Strand WallRack trips

[Munro]

Hi folks,

 

At risk of posting before I fully investigate this issue, I was looking for a few points.

 

I've two 24 channel Strand Wall Racks hard patched to my current rig. Dimmer 1 carries channels 1 through 24, and dimmer 2 channels 25 through 48. Quite frequently when I boot our console up and select the whole rig and set it to 80%, dimmer 1 will trip the main distribution board, despite dimmer 2 having a much greater overall load. I initially put it down to there being a couple of dead lamps in the rig possibly knocking it out, but as these have now been replaced, I'm starting to wonder if there is a more severe issue. Normally after it trips, a quick reset does the trick and then everything works perfectly for the rest of the day.

 

On Friday, I went to reset the distribution box and found that in the time it took me to get back to the control box, the dimmer had tripped again. Also, the smell of burning plastic was evident in the room where the dimmers are. Before I empty out this props cupboard to get to them (don't ask - the maintenance guys see it as a cupboard) I was wondering if anybody had any idea what may be wrong... I'm thinking maybe there is an MCB gone bad in the dimmer, but that's pure guesswork! (Can they go bad?)

 

Cheers

JM

[J Pearce]

What is the need to take the entire rig to 80% anyway? You can rig check quite easily at 30% and even then I tend to phase my rig checks either on subs or automated cuelists.

 

80% control is still very nearly 100% current due to the dimming law.

 

If you are going straight to 80% from cold that could be one enormous inrush current particularly if you have PARs or cyc floods, which seem to have greater inrush than T class lamps.

 

MCBs can go bad, but your description of it working fine the rest of the day would lead me to think the issue is your practices rather than an electrical fault. Warm your rig first if you must snap the whole thing to nearly full current for some reason.

 

Finally, burning plastic smell in a room full of props already sounds worrying.

[Munro]

Yeah, I am aware of the huge inrush of current, which is what I put it down to initially.

 

The latest issue though appears to be more severe. The circuit breaker trips, I reset the circuit, and even with the rig completely off, it trips again.

[Tomo]

The vast majority of theatrical dimming installations CANNOT run all circuits at a high level simultaneously, and doing so WILL trip a supply breaker.

- In some cases doing this would trip the really big breaker (or blow the fuse) supplying the entire building, and perhaps require a callout from the local electricity board to reset/replace it.

 

This is by design, because it is extremely rare that any show would actually need to do this - a show nearly always has specials, different washes (warm/cold/etc), and never uses all of them at a high level simultaneously, instead some are faded up while others fade down.

 

The technical term for this is "Diversity".

- For example, count the number of 13A sockets in your house and the number of 32A ring-main breakers. You'll find that you can't plug a 13A load into every socket without tripping your ring main breakers.

 

So you should simply never do "All@80%", because the theatre isn't designed to.

 

- As Jon said, there is simply no need. To rig check you're better off with All@20%, or a slow-fading, low intensity chase across the luminaires - perhaps running along each lighting bar so you can watch it sweep along and any "jumps" mean there's a dead lamp.

 

This is less blinding (easier to see which lamp's failed), uses less power (so you don't trip breakers) and you're less likely to blow lamps - or damage your MCBs/wiring.

 

For example, one theatre I used to do shows at only had a 100A single-phase supply (same as my house) - with an ammeter display in the control room so I could see if I was getting close to it and ride the fade to stay below the yellow line - and well away from the red one!

 

Ask your tutor how much current you've actually got, and check the MCB ratings yourself - but remember that the breakers feeding the dimmers don't tell the whole story!

 

[Edit] - The burning plasticy smell may well be that the MCB/RCBO (or even supply wiring) has been damaged through repeated overload, and now it won't stay in.

MCBs are only designed for a relatively small number of overloads, and will eventually die if repeatedly overloaded.

Thermal-magnetic types (most common) also trip at a much lower current when warm (eg from a previous overload), so letting it cool might solve it.

 

Either way, I'd really recommend talking to your tutor about this - they'll know what current you can actually draw, and if it is damaged they'll much prefer it if you tell them, rather than they find out later - perhaps without the time to get it fixed!

[Munro]

Thankyou for your informative post Tomo particularly with regards to breakers.

 

I'll have a chat to our on-site electrian when he returns from his holiday and hopefully thanks to you, we'll be able to have a knowledgable conversation!

 

Considering the main circuit breaker trips moments after being turned on, would you be expecting that there is an electrical fault within the dimmer itself?

[Gareth A]

Also get the electrician to take the cover off the dimmer and check that the electrical connections are tight on the incoming cables. If they are loose, they could be arcing and burning which could cause all the above issues.

[bigclive]

Also get the electrician to take the cover off the dimmer and check that the electrical connections are tight on the incoming cables. If they are loose, they could be arcing and burning which could cause all the above issues.

 

I'll second that. It sounds like the supply needs checked out, particularly given the burning smell. Until it's been checked out I'd suggest not trying to reset the main breaker feeding it. High current shorts can degrade breakers with repeated resets until they themselves fail with a bang. (Not something you want to have your hand near when it happens.)

[Dmills]

And get that cupboard cleared out and clearly labelled as "SWITCH ROOM, NOT A STORE", keeping potentially flammable objects in a dimmer room is stupid.

 

My guess is a loose connection producing a lot of heat, leave the dimmers powered down until you can get someone to look into it, you don't want a fire on your hands, and neither does the building owner.

 

Regards, Dan.

[dmxlights]

Also get the electrician to take the cover off the dimmer and check that the electrical connections are tight on the incoming cables. If they are loose, they could be arcing and burning which could cause all the above issues.

 

I'll second that. It sounds like the supply needs checked out, particularly given the burning smell. Until it's been checked out I'd suggest not trying to reset the main breaker feeding it. High current shorts can degrade breakers with repeated resets until they themselves fail with a bang. (Not something you want to have your hand near when it happens.)

 

 

I have been there when a breaker failed or I should say one had given up as I reset it as some clown turned on all dimmer packs as I reset it. It was a 128A trip and as I reset it give up in my hand and left with the switch in my hang which also had a burn across my fingers that were on the switch part of the breaker and a spring that had come out of the breaker stuck in my hair as it melted my hair as it touch it.

 

I would not like to see another 3 phase breaker give up one is plenty and the bang out of it was that loud I though I had gone completly deaf after it had happen

[Brian]

...the main circuit breaker trips moments after being turned on...

 

What sort of 'circuit breaker' is it?

 

MCB - Miniature Circuit Breaker - Trips on current overload only.

MCCB - Moulded Case Circuit Breaker - Trips on current overload only.

RCD - Residual Current Device - Trips on current imbalance between L and N (earth fault), also called earth trip or other incorrect names

RCBO - Residual Current Breaker Overload - Trips on earth fault or overcurrent

 

If you aren't sure, post a photo or details of part numbers visible.

[bigclive]

Oh there's a point. The main breaker that's tripping may be an RCD.

[adamharman]

Being a college venue, it could well be an RCD.

 

99% sure the wallrack is fitted with double pole MCBs, so you can completely isolate each channel and it's outgoing wiring individually. That's very useful when you're trying to trace a fault.

 

Turn off all the MCBs on the wallrack and see if the main breaker still trips. If it doesn't, turn them on one at a time and see which channel is tripping it.

 

If it still trips with all the channels isolated, the fault has to be between the main breaker and the channel MCBs.