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switching on power


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Hi everyone,

as always pardon my english, I hope I'll be clear.


What I'm going to discuss will apper very very stupid.

But it is a fair question to me.


I know as a rule to switch on power switch by switch: first the main, then the secondary ones, then the signle ones, when you have to give power to an entire show. Dimmers and movind heads.

The same, in the opposite direction, when you switch off. First the single ones, then the principals.


Unfortunally I have never studied in a technical school, nor I have been instructed by anyone, so I still have questions.


Why do we have to switch things this way?

Why switching on or off the pricipal switch with all the others on is a problem?

What's the explanation for dummies?

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Some items (certain power amps and moving head lights in particular) draw much more power during the initial turn-on than when they are in normal use.


If you were to leave all the secondary switches on, and just threw the main breaker to turn everything on at once, you could get a large inrush of current which might be enough to overload your supply.


A friend of mine was at the secondary stage of a festival in the US a few years ago, when someone working on the mainstage turned on the entire PA rig (many Crown MacroTech amps) all at once. The draw actually caused the transformer feeding the entire site to fail, but not before there had been some fairly drastic voltage fluctuations, enough to destroy the lighting board and take out most of the drivers on the PA.

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To take another (less dramatic) example, I've seen an installation where the stage lighting was routinely turned off and on using the isolator, whether there was power being drawn or not. Eventually, the isolator got fed up with being asked to routinely interrupt such heavy loads and decided it didn't want to turn one of the phases back on.


Nowadays, the venue are careful to power off the dimmers downstream of the isolator before using the isolator.

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Agree with pritch - it's worth remembering that *isolators* are exactly that, they are not designed to be operated under load - Ideally everything connected to it should be switched off first and the isolator used solely to safely isolate power afterwards.


Inrush current is another good point, particularly if you have a lot of PA or discharge lamps (e.g. movers)

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