Installing Earth "Spikes" for TV Systems

[JamieK]

Hi Guys

I have just passed my 2391 after a 4 week course and my background is in the installation of Digital TV Systems, predominantly Integrated Reception Systems which deliver satellite and terrestrial signals to blocks of flats, etc.

The reason I am posting on the electrical forum is I am in the middle of a huge debate over the use of earth electrodes (or "spikes" as my colleagues call them!).

The TV industry is regulated by the Confederation of Aerial Industries (CAI) who state you must install a supplementary bond back to the MET of the building to make the TV System electrically safe.

This is all well and good in blocks of flats which have a communal landlord supply, but many of the blocks I am upgrading are nothing more than rows of bungalows which do not have an MET as such, rather there is an MET in the CU of each bungalow/flat.

Its usually impossible to access any of these properties at the time of install and the customers are reluctant to have you route a 4mm2 earth cable into their CU anyway, so the "solution" according to many is install an earth spike.

The earth electrodes are being installed by non electrically qualified individuals, not tested and are literally drilled into the concrete or jabbed into the grass and then a lug is used to connect the 4mm2 earth cable to the electrode, forming the supplementary bond, no mechanical protection or anything. Not to mention running the risk of hitting services such as gas and water pipes, etc.

I understand the resistance of an earth electrode must be under 200 ohms, this isn't checked most of the time and when it is I usually find that several earth electrodes have to be daisy chained together in order to reduce resistance under 200 ohms.

The CAI state that the resistance of the outer conductor of the coaxial cable to the MET must not exceed 4 ohms.

Is it just me or does anyone else see an issue here?

I've always had trouble getting my head around the need to earth bond TV systems, I understand that by running coaxial cables to each property in a block of flats you essentially create an electrical circuit that needs to be made safe buy typically the max voltage you are working with is 18V.

 

[mayfair]

The last paragraph dont add up m8, what if 240v gets on the coax braid and goes to all the flats, It only needs a leaky power supply in a Sky box and you have a direct conection between skybox to tv (RF or Scart) then from chasis to RF input and your on your way.

 

[JamieK]

Hear you Mayfair, but how would 240V get onto the braid?

The likely scenario for that must be if the cable gets damaged and the braid touches something "live", but wouldn't that just short out?

If the appliance such as the SKY box or Freeview Box becomes faulty wouldn't the CPC take care of any danger in fact I think the appliances are mostly Class 2.

If 240V did get into the system this would then be sending really dangerous voltages through the coaxial cables to all other properties and I'd assumes damage the appliance in other properties?

I've just never come across it in years and years and nobody I've every spoken to has either, I rally appreciate your feedback as its one that drives me nuts even though I know I should understand it!

Even the experts cant give you a reasonable explanation.

The earthing solution is to install a bus bar across the inputs (from the TV Antenna and Satellite Dish) and a bus bar across all the outputs (coax to TV socket in each property) of the main head end amplifier and then link that back to the MET, this would get rid of dangerous voltages and ensure that any installer working on the system would avoid a shock?

Would the earth electrode work even under this fault condition if it happened?

 

[JamieK]

MJ, it hasn't been solved -virtually all businesses I am involved with use this method

 

[malcolmsanford]

MJsusan is a spammer jamie just forget that.

The reason communal system "need" an earth bond is the protective conductor currents all these digital equipment leak. It is felt that in a communal situation a large amount of these digital appliances could leak the milliamps into the outer sheaf of the coax cable and then upto the dish/aerial on the roof. As you know just 30mA is deemd high enough to kill someone and 10mA can difib the heart, so for safety it is felt the need to bond.

I'm not 100% convinced about this, I'm still of the mind is are you making that pole more dangerous by bonding it, as there is a chance that you could make it "LIVE" during a fault within the equipotential zone. Time will tell on that one.

As for the rods, well I think you answered your own question, you get what you pay for and really the way to install a rod, and get the desired value is by obtaining you Ra with a proper earth electrode tester, which is a complicate test, that mainly only qualified electricians can perform, and to this would cost money.

 

[JamieK]

Thanks for the heads up on MJ Malc, thought it was a weird post but Im new to the forum so I went with it!


Thanks for the brief on the leakage currents, well explained this forum has alot of knoweldge in the couple of posts of Ive made thanks alot for the response.