Used a neon screwdriver type many many years ago while working on a immersion heater -as I tested the live I inadvertently Leaned on one of the water pipes and received a belt of 240 volts and promptly threw the tester away - never used one since .
Electricity detector
- Thread starter LittleEars
- Start date
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They sense an electrical field - I suppose akin to a radio.
The body of most of these is totally plastic. The user doesn't need to touch a metal cap or clip to make it work and isn't part of the electrical circuit. Much safer in that regard.
But as Spectric noted, these aren't suitable for proving anything dead. Even the better ones.
Electricians follow HSE guidance on this
https://www.hse.gov.uk/pubns/priced/gs38.pdf
Oscar43634
Established Member
Interesting thread; I feel a bit silly having been in the loft over the weekend with my neon tester.
John Brown
Freeloading Social media influenza
Again, I wasn't talking about safety from the aspect of electric shocks, I was talking about false negatives.
But no matter, I can't be bothered to debate the point.
John Brown
Freeloading Social media influenza
From the Fluke website:
"A built-in sensor at the tip of the tester detects the presence of voltage when touching a conductor, outlet, or supply cord. By holding the tool, you are the ground reference through capacitive coupling. When the tip glows red and the unit beeps, you know voltage is present."
So the user is part of the circuit, albeit capacitive rather than resistive.
I have a LAP live cable tester plus several neon screwdrivers. I isolate the circuit at the CU then use the tester to check for live and then the screwdriver before I proceed. However, this is only for circuits I am familiar with, if I found a floating bare wire, I wouldn't go further without knocking off the main breaker on the CSU.
johna.clements
Established Member
You should treat all cables as live. As other people have noted the cable could be connected to something that only comes on now and then, with controls that draw little current or are non electrical. You could also have a cable that is connected to the mains at one end but goes nowhere, when you cut it you complete the circuit. I have been on a number of sites where supposedly dead cable were live. On one site the clients representative was a bit upset that we would not excavate around a cable with the machine which he said was dead until he got it spiked. He was paying for the delay. Rather get the cable spiked, since he was sure it was dead, he took a hacksaw too it. He was very lucky that the 450V cable did not hurt him.
Do not believe people who say a cable is in a particular position. Was on a site in Crayford, London where the client "had" installed a 11kV cable behind a footpath. My cable detector would not pick it up because there was a 132kV overhead running parallel. I was knocking in crash barrier posts at the front of the footpath. I personally went and saw the resident engineer who insisted that the cable was at the back of the footpath and did not want to pay for hand dug holes. We drove a post into the cable and the explosion lifted the lorry up on its suspension. No one was hurt but I doubt that the people who lived in a mile radius were too happy when the substation tripped out.
Dead cables (and metal pipes and other things) that are running parallel to live cables will have a current induced in them so they will show up as live.
Do not believe people who say a cable is in a particular position. Was on a site in Crayford, London where the client "had" installed a 11kV cable behind a footpath. My cable detector would not pick it up because there was a 132kV overhead running parallel. I was knocking in crash barrier posts at the front of the footpath. I personally went and saw the resident engineer who insisted that the cable was at the back of the footpath and did not want to pay for hand dug holes. We drove a post into the cable and the explosion lifted the lorry up on its suspension. No one was hurt but I doubt that the people who lived in a mile radius were too happy when the substation tripped out.
Dead cables (and metal pipes and other things) that are running parallel to live cables will have a current induced in them so they will show up as live.
I had a 50 cubic metre reinforced concrete machine foundation installed on one of my projects.
When the contractor removed the floor slab before the excavation (diamond saw into squares, rawl bolt and pluck them out with the crane) we found a conduit with the best part of 200 ethernet cables crossing diagonally just under it.
Not on any of the as-builts !
Fortunately we only nicked the conduit without cutting a single cable. Rejointing unmarked cable bundles like that takes an age :-(
On the non contact voltage testers, this is probably the best I have found: Volt Stick PRO230 | 230v AC non-contact instant voltage tester with shielded fork tip very sensitive when using the fork end, even with a live cable beside it, but as others have said it should only be used as an indicator and not relied upon for proving a circuit is dead.
johna.clements
Established Member
As builts are often produced after a project is completed when the contractors and junior RE staff have gone. The people who are left just want to wrap up the job and leave.
The drawings are not up dated as the works progress because there are always something to do now. When the job slows down they get rid of most of the draftsmen because its all finished.
In a perfect world the drawings would be updated as the works progressed and then the draft as builts would be checked by the people who supervised the construction. I have never been asked to check as builts. The as builts I have seen did not include all the changes.
This is an important lesson. Never ever trust an isolator. Always double & treble check everything for safety. Always assume everything is live or may become unexpectedly live as you are working on it. Maintain the highest possible self-preservation factor! Little details are important and may save a life.
As an aside on this, you should also interlock (padlock) the isolator if possible and if not, cover it with tape and mark it as needing to stay off until you personally say otherwise! Remove the fuses if you can too... there's a reason electricians' stepladders are made of fiberglass
A problem with very large high voltage cables that are burried is that they may be shown as an ideal line on the drawing but in reality there can be several metres difference because of the required bend radius, so digger beware.
johna.clements
Established Member
11kV is not particularly big but I take your point that they can not turn at right angles.
This cable was installed when the area was just a 100m diameter area of gravel, the cable will have no problem with bending around that. It was being left for six months to settle into the marsh below. The REs staff obviously did set out something as the cable was not under the road but they did not supervise the works.
My old boss (this was a few decades ago) hit a phone cable and took out the entire roads phones- he'd even phoned 'dial-a-dig' (shows how long ago it was)
He was digging about 20m away from where it was supposed to be (running down the front of the property) to build a new shed
Turned out that it was well known in that area that the phone lines could 'walk'- a sloped clay block, and they could move underground- the head office boys didn't know about it, but the crew that came out to repair it said it was quite common- depending on whether it was a drought year or a wet one, the lines would move back and forth...
It was funny, during the time I was working there, got to see it in action- there was a covered concrete walkway between the 'old building' and the new shed- at one point it was as built- touching both buildings, then later on it had about a 6" gap at each end of it (roof and concrete) then it closed back up again by the time I left...
johna.clements
Established Member
The telephone line that was 20m out of line is likely to have been laid in that position or the whole hillside has slid down towards the valley bottom. Telephone cables are not very stretchy neither are the ducts they are normally laid in. If the ground moves then the cable has to get longer or they have to have coiled up cable in the drawpits. The ducts that the cables are normally laid in will open up at the joints soil will enter the duct and the cable will be clamped in position eventually, then break when more movement happens.
The ground normally moves down a hillside in two ways. A slab moves down the hillside because the friction between the slab and the soil below will not resist the weight of the slab. This could be caused by the clay at the surface being more permeable than the clay at depth. When it rains you can get a layer of wet clay on top of the harder less permeable clay that shears. When the moves buildings etc could well stay up right so it may not be noticed.
The other common way is for the slope to rotate in section. The weight at the top of the slope is not resisted by a "slip circle" down through the slope. The top of the slop goes down and leans backward and the bottom comes up to balance the weight with the friction. You would certainly notice this as your building would start leaning.
For a cable and the soil to move below the ground but the surface to remain in position something else would have to be happening. There would have to be something to hold the surface whilst the cable and the soil around it moved; otherwise it will just move with the soil below. You do get underground streams that could move the soil locally, with the soil above arching over the soil below. But if it was happening over a whole hillside the surface would just float down of the saturated ground below.
Clay will also expand and contract as it gets wet and drys out, But this is a bit like timber expanding and contracting. The relative position of things will not change much. My parents garden gets 50mm wide cracks in the lawn when it has been dry for a while.
British Telecom use cable detectors when marking their services. These would work anywhere unless the cable was a fibre with no tracing wire laid with it.
It does take time to mark out cables in this way.
Drawings are also used but there could be errors if there are no features to measure off, or features have moved (house knocked down and shifted etc).
The drawings can also be wrong as discussed above. drawing says the cable is on the south side of the road but it is 20m away on the north side.
This is country Australia- no ducts used, just direct buried cables
It was 'relatively flat' but with a very slight slope up to a rock ridge behind, but the movement on the surface was noticeable enough that you could see the gaps opening and closing over the years I worked there- sometimes the concrete path between the two buildings (and free standing roof above it) would be butted up hard against the buildings, other times it was a noticeable (about 5cm or so) gap at each end- I often wondered if they had built the roof and path at the wrong time- instead of the gap, would they simply have been crushed instead
The plans showed the cable running along the edge of the property, at the edge of the road basically, but they were well inside instead (and next door would have been under the building itself as it ran almost to the road- an old store that was there well before the phone lines were installed)
Luckily the electric was overhead, and there was no town water or sewerage there either (rain water tanks and septic as was- and still is- common in rural areas- the pipes would not have been as forgiving as the cables (which can and do stretch quite considerably before they break- at least ours do)
johna.clements
Established Member
It is largely a matter of luck which sections of the slope decide to move at one time. The crack could have happened to form under the house and unless it had flexible timber foundation so the ground could move below it or a shallow concrete raft then the house could have split.
I would guess that they laid the cable to avoid next door but did not update the plans. If they would have to dig in the road to get passed next door it would have been easier to go around the back. If it was not done officially with a legal agreement to put the cable on the land your boss is unlikely to guess it was there.
Jacob
What goes around comes around.
Wiring and plumbing get ever more complicated to meet modern demands, and hence ever more difficult to locate precisely. I wonder if more should simply be surface mounted to avoid doubt.
Also gives easy access for repair, modifications, alterations.
Just a bit of a design challenge to make it acceptable, but we'd soon get used to it. Could be in flush-to-surface conduits with easy removable covers? Behind skirting boards etc.
Also gives easy access for repair, modifications, alterations.
Just a bit of a design challenge to make it acceptable, but we'd soon get used to it. Could be in flush-to-surface conduits with easy removable covers? Behind skirting boards etc.
Well the plans showed it as being roadside- but it most certainly wasn't...
(the original building where i was working (originally a workshop, then converted to admin as the workshop had become too small) the path, and the 'new workshop' were decades apart in construction dates and were all separate concrete pads
The thing was the plans showed it roadside- but it was well inside the property boundaries (which wouldn't have been done) and it was disappearing directly towards the old building (over a century old) and built well before the telephone line installation...
This area was several layers of clay, with some other soils inbetween (I remember that it caused issues for the new building as they wanted a more expensive 'raft' concrete floor/foundations rather than just a 'slab' because of the soil conditions)
It was also noticable that depending on the soil moisture levels, you could see the powerpoles outside either leaning towards the road, standing upright, or leaning away from the road- depending on whether we had had a drought lately or not lol
So there was a lot of movement going on there...
