Strange earthing connection on an outdoor exihbition unit.

Camlok fell out of use in the industry a while ago, powerlock is the common high current connector these days.

Either way either would be overkill for this situation. One hopes the event electrician will be on top of this kind of thing, I certainly would be on one of my sites.

Speaking of event trailers and effectiveness.....




Well the thought was there???...

Apart from the fact the plate should be under the wheel to maintain pressure in it what's wrong with it? Standard means of earthing vehicle mounted trailers in teh outside broadcast industry and i'm pretty sure you'll find it in BS7909. Don't forget they aren't creating an earth connection like in a TT system, just making a reference connection to the local physical ground.

davethsparky said:

Apart from the fact the plate should be under the wheel to maintain pressure in it what's wrong with it? Standard means of earthing vehicle mounted trailers in teh outside broadcast industry and i'm pretty sure you'll find it in BS7909. Don't forget they aren't creating an earth connection like in a TT system, just making a reference connection to the local physical ground.

Click to expand...

I dont know, I dont work on them so have no idea just looked a little funny. I was saving it for when a TT thread came up and to say "TT have I done it right" but I got excited and posted it early ha ha.

Only for making a reference to earth is it?, do most of them have this type of thing then?. Every days a school day.

I try to steer clear of the myself, but sometimes you have to associate with them.

When you set up temporary distribution on generator supplies you are creating a TNS network and the connection to physical ground is effectively a bond to the ground. Despite there being an earth rod it is not a TT system, the earthing/CPC is a seperate conductor in all of the cables with a N - E link at the generator.

Carefully selected RCD's (usually variables) are used to provide protection in a similar manner to how they are used in a TT system.

I often run into the plate-under-the-wheel thing and it is sometimes a big deal. Tar is a jolly good insulator and on a hot, dry day I clocked a plate under an artic wheel on tarmac at some megohms (can't remember how many). I take your point about merely establishing a reference for a TN-S system, which in an ideal world would never carry current, so the resistance might not be very important. Trouble sets in when (for example) class-1 and class-2 equipment come out to play together on the same genny, and the class-2 equipment has external connections that make better contact to the ground than the earth plate. Suddenly the reference is half-way up the phase voltage by way of the delta caps in the PSUs and people start getting tingles between exposed conductive parts of the whole system and earth. That's approximately when I get the phone call and I have to go out and sort out the argument between the genny op, the sparks and the tech about whose 'fault' it is that people are getting shocked.

One of my friends came up with a nifty little device we call the Ward Patent Earth Fork. It's a regular table fork with a piece of 2.5 G/Y connected with a crimp eyelet and screw/nut through a hole in the handle, terminating in a croc clip. In sight of the genny op, he puts one probe of his meter in the ground, the other on the MET or a distro stud. He then pokes the fork in (and it fits between paving slabs) and clips that on too. Invariably, the nasty stray voltage collapses. When he points out that his little fork is a better earth reference than the big chunk of steel connected to the MET by 70mm², they tend to stop slagging off his equipment as 'dangerous'.

Ive never personally used the plate method, I always find away to get a rod in. I know it is used widely and was under the impression it only normally got used in OB truck situations where the generator is truck mounted and and insulated from the ground and also the mobile studio or whatever is also in a truck isolated from the ground.

In your situation if there's space to get that fork in why on earth didn't they get a twig type earth rod in there?

Dunno, I guess it's mostly habit. I've seen a plate under the wheel on tarmac when there was a recently watered flower bed 6 feet away. The old line comes out about 'the BBC say it's OK so it must be' and no-one actually bothers to find the best available solution in the circumstances. But it gets used for setups where the kit is not all inside the equipotential zone of the trucks, e.g. cables snaking off to cameras here and there and some of the time they get away with it. Then somebody rigs a TV and satellite decoder in the back of a lorry and attaches the dish to a scaffold... Result, the combined leakage current of the two returns via the next person to grab the pan bar on the camera, and of course the camera gets the blame.

Oh yes, 'the BBC do it like this' or the 'I used to work for the BBC and you're doing it wrong' two of the most irritating phrases to be heard for the sound/tv/vidiot world