Hi. I'm putting an extra TV aeirial outlet in a bedroom. The backbox is metal. The faceplate is plastic and the TV co-ax cable screws into terminals on the back of the faceplate.
Is there a requirement (regs), or is it good practice, to:
- earth the backbox? (if so, what's the best way?)
- earth the outer shield of the co-ax?
Thanks
In article , Steve writes
Not required or particularly desirable in a single domestic installation.
If the aerial system was feeding two or more properties (or flats) then it would be required but would be connected at a single point in the loft.
Backbox earthing is for electrical safety on mains power circuits.
Thanks Fred. That'll save me a bit of work too.
Steve
Also, if you every put in Sky and want to pipe it around the house and control it from other TV's, the Sky box outputs 9v on the outer cable/ sheath.
Alan
Thanks Alan. I'm not using Sky (though potentially someone else could in the future).
What a peculiar system though - surely the outer cable is connected, via the outer part of the plug, to the metalwork of any boxes it's plugged into. The best way would be to have the outer cable at ground and have the 9V on the inner cable wouldn't it? Do you know the reason for doing it?
Steve
There would still be 9V between the core and the outer. The 9V is not referenced to earth in the Sky box AFAIK.
_MASK_i?a63jfAD$z_ snipped-for-privacy@o40g2000prn.googlegroups.com...
Yes, it is. I was in the loft this weekend trying to sort out some of my RF wiring and I got a few small tingles when I was unplugging some of the connectors. If you are making changes then you are really meant to switch off the 9v signal (its in the Sky config/setup menu).
I dont know how or if it would affect the TV signal by applying 9v to the inner wire. I may be wrong but I beleive that the inner wire carries the signal. I am not sure what the outer wire does.
Alan
The inner wire carries signals and power to operate the LNB. The outer cable may be earthed either deliberately or extraniously.
The socket on the receiver is probably live at half the mains voltage but low current (so not very dangerous) but can give a tingle which could be dangerous aloft.
On Mon, 12 Jan 2009 08:25:46 -0800 (PST) someone who may be AlanC wrote this:-
Both do.
shows the simplest aerial, a half-wave dipole, scroll down a little from the top. The first two photos in show one, although it looks like one rod it is in fact two, separated by insulation (an air gap).
For an FM aerial the cable to the receiver can either be a co-axial cable or two conductors in a ribbon cable. With a co-axial cable the inner is connected to one element, the outer is connected to the other element.
The outer ends of a half-wave dipole can be bent through 180 degrees and joined together to produce a (folded) full-wave dipole. The first photograph at shows one of these too and it is described at the bottom of . Although this might look like a dead short, it is only a dead short at low frequencies. At high frequencies the thing has an impedance. The driven element in television aerials is generally one of these.
Add a reflector behind the dipole and one or more directors in the front and one has what looks like a (Yagi) television aerial. The first photograph at shows the type for FM and DAB radio (the latter is smaller).
(Yagi) television aerials are built the same way but are smaller in width, due to the higher frequency.
Power supplies for masthead equipment put a DC voltage on the cable. However, this is removed by the equipment and not passed through to the dipole. If it was not removed then the DC supply would be shorted out. All sorts of signals can be sent along co-axial cables, which are very "broadband", but may need to be confined to particular bits of the cable.
Half right. RF2 power is +9v on the inner, 0v sheath. The PSU is double insulated so the case, and consequently the cable sheath is normally not earthed, but could be so if connected to other earthed equipment.
Some early Sky boxes *were* earthed via their mains cables, the Amstrad DRX100 certainly, and I think those slim Sony ones too.
Earthed or not, it makes no difference to the 9v RF2 power.
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