Proudly making Quest AV equipment in Australia since 1988
also t/a Quest Electronics ® and Quest AV ®
Phone and SMS ... 0431 920 667 Mail ... POB 348 Woy Woy NSW 2256
In the "good old days" all electrical appliances were "earthed" (aka "grounded") which means 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 the 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 mains 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 usually cause the equipment's protective fuse to blow, disconnecting the mains supply and thereby signalling that something is very wrong. In HiFi setups and Public Address systems it is not uncommon to have an audible hum ever-present. In distributed Video systems such a hum can become visible as a light and a dark horizontal bar moving 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 is a very dangerous, potentially lethal, thing to do! An example:- Many Rock n' Roll performers have suffered shocks or electrical burns to their lips due to the ignorance of the "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 side of the mains to the chassis and the other from the Neutral side to the chassis. These are fairly effective at preventing spikey interference from entering or leaving the amplifier, but when the earth connection is broken they form a voltage divider with the centre-tap on the chassis! In Australia and the UK where the mains voltage is around the 240V mark, this places about 120V on the chassis and the capacitors are usually of such a value that roughly 1 to 10mA of current can flow from the electrified, unearthed chassis to anything that is properly earthed. So when the performer, holding a guitar solidly connected to the unearthed amp, touches their lips to the microphone which is just as solidly earthed by the PA system, they get a major shock in a very sensitive part of the body - their lips! The current flows from their hands through their chest (and heart!) to their lips. The natural assumption is to blame the Microphone or the PA system, but all tests of that equipment show no problem. If the guitar amp suffers a serious electrical fault or the capacitor in the Active line shorts out (and they do short out sometimes, by the way) the performer could easily be killed. DON'T EVER DISCONNECT A MAINS EARTH!!! Double-insulated equipment is not connected to the mains earth and most of the HiFi and Video gear intended for the domestic user is double-insulated. In double-insulated gear the metal chassis is connected only to the "common" side of the circuitry to achieve some shielding to prevent RF 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 symbol on the chassis - two concentric squares. So what can go wrong with double - insulated gear? Plenty, as we shall now see. All electrical equipment has a power supply which may be internal or external. The power supply converts the raw mains into low voltages for the equipment's circuitry and isolates the mains from the low voltage side... 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, or to drain the leakage to ground, 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 50 or 60Hz. The leakage current is usually less than 1mA and is often as low as 10uA, but it is still present and can cause problems. Generally, the more power something uses, the higher the leakage will be. Things 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 leak just like anything else connected to the mains. Insects, dust, moisture, food particles and all sorts of detritus
finds it's way into electrical equipment. Electrolytic capacitors and batteries can leak their corrosive contents and components can overheat or be damaged in a million other ways. Insulation gradually loses it's 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's not insulated at all! 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 "interesting" problems. Let's say you have a DVD player, a notebook computer, a media player, a tv/monitor, a CD player, a cassette deck, a graphic equaliser, get the idea. Let’s also say that all these things are double-insulated. Now 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 measure the voltage 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 ac voltage, probably at least half the mains supply or even higher. Don't try to measure the current directly, you might damage your multimeter. Now let's say you have the metal shell of an RCA on an audio cable which 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 is this might be unnoticeable or 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 - from that item - the others are still ‘floating’! You might notice though, that your previously hum-free system is now humming. The hum might be affected by the volume control on your amp or not or it might only be recorded or it might be visible on your TV or... 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, but disconnect all the gear from the mains first, otherwise if you've been unlucky, you might also notice that that first input you plugged into is noisy, distorted or dead. 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 save your life (or at least that of your other gear) if one of your double - insulated 'whatevers' happens to have a serious electrical fault or a bit more leakage than your other equipment can tolerate. Lastly, a tale of excess. A customer of ours rang one day 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, 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 less than sixty domestic vcr's 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". I declined to make the leads as I had no desire to be a party to the electrocution of some unsuspecting operator. An electrician colleague checked over their system and ended up having to attach earth straps to all the vcr's to get rid of the considerable leakage of 60 vcr's. He also re - attached all the lifted grounds and made their system safe. A few relocated mains plugs and some isolating transformers for audio and video lines (our VIT1 ) from other rooms (to break earth loops) and the system was clean and safe. You really can't beat the results of doing things properly (and safely)! (c) 2000 Quest Electronics abn 99 064 323 255
I'm Shocked - Why Am I Getting A Tingle?