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Safety and DMX512

There is a particular group of nightmare situations that regularly crop up in entertainment lighting (and sound reinforcement) systems that can lead to all sorts of safety and operational problems. These are all centred around the concept of (pick your word here) grounding and/or earthing.

These problems occur when you have two points that both have a 'ground' connection, yet those grounds are at different potentials. This potential difference:

The magnitude of these problems varies by the type of situation. An acceptable situation is to have a dimmer pack with a well grounded input socket, and a console with a well grounded output socket. Provided that the potential difference between these two grounds (and hence the devices) is zero, the no current will flow.

But why should there be a potential difference between two points both called 'ground'. Isn't a ground a ground?

The simple answer is no.

The worst possible case is when the camlocs on the dimmer rack get connected incorrectly, such as the ground post is connected to a live source, making the case of the dimmer rack (ie the dimmer rack's idea of "ground") live. Sure, this is a cockup, but can happen, and is a true nightmare (This is why many hire companies put the earth camloc on the other way round to the phases and neutral). In a fully rigged show, there should be enough bonded ground points to blow the breakers instantly.

Now imagine a rack that as of yet has no loads connected to it; just the incorect mains connection (making the case live). Now imaging plugging into that rack a DMX512 connector which has a 'true' earth connection at the other end of 70 meters of cable, connected to that plug in your hand. You insert that connector, what will happen?

Bad things, thats for sure. What you have here is a short circuit with hundreds of volts across it. The weakest point will become the fuse, and it will "blow". What could that fuse be? Possibilities include:

None of these possibilities are appealing. Therefore it follows that precautions have to be taken to prevent this sort of thing happening.

However, all these nightmares are worst case scenarios, that don't crop up every day. More common cases is where there is just a few volts between the mains sockets in the lighting booth, and the on-stage power. This few volts will cause a current to flow, which may be enought to warm the cable, or could just cause interference to the DMX512 signal, making your lights flicker or your mirrors waggle.

The same steps you take to ensure you (and your show's) safety will actually improve the performance of your DMX512 network, and this will often be noticeable to the show's audience.

Stuff you need to do for safety and show

  1. Sort your mains out.
  2. Reverse your earth camlocs to prevent really bad accidents.
  3. Don't be tempted to connect the shell of XLRs to the screen. Leave them floating. Stops "people in the way" accidents, and means that a pair of XLRs scratching on a scaffolding pipe isn't going to generate interference or sparks (or both).
  4. Use equipment that has opto-isolated (sometimes called " floating", or maybe "galvanically isolated") inputs.
  5. Use splitters with opto-isolated inputs and outputs.

And while you are at it, for show

  1. Be wary of daisy-chaining, use splitters (opto-isolated, of course) in preference
  2. Ensure your DMX lines are terminated!

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