Staying safe on stage is more than a matter of simply making sure that willing hands are available before taking a dive. Knowing how to properly handle the mains power we all need is also crucial to performance health...
Whatever the size, complexity or cost of your live sound rig, one of the first — if not the first — question on your mind when you get to a venue will usually be "where do I plug it in?" Depending on the venue, the answer can vary from a wall-socket behind a plant pot to a dedicated and professionally-installed supply that is reserved for your exclusive use, fully tested and certificated, and for which (with any luck) you'll have brought an appropriate connector. Whatever you encounter, you'll need to know some basic rules. When it comes to portable live-sound systems, this means firstly, using a suitable electrical supply; secondly, using suitable equipment; and, thirdly, connecting and using that equipment safely.
How much power will the average band's gear actually need? The only way to know for sure is to add up the power requirements of each individual item.Photo: Mike Crofts
How Much Mains Power?
What constitutes a suitable supply will depend, of course, on what you need to plug into it: if it's your own equipment you'll presumably know what supply capacity is required, but there may be other factors to consider if additional gear needs to be connected to the same supply. Such gear might include a visiting disco, a lighting rig, or other event equipment — for example, fridges at summer events.A good first step, then, is working out what current your equipment will draw from the mains. The power rating of each piece of gear should be stated on a panel fixed close to the mains connector, or where a fixed mains lead enters the equipment. The power rating may be expressed as a current (in Amps) or as a power figure in Watts. It's generally best to work out the total current your gear will draw, adding up all the individual figures to find the total load you'll be connecting to the mains. To convert Watts to Amps, divide the Wattage figure by 230 (mains voltage). As an example, a piece of equipment with a mains power rating of 100 Watts (not 100W of audio power) will draw a little under half an Amp. In a small venue that is only offering 13-Amp sockets of the normal domestic type, you can then work out how you need to wire up. If the total connected load of your system — including the backline equipment — is comfortably within the rating of a single or double 13-Amp socket, it's perfectly alright to connect it all from a single point. After all, that's what they're designed for! Try to avoid too many connections between this point and your equipment. It's much better to have a single power lead of the required length than two shorter ones joined together: less to go wrong!
A professionally made distribution box with meters to indicate AC mains voltage and current.Photo: Mike Crofts
Working It Out
One common mistake is assuming that audio output power is the same as the mains power required to operate the gear. If an amplifier were 100 percent efficient, you could, in theory, use all the mains power as audio output power, but this is not the case in practice, as some of the power used by the amplifier is dissipated as heat. A typical full-range 'active' speaker with built-in amp modules, rated at 240 Watts audio output, would have a mains power rating somewhere around 350 Watts. A useful rule of thumb (if you don't have the manufacturer's stated figures) is to multiply the audio output power by 1.4 to get an idea of how much mains power would be needed, then divide by 230 to find out the current consumption.The table below gives a rough guide to the supply current likely to be required by a band with three backline amps and a vocal PA (based on UK voltage). Bear in mind that equipment may demand a much bigger supply current when it is first switched on, so don't be tempted to turn everything on from a single switched socket — you wouldn't want to do this anyway, for many other reasons, such as risking a huge pop through your speakers! Also consider that the power that you can safely run your system on may not be enough to realise its full performance capability. Any system capable of delivering good bass power will need to draw a hefty current from the mains, and if, in the above example, we were to replace our typical small speakers with, say, a pair of Mackie SA1521s, the makers recommend that each speaker's mains supply is capable of providing seven Amps at 230 Volts! This is, of course, not a constant current requirement, but it does illustrate how important a good power source is for getting the best from your gear.
Equipment | Mains Power needed | Mains Current needed | Amps |
| 2 x 240W active PA speakers | 480W x 1.4 = 672W | 672 / 230 = 2.92 Amps | 2.92 |
| Mixer | 100W (stated) | 100 / 230 = 0.44 Amps | 0.44 |
| 2 x rack processors | 20W each (stated) = 40W | 40 / 230 = 0.17 Amps | 0.17 |
| 3 x 100W backline amps | 300W audio x 1.4 = 420W | 420 / 230 = 1.83 Amps | 1.83 |
| TOTAL = | 5.36 | ||
| ROUNDED UP = | 5.5 Amps |
User Beware
While we're talking in Amperes, it's worth remembering that electrical current is a dangerous animal; a current of only 50 Milliamps (0.005 Amps) can be fatal, and our typical small rig above is using over a thousand times more current than this. Safety is thus a huge consideration, and the use of a suitably rated supply is only the beginning. The best way to stay safe is to use only well-maintained equipment (including cables and connectors) that are properly designed for the task in hand, and to make sure that they are used as the manufacturers intended.
A professionally made distribution box with four 16-Amp outputs, all RCD protected, for limited outdoor use.Photo: Mike Crofts
If the venue in question is unfamiliar to you and you are responsible for providing and operating the PA, always check that the supply you're asked to use is suitable. Just because it's a 13-Amp socket doesn't mean that it's capable of supplying 13 Amps: it may have been DIY-installed as a spur from a domestic ring main, originally to light a garden shed or run a fountain or something! If you're operating in any kind of business or commercial premises, they should have an up-to-date electrical safety certificate. A quick look at the distribution board or consumer unit should show the overall current rating of the circuit you'll be using, and you can also see if it uses old-style wired fuses or the more modern MCBs (Miniature Circuit Breakers), which react more quickly if the rated current is exceeded.
Fuses and MCBs do not protect you from electric shock, so always make sure that your system is fed via a residual current device (RCD). This could be at the main board/box, on the socket itself, or at the point where a separate spur is fed. If you're not sure that this is the case, use your own RCD, either as a plug type or one of the RCD plug-in adaptors readily available for a few quid from any electrical retailer. The RCD should be as far 'upstream' as possible so that it protects as much as possible, and wherever it is, make sure you test it before use, by using the built-in test button. If it doesn't seem to work, find another!
A final word on RCDs: they are there as a backup in case anything goes wrong, not as a substitute for poorly-maintained, faulty or unsuitable equipment.
Going Through A Phase
Most small venues are likely to have a single-phase supply, as will normal domestic premises, and for the purposes of this basic article we won't be looking at the whys and wherefores of 'three-phase' systems, other than to point out that all the sound equipment should be connected to the same phase. Any other electrical equipment, such as lighting, should also share the sound-system phase if it is possible for a person to come into physical contact with both systems — for example, to touch the lights and a guitar at the same time. It's a given that the venue's technical staff should supervise any connection to a three-phase supply.Distribution Deal
A damaged mains plug recently discovered at the bottom of the cable trunk — to be disposed of straight away!Photo: Mike CroftsHaving found a suitable supply point, you now have to feed it to all your equipment. For all gigs where a 'proper' supply is available, I use a professionally made portable distribution box, which has a single 32-Amp inlet and 32-Amp breaker, feeding four 16-Amp outlets, all via separate combined RCD/MCBs. Although, on the face of it, I've got four 16-Amp outlets, giving a total of 64 amps, I can only use 32 Amps overall, with each outlet limited to 16 Amps. I run my front-of-house speakers from two of these feeds, the monitors and desk from the third, and the stage backline from the fourth. This splits up the load and ensures that each feed is fully RCD protected. As mentioned earlier, it is always best to have an RCD as far upstream as possible, and I would ensure that my original 32-Amp source incorporated suitable protection if available.
For smaller indoor events, a single 13-Amp fused RCD plug feeding into a multi-way distribution board (four or six sockets) is fine, as the total current can't exceed the 13-Amp fuse rating in your RCD plug. From this distribution board you should try, where possible, to connect direct to equipment, or feed the equipment in logical groups. Normally, you can take the initial feed from a socket at the back of the stage and run all your backline straight from this, with one feed going off to the PA system. If you need to use more than one socket in a small venue, ensure that all your signal connections are balanced, and never, ever remove an earth connection to get rid of hum or noise. Also take care when using those 'flying saucer' extension reels. They are very useful and neat, but remember that their maximum current-carrying rating only applies when the cable is fully unwound.
Care & Maintenance
All leads, connectors and equipment should always be checked before use, even if this is a quick visual check for any obvious signs of damage. If it's your own gear, you'll know it's all correctly fused, but it's best to check if you're not sure. Cables should be undamaged along their entire length and plugs should be securely clamped on, with no inner conductors visible. Cables with moulded plugs are a common sight nowadays, but these plugs cannot ever be re-used, and if damaged or removed for any reason they must be thrown away — preferably after destroying them so that an unaware person can't find one and plug it in. If anything looks faulty, then it probably is. Remove it from service and make sure it can't be used again until it has been repaired and tested.All electrical equipment, including cables and connectors should be stored and used in dry conditions unless it is designed for outdoor wet weather use and carries an appropriate IP rating (for mains connectors this will usually mean industrial 'Ceeform' types — coloured blue — either rated IP44 (which is splashproof) or IP67 (which is waterproof).
Never be tempted to 'lift' the earth wire for any reason. If you find a plug like this one, with the earth disconnected, don't use it.Photo: Mike Crofts
Summing Up
We've covered the basics of finding a suitable supply and connecting the gear to it, but there are other things to consider when rigging. Cable runs need to be thought out to avoid or minimise trip hazards, and a generally neat cabling job will be much easier to troubleshoot than a spaghetti surprise. Don't forget the rule 'signal before mains'. Connect the power leads last and switch on after everything has been connected in the signal path (with the master levels down, of course). Turn on your power amps last of all, and switch them off first when powering down the system.In this article I've taken a very basic and superficial look at the power side of live sound. There's a lot of additional good advice to be found, and it's well worth taking a professional approach and discovering as much as you can. After all, if you were going to jump out of an aeroplane you would, presumably, want to know that your parachute was (a) of the correct type; (b) correctly installed on your person; and (c) recently tested! Electrical power is a serious business, so if in doubt, ask a qualified electrician. If you don't know one personally, someone you know will, or you can look one up in the phone book.
There are also plenty of useful pages on the Internet. The UK's Health and Safety Executive web site, for example, has a lot of relevant information and links to some very good guidance publications. Check out www.hse.gov.uk.
Safety Checks
Your visual examination, before connecting any equipment, every time you're about to use it, should include checks for:
All portable electrical equipment should be periodically tested for electrical safety, and 'PAT' (Portable Appliance Testing) records kept. Some venues will not allow you to use anything which hasn't been properly tested. Get a quote from your local electrician for testing; it's not expensive and is well worth it for the peace of mind.Photo: Mike Crofts
- Damage to cables or plugs, including cuts, cracks, abrasions, bent or missing pins.
- Previous repairs or modifications, including exposed or taped-up cable joins and unsuitable connectors.
- Exposed inner conductors where the cable enters the mains plug.
- Signs of damage to casing and covers.
- Obvious signs of previous problems; for example, signs of water, moisture or heat damage.
- All wires are firmly attached (screws nice and tight) to the correct terminals, with no bare wires showing.
- The cable outer sheath is firmly gripped by the cord grip.
- There is no debris or signs of damage internally.
- Additional testing of earth integrity and insulation.
- Test results recorded and appropriate labels attached to the equipment.
- Failed equipment identified for disposal or repair.
No comments:
Post a Comment