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Mains Distribution Board

A super-screened distribution board that protects against mains transients, ground loops and short circuits - ideal for every electro-musician

Clean up your mains with E&MM's Super Screened Mains Distribution Board

This feature deals with the cautionary subject of AC mains. Under no circumstances should you attempt to work on circuits to which live mains is attached. Readers are not so plentiful that we can afford to terminate them in such a perfunctory manner!

Distributing the mains may seem like a relatively tedious and uninspiring business when compared to the unashamed esoteric nature of the modern electro-musician's battery of high technology - yet it's still one of the most potentially embarrassing and hazardous areas for any musical enterprise. No mains distribution system has been designed to cope with the requirements of the numbers of mains operated pieces of equipment in the average living room - let alone the considerations of a performer: mixers, amplifiers, keyboards, computers, effects boxes, lighting - the list is not difficult to extend. Thus we all need some means of turning one 13 amp mains lead into many.

Then there are the apocryphal tales of musicians who subscribe to the school of electrical engineers that rely on poking the wires into the mains sockets using matchsticks. A practice which gives a whole new dimension to the expression 'electric guitar'.

Performers frequently get hot, sweaty and rather enthusiastic. Tugging at cables, draping invitingly conductive flesh within millimetres of the National Grid. Such events may contribute to the performance in ways hitherto unplanned and unimagined by audience and artist alike.

Many of the 13 amp distribution units available are less than adequate after the sort of pounding received at the hands of the brutal musician, since all 13 amp mains plugs are not alike, and the variety of dimensional tolerances can lead to slackness that does wonders to equipment through the induced arcing and intermittency that most readers will have chanced across 'in their time'.

We're not finished with this cautionary tale just yet, since it is as well to remember that the flashing of the obligatory light show, the switching of the triacs and the delicacy of the 64K dynamic random access memory in the latest computerised equipment are about as compatible as Frank Zappa and Dame Myra Hess. (Who's he?). In brief, a good tweak of the mains supply will readily reset a computer and reduce a carefully preprogrammed synthesised musical extravaganza into something that would shame a novice stylophone operator.

What you need is an E&MM mains transient protection distribution unit. Or the EMMMTPDU, for short.

You and the EMMTPDU

The circuit of the Mains Distribution Board is displayed in Figure 1.

Figure 1. Mains Distribution Board circuit diagram.
(Click image for higher resolution version)

Following through from the mains input end of the circuit, you will note that there's a fuse. It's a great deal easier to change this type of fuse than to fiddle about unscrewing the plug, so make certain this fuse is rated below the main 13A plug fuse. You should not attempt to distribute the mains if your 'Christmas' tree is likely to be taking in excess of 10A anyway. The ratings of the switch are the prime consideration, but so are the ratings of the individual outlets. No single outlet should exceed 6 amps.

"But why?" you ask. After all, a piece of 6A fuse wire is only a fraction of the size of the pins on the 6A plus/socket.

A practical demonstration is to unplug a 2.5-3kW heater from a 13A socket and just check how warm the pins on the plug have become, be careful, since many of the less stoutly constructed arrangements will actually be far too hot to touch, even at 'only' 10A. The answer is simply to be found by reference to Ohm's law, one derivative of which is:

W = I.I.R (W = power in watts, I = current in amps, R = resistance in ohms)

In other words, if you introduce even a small resistance in the path of a large current, it generates much heat. Ultimately in mains applications, that heat will cause the cable or connector to get so hot it deforms, and may then arc, causing carbonisation and a situation which is virtually irrecoverable. If you have a 13A mains outlet that is blackened around the Live pin, then the chances are that you will have to replace it entirely if you want to use it to full capacity again. Never unplug high capacity equipment without first using the outlet switch to disconnect the circuit without a flash and a bang.

Back to the rating of the distribution block: even the relatively small resistance offered by the contacts in the outlet sockets means that you must not exceed the ratings to ensure reliability and longevity. If you dismantle most of the moulded 13A distribution strips, you may well be amazed/unnerved at the relatively insubstantial nature of the conductor and contact surfaces. Most such blocks are not really recommended for more than 3A per outlet.

All that, and we're only as far as the fuse! On past the switch to the mains 'on' neon indicator. The second neon (Ne2) is placed between the neutral and earth, and if this should light, then you should not connect anything but attempt to trace the reason why there is a significant voltage present on a line where there should be none. 13A mains wiring is a vexed enough subject not to be covered here: buy the Readers Digest DIY book or whatever...

The coils (L1-L5) are wound on dust iron toroids. Similar to ferrite toroids, but better suited in most cases to the function of interference suppression whilst under the influence of current. The manufacturer produces a range of technical support literature for those interested, otherwise take it as read that the details given here are a suitable general purpose compromise for an application such as this. Purists with computer programs may want to match source and load impedance, current ratings etc., but that's beyond our scope here, although we shall concede that such efforts may be rewarded - marginally.

Thus each outlet has a degree of isolation from the next. The circuit is shown wired in series, and it's arguable that it may be better to run them in parallel from the main feed points (A and B) after the prime input filter formed by L1, C1 and C2. But now we move into the most vexed question of all (as far as the electric musician is concerned): "When is an earth not an earth?"

Briefly, certainly not necessarily when it's connected to the earth point of the mains. The impedance of the earth is a vitally important consideration in all AC engineering, and the earth connection should always be the heaviest gauge possible in order to keep its resistance to the barest possible minimum. Note that the earth is drawn as a bus bar function, not a direct series line.

When connecting equipment arrays you are likely to find that it's important to isolate certain items from their own mains earth, if the connection is made via another piece of gear. The sport of tracing the 'hum loop' is one which few of you will not have participated in. ALWAYS mark sockets and plugs with an earth pin present, but without the mains earth connected accordingly. It will save you trouble, time and possibly your neck should you subsequently change the equipment array and lose the earth.

Figure 3. DC - 20MHz attenuation.
Figure 2. DC - 200kHz attenuation of a typical mains filter network.

Building and testing

Assembling and testing the distribution block may not seem like the most complicated construction project you've ever undertaken, but don't allow a blase approach to cause you to forget the basics. Check the continuities with a reliable ohmmeter before plugging in.

Enhancements and add-ons

There is an excellent chance that you will be operating your distribution system on the very distant end of a 13A circuit. So distant that the cable resistances will become significant under load. Fitting a 240V AC meter on the box (possibly the variety with the scale expansion to read 210-250v only) will give you a good idea if the strange hum apparent on the equipment arises simply from the fact that the rectified voltage in the equipment PSU has fallen below the threshold required for operation of the voltage regulation circuitry. It happens frequently, and you might like to watch for it.

An alternative water resistant/'yank' resistant mains connector range - Bulgin's 'Buccaneer' range.

The box described here uses the Ambit 10-02024 and 10-02035 plug and socket system, which are part of the internationally approved IEC system. If you want to provide a degree of security that the cables cannot be pulled out under mild strain, then the Bulgin "Bucanneer" range of water resistant screw-down mains connectors will provide both security against accidental disconnection and (provided the rest of your construction does as well...) a good degree of environmental isolation if you ever need to operate 'al fresco'.

Figure 4. Mains filter network.


R1,2 Integral with mains neon assembly
C1/C8 10nF/400vAC - MUST BE High voltage (mains rated!!) Ambit 01-10312
Ne1/2 Ambit 15-71201 nut-fitting panel bezel Neon
L1 20t + 20t on Ambit 55-10626 dust iron EMI core. Use 20 SWG wire
L2-L5 10t + 10t on Ambit 55-08026 or 55-09440 core. Use 20 SWG wire.
Sk1-4 Ambit 10-02024 IEC outlet (use plug 10-02033)
Box Ambit 21-06043 ABS plastic box with tight fitting lid

E&MM's special offer price for a complete kit of parts for the MAINS DISTRIBUTION BOARD is £17.50 inc. VAT and P&P. Please note that the supplied sockets are Euro type, without plug section. Send your order to (Contact Details).

Previous Article in this issue

Advanced Music Synthesis

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


Electronics & Music Maker - Oct 1983


Previous article in this issue:

> Advanced Music Synthesis

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