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Running shows on “generators”.
Contains:| The Machine, Earthing, Distribution, Fuel consumption, Fire, ELB's, Mixed systems, Operational problems |
From time to time someone decides to give a show somewhere out in the Mulga where there is no electric power, or an insufficient supply.
And not just in the Mulga. Regional showgrounds and race courses where shows are sometimes held are frequently very poorly supplied with electric power.
I recall wandering around the Melbourne Cricket Ground during a Military Tattoo looking at a large number of big truck-mounted diesel units around 200kVA each, ranging from sublime fully shrouded film set units to clattering exposed turbo-charged bulldozer engines, all rung in to provide an extra megawatt or two not available inside even this major venue.
Similarly behind sideshow alley at any regional Show there will be several big machines powering the whole kaboodle. These range from the vintage to brand-new.
In my youth my primary interest in the visting fair was the preposterous industrial revolution generator that powered the Dodgems with its exposed sparking commutator and huge front mounting meters announcing something like 400 amps peaks at 40-odd volts. The Dodgem cars themselves seemed to use starter motors.
So if you put up your average rock show for about a thousand audience with an average lighting rig and PA you have quickly spent the best part of 20 to 30 kilowatts which is fairly serious power territory.
Even a small stage with a few lighting Par cans and common band gear can quickly outrun the capacity of smaller machines if you are not careful.
A key point is safety. With DIY power you are your own Supply Authority, and as such there are a few small but important details you have to take care of to ensure a fun time for all.
Another key point is voltage stability. People with amplifers tend to be a bit suspicious of plugging into DIY power, often with good reason.
Everybody calls them “generators” which drives me nuts because generators are DC machines and these are AC machines or alternators. They may have a DC output too but it is the 240VAC we are interested in. So “machine” here means the engine and generator, er, alternator, and its switchboard. It's just that alternators are so much more elegant machines than generators, but Edison had better PR than Tesla.
Machines come from a few hundred watt two-stroke up to fully shrouded 200kVA truck mounted units for film sets. In between are a range of machines of various ratings and quality. Generally the small ones run on petrol and the biggies on diesel, and that also tends to mark the difference between the toys and the professional.
“kVA” is a pedantic way of saying “kilowatts” in AC engineering. Since this is part of fakin' it you will now know what the lighting guy means when he says you can't have [blah] because he's only got a “20 kayveeay feed”. It doesn't mean he missed dinner.
This is often overlooked, particularly when smaller petrol generators are in use for a small stage - but an genuine earth point is vital.
In all cases it is essential to establish a solid physical ground point at least where the power arrives at the stage. If the stage is metallic such as scaffolding then it should also be electrically bonded to the earth spike, but independently with its own lead and clamp, not back via the stage switchboard.
This is particularly important if the same metal structure is being used to carry the lights. Apart from shock safety, a dimming rig may propagate buzz through an ungrounded structure. Don't assume that just because metal touches the ground somewhere it is effectively grounded; riggers often use wooden pillows or footings under each leg which can pretty effectively insulate it from ground.
If the power cable run to the machine is more than about 30m, has any joins, or if three-phase is in use, then a second ground must be established at the machine as well. Connecting to the machine ground may require some investigation and improvisation. It should require tools to connect and disconnect, and should be green with a yellow stripe so everyone knows what it is.
Even with all this done I have still had problems with people getting leakage tings off equipment at the ends of long extension runs and now keep a couple of home-made plugboards that have a hank of earth wire hanging out the back for connection to a local earth spike, which is very effective.
Electricity Supply works depots sell copper-plated steel earth spikes for about $A20 each. These or similar should be driven at least a foot (300mm) into the ground and the earth cable firmly attached using two screw-up hose clamps or the like. Stage or other structure grounds, such as nearby light or speaker towers, should each have their own earth cable and clamps. Earth cables should be no lighter guage than the load cables.
Starting a small bushfire with a string of brummy old extension leads seems to almost be an Australian national tradition in my experience.
When you consider domestic fittings run out at around the 2-2.5kVA power level, getting the juice from even a 6.5kVA machine down in the gully up to the stage can be a problem. A suitable cable will set you back a few bob and need two guys to lift.
In this case we used a pair of 100m 15 amp cables in parallel. If plugged-in rather than wired-in this shares the load current between the connectors, BUT the plug of one lead will be live if unplugged while the other is still connected. Not good. I therefore prefer to make solid screw-up connections, rather than simply plugging in at the machine, if at all possible.
In higher power machines the answer is to hire a length of three-phase cable and suitable break-out switchboard for the duration, and may even be available from the same hirer as a “3-phase pendant”.
It is not uncommon for big machines to have a three-phase socket or two, and some normal power points. While these power points are useful, they are not the main power output of the machine. For that you need a three-phase plug.
Apart from the astounding expense of these connectors, if you are hiring a machine at this level it is important to know that there are two kinds of three-phase plug, one with a neutral pin, a five-pin socket which you need, and a more common four-pin without neutral that won't accept a five-pin plug. And the neutral is a “must have”.
The first time I was caught by this I ended up taking the plug off and wiring the cable directly into the fuses on the back of the machine switchboard. I have since seen a couple of others, plug in hand, turning the air blue at this last-minute discovery.
Once you get the power to the stage, even at 6kVA level, you have an interesting problem making the initial transition to normal fittings. Your first stage switchboard/powerbox needs to be a brute. At 2kVA max per socket 6kVA requires at least three double outlets, better more.
At 50kVA you are looking at a big board with three rows of outlets, one row for each phase, and these are primary circuit outlets to be fully loaded and not wasted. The next level down can be a clutch of conventional domestic plugboards on each circuit.
The stage switchboard often will have some protection, normally in the form of a number of circuit-breakers; at least one for each phase, sometimes more.
One of the problems with running a stage on DIY power is stability, and this is not helped at all by the nature of the stage as a load.
Both the amplifiers and light rig tend to draw highly variable loads. The lights generally consume the most power so going from blackout to full up and back again can ask a lot of the machine.
One trick is to avoid very light loading because this is when the machine is apt to over-speed.
To avoid this we provide some base load which draws a constant minimum. A satisfactory base load is no less than 10% of the machine rating, better 15-20%. Generally machines will be most stable running at between 25% and 75% of rated capacity. Area lighting makes a good base load and can be strings of festoon and high-power QI's pointing upward in the trees (three equal sets with three phase). It's an opportunity for someone to get creative painting with light.
The base load must be pretty directly connected to the machine, not least because well-meaning people are apt to turn off the lights to “save electricity” during the day. These circuits will have only minimal protection so should be kept up away from casual contact.
Beit splitting up single phase or fanning out three phase, you need to take care of balance - the loads have to be balanced and distributed as evenly across the circuits/phases as possible. After getting the base loads right the next most important thing is the understanding and co-operation of the lighting operator to avoid popping circuit-breakers mid-show.
Load balance between phases isn't a critical matter and loading can be highly variable. It's more a thermal issue of trying to distribute the work evenly across each of the three phases on average.
The voltage control circuit in most machines will produce a noticeable buzz in most sound gear, similar to light dimmer buzz since it works the same way. This should not be confused with lighting rig buzz and can be demonstrated using a blow-heater.
This is where a line filter or two comes in really handy. All the backline and PA gear can be supplied via one each and this will at least kill power-borne noise, if not direct pickup.
Lights, heaters and other high loads should NOT be run through a line filter as they don't need it and may burn it out. Light dimming rigs generally have their own power splitting and filtering acts already together internally.
Most machines, small or large, consume little fuel on no-load so consumption tends to be a very direct function of electric power use.
To estimate fuel consumption, how many barrels of diesel to buy before the holidays start, you first need to estimate the power consumption in watt-hours, how many watts for how many hours.
After you get all your guesses for the stage, kitchen, market, base load and whatnot in kVA, multiply by the number of hours, which gives total watt-hours.
Now take my guess that fuel contains roughly 10kW/Hrs per litre, and that the machine is about 28% efficient (less if small, and depends a lot on the condition of the engine) gives about 2.5kVA per litre per hour. So at 25kVA that's 10 litres/hr. That's why they have big tanks. :)
The same machine running under hot, high and dry conditions will consume more fuel than cool, low, and humid.
Fires are very common around small machines. The proximity of petrol to the exhaust is a real danger and spills more likely than with a big machine. But in either case the correct proceedure is to allow the machine at least five minutes cool-down after shut-down before attempting to refuel.
During actual refueling another person should be standing nearby with a suitable fire extinguisher, not close assisting.
NEVER refuel a running machine of any sort. NEVER.
The machine should be shut down at sundown, fully refueled and oil and water checked, for the evening run. Big machines will make it through the evening easily, but smaller machines will require refueling stops. Don't attempt to hurry the proceedure as audiences generally show great understanding when they know what is going on. If you have to shut down mid show, try to do it in an orderly manner and not just let the fuel run out mid-song.
- fuel-out is when engines surge and damage gear
- it risks sucking up crud from the bottom of the tank
- diesels should never be run dry or the injectors may requre bleeding to restart
- earmuffs and suitable fire extinguisher are a must for the genny crew
Once restarted any adjustments should be made on base load only. Don't fiddle with a loaded machine. Against all advice one site electrician decided to make a tiny unneeded adjustment to the frequency, or engine speed. The result was wild surging that caused the PA guy to go ballistic. So if it ain't broke, don't mess with it.
You want the machine as close to the stage load as possible, but you don't want its noise and smell.
- site it behind something solid, tree, house
- down in a gully or depression
- point the exhaust away (small), the radiator/cooling away (big)
- make a barrier, van, truck, curved haybail wall (but not too close!)
- site it somewhere generally behind the stage to the audience
I have seen one bigish machine do a bad dummy-spit and start a grassfire at a festival.
In poor condition to start with, after days of excessive and unbalanced loads and 40+°C heat it had had enough. Due to the worn rings (I am told) and the very hot conditions, it started running on its own sump oil, ran away to some frighteningly large number of revs, promptly blowing its whole switchboard off the end in a shower of sparks that started a grass fire in the middle of the crowded camp.
An engineer who understood what was happening, and fearing the machine would disintegrate at any moment, ripped the air cleaner off and looking about for something to choke it grabbed a number of womens panties drying on a nearby tent guy, wadded them into a ball and stuffed it into the air intake.
In a fraction of a second they were sucked in, digested and spat out the exhaust as a puff of soot, without missing a beat.
Somebody then arrived with a childs hard ball and that finally choked it into silence. But a very dead parrot. Then we put out the grass fire.
So a firebreak around the machine and fuel storage is also a good idea.
Earth Leakage Breakers or Residual Current Devices are generally a good idea and in fact compulsory in new installations. But around informal electrics they can be a real pain, prone to dropping part of the show off without reason.
I have found well-bonded grounds much more effective at stopping on-stage shocks from the mikes and so on, so I tend to see ELB's on stage as more of a liability than asset.
I have had sites where mains as well as a couple of 50kVA machine were running. Whenever there is more than one source of power you have a mixed system and this has particular risks.
To avoid these risks power from mixed systems must not be available in the same area. There must be no chance, for example, that the PA can be plugged into one machine and the backline into another.
I've sited machines together for fueling and noise reasons, even with common grounding, but the power circuits run off in opposite directions to the stage and market so there is no chance of casual interconnection.
Because of their organised nature stage electrics once going don't present many operational problems. The health of the system can be judged by going around feeling plugs to see how hot they are and very hot connectors need to be attended to before they produce smoke.
Market areas and such on the other hand can be a barrel of monkeys for a site electrician. Problems in this area fall into three main groups:
- Overloading - buying one unit of power (10 amp breaker and double outlet) then hooking up two urns, sandwich maker, jug, toster and fridge, not to mention several lights and a stereo, then complaining their breaker keeps popping.
- Faulty leads - opens, shorts, crossed wires, twinflex extensions with no earth.
- Faulty equipment - sundry, including Chi Mick's shorting urn that blew electrics at several festivals.
One complaint of “power cutting out” turned out to be internal thermal protectors in the domestic juicers being used, only rated for 30% duty-cycle so another get-rich-quick scheme bites the dust on a technicality.
Towards the end of one long festival I noticed an extension lead snaking off up into the hillside camping area and following it found a number of tents had connected themselves unnoticed to the system. Mostly small rechargers, but out of 5000 people somebody always brings a blow-heater. ;)
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<<<OzValveAmps
http://www.ozvalveamps.org/diypower.html | Last update: 01:57 14/01/06 |