss electrocution alone. You can also search on many other specific areas within the topic - this I leave up to you.
Even with relatively mild shocks, anyone with a heart pacemaker or a chronic heart condition is at risk of suffering cardiac arrest as an indirect result of an electric shock. I was unable to find any statistics on this, but I'm sure they are out there somewhere.
If you are working with mains powered items (such as audio equipment), use a safety switch. These are variously known as RCDs (residual current detectors/devices), ELCBs (earth leakage circuit breakers), core balance relays, or just safety switches. In the US, you may see them referred to as Ground Fault Circuit Interrupter (GFCI) or an Appliance Leakage Current Interrupter (ALCI). Regardless of what it's called, test it regularly (they have a self-test button), and make sure that you use it. Always. No excuses.
Your entire workbench should be protected, but be aware that a safety switch will not work if you get yourself across the active and neutral wires but have no path to earth (ground). For this reason, never disconnect the safety earth pin or wire on any piece of equipment - especially while you are working on it.
Safety switches operate by comparing the current in each mains conductor - active (live, hot, line, etc.) and neutral. Provided the two currents are exactly equal, the safety switch will not operate. When a person contacts the live conductor, some current passes through the person's body. This unbalances the current (because that current is not returned via the neutral conductor) and the power is interrupted. RCDs do not protect against overloads, short circuits between active and neutral or any fault condition other than a current imbalance between active and neutral. Normal trigger conditions may be as little as 5mA, and the RCD should disconnect the power within as little as 25ms. Actual specifications vary, but are usually regulated by the electrical authorities for each country. Most RCDs are less sensitive than indicated above, because such a high sensitivity and fast switching will cause nuisance tripping - a small amount of leakage (or even capacitance) can cause the RCD to interrupt the supply.
Typical portable RCDs will have a sensitivity of between 15-30mA, and will switch off if this condition is maintained for more than around 40ms. While the current may seem a little high, it's a reasonable balance between safety and lack of nuisance tripping. If the unit switches off the power for no apparent reason, people are less likely to use it, and then have no protection at all.
Just because you have an RCD installed, this does not mean that normal safety precautions can be neglected. Remember that anything beyond a transformer is not protected - regardless of voltage, so power supplies still present a risk. Small though it may be, the risk is still there, and ignoring it is not recommended.
This applies especially to the use of isolation transformers (sometimes erroneously called 'safety' transformers). Use an isolation transformer only when absolutely necessary, such as working on a switchmode power supply or hot chassis equipment. These are potential killers even with an isolation transformer, so don't think for an instant that you are 'safe' - you most certainly are not. Remember that your safety switch will not operate if there is a transformer in the circuit and you contact the transformer secondary!
Remember - Even with a safety switch, there is still a risk of electrocution as noted above. There is no technology that will keep you completely safe while working on mains powered equipment. Your survival depends on you ... employ safe working practices, and never assume anything!
In many workplaces, it would seem that electrical safety is compromised by the use of conductive wristbands (or sometimes ankle bands or similar). These are used to prevent ESD (electrostatic discharge) damage to sensitive components. If the ESD protection equipment is either not made to a high standard or not tested regularly, there is indeed a risk.
Most ESD protection systems use a resistor to limit the current. 1MΩ is the most common, and this limits the current to 250uA at the maximum recommended working voltage of 250V. Where voltages above 250V are encountered, wristbands or other methods of connecting the technician to ground should not be used! Alternative methods of static reduction must be employed to minimise the risk to those working on the equipment.
The table below provides the US DOD (Department of Defence) guidelines, based on MIL-STD-454. Bear in mind that these are guidelines, and different people may react differently. These figures may be somewhat higher than those accepted by many other authorities - perhaps defence personnel are tougher than the rest of us .
|Current in mA|
|0 - 1||0 - 4||Perception|
|1 - 4||4 - 15||Surprise|
|4 - 21||15 - 80||Reflex Action|
|21 - 40||80 - 160||Muscular Inhibition|
|40 - 100||160 - 300||Respiratory Block|
|Over 100||Over 300||Usually Fatal|
60Hz is referenced because that is the frequency in the US (where the data originated). Expect the results for 50Hz to be fairly similar though. As always, different countries will have differing regulations and requirements, although the anti-static wristbands (and other anti-static equipment) are fairly standard these days. If you do need to use any form of anti-static device, ensure that it is tested regularly and maintained in good condition. Damaged insulation or a shorted safety resistor (for example) could place you at serious risk.
Many people wonder why the neutral conductor (aka grounded conductor in the US) is earthed. Surely it would be safer if both AC lines were floating, so that contact with either one (but not both at once) would not give anyone an electric shock. This is the same reasoning behind using an isolation transformer when working on equipment with a live chassis - it becomes "safe". This "safety" should extend no further than one individual piece of equipment, and only while it is being repaired or tested.
There is one major problem with a floating mains supply, and it turns out that leaving it floating is actually incredibly dangerous. It must be remembered that a great many houses and business premises will be connected to the same circuit. Should a fault develop in the wiring or in any piece of equipment in any of the connected premises such that one conductor contacted earth, then the situation is as it is now.
The problem is that no-one knows about the fault, because nothing happens. Fuses/ circuit breakers don't blow, residual current devices can't be used effectively with a floating supply, and now one conductor is 'hot' and the other is 'cold'. BUT WHICH ONE?
Unlike the situation now where everyone knows which mains lead is active ('hot', which can kill you) and which is neutral ('cold', which is generally 'benign') because they are colour-coded, we have a condition where no-one knows about the fault, no-one knows that one lead is now 'hot', nor do they know which one! Because circuit breakers/ fuses remain operational, there is nothing to warn anyone or disconnect the fault. This situation could remain for quite some time, without anyone being aware there was a problem.
Some time later, a similar fault may develop in another piece of equipment in another house, but with the other lead now connected to earth. This is a short circuit, with both 'floating' mains leads connected to earth, but in different premises. Circuit breakers may or may not operate, depending on the resistance of the earth connection, and meanwhile we may have a voltage gradient across the ground itself, between the two faults.
Likewise, the exact same AC line may be connected to earth in several places due to faults. It would be incredibly difficult for any electrician to try to isolate the faults, because they could be in any house(s) on that section of the distribution grid. It should be clear that this is a nightmare scenario - in every significant respect. Even when everything is ok (no faults) the capacitance to earth from the distribution transformer and all distribution and household wiring will be significant, and may be enough to create an artificial 'centre-tap', so both mains leads are 'hot' with respect to earth, albeit at half the normal voltage. Doesn't sound very safe to me!
The current system is used in one form or another everywhere. I don't know of any country that allows the use of floating mains supplies, because everyone in the industry knows that the risks are unacceptably high if one mains conductor is not designated as a neutral, and securely connected to earth/ ground via water pipes, dedicated grounding stakes, or other means that ensure that the neutral conductor remains at (or near) 0V AC with respect to earth.
A large number of sites were scanned for information, and it is not possible to list them all. Some have minimal basic information, others go into great detail about specific incidents. Those sites that had material that was used in this article are listed within the text. As noted in the warning panel in the introduction, the material presented in this article is largely common sense, with much based on personal experience. It is definitely a topic worthy of your own further research.
If anyone has information they would like included, please let me know. While I have taken every care to ensure that the material is correct, there will be errors and omissions. I welcome further input, but no anecdotal 'evidence' please.
Several suggestions have been included already, either adding to existing information or providing new details. This is a serious topic, and it is my intention to add updates or additional warnings when they are received. Any material submitted should have references if possible, although this isn't always necessary.
Information on electrostatic discharge protection was obtained from ...
ESD Around High Voltage - August, 1996 Ryne C. Allen, NARTE certified ESD Control Engineer, Desco Industries Inc.
The document was supplied to me by a reader.
|Copyright Notice. This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 2007. In the interests of safety, reproduction or re-publication is permitted, provided this copyright notice and a reference and link to the original are included. Please contact the author (Rod Elliott) before using the material herein in any publication, website or other publishing medium.|