Serious shock via Coax at least 120 AC Volts

T

tthecableguy

Joined
9 Sep 2009
Messages
1
Reaction score
0
Country
United Kingdom
This question has been on my mind for quite sometime now. What can cause serious voltage to short coax. I have done cable for 4 years now and know a good deal more than most but, do not know the 1st thing about electricity. It started 4 months ago when I was unscrewing a coax line from the back of a cable modem and the wire fell on my arm. When it hit my arm it clinched up as if I had my fingre in a wall socket. I did not think too much of it and screwed it back in, it sparked, and the surge protector shut down the rest of the equipment. Told customer to call electrician. She called me back and said he stated our wires were not grounded but, they were. And even if they were not they still should not have had that much voltage running thought them.

Next incident I was cutting a line on a splitter outside and soon as I did it threw a breaker. Quit what I was doing and said call an electrician.

I this point I said its time to buy a voltage tester.

3rd incident- at the side of a house hooking up the service and check the homerun going into the home for voltage, bingo, my meter checks for a min of 50 volts, by the way. So I told the customer to unplug the TV's one by one until it ceased and it did. Easy fix here.

4th incident skip- not too relevant

5th incident (the whopper)- unscrewing a line off of a splitter going to a modem, soon as I unscrew it WHAM, I feel the juice pulsating through me I blackout for about 3 secs and wake up on the ground. Come to my senses and tell the home owner to go unplug all tvs vcrs etc from the power outlet incase something is backfeeding power into the line. Checked the line still a whoping amount of juice, my tester going crazy when I even put it close to the coax. Then proceed to tell them call and electrician.

Now, I am VERY curious what is causing this. Ideas, staple through romex and coax(unlikely) - rat chewing two shorted (HUMM) - power not grounded trying to find a ground, coax feeding through romex hole in wallfish stud shorting somewhere.

IDEAS PLEASE!!!!!!!!!!!!
 
For some reason, cable co's in this country do not bond down incoming lines to ground, so these are all accidents waiting to happen. As you say, it can be caused by a faulty set in the customers home. If your splitters/taps at the green box down the street don't provide electrical isolation, a fault on another customer's TV set in a different property could also cause problems elsewhere.

There's also the possibility that two faults within a customer's electrical installation (not necessarily in the TV set) could cause the local earth potential to rise about true earth. This might happen, for example, if a live cable chafes and makes contact with a water pipe, and no RCDs and defective earth mean that no protective devices operate. Now, if you come into contact with 'live' earth from metalwork in the property, and near true earth from the screen on your coax, there's going to be trouble. However, in most cases I would have expected the customer to notice such a fault before you.

In any case, there must be some large difference between earth potential at the property and the origin of the coax, it's just a case of finding out why and where!
 
You should report ALL such incidents to your employer - H&S law requires you to do so and failure to do so is in itself a criminal offence (H&S law is part of CRIMINAL law and is one of the few that imposes joint and several liability - that means that the individual is liable as well as the company and can be fined and/or JAILED.

The companay must provide you with suitable equipment and training to be able to do your job safely. Clearly at the moment they do not and you cannot.

They are failing in their duty to their employees under H&S law and the company and individual employees could/will be liable to huge fines/jail and to pay compensation if you are injured.

You need to sort this out ASAP. Don't just go and buy yourself a voltage tester - your company must provide whatever is needed. They need to carry out a full risk assessment to determine what steps need to be taken

This is serious - someone could be killed, you MUST pursue it.

If you are self employed, then this still applies and you need to get someone to help you.
 
The problem you relate to seems similar to that of electricians when removing neutral cables while there is current flowing.

While connected there is no voltage but on disconnection the voltage can rise to line voltage.

Only by measuring current using for example a clip on ammeter can one predict that there will be voltage on disconnection.

Since this is UK and we have different system we can only guess but it is likely that there is a switch mode power supply in the TV and this will have some filters in the line. These filters will leak some power to earth or chassis if no earth (ground) is connected and the cable can store some of this power. Under normal use the energy stored is not enough to do any harm but where the cables them selves are storing the energy it can give quite a belt.

So step one would be to isolate all TV and video units and step to wait until any voltage has leaked away and then work on item. There may be other methods such as wearing PPE. (Rubber gloves)

But you should be trained as to correct procedure and you should not need to ask on here.

Also it would be better to post on Non UK site.
 
After reading your post I wondered about our own regulations.
411.3.1.2 Protective equipotential bonding
In each installation main protective bonding conductors complying with Chapter 54 shall connect to the main earthing terminal extraneous-conductive-parts including the following:
(i) Water installation pipes
(ii) Gas installation pipes
(iii) Other installation pipework and ducting
(iv) Central heating and air conditioning systems
(v) Exposed metallic structural parts of the building.
Connection of a lightning protection system to the protective equipotential bonding shall be made in accordance with BS EN 62305.
Where an installation serves more than one building the above requirement shall be applied to each building.
To comply with the requirements of these Regulations it is also necessary to apply equipotential bonding to any metallic sheath of a telecommunication cable. However, the consent of the owner or operator of the cable shall be obtained.

The earthing of coax outers for cable TV systems it seems is required in UK. The "However, the consent of the owner or operator of the cable shall be obtained." does seem to be a stumbling block and I would be interested to see what is in fact done. Where I live we don't have cable so the problem does not arise.
 
My understanding is that the cablecos in the UK put in an isolation unit so that the cable line is not an ECP and doen't need bonding.
 
ericmark said:
After reading your post I wondered about our own regulations.
411.3.1.2 Protective equipotential bonding
Click to expand...

Similar provisions are found within the American NEC (National Electrical Code). Article 820 deals specifically with cable TV systems:

820.33 Grounding of Outer Conductive Shield of a Coaxial Cable. The outer conductive shield of the coaxial cable shall be grounded at the building premises as close to the point of cable entrance or attachment as practicable. {.....}
Click to expand...

To summarize the other key points, that section goes on to say that the bonding conductor should be sized to be approximately equal to the size of the coax shield, subject to a minimum of 14 AWG and a maximum of 6 AWG, that it should be run as directly as possible to the grounding point. If the cable is attached to the house near the electrical service, it's typically bonded to the service riser and thus to the home's main electrical ground. If a separate electrode is used, then a #6 cross-bond to the electrical service ground is required.
 
You must log in or register to reply here.
Back
Top