Tuesday, January 12, 2010

The importance of a consistent earth with technical mains

This is a paragraph from my standard Scope of Works document that goes to customers of build where I'm not responsible for the technical power;

Electrical requirements for all rooms part of the installation
We recommend an MCB-protected 16A mains feed terminated in a Commando connector. It is vital that the customer’s electrician runs the earths for the rooms back to the same earth bus-bar as the mains feeds to the bays thus creating a technical supply for all production/editing equipment. Failure to observe this request will cause mains hum on all video signal distribution around the new facility.

So we're quite explicit about the need for the need for a proper technical earth and what will probably happen if it's neglected. One of our recent builds has been having niggling problems with corrupt video captures and despite me having twice tested the physical layer performance of all the cabling (using the eye pattern on a Tek WFM7120) the attitude from the customer has been "…it must be the cabling or the routers you provided". After lots of haggling I was there recently and I measured a full 400mV of hum between the 'technical'(!) earth in the suites and the power distribution in the machine room. Given that an HD/SDi signal is only a volt I'm amazed they weren't seeing more corruption.
Now it's the inevitable "..why didn't you test for this and spot it earlier"? Perhaps there's a lesson here in not splitting the job up into many parts and using the cheapest contractor for each. If they'd been my electricians I'd have briefed them and made sure they ran proper tech earths - and I'd have made sure they tested them before handing over to the customer. That's why we'd have been a tad more expensive. As it is we'll no doubt wind up fixing this for free.

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