3 phase mains disconnector

[Mell]

Hi everyone just a general question I came across a 3 phase board at a university and it was running 12 stage lighting dimmer packs 3 phase all on 32a 3 pole mcbs and 2 circuits on 64a mcbs, my question is if the dimmer packs was to pull 30a each the mcbs would be fine but would the 100a main switch start glowing due to the excessive amperage pulled through it, I've noticed this on single phase boards and often thought the amount of amps that could potentially be pulled without exceeding their individual breakers is more than the isolation switch.

 

[Mell]

I think I've answered my own question by explaining it to my friend its got to depend on the distributors main fuse by the meter I would assume the main switch on a dB board has got to be equal too or greater than the fuse rating so to much demand just blows the fuse and not damage any of switchgear.

 

[Geoff1946]

Yes, you've worked it out correctly. The supply fuse protects wiring and switchgear downstream.

The disparity between supply capacity and the sum of possible loads is called "diversity" and amounts to an assumption that not everything will be switched on at once. Actually calculating it is a bit of a black art.

That's one hell of a stage lighting set up which is probably way over necessary capacity. On top of that most stage lighting has now been updated to led fittings which will reduce demand by at least 75% relative to old incandescent lamps. 

 

[Mell]

Hi Geoff thanks for your reply yes it is a big set up of incandescent lighting well over 100 fixtures and yes i learnt about diversity but always wondered what if though lol, thanks again 

 

[Andy™]

Yes, you've worked it out correctly. The supply fuse protects wiring and switchgear downstream.

The disparity between supply capacity and the sum of possible loads is called "diversity" and amounts to an assumption that not everything will be switched on at once. Actually calculating it is a bit of a black art.

That's one hell of a stage lighting set up which is probably way over necessary capacity. On top of that most stage lighting has now been updated to led fittings which will reduce demand by at least 75% relative to old incandescent lamps. 

one place i look after has 200a 3 phase going to their dimmer rack. rarely loaded to 200a but its capable of it. during a show its probably 100a

and a lot of stage lighting hasnt been changed to LED, you dont get the same effect from LED as incandescent, and some dimmer racks dont play nicely with LED

 

[Mell]

Hi Andy yes I find alot of places still have incandescent lighting and generally I'll wire in a separate hot mains to the bar to bypass the dimmers.