I'm Shocked—Why am I Getting a Tingle?
These days, hi-fi and video components are technically electrically safer than they’ve ever been, and so are the various cords, cables and interconnects that link them together. Why is it, then, that it’s now very common to get an electrical shock when installing equipment? Experienced electronic designer and technician Joe Raine reveals some shocking secrets…
In the ‘good old days’ all electrical appliances were what is known as ‘earthed’, or ‘grounded’. This meant that all exposed metal parts were connected to the ground pin of the mains cable attached to the equipment. This was—and still is—a pretty good scheme, which is why professional equipment makers have stuck to it! If something happens to cause any sort of conductive path from the ‘Live’ (also known as ‘Active’) side of the 240V mains power supply to the metal chassis, then that ‘fault’ current is conducted to ground. If the impedance of the fault current is low enough, a large current will flow and this will usually cause the equipment's protective fuse to blow, disconnecting the mains supply and thereby signalling that something is very wrong.
In many hi-fi and video systems and also in public address (PA) systems it is not uncommon to have an audible hum ever-present. In distributed video systems such a hum can actually manifest itself not as sound, but on the picture itself, becoming visible as light and dark horizontal bars that move slowly up and/or down the video screen. If the hum is severe it can disrupt the synchronisation signals and cause major distortion of the image. It was (and unfortunately still is!) common practise for ignorant persons to disconnect the mains earths from various items of equipment until the hum ceased. This practise of disconnecting the earth is very dangerous… and potentially lethal!
Many musicians have suffered shocks or electrical burns to their lips when they have come into contact with a microphone due to the ignorance of a ‘technician’ who removed the earth connection from their guitar amp to stop it humming. What they did not know was that there is often a mains filter consisting mainly of two capacitors, one from the ‘live’ or ‘active’ side of the 240V mains to the chassis of the guitar amp, and the other from the ‘neutral’ side of the mains supply to the chassis. These are fairly effective at preventing spiky interference from entering or leaving the amplifier, but when the earth connection is broken (as in this example), the result is that the chassis of the guitar amp becomes ‘live’ with 120 volts. This is because they form what’s called a ‘voltage divider’ with the centre-tap on the chassis, and because here in Australia the mains voltage is around the 240V mark, this places half this voltage (120V) on the chassis. The two capacitors involved are usually of such a value that roughly 1–10mA of current can flow from the electrified, unearthed chassis to anything that is properly earthed. So when the performer, holding a guitar that is connected to the unearthed amp, touches their lips to the microphone which is solidly earthed by the PA system, that performer will get a major shock in a very sensitive part of their anatomy—their lips! The current flows from their hands through their chest (and heart!) to their lips.
In such cases, most performers make the natural assumption that the fault is with the microphone or the PA system, since they were shocked or injured only when they came in contact with the microphone, but all tests of that equipment will show no problem, because the fault was in the guitar amp, due to the missing earth. There is another scenario, which is that if the (unearthed) guitar amp suffers a serious electrical fault or the capacitor on the active side of the mains shorts out (and they do sometimes) the performer could easily be killed. DO NOT, EVER, DISCONNECT A MAINS EARTH!
Double-insulated equipment is not connected to the mains earth and these days, more and more domestic hi-fi and video equipment is double-insulated. In double-insulated equipment the metal chassis is connected only to the ‘common’ side of the circuitry to achieve some shielding in order to prevent r.f. interference from radiating into or out of the item. There can be no mains filters which connect to the chassis, so at least the chance of a shock from this cause is eliminated… or at least minimised. It is easy to identify double-insulated equipment because the mains power lead will often be flat instead of round and the mains plug will only have two pins and there may also be a specific symbol on the chassis or label to indicate that it is double insulated—that symbol comprises two concentric squares (visible in the illustration above just over the words 'in China'). So what can go wrong with double-insulated gear? Plenty, as we shall now see.
All electrical equipment contains a power supply. The power supply converts the raw 240V mains down to low voltages for the equipment's circuitry and also isolates the mains voltage side of the supply from the low voltage side… or almost. You see, all power transformers, whether they are designed for a linear supply or a switch-mode supply, leak a small amount of current from the primary to the secondary due to stray capacitance. In double-insulated items there is no earth available to use as a shield between input and output, so the power supply is designed as far as possible to minimise the leakage due to this unavoidable effect.
Unfortunately, switch-mode supplies run at very high frequencies, so the smaller capacitance in the smaller transformer ends up leaking just as much as the bigger capacitance in the bigger transformer running at 50Hz. The leakage current is usually less than 1mA and is often as low as 10µA, but it is still present and it can cause problems. Generally, the more power a component uses, the higher the leakage current will be. Components that use an external power supply (also known as a ‘wall-wart’ or ‘plug-pack’) are not exempt either. These power supplies are usually double-insulated and will also leak just like anything else connected to the mains.
Insects, dust, moisture, food particles and all sorts of detritus finds its way into electrical equipment. Electrolytic capacitors and batteries can leak their corrosive contents, and internal electrical components such as resistors, diodes, transistors and other semiconductors can overheat or be damaged in a million other ways. Insulation on wires gradually loses its plasticiser due to age, exposure to light and thermal cycling to mention a few causes, and becomes brittle. Insulation in transformer windings degrades over time due to vibration and heat. All these things can conspire together to cause electrical leakage (or even short-circuits) to the unearthed chassis of double-insulated gear and, guess what? All of a sudden it is no longer insulated!
The introduction of an earthed item into a system made up of double-insulated gear is not unlikely, but it is likely to be the start of some, let us say, ‘interesting’ problems. Let's say you have a DVD player, a DVD recorder, a set-top box, a TV/monitor, a CD player, and maybe even an old cassette deck or VCR as well… you get the idea. All these components are double-insulated and they are all connected to each other either directly or through one of the others. It would not be unreasonable to expect 0.1 to 1mA (or even more!) of leakage from the combined equipment because the leakage currents simply add together. If you were to connect a digital multimeter between this pile of gear and the chassis of your new item, let's say it's a big brute of a surround sound amplifier which just happens to be earthed, you will measure a very significant a.c. voltage, probably at least half the mains supply or even higher. (DO NOT try to measure the current, you might blow up your multimeter if there's a bad fault).
Now let's say you have the metal shell of an RCA lead on an audio cable that is connected to an output on the unearthed gear in one hand and you steady the amplifier with your other hand as you start to plug it into an input. As soon as you complete the circuit between them you will get a mild electric shock. Depending on how moist your skin is and how high the leakage current, this might be so low as to be unnoticeable or so high that it might cause you to say a bad word! Unless you have a bad heart it is unlikely to hurt you physically, but it can be quite unpleasant… especially when you don't expect it. Once the first connection is made and the leakage is flowing to ground, there will be no more tingles.
You might notice though, that your previously hum-free system is now humming. The level of the hum might be affected by the volume control on your amp or not or, it might only record on tape or it might be visible on your TV. So what do you do? That's a good question and one which deserves a really good answer. Unfortunately, there is no easy answer, but now that you know a bit more about where the hum comes from, an orderly and logical disconnection and re-connection of the whole system will locate the major culprit. If you've been unlucky, you might also notice that that first input you plugged in is now noisy or distorted or not working at all.
Ever wondered why they always say in the instruction manuals that you should disconnect the power when making or breaking connections to anything? Well now you know! It's to avoid shocks, huge hums, loud crackles and damage caused by high voltages being connected to sensitive inputs before the common/earth/ground side manages to short out the dreaded mains leakage (or your own static electricity build-up). It might be a nuisance, but it does avoid all these problems, and could even save your life in the event that one of your double-insulated components happens to have a serious electrical fault or a bit more leakage than your other equipment can tolerate.
Lastly, a tale of excess. Some years ago, before the ascendency of DVD, a customer of ours rang to order some audio and video patch cables. Nothing unusual about that, but the next request had me spinning out—they wanted them totally insulated, so that there was no exposed metal anywhere. ‘Why?’ I asked. ‘Because we get really, really bad shocks from exposed connectors in our duplicating setup,’ came the answer. Upon further investigation I was informed that they had no fewer than sixty domestic VCRs connected to a rack of distribution amplifiers.
'Domestic' was the key word. ‘Are they earthed?’ I asked. ‘No, nothing is, we had to un-earth everything to try and get rid of all the hums,’ was the answer. I declined to make the leads as I had no desire to be a party to the electrocution of some unsuspecting soul. A colleague subsequently checked over their system and ended up having to attach earth straps to every single one of those 60 VCRs in order to get rid of what was considerable leakage, after which he then re-attached all the lifted grounds to make their system electrically safe. After that, it was a matter of re-located some mains plugs and adding some isolating transformers to the audio and video lines (to break earth loops) and the system ended up being not only safe, but also completely free from hum and noise. You really can't beat the results of doing things properly (and safely)! # Joe Raine*
About The Author
Joe Raine has worked in professional audio since 1972 when he serviced Studer, Revox, AKG, Denon and Tannoy products at AWA. In 1976 Raine moved to Festival Records, at that time a major player in the Australian Record Industry. After what he describes warmly as “a very interesting and fulfilling ten years in the studio as the technician and a recording engineer”, he moved to AAV's then-new Sydney facilities as the technical manager for The Production Group (Video Production), Soundfirm (Film Sound), and Intercity Hire (Event Management and AV Hire). When those divisions split apart in 1988, Raine started his own company Quest Electronics (which also trades under the names Questronix and Quest AV). He has designed many items of analogue audio and video electronics for recording studios, disc mastering, audio-visual, outside broadcasting, film sound, etc., and continues with that to the present. He can be contacted here.