Studio Mains Supplies (Part 5)
From the immortal pen of Ben Duncan comes the penultimate part of this epic reference work.
In this penultimate part, Ben Duncan goes into earthing and earth safety — after filling in on the practical details of protecting sensitive electronics from manic mains over-voltages.
To protect studio gear from the majority of transient over-voltages, we need only fit a Voltage Dependent Resistor (VDR) across the supply, close up to all the individual power supply transformers. The VDR looks like a ceramic disc capacitor (Figure 1), but it's a white or red disc (not brown or blue), with a pair of lead out wires.
It can be connected either way round, and is simply soldered in place (Figure 2). If you're uncertain about exactly where to put it, ask a boffin or your equipment dealer to help you out. It's quick and painless mod, and shouldn't cost more than the basic hourly rate. If you feel confident about locating the power supply transformers live and neutral connections I expect you won't need reminding to sleeve the VDR's leads and keep them as short as possible and not to bend the leads too close to the seal, without support from long-nosed pliers, otherwise the seal may be cracked, or worse, the VDR may split.
Aside from internal fitting, a second optional VDR can be fitted at the plug end of the mains lead. This is a much simpler task, no harder than wiring a plug. There's a short DIY article on this in the March 84 edition of Electronics and Music Maker, back issues of which are still available.
VDRs are rated by voltage and energy (in joules or J). Normally, you'll want one rated 230 or 240 volts, but 120 volt VDRs can come in handy for protecting any US gear which runs from external autotransformers. Placing a 120 volt VDR upstream from this transformer winding (Figure 3b) greatly enhances the effectiveness of the suppression, again because of the extra source impedance seen by the VDR, which wouldn't apply if we choose to fit a 240 volt VDR on the auto-transformer's input side (Figure 3a). The VDR energy rating is normally 10 joules for domestic equipment, but for a stiff (low impedance) mains supply, 20 or 40 joule VDRs are better able to cope, and cost only a little more.
If any of your VDRs should periodically blow up, this is a sign that they're underrated, relative to the prospective fault current, ie. the maximum current that can be pulled through the mains conductors, wherever the VDR acts and presents momentary short circuit.
Unless your 13 amp sockets are fed from an unusually short and stout cable, it's unlikely that a 10 or 20 joule VDR will fail to cope at the equipment position. However, once we start to put VDRs in more up-front positions, such as across the supply at the fusebox (which is a good position for some extra front-line defence), then bigger, 40 or 100 joule VDR's will be called for.
Figures 4 and 5 round off our discussion of protective elements at the interface between the mains and the equipment power supply. The diagram and picture reveal the protective components on a simple, but refined power unit, used to supply precision instruments and frame-mounted processor cards, with their own power regulation on board. Though inexpensive relative to the equipment value, the protective components shown here won't be found on the vast majority of professional audio equipment: it's not (yet!) a big selling point. Apart from the VDR (and the filter, which is wired off the PCB) note our old friend, the snubber, comprising R1 and C1. Here, it's used to help the VDR attenuate fast-rising voltages. Note that both devices are placed after the mains filter, so the series impedance of the inductors in the filter increases the available attenuation. C2 and C3 are strictly part of the equipment's low voltage side, but they provide a 2nd (or 3rd!) line of defence. In particular, they cut out recurring stresses at switch-off. Every time the juice to a transformer is abruptly cut, a large kickback voltage (called Back-EMF) would otherwise appear across the secondary windings. Ouch! For most small transformers, we can make C2, C3 = 100nF to quell this misbehaviour.
Make sure the capacitor's AC voltage rating is adequate; it's usually 40% of the DC rating, (for example 40V for a capacitor rated at 100V DC). Self-healing metallised polyester capacitors are best for all types of suppression duty, because an excessive voltage won't blow up the capacitor outright.
The mains earth connection is primarily a safety net. If a live wire comes loose, and touches any external metalwork, current is diverted to ground. Ideally this blows out the fuse which is preferable to the live metalwork hanging around with a smug grin, until some poor B grasps hold of it, or leans on it! 'But how does the current get back to the neutral, where it normally goes?' you ask. This is not actually a silly question. Only if your electricity installation had come with a full wiring diagram, could you be accused of stupidity. Neutral and earth conductors are in fact tied together at every substation transformer (Figure 6), with the soil acting as a literal earth conductor between the two earthing stakes. Alternatively there's a PME (Protective Multiple Earthing), system. This differs in that the earth wire is additionally joined (bonded) to the neutral at your electricity meter and again at regular intervals down the road. When this system is installed, there's often a 'PME' sign tacked onto every other electricity pole (Figure 7).
Problems begin when working with audio, or indeed, any sort of sensitive electronics. Common to all electronics is the concept of ground; (not necessarily earth) the common or 'zero volt' (0V) connection which also surrounds and shields the circuitry. As soon as we add mains power to the overcoursing picture, a conflict arises: the exterior metalwork must be connected to the mains earth (because it's exposed) and it must also be tied to the signal common. If left floating, the metal won't shield the electronics, but instead will propagate surrounding interference fields. We'd then create a loud hum just by standing next to the enclosure!
Connecting signal ground to the same metal work as the mains earth is not in itself wrong. Not that is, until we hook up the inputs and outputs to other gear, we assume to be earthed in the same fashion. The outcome is the infamous 1 hum (or ground) loop, illustrated in Figure 7.
Three independent mechanisms are at work here. Firstly, the main portion of earth conductor is shared by all the other appliances on the same ring main, so as soon as any current leaks down the earth (the result of a defective appliance or maybe a spot of water down the back of the washing machine), there's a voltage developed across points X and Y. If for example X and Y are separated by a metre length of wiring cable, and the earth conductor is 1mm2 (as on 5 amp lighting cable), the resistance will be 0.16 ohms. If the leakage is just 5mA, this produces an error voltage of (5mA x 0.16) ohms = 0.8mV or 800 microvolts. If this voltage is imposed directly onto an unbalanced line interconnections, the resulting hum will be (0.8/776) = -60dB below the signal's zero level (0dBu) or -50dB below a -10dBu line connection.
Secondly, there's capacitance between the earth wire and the adjacent conductors, ie C (Live) and C (Neutral) in Figure 7. At low frequencies (ie. 50Hz), this capacitance presents a high impedance, and the earth conductor's low impedance acts to keep any 'crosstalk' at bay. At higher, audio and radio frequencies, these impedance relations are transposed. Then the coupling of asymmetric interference between the earth and live (or neutral) wires becomes much more intimate. This is especially true for conduit wiring where there's an intrinsic imbalance in capacitance between earth-to-live, and earth-to-neutral. To avoid noise and interference (of the RF variety, this time) being superimposed on unbalanced audio connections, we need a ground which maintains a low impedance at high frequencies. This means a short, stout earth wire that has a low inductance.
Thirdly, there's a single-turn 'transformer' at large in the earth circuit, comprising either of the two loops between the mixer and tape deck (for example). Think carefully about the nature of the ring main's earth connections if you don't see this. Any magnetic fields cutting the closed circle of wire sets up a circulating current. Because there's only a single turn, and the 'transformer's' coupling isn't terribly good (it's not, after all, an intentional effect) the induced voltage will be small, but the potential current is surprisingly high. This is the outcome of the earth conductor's low resistance, necessary for safety. Again, the circulating current sets up an error voltage across the earth conductor, this time mostly the edgy, 75Hz or 50Hz buzz of magnetically induced harmonics.
There are several ways to overcome the conflict between signal ground and mains earth: 1) Number one is to stick resolutely to balanced audio interconnections. We can then break the pin 1 (signal ground) connection at the source end of every interconnect, so there are no loops. Of course theory can lead us astray, but nevertheless, a colleague of mine, console designer Harry, recently used just this technique to install 16Km(!) of balanced cables in a palatial 'home' studio in Surrey. By religiously connecting every single unbalanced source through a balanced-to-unbalanced convertor box. The whole set-up, all 5000+ individual connections, worked first time without hum. Moral: don't underestimate this method. Hint: he kept all the gear switched on, so any hums could be heard immediately they developed, as the looms were being installed. Failing this, we can separate out the chassis and signal-ground on unbalanced gear, placing a 1K groundlift resistor between these points. We also need to take care that unearthed oriental and US equipment is suitably modified or isolated from the rack metalwork, because chassis and signal grounds on this gear are often one and the same, ie. bonded. Both these techniques have been described in greater detail in previous issues (Studio Earthing Techniques, HSR September and October 84).
More sledge-hammer techniques include ground chokes, which are placed in line with individual mains earth wires. These are heavyweight inductors, presenting a high impedance to high frequency currents, but not to the 50Hz mains. They're heavyweight because they may have to pass heavy peak fault currents, up to 500 amps, without destruction: any failure would naturally defeat the purpose of the safety earth connection! With these stringent safety requirements, ground chokes are expensive, and they don't do much for audio band interference below 1 kHz. Nevertheless, you could consider them if your mains is incurably unclean.
Abolition of the mains earth is another method. In theory, a residual current balance circuit breaker (RCBB) capable of detecting a 30mA or greater difference between the currents flowing in the live and neutral conductors provides a far greater protection against fire or electrocution, than any green/yellow earth wire. After all, the earthing concept is over 50 years old, and technology has moved on a bit since. And even though the mains earth connection has the benefit of reassuring physical solidity and simplicity, a modern RCCB is no more likely to fail. Consider the weed grown dereliction of many electrical earthing stakes...
Before you charge off to cut out the earth on your ring main, it's sobering to know that any insurance on your premises operates on the assumption that the permanent wiring meets regulations of the IEE (Institution of Electrical Engineers). Therefore, any unapproved mods in this area could void your insurance. As it happens, the IEE wiring rules for earthing have undergone profound revision in recent years, but not to the lengths of omitting the earth connection on a 13 amp socket: this is certainly out of order. On the other hand whatever you choose to plug into a wall socket is your own business, so it's less naughty to drop the earth connections on your individual 13 amp plugs, but only safe if you've fitted a 30mA RCBB, and had the new installation checked over by an experienced electrician.
Remember the concept of a separate technical supply outlined in part 4? Well, a technical earth is just an extension of this. Not only is the technical earth wire kept separate from the house supply's earth wire, it also continues back to it's own earth stake.
Because this method is effective, safe, (provided it's tested by a knowledgeable electrician before you turn on the power again) and inexpensive, we'll go into the DIY instructions in detail. Before doing so, a quick look at Figure 8 will help us spot the advantages and safety aspects. Firstly, residual leakage currents and interference in the house supply aren't superimposed on the technical earth - at least, not directly. But we should take care to keep the technical, earthing stake well away from the house supply's stake, so the path back to the substation can be as separate as possible. We can also fend off superimposition by ensuring the technical earthing stake has a good, low resistance contact to the soil. This, above anything else, is the secret of a clean, noiseless earth conductor for the studio. The equivalent circuit shows how the substation earth forms a great earth point,the ultimate grounding node!
Regarding safety, your existing domestic supply will have either a PME system or an earth leakage circuit breaker. This mouthful (usually abbreviated to ELCB) is wired in line with the earth conductor (see Figure 6) and serves current flowing into the earth. Alas, unlike the RCCB it won't trip out if the earth connection is severed, or the soil impedance is high because of the long, hot summer.
Because our new technical earth bypasses the existing safety devices we'll need to install a separate circuit-breaker and/or PME connection. Whether your electrician will advise on this, much depends on the local Electricity Board's practice. But if possible, you should avoid installing an ELCB, because it imposes extra impedance in line with the technical earth. An RCCB can provide superior protection against shock and fire, and puts no resistance or inductance in the earth conductor.
Feature by Ben Duncan
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