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Electro-Music Engineer

The Taming of the Juice

Mains Safety - the Taming of the Juice!


Most 240 volt mains fatalities occur as a result of housefires. However, for musicians and sound engineers, electrocution is a far greater menace. Death occurs when high currents flow through the heart muscles and respiratory system; Table 1 delineates the effects of various magnitudes of current. Note that currents in excess of 20mA, whilst not inherently lethal, may result in muscular contraction, so that you're unable to let go of the live object. This situation is especially dangerous, as an initially small current can build up to a lethal dose as a result of skin burns.

Table 1. Physiological effects of AC (50/60Hz) fault currents.
Current level Effect
1mA Threshold of perception.
5mA Maximum harmless current level.
10-20mA Unpleasant shock but no sustained muscular contraction.
50mA Pain, exhaustion, possible fainting, physical injury.
100-300mA Ventricular fibrillation starts, but respiratory centre remains intact.
6A Sustained myocardial contraction and respiratory paralysis. Burns if current is unduly concentrated in one area.


The body's main line of defence against excessive current flow is skin resistance. Dry, callosed skin, with a typical resistance of 250k implies a current circa 1mA at 240 volts and countless engineers owe their lives to the thick, horny calluses that develop on the palms and fingertips after years of soldering and rough manual tasks. However, it should be borne in mind that resistance bears a crucial relationship to the area of skin contact. Moreover, the passage of even quite low levels of current can result in burns, and as carbon is a good conductor, the current can rapidly snowball. The addition of sweat greatly reduces skin resistance, which makes musicians particularly vulnerable, especially when they're involved in the hyper-activity of a live performance.

Before considering mechanical means of protection against electric shock, it should be noted that the passage of mains-derived current will only normally be lethal if the heart/respiratory centre lies in the way. With this in mind, many engineers automatically place their left hand in their pocket whilst working in proximity to live circuitry. Then, if they do touch a 'hot' wire, a lethal current is unlikely to flow near the heart; rather (assuming there's no direct bodily contact with mains earth or neutral) the current will usually find its way to ground through the feet, via the right-hand side of the chest and legs. Of course, in no way does this precautionary measure guarantee that a shock will be harmless nor is this tactic of practical use to performing musicians. But then the nature of the hazard they face is different to that encountered in workshops.

Earth Leakage



Assuming that the mains terminations in stage and studio equipment are competently enclosed, then the prime hazard boils down to simultaneous and inadvertent electrical connections between the live side of the mains and exposed metal which happens to have a poor earth connection, say a microphone. As exposed metal in any mains powered equipment is normally assumed to be earthed, it follows that a musician holding this microphone is bound to receive a serious shock immediately he touches other areas of exposed metal such as the aluminium knobs on his amplifier. Ideally, with the neutral side of the mains being earthed, contact between live and earthed conductors should draw a large current and blow fuses. However, even if the earth connection is apparently sound all the way from the equipment to the mains socket, a high impedance or open circuit may appear either in the permanent mains wiring or in the vital connection between earth and neutral. In this case, heavy-duty supply fuses are unlikely to blow, and there's no guarantee that the vaguely defined fault current will blow even the more sensitive mains fuses incorporated in the equipment, at least in time to save a life. To an extent, the highly dangerous 'Bad Earth' is guarded against by PME (Protective Multiple Earthing) systems wherein the neutral and earth conductors are bonded at several points between the local substation transformer and the mains outlets, and by earth leakage circuit breakers (ELCBs). There are two types in common use. 'Voltage operated' ELCBs detect small fault currents (which wouldn't normally blow fuses) flowing down the earth cable. 'Current balance' ELCBs compare the magnitude of the supply current in the live and neutral conductors; obviously, if the current differs, then some must be flowing to earth (or at least, somewhere it shouldn't!) and the circuit breaker promptly opens (see Figure 1).

The current-balance ELCB compares the current levels in each side of the mains supply. When these differ, an error voltage appears across the output of the current transformer, causing the trip mechanism to disconnect the mains supply.


Active Protection



MK current balance ELCB.

Although one or other of these types of circuit breaker appear in the electrical installations of many venues, it's exceedingly naughty to assume as a matter of course that they exist and that they're functional! Also, the voltage operated ELCB is readily defeated if the fault current fails to flow down the earth wire to which the ELCB is connected; if a musician is standing on a damp concrete floor, or touches a brass water pipe, then the fault current will bypass the ELCB. By comparison, the current balance style of ELCB is relatively foolproof, but one can't be sure that the current level at which a permanently installed one 'trips' is low enough to be a lifesaver. The answer to these niggles is to buy your own, with a 50mA or (even better) 30mA trip current, and wire it into the master plugboard. You can then be absolutely certain that an earth fault appearing anywhere amongst your gear will cause the mains to be promptly disconnected whether you're playing at home, at a gig or in a field. Obviously, if you use several plugboards any one of which could be connected to a wall socket, a current balance ELCB should be fitted in each, otherwise a false sense of security can arise. Alternatively, ready-wired ELCBs are available built into the socket end of a short extension cable. For home studio use, double wall sockets can be replaced by single ELCB protected sockets, e.g. the MK 'Sentry Socket'.

MK 'Sentry Socket' with integral ELCB replaces a standard wall socket.


From a practical viewpoint, careful enclosure design is demanded for use on the road. A suitable box must guard the ELCB's delicate plastic body and prevent the 'trip', 'test' and 'reset' buttons being knocked accidentally; but at the same time, all the buttons must be readily accessible. One way to achieve this is to mount the ELCB in a diecast box with the buttons almost flush with the lid (see photograph in March 1982 'Electro-Music Engineer'). Holes marginally smaller than a fingertip are then drilled to expose the buttons, allowing these to be pressed with the help of a handy stave-like object such as a drumstick or car key! Of course, in large rigs the breaker can be conveniently and safely placed inside a proprietary 'cupboard' within the mains distribution trunk.

Having installed a current balance ELCB, it's sensible to be aware of its capabilities. Firstly, it offers no protection if you make contact between live and neutral conductors. Fortunately, this mode of shock is rare unless you probe inside equipment using both hands (which is unlikely on stage), or are silly enough to place both hands across a half-withdrawn 13 amp plug! Secondly, because excessive sensitivity can result in nuisance tripping, when say, equipment is slightly damp, or there's a lot of RFI or other random 'noise' on the supply, usable ELCBs for stage and studio use are generally restricted to a 20 or 30mA minimum trip current. So protection with 100% certainty cannot be assured and should never be assumed. In particular, shocks can be suffered without tripping the unit if the leakage current hovers between 1 and say 29mA.

In mitigation it's fair to say that the presence of a correctly wired and regularly tested ELCB makes serious shock or electrocution highly unlikely, but more to the point, provided you give regular attention to the integrity of the earth connections on your mains cables, a potentially lethal fault will be detected the moment it occurs, and usually long before it has a chance to cause harm. So the golden rule with ELCBs is to heed them when they trip even if everything seems in order. Use a meter with one lead connected to a known ground (e.g. the earth pin on a permanently wired 13 amp outlet) to check for live potentials on exposed metalwork, or try to isolate the faulty equipment by progressive unplugging.

Figure 2. A simple mains wiring fault indicator. 'Asterisked neons are normally lit. #Red neon lights to indicate an earth fault.


Figure 2 depicts a cheap and simple circuit which offers no active protection but provides confirmation that all three mains conductors are intact. Note in particular that the earth side of the upper pair of neons is connected not directly to the earth conductor, but rather to the equipment chassis, to which the earth conductor is assumed to be connected. If this arrangement isn't adopted then the neons will fail to show a fault if the earth to chassis connection fails. However, 'LNE neons' obviously can't monitor the earthing of all the panels and sub-frames in a large chassis, so this circuit should be regarded essentially as a means of spotting decrepit connections inside 13 amp plugs. Testers working along similar lines, but built into a dummy 13 amp plug, are also available commercially, e.g. The Martindale mains tester, available from Turnkey.

Though Britain's mains voltage is uniquely high and especially waspish, at the same time, U.K. standards of mains distribution are widely regarded as the world's most scrupulous.

Fusible Materials — a Precautionary Pot-Pourri



Regrettably, our most elegant masterpiece, the fused 13 amp plug is widely abused. In theory, square pin plugs should contain a fuse commensurate with the cable rating, but public nonchalance and the irresponsibility of 'shifting units at all costs' as a selling technique combine to ensure that most 13 amp plugs contain 13 amp fuses. Thus weedy 2 amp cables commonly fitted to musical gear become potential fire risks. To make matters worse, conscientious musicians often report great difficulty in obtaining suitable fuses: 1, 2, 3, 5, 7 and 10 amp fuses are certainly manufactured, but they rarely appear in the average high street shop, though to be fair, electronic component retailers do stock most of the values specified in the BS1363 standard.

The plethora of fuse types found in the panel fuseholders of musical gear is strangely much less of a problem; if your local music shop or an electronic equipment retailer can't help you, try a shop which handles radio and TV repairs.

A more pressing quandary is widespread misapprehension of the abilities of various fuse styles. Strictly all mains fuses should be High Rupture Capacity (HRC) types — the ones with ceramic bodies. These fuses are designed to clear the massive peak fault currents caused by short circuits (which can lie between 500 and 10,000 amps on a single phase mains supply) without exploding or setting fire to anything. If a glass-bodied mains fuse is used in musical equipment, it's unlikely that any great alarm will be raised. However, if something does short circuit, the current surge will probably cause the fuse body to shatter, an event which leaves a myriad of glass fragments plus a layer of vaporised copper inside the fuseholder.

Contrary to popular belief, a 2 amp 20mm glass fuse, say, does not 'blow' at two amps. The minimum current excess which will reliably blow the fuse is approximately 1.85 times the fuse's normal rating, thus a 2 amp fuse can last an hour or more when passing a steady 3.7 amps. At the same time, standard glass-bodied fuses exhibit wide tolerances — another sample of the same fuse may blow in two seconds when passing the same current, and fuse blowing times can also be surprisingly dependant upon ambient temperature. This prelude offers some answers as to why fuses sometimes blow spuriously, and at other times don't blow when their rating suggests they ought to. At the same time it underscores the dangerous sophistry of 'commonsense' action such as doubling the fuse value when nuisance blowing is experienced. Yet another reason for a sudden dearth of fuse problems is a lowering of the mains supply impedance. The wiring in a domestic house, particularly if it dates from the 30s or 40s, can have a relatively high impedance, which limits the inrush current to amplifiers when they're initially switched on. If your house is then rewired or you use your amplifier in a public venue with chunky mains cables, the impedance of the mains supply can be low enough to double the inrush current you previously experienced — and a fuse value that previously gave no problems may then blow capriciously.

Fuses can also fail quite randomly as a result of metal fatigue brought on by long term thermal-cycling stresses (e.g. the alternate heating and cooling inside a valve guitar amplifier), or by vibration. Naturally, low capacity, quick-blow types are most prone, and if the fuse is in a hard-to-reach-in-a-panic-situation, it may be worthwhile replacing it as a matter of course with an anti-surge fuse of equivalent rating; the 'spring' inside these fuses tends to absorb shocks and retard metal fatigue.

The most satisfactory answer to all these problems is to adopt an empirical approach, testing a series of marginally greater fuse values until a compromise between reliability and the retention of effective protection is attained. To complete this discussion, readers who aren't au fait with the less esoteric aspects of fusing the mains such as the difference between 'quick blow' and 'anti-surge' fuses will find Robert Penfold's article useful ('Fuses', E&MM December'81).

Foreign Policy



Last of all, a brief look at some of the hazards brought on by using foreign equipment designed for 110-120 volt mains. Sometimes, such gear is adopted, by a change of mains transformer, for export to the U.K. and Europe, in which case it will operate from 220-240 volt mains, but won't necessarily meet U.K. safety standards. At the same time, there are many items of sound equipment residing in the U.K. which don't even boast a 220-240 volt transformer tapping. Ironically, this unmodified 110 volt equipment, operated via an external isolating transformer tends to be less hazardous, principally because the mains hardware (switches, filter capacitors, fuses and connectors) sees the relatively low voltages it was designed to suit. Unfortunately, 110 volt equipment is often powered via cheaper auto-transformers, which don't offer the same degree of protection and isolation as a transformer with a wholly separate 110 volt secondary. In this case, and whenever 110 volt equipment is modified to accept 240 volt mains, it's sensible to carefully check all the mains hardware, ruthlessly exchanging switches, fuseholders etc. for U.K./European approved versions if there's any doubt. When carrying out modifications, look out for the mains switch. In 110 volt equipment their rating will usually be 350 volt or 500 volt DC, which may seem adequate on 240 volt mains, but isn't safe in practice. Replace these with 1000 volt DC or 240 AC capacitors. Either type should be designed especially for direct connection across the mains.

Next, a new 3 core cable and mains input connector (where required) should be fitted, and one or more firm earth connection made to the chassis; remember to leave the earth wire a little slacker than the other conductors, so that if the cable is strained, the earth connection will be the last to part company!

Then check earth continuity between the equipment chassis and all exposed metalwork, and make subsidiary earth connections where this seems wise. Also check the mains circuitry very carefully, looking for evidence of the bizarre and potentially very nasty American-style bonded earth system. This is normally manifest as a 'ground' switch, which connected the (unearthed) chassis to either the neutral or live side of the 110 volt mains. Remove and joyously destroy the capacitor and all vestiges of the 'ground switch' wiring. This circuitry serves no practical purpose whatsoever except to maim and kill musicians. Finally, check that the mains switch (if a single pole type) and the fuseholder are wired in series with the live conductor.

As implied, 110 volt gear operated at its native potential isn't entirely obnoxious. One of its greatest attributes is the intrinsic safety that can be gleaned by powering it from a 110 volt transformer with an earthed centre-tap. With this arrangement, both sides of the supply become live (relative to earth), but at the same time, they're only at 55 volts (110 volt/2), a potential which is a little more dangerous than a couple of pairs of 12 volt car batteries as used in some outdoor public address systems. So gear set up for 110 volts running from a 55-0-55 volt supply can be expedient when music has to be played in damp conditions (e.g. waterlogged festivals), where nuisance tripping renders 30mA ELCBs useless. Hefty centre-tapped 110 volt transformers are readily available from plant hire contractors.



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Universal Trigger Interface

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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.

 

Electronics & Music Maker - Jul 1982

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