Virgin Media - Equipotential Bonding

[Click4]

On a roll it seems... question time lol

Interesting thing ive been thinking about for a while.. but unable to find anybody who can confirm exactly.

But we bond water and gas to the MET as incoming services.

However there has been no mention about whether Virgin Media drop cables should be also attached to the MET? and im yet to find anybody who has... Virgin Media certainly dont bother.. but then again their idea of safely burying a cable is 2" below the surface... then when I buried ours in the garden at the minimum 12".. the guys who turned up and was supposed to be doing it seemed a bit annoyed that I had done their job for them "properly" and admitted to me they wouldnt of buried it as deep.

after all, the cable coming into your house goes back to a giant splitter which they call taps.. and from that tap it connects to every other house on the street which in turn is connected to the metallic chassis of the box.

So should it be also bonded? a few times on ours, I have felt a tingle when ive disconnected the coax from the back of the box.

Cant find any references to it, seems nobody has thought about it.. hmm.

I would of thought the answer should be yes? but could be wrong, there are earthing points on their splitters.

I know while contraversial you are supposed to bond TV amps and splitters inside the property.. Arguements against are it can turn the aerial into a lightening rod.. but it is a guideline that it is done.

So I would of thought VM cables should also be bonded back to MET, to me anyway it makes logical sense and is like bonding the gas and water.

just cant find any references to anybody thinking of doing it, let alone doing it.

 

[bernardgreen]

I sense a long thread.

The tingle is between the aerial coax cable which is roughly at ground potential due to capacitive coupling between cable and building structure while the TV set's 0 volt ( on the connectors ) is possibly floating somewhere between Neutral and Live potentials due to the way the power supplies are designed. It will be floating if the mains supply is via a two wire cable without an earth.

The problem for the telecoms companies comes if their cables join together the "earths" from two different buildings. If those "earths" are not at the same potential then currents will flow through the communication cables. As we know the regulations recognise that this current may be larger enough to require 10 mm earth cables ( water pipe bonding ) so the same requirement would have to apply to communication cables if they were to be bonded. What would ( and does ) happen is that when there is a serious difference in "earth" potentials the comms cables that are connected to more than one earth have been damaged by the high currents flowing in them.

 

[Maxatoria]

Our VM cable modem has an isolator screwed onto the wall in a little white box so it should be isolated from the rest of the world once its passed that but i suppose its the 6 inches of cable poking through the wall that's the concern but ours has another one outside on the wall in a brown box so i'd imagine its safe since only VM engineers should be poking in there

 

[Click4]

Our VM cable modem has an isolator screwed onto the wall in a little white box so it should be isolated from the rest of the world once its passed that but i suppose its the 6 inches of cable poking through the wall that's the concern but ours has another one outside on the wall in a brown box so i'd imagine its safe since only VM engineers should be poking in there

Isolator doesn't isolate the outer braid of the coax, it only isolates the inner core.

Has metal connector on their drop cable on 1 side of isolator, isolator is made of metal, and connector other side is metal, so the path is continued.

If you get a multimeter and touch it on the connectors each side of the isolator you will find continuity

 

[Spark123]

I haven't seen bonding on BT telephone cables as they enter houses either. I believe this is a BT decision not to bond it. I suspect VM may also have a similar ruling- only way to find out is to check with them.

 

[ericmark]

I have said many times the master socket used with our phone line has a spark gap and discharge resistors yet I have rarely seen an earth connected for it to discharge to.

It may be this is a left over from the old party line where the GPO as it was called then fitted an earth rod which the call button used to allow one to select the line onto your phone.

Static on phone lines has long been a problem especially when using earth return high winds on the Falklands would stop one using the local phone system in the camp. Worked OK in the town but in UK only two parties shared a phone line on Falkland up to 6 shared a line.

With fibre optics the earthing problem is fast going away and so likely we don't need earth bonding in the same way. But in general we do seem to have gone down the risk assessment route and although in an extreme case we may get a shock from a metal window frame the idea of bonding all metal parts is no longer done.

However there is nothing to separate the rules from a water pipe, gas pipe, fuel oil pipe, drain pipe or any other metal pipe coming into the house so I suppose it should be bonded.

 

[Click4]

Phone cables are different in the sense that the wires connect into plastic faceplates, and have no extraneous conductive parts.

The VM coax cables are the opposite, metal splitters, metal casings on their boxes, metal connectors.. And because the boxes have 230v running through them.. IF and it is an IF as they are supposed to be double insulated but if voltage somehow connected to the outside of the box, or got onto the outside braid of the cable.. It would be connected to every other house on that cabinet, and voltage must be getting into the braid somehow because a few times I've felt a tingle through potential difference... Possibly it's like computers the boxes have a slight earth leakage.

Fibre optic component of VM network is also only a small proportion.. Because fibre cable it may go to 1 green box then off that green box they may daisy chain a Dozen or more cabinets or pits and each pit / cabinet may serve anywhere between 8 customers and 48+ and that part is all coax with braid connected to the outer part of the metal coax connectors and box.

Isolator only really isolates the internal core, probably because the inner core is the bit which goes through their equipment whereas outer core just connects to outer casing of the box.

I suspect reason vm don't earth is for same reason gas and water suppliers don't earth their connections back to the met, they leave it to the electrician to do and just look after their side.. Which is what VM do because they earth their cabinets... But like here the earthed cabinet is at 1 end of the road, with pits in between which customers connect to...which aren't earthed. Nobody here connects directly to the earthed cabinet, only pits... And each pit can connect 8 customers and they are dotted along this road every 30m possibly

There is also, while controversial guidelines that state tv aerial distribution systems to be bonded back to met, like loft boxes which provide earth terminals.. Splitters etc.. So that in the event 1 tv on the system sends voltage down the coax it doesn't pose a danger in the same way as if no earth connection existed.

So it should apply also to VM, it is an incoming service also like gas and water... And does have a means of earthing.

Just can't see any cases of anybody doing it lol and everyone I've asked doesn't know the answer.

 

[Maxatoria]

I doubt that there's any regulations and Virgin would prefer it that way as you can imagine how many problems they'd have to sort out should they have to start running cables to the CU and what happens when they find a potentially dangerous fault and the extra training/gear required before their insurance company would allow the average engineer to go anywhere near a CU and the people complaining about them running trunking all across the place etc especially in a multi room setup where each rooms cables are fed from the outside box and never mind the price hike on the installation costs as they may have to replace anything that gets damaged while doing the work

 

[bernardgreen]

I haven't seen bonding on BT telephone cables as they enter houses either.

If a wire in a phone cable was bonded then the phone would appear at the exchange to have a line fault and if the battery side ( -50 volt ) was bonded to earth then the exchange would "see" the phone as being permanently off hook and thus unable to receive calls.

 

[Gees]

I am sure that the outer of the coax will already be at or very near to earth poential, due to the many many earthed metalic boxes the cables are srewed into and the numerous splitters and back to back couplers invoved in a cable distribution network, Has anyone ever done a continuity test between the MET and the coax outer ?????

 

[JohnW2]

I am sure that the outer of the coax will already be at or very near to earth poential, due to the many many earthed metalic boxes .... etc.

Indeed - and that is the very reason why the regulations theoretically require it to be main bonded to the MET - just as with any extraneous-conductive-part which is 'liable to introduce true earth potential into the premises".

Don't forget that the reason for main protective bonding is to avoid dangerous potential differences between the MET (hence all the CPCs and exposed-conductive-parts of the electrical installation) and a 'true earth potential' introduced by an extraneous-conductive-part', in the event that the potential of the MET rises significantly above true earth because of a fault.

Kind Regards, John

 

[dinopower]

i work for a dno, i know we had a similar problem when local council contractors wiring in new coax for communal aerial system which sheath was bonded at the booster, when plugin to telly sparks and about 30 volts between, called us we checked loop impedance, all good left it to them to sot out, the supply to the flat was pme, but supply to booster was cable sheath, not sure how they got over the problem.

 

[AdrianUK]

VM do/should/are-supposed-to fit a galvanic isolator in the outside box where the cable from the street cabinet joins the cable which goes into the house. This is a true galvanic isolator & DOES separate both the braid & the inner core from the street cabling. (If you look at one carefully, you'll observe that one of the F connectors actually enters the metal can via an insulating bush).

If everything has been done properly then the coax braid in the house should be floating & not electrically connected to the street coax. Hence you can, if you wish bond it your your MET but you dont NEED to because it doesnt actually leave the confinds of your equipotential zone. The tingle that you feel is actually probably coming from YOUR VM box or your TV not from the street. HOWEVER, these isolators often fail (I wonder why.......) and are often replaced by thru connectors thus removing the galvanic isolation.

If these isolators are not present then consider what would happen ..... if the braid in the house is connected to a piece of earthed equipment then the braid will become connected to your mains earth...... if you are on a PME system then this earth is also connected to the mains neutral. Now consider the house next door with the same arrangement ....... Result - the braid of the coax is actually in parallel with the mains neutral & hence will carry a proportion of the returning load current, something which it was never intended to do!!!


Adrian

 

[TicklyT]

In telecommunications it's not unusual for circuits carrying just signal levels to also have a DC voltage or 'wetting current', usually limited to a few miliamps, applied to them, for a variety of purposes:-

It mitigates the effects of oxidation in cable joints and plug/socket connections,
It can identify the cable pairs carrying the signal as a 'live' circuit,
It provides power to network terminating equipment,
It's presence (or absence) can assist in fault finding / localising.

The DC voltage is usually 'blocked' by a transformer or other circuitry in the network terminating arrangements, passing just the 'AC' signal component onwards to the customers' interface.

POTS (Plain Old Telephone Service) lines don't feature any DC blocking, as the DC component is needed to power the telephone handset, and send the 'off hook' signal to the exchange.

Incidentally, party lines, which had earthed ringing circuits, and electrical installations with VO ELCBs sometimes made for some interesting fun and games.....

 

[Click4]

Thanks for the replies

That has cleared it up for me now lol.

I was under the assumption that the braid was not isolated, because from the isolators fitted in the house here... the outer braid connects to the metal connector.. which then screws onto a metal isolator.. and the connector the other end also screws onto the metal isolator.

which doesnt look isolated on the braid... but you have now cleared that up for me