Communal TV Aerial - Earthing Coax Cables

[andrewc1]

Hello,

I live in a small block of flats that we manage ourselves. There's a shared TV aerial on the roof connected to a passive splitter whose cable connectors sit out exposed to the elements. At the moment, none of the cables, splitter nor aerial mast are bonded to earth, but I anticipate that sometime in the near future, we'll have scaffolding over the building, at which point I can call in an electrician to run an earth wire.

Recently the picture quality has started to degrade, so I ordered a new outdoor masthead splitter - a Vision V24-140NG. It comes inside an integrated plastic housing to protect the splitter and F connectors from the weather. On opening the box, I was a bit surprised to see that there doesn't seem to be any earth bonding terminal on the splitter.

Is this normal/to be expected? From the pictures on various websites, I can see that metal indoor splitters generally have an earth terminal, but for outdoor ones, it's hard to tell. Otherwise, what is the usual way of bonding these cables to earth?

 

[OwainDIYer]

equipotential bonding bars are used between the amplifier and the downleads

http://www.blake-uk.com/rf-signal-distribution/equipotential-bonding-bars/

(Morgan Scott)

 

[andrewc1]

I see how the bonding works in this particular case as shown in the picture, but I now have two questions about this particular splitter that I have:

- in my case, the splitter has five F connectors, but not arranged in a single row. Rather, they're arranged in one row of two and one row of three, something like this:

x x x
x x

I suppose it's not a standard layout, so does this mean that I'd need to run cables between the splitter and the bonding bar, and then connect the downleads at the other end of the bonding bar?

- also, in our case there's no amplifier and the splitter is located outdoors on the roof, with the downleads running directly down the front of the building and into each person's flat. Are there outdoor versions of equipotential bonding bars available? A quick search of the internet doesn't yield anything obvious.

 

[OwainDIYer]

put the splitter and bonding bar in an appropriate IP rated enclosure and gland the cables accordingly.

 

[Sam Gangee]

In case it helps, a passive splitter should be mounted as far down the "drop cable" as possible. This arrangement ensures that the strongest signal is travelling along the cable for as far as possible. The splitter reduces the signal by about 10dB per output so the output cables should be kept as short as possible in order to minimise the risk of interference pickup.

If the splitter can therefore be mounted in the roof space, or sheltered area, you no longer have the bother of weatherproofing.

If the earth bonding cable has to be run outdoors, make sure that it's protected against UV (sunlight), wind and rain. Wind causes vibration that can wear away the insulation. UV makes the insulation crack. Both effects allow water inside and the copper rots away, leaving no connection.

Since there's a tiny risk of a lightning strike on a TV aerial, the earthing cable should ideally be run down to ground level then back upwards to ensure that lightning will go into the earth and not into the building.

That applies to the coaxial cables, too, but it's rarely possible to achieve it.

 

[andrewc1]

Thanks for the responses, they now clear things up for me. It looks like I've probably wasted a bit of money and effort by specially seeking out an splitter with an integrated enclosure instead of a simple one, as I'll need to eventually buy a separate outdoor enclosure anyway. But oh well.

And thanks for the info about the earth cable. This is something I'll leave the electrician to do, when I eventually get around to calling him in, but it's good to have some context around what to expect from the job.

The nearest indoor or sheltered common area to keep any equipment in is the basement of the building - there are no cubby holes of any kind on the roof. So I think everything will have to be outdoors.

 

[ericmark]

Many years ago I attended a lecture on earthing and aerials. What he pointed out any lighting strike is attracted by any earthed item. He said how aerials should either be very well earthed with at least 150mm² cable or not earthed at all in fact using capacitor couplers so there was no DC connection.

He continued to point out not to fit an aerial to gable end or soot lined chimney which it would seem are where most aerials are fitted. He advocated fitting a spark gap and leak off resistors and showed us pictures of what was his G5RV now a line of copper gobbles lined across his lawn where the lighting got it.

OK the Yargi beam is not as big as a G5RV or inverted V but principal is the same. Do NOT earth an aerial. Most boxes do have capacitors which it is hoped will stop belts when plugging in or unplugging an aerial lead. Earth the pre-amp OK but not the aerial. Decouple the aerial from the pre-amp.

The DTi published this

diagram of how to build a de-coupling or braid breaker and high pass filter. Today you can buy them ready built. Many years ago I scanned in the book see here no longer have assess to page could not remove if I wanted to. It was pre-digital but still has some good advice.

 

[Sam Gangee]

Looks like amateurish radio-speak to me. Not relevant to a domestic situation and designed to minimise damage from build up of static electricity on a LARGE antenna.

For a domestic aerial with masthead amplifier you can't avoid a DC connection for the power supply so forget decoupling.

In the rare event of a direct strike, the best way to mitigate the effect is to loop the cable down to ground level where the bulk of the discharge will jump to earth, rather than navigating the loop into the house.

See http://www.satcure.co.uk/tech/lightning.htm

 

[ericmark]

In the rare event of a direct strike you need insurance for a domestic house there is no way one is going to fit a lighting conductor.

For a lighting conductor at least 120 mm² cable is required so forget it.

Idea is to not attract lighting in the first place, so don't earth exposed metal parts.

I know the closer to the aerial you put the mast head amplifier the better, but it really does not need to be on the mast its self. So place amplifier where cable enters the property, and de-coupling between mast head amplifier and aerial one would hope built into the amplifier.

If you run an earth to an aerial and it does get a strike then likely you will have to have whole house re-wired.

If the aerial is hit the de-coupling will not help. If the pre-amp is earthed with an external cable to an earth stake and that earth is not bonded to any other earth in the house then it may direct the energy down that cable and away from the internal parts of the house. It likely will burn out that cable but little else.

But main idea is not to attract the lighting in the first place so simply do not earth aerials.

The problem is during a storm there is a voltage gradient in the air. So unless the socket is earthed unplugging the aerial lead could result in a shock. Even high winds can cause this. The de-coupling is not to stop lighting it is to stop the house holder getting a shock when unplugging.

In the main there is no reason for a satellite dish to be mounted very high. Mounted to side of the house there is little chance of a lighting strike so the DC supply to LNB is not a problem.

Yes "amateurish radio-speak" since our aerials are bigger we have more of a problem with electrical storms. We have learn from mistakes and the biggest mistake is to earth an aerial.

 

[andrewc1]

Sorry if this is a silly question, as I have no practical experience of this, and only a limited grasp of the fundamentals, but isn't it a building regulation that, for shared aerial installations in a residential setting, the following must be done:

- earth the outer sheaths of all coax cables leading to the individual flats using minimum 4 sq mm wire
- earth the mast that holds the aerial using minimum 4 sq mm wire

Now I understand that such a wire is useless in the face of a direct lightning strike, and I also understand that Eric is questioning whether this regulation is sensible given the case of lightning. But isn't it still a requirement nonetheless according to current regulations? I seem to remember stumbing across it somewhere.

 

[ericmark]

- earth the outer sheaths of all coax cables leading to the individual flats using minimum 4 sq mm wire

Yes that is correct pre-17th Edition installations.

- earth the mast that holds the aerial using minimum 4 sq mm wire

The mast is another thing.

443.2.2 Where an installation is supplied by a low voltage network which includes overhead lines or where installation includes an overhead line and in either case the condition of external influences AQ1(≤25 thunderstorm days per year) exists, no additional protection against overvoltages of atmospheric origin is required if the impulse withstand voltage of equipment is in accordance with Table 44.3.

So likely we don't need protection. Clearly churches have lighting conductors and so will high rise flats. Connecting a mast to an earth not able to take the power is clearly wrong and other than electrical storms there is nothing on the roof which could in any way result in power being put onto the mast. So regard it as Class II equipment requiring no earth.

In 2008 the regulations changed up to that point it had been a case of earth everything. At one point even metal window frames were earthed but with the introduction of RCD protection in 2008 even the bathroom did not require bonding. Clearly an earth still goes to the shower but no earth onto any metal pipework.

Basic idea is you don't want an external fault making something live in the house as even if the RCD trips it would not remove the fault. So water, gas, and oil pipes are still bonded as they enter the property. But if wired to 17th Edition that's where it stops.

We clearly still have houses wired to the 14th Edition (1966) and before that 12th and 13th did not even required lights to be earthed. The problem is of course you can't now buy a 12th Edition so checking something complies is a problem. And nothing says you must up-grade.

So to recap on a modern house after 2008 wired to BS7671:2008 no need for any earth wires other than incoming services and what is included in the supply system. Before that date you may need to earth (real name is bond) aerial cables but not the mast. But in the main that is only when they use shared risers. There should be rule of thumb 300 mm between extra low voltage coms cables and low voltage cables so to bond is really not required if all pre-amps are class II.

In fact it is against the regulations to earth Class II equipment so just fitting earth wires willy nilly could breach the regulations.