How much electrical knowledge do you need?

[Jerome]

JDP- I can see why you thought I said I had made a 3 phase system but I didn't as I replied... my bad I'm sorry.

 

About working to a plan, I did on the gig I did but this was already made previous by another technician. This is why I was asking about how much I need to know because I can safely say I have learn't nothing so far about electrical phasing on my college course and am not likely to because college don't allow us to work with this due to insurance purposes, the closest we go to any electric units is plugging 15A cables into a set of racks or the grid with patch bays. Not exactly the workings of an electrician, we do fix lights and pat test them along with sound desks and all sorts of other things. On outside gigs however it comes without saying people need a little knowledge in what there doing with electrical systems and plans, but a little knowledge is dangerous is it not? Going back to 'the' word, I'm not competent enough to know if a system is balanced... but I want to be, hence the shout out for books or experience. Basically I know how something should look but not how it actually works. I couldn't look at a set up and do calculations to see if it was safe... but do I need to? I don't know.

 

I ask again for books or any websites even I can look at so I can do some good self-education.

 

Thanks, Jerome

[Dmills]

I think that for an entry level post, you probably don't need to be competent to design a sub distribution system, that is what the chief is for! Of course if you are competent then that is a nice bonus, but it is probably not required.

 

What is helpful, is to have seen enough to have a feel for when something looks wrong, for which reason I think your collage are wrong to limit you to plugging up smallish loads, as while it is probably unreasonable to include all that you need to know about BS7671/7909 in a stage lighting course at lower then university level, developing a bit of a feel for what is involved would be a real bonus. In the same way, having handled all the bits is good, as it makes things instantly recognisable which speeds things up on a job and is always welcome.

 

This sort of experience is surely why you are on the casual books of a whole pile of local venues?

Most of such work is poorly paid box shifting at unsociable hours, but it gets you hands on in a real work environment and the conversations in the local afterwards are a good opportunity to ask the how and why (and why not) questions that would be inappropriate while doing the build. Most venue crew will take the time to explain things to a new casual if you ask at the right time (20 minutes before the doors open for the preview is probably not it!).

 

Just don't take all the answers (from anybody) as gospel, there is a scary amount of misinformation (some of it strongly held by experienced professionals) out there.

 

I don't have much in the way of good book recommendations, but a poke round the stuff from 'focal press' and 'entertainment technology' may reveal interesting industry specific things. I seem to recall that the ABTT also run some electrical courses aimed at the stage.

 

Regards, Dan.

[Simon Lewis]

There is this book (from the ET series) by Marco Van Beek. It's quite readable, but perhaps doesn't provide all the background that is needed.

 

Although meant to cover d0m3zt1k and industrial electrical installations, there are numerous books on the "17th edition", notably those by Brian Scaddan.

 

Simon

[The Gaffa]

A couple of useful things to know are:

 

Ohms Law. V=IR, R= V / I, I = V / R , V=Volts, I= Current (Amps), R= Resistance (Ohms)

 

And Watts Law P=VI, V= P / I, I=P / V, P=Power (Watts), V=Volts, I= Current (Amps)

 

E.G. Watts law, 1000 / 230 = 4.35, 1000 / 110 = 9.1

 

HTH.

[adam2]

BTW some employers in theatre dont want technicians to learn too much about electricity, or so I am told !

Firstly they might be better informed than management and therefore query some of the dafter choices made by said management, and secondly they might decide to leave and become electricians! Although pay rates vary a lot, I suspect that in general most electricians earn more than most theatre technicians.

[smalljoshua]

On a side note even though the Voltage is meant to be 230v what do you use when working out I=P/V?

 

I use 220v so there is a safety factor. What do you use?

 

Josh

[Ike]

And Watts Law P=VI, V= P / I, I=P / V, P=Power (Watts), V=Volts, I= Current (Amps)

That isn't Watt's law it's simply the relationship between voltage, current and power. If Watt even had a law it's bound to have been something steam engine or pressure related so probably not much use to your average theatre technician. :)

 

</pedant>

[HobitLight]

what do you use when working out I=P/V?

I use 230v for two reasons:

 

1) 230v is the average of 220v and 240v

I use 220v so there is a safety factor

2) I use 230v as a safety factor over 240v! I suppose that 220v is even beter though so that you don't over load. :)

[Ike]

Selecting a voltage to give you a safety margin doesn't make much sense.

 

With a fixed power appliance (eg a laptop power supply) the higher the voltage the lower current. For a fixed resistance device (eg a par lamp) the higher the voltage the higher the current.

 

Par lamps also have the additional problem of coming in both 230V and 240V flavors. A 1kW 230V lamp will draw 4.35A on 230V and 4.54A at 240V while a 240V lamp will draw 3.99A at 230V and 4.17A 240V.

 

Personally I'll use 230V unless I know better with regards the installation in question.

[smalljoshua]

Fair enough I see your reasoning behind 230V.

 

I use 220V to add a safety margin into the result.

 

Josh

[bruce]

And Watts Law P=VI, V= P / I, I=P / V, P=Power (Watts), V=Volts, I= Current (Amps)

E.G. Watts law, 1000 / 230 = 4.35, 1000 / 110 = 9.1

 

I think you'll find it's actually known as Joule's law, despite Watt being the SI unit of power...

[Ike]

Fair enough I see your reasoning behind 230V.

 

I use 220V to add a safety margin into the result.

 

Josh

I don't think you do. A safety margin needs to be a known quantity, what you've got is an error.

[smalljoshua]

I do see what you are saying and I will be using 230v with a known safety factor from now on.

 

Thanks.

 

Josh

[themadhippy]

As a rough guide,when a questimate is close enough,4A a kw :)

[adam2]

On a side note even though the Voltage is meant to be 230v what do you use when working out I=P/V?

 

I use 220v so there is a safety factor. What do you use?

 

Josh

 

There is no simple answer to this question, the UK supply voltage is now officialy 230 volts, though the actual measured voltage is generally closer to 240 volts.

 

To be strictly accurate, it depends on at what voltage the load was intended to operate, i.e. the design voltage of the lamp or other load.

It also depends on whether the load will draw more, or less amps on a reduced voltage.

 

As an example consider a 1KW par lamp, if the lamp was designed for 240 volts, and is used on 240 volts it will consume 4.167 amps.

If the same 240 volt, 1KW lamp is connected to a 230 volt supply, then it will consume LESS than 4.167 amps and not more, as is widely believed.

Therefore if (as is likely) the load consists of incandescent lamps designed for 240 volts, then 240 volts may be used in calculations. Any reduction in the supply voltage to say 230 volts will result in LESS current being used and not more.

 

If however lamps actually designed for 230 volts are used, then the current will be slightly greater, as an example consider a 230 volt 1KW par lamp, this if connected to a 230 volt supply, will draw 4.348 amps. If the same 230 volt 1KW lamp is connected to 240 volts, it will consume slightly more than 4.348 amps.

In practice though lamps actually designed for 230 volts are almost unknown, they would have a very short life on most UK supplies which in practice are 240 volts.

 

 

(Certain loads including some types of motor, switch mode power supplies, and some electronic ballasts, will draw more current on a reduced voltage, such loads are not however common in theatre lighting, and if used at all, will typically be a small % of the total load)