Clean earths


Postby Sparkydelux » Mon Jun 16, 2008 9:07 pm

Hi,

I am running a new supply for a server room. Also correcting a few issues that the A/C guys done ie: split unit off a 13A plug.

I spoke to the employees there and they say that their PCs experience interference at some points. I looked to see if there was a 'clean earth. But it appears that the earth used is coming off the Earth terminal in the DB. I looked in the other electrical risers and found a SUB-M.E.T on the floor above. I can take a earth run off here.

I also went to the main incoming riser and there is a 3 phase system coming in with an earth. Does this constitue as a Clean Earth? Can I use this if connected to the SUB-M.E.T?

As per usual,no-one knows what is what in the building services side of things. With not seeing the job I know it is hard to comment on,but I was keen to see what other electricians have done in relation to overcoming this type of problem.

Would an RCD 'clean' up interference?I can use a wylex RCBO if 'clean earth' cannot be clarified.

Any info always grateful,

Regards,

Sparkydelux
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Postby ericmark » Tue Jun 17, 2008 1:39 am

I have had problems with EMC in the past and it is impossible to predict what is going to happen. Main thing is use as much shielded cable as you can. And keep the 300mm separation between mains and comms cables.
RCD's and RCBO's will not help with EMC in fact they become a problem as the filters used to stop EMC tend to trip RCD's. To separate earths into clean and dirty can also cause problems as you can get a potential difference between to two and so introduce danger much depends on the building. Switch mode units are the biggest problem especially with motor control keeping the control as close to the motor as you can and using metal boxes helps.
One big problem is where wireless networks are used as all the efforts to stop interference can create a Faraday cage and reduce signals to an unusable level. Also insulating plaster board both for sound and heat often has a metallic film and the problems is not so much interference as weak signals again where Faraday cages are formed throughout the building.
Where this is a problem hard wiring all the networks is really only way out.
Different systems seem to be very different as to EMC problems. ASii was a real problem but Probus seemed much more immune. So any long runs were in Probus and ASii only used in local areas. Also banned all mobile phones the wireless phone did not seem to cause a problem but mobiles were found to upset things.
ericmark


Postby sparx » Tue Jun 17, 2008 8:19 pm

Hi.
got to be a bit careful here...
I am putting in supply for server installation in military installation & by their definition a clean earth will mean running a 10mm2 seperate from all other earths direct from main intake in building to an earth bar near server cabinets. In fact have to run two as 2 sets of cabinets running different security level data in same room. A new main supply goes to a main fuse board, then a sub main goes to a second fuse bd via a low-pass filter to prevent data back feeding down mains cable!!
type in 'tempest filter' to google and read BBC article, second on page, very interesting!
However normal computor type clean earth is in regs. basically using 2 earths to each socket returning to 2 seperate terms in consumer unit, comes under circuit with 'high earth leakage currents'
regards SPARX
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