Testing for isolation

[Mike PooleParticipant @mikepoole82104]

What you have to remember is that it is the current passing though the body is what gives you a problem, we have probably all experienced a high voltage shock from a spark plug cable and lived to tell the tale. Earth leakage trips are rated in milliamps because they should disconnect before we suffer serious damage from passing too much current. Working safely we should never need to find out if an earth trip disconnects before we die, it is there as a last resort to save us from ourselves or the unexpected. Live working and testing according to the HSE should be exceptional but in real life shutting down a high production facility to dead test is not going to be popular with management so you can actually do much live testing with a minimal risk to the tester. It is actually quite difficult to touch any live parts in an electrical panel built to recent standards and even test probes struggle to contact live parts for testing. In 44 years of electrical and electronic maintenance I have had a few shocks and bangs but in all honesty they should not have happened, only one or two were a surprise and others were when I should have known better and defeated things to make life easier. A man who I have the greatest respect for who taught me a huge amount when I was an apprentice was twisting the ends of a cable to terminate them when I realised the were live, I said Alan they are alive, he said it’s only 110V and I have a dry skin and hardly feel anything at all, it would have me leaping around if I was touching 110V. A 240V shock is always going to have the potential to kill you, if you survive you are lucky because the circumstances didn’t drive a fatal current through you or you disconnected before damage was done, count yourself lucky and if you are a cat tick off one of your nine lives.

Mike

[Tim Taylor 2Participant @timtaylor2]

Get a wiggy solenoid type tester – accurate, reliable, inexpensive, and will test anything up to 600vac.

[not done it yetParticipant @notdoneityet]

One point to remember is that many electricians have a mate with them. When you are on your own, there is no immediate help.

[EmgeeParticipant @emgee]

A mains indicator/tester I used was made by Square D, indicated AC or DC voltage to 600v, it was a solenoid type so could be seen to work in a graduated window, but always prove any mains indicator on a known source before relying on the test result.

Emgee

[Mark RandParticipant @markrand96270]

Folks may raise their eybrows over the comments about keeping one hand in one's pocket, but when I started at GEC Machines, Rugby and did a stint in the test department, the instruction booklet (Which was still entitled BTH test department) stated quite clearly that:-

"When working on open switchgear, operatives will keep one hand in their pocket at all times".

This isn't an oddity, it'll save you from the very real danger of touching a live conductor with one hand, while holding onto something that's earthed with the other one. It has stuck with me for over 40 years.

Personally, 110V scares me far more than 240 or even 11kV. For 240V or higher, I'm paranoid and make sure that I don't work on live equipment. On Higher voltages, Lockout/tag out was always available. But 110V often got worked on as-is. It stings, even when centre tap earthed! And when some morons in a foreign power station wired 50 odd 220-110V local supply transformers to accidentally give 220-330V, instead of 55-0-55, half the time, it got down right scary.

Edited By Mark Rand on 02/02/2019 01:03:04

[Nick HulmeParticipant @nickhulme30114]

No one here heard of or have a Test Lamp?
You can choose the load to suit your requirements and unlike a volt meter it shows you have current delivery capability and not just leakage.

[Harry WilkesParticipant @harrywilkes58467]

Nick

I'm with you on the use of a test lamp during my years as a steelworks electrician working on voltages both AC and DC up to 11KV if I had to put my trust into any thing it would have been a test lamp, Ive had Martindale / SqD voltage tester along with multimeters but if in dought test lamp out and most of all where possible fuse's in my pocket !

H

[Anthony KnightsParticipant @anthonyknights16741]

I can remember having two electric shocks in my lifetime. The first was when I was about 14 years old. I was plugging some thing in and touched the the unshrouded live pin of the 13 amp plug. I couldn't do it now as they have changed the design of the plugs. The second time, I was working on a monochrome TV set, when 11kv jumped from a damaged EHT cable and burnt a small crater in the back of my hand. My other hand was stuck firmly in my pocket.

These experiences have no doubt contributed to me being careful to not get any more, which is pretty good considering I am now in my seventies.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Anthony Knights on 02/02/2019 12:01:03:

… I was working on a monochrome TV set, when 11kv jumped from a damaged EHT cable and burnt a small crater in the back of my hand. My other hand was stuck firmly in my pocket.

.

… perhaps risky, in the circumstances

MichaelG.

[gerry maddenParticipant @gerrymadden53711]

I have always wondered why we feel anything at all if we are isolated from earth and touch a live wire with one finger only ? Is it a capacitance effect allowing a tiny current to flow momentarily till we are raised to the live potential ?

Birds land on HT cables and clearly don't even get a tingle of they probably wouldn't do it.

Any thoughts ?

[peak4Participant @peak4]

Posted by gerry madden on 02/02/2019 15:07:33:

I have always wondered why we feel anything at all if we are isolated from earth and touch a live wire with one finger only ? Is it a capacitance effect allowing a tiny current to flow momentarily till we are raised to the live potential ?

Birds land on HT cables and clearly don't even get a tingle of they probably wouldn't do it.

Any thoughts ?

Under normal circumstances, we aren't really fully isolated from earth. There is enough current flow through the body, via the soles of your feet, and thus to ground, to be able to perceive the shock.

How dangerous it is depends on, amongst other things, how much current flows. There's less flow through your body from finger-to foot-to ground, than there is from right hand to left hand to the chassis earth.

Anyone who's worked on live telephone stuff will be able to relate to a belt from ringing current. From memory, about 70v but @ 17Hz in the UK, and you don't half know when you've been shocked.

The latter is also more dangerous as the current flow directly crossed the heart, and it only needs a tiny level to disrupt the heart's rhythm. This why ELCB/RCCD need to trigger so quickly, i.e. in less than a heartbeat.

There are earth free environments in some electronics labs/workshops, or depending on one's circumstances a 240v-240v isolating transformer can be used. Here's a youtube link explaining it quite well

Lots of interesting videos on the internet, have a look at the sparks flying at the end of this one, so it works for people as well as birds.

A much safer way of doing stuff than my previous video link on Page 1 of this thread.

 

Bill

Edited By peak4 on 02/02/2019 16:20:24

[not done it yetParticipant @notdoneityet]

Posted by gerry madden on 02/02/2019 15:07:33:

Birds land on HT cables and clearly don't even get a tingle of they probably wouldn't do it.

Any thoughts ?

There will be a potential difference between the birds feet. Simple physics of a potential dividing circuit. However, the voltage drop between the feet is miniscule as the conductor (here regarded as a resistor of very low ohmage) will be umpteen miles long, with even a voltage of several thousands across the ends.

Added to that, the bird would only be acting as a “shunt” between the two points on the conductor and any current shared would be in inverse proportion to the two resistances, so virtually all the current would still pass through the conductor.

Now, an unlucky pheasant that tries to fly between the high voltage conductors would get fried if it contacted both simultaneously!

[peak4Participant @peak4]

Posted by not done it yet on 02/02/2019 16:31:27:

Posted by gerry madden on 02/02/2019 15:07:33:

Birds land on HT cables and clearly don't even get a tingle of they probably wouldn't do it.

Any thoughts ?

There will be a potential difference between the birds feet. Simple physics of a potential dividing circuit. However, the voltage drop between the feet is miniscule as the conductor (here regarded as a resistor of very low ohmage) will be umpteen miles long, with even a voltage of several thousands across the ends.

Added to that, the bird would only be acting as a “shunt” between the two points on the conductor and any current shared would be in inverse proportion to the two resistances, so virtually all the current would still pass through the conductor.

Now, an unlucky pheasant that tries to fly between the high voltage conductors would get fried if it contacted both simultaneously!

Indeed, if you're ever near when someone brings an overhead power line down and you need to vacate the area, it's absolutely vital that you shuffle or bunny hop out of the vicinity.

Stepping just half a metre can be enough to cause you a serious problem as the current will take the path of least resistance, which is likely to be you, rather than the ground on which you travel.

This is why the guy on the power line is OK as he's wearing a conductive suit and the metal of the wire is a much lower resistance than his body.

Bill

[Neil WyattModerator @neilwyatt]

I don't think we should treat the sensible 'hand in pocket' precaution as a substitute for checking.

I do think mains tester screwdrivers being cheap and easily kept handy are the simplest solution. If it lights up before you isolate and doesn' t after you isolate you can be pretty sure things are OK.

Neil

[mark costello 1Participant @markcostello1]

Worked with a chap who undid a 2000 volt extension cord. He said it blew out 4 caps from His teeth and did a long list of damages to Him. Took 2 years to recover,thought He was careless.Found out later some one plugged in the other end.

Neighbor had a 440 transformer located on His house, put a ladder upon the house to paint it, the ladder slid and He hit the 440 line and it burned all the hair off His body. Docs said the only thing saved Him was He was a weightlifter and had enough strength to last through the initial recovery process.

[Howard LewisParticipant @howardlewis46836]

+1 for always having the fuse in your pocket (A former colleague of mine didn't and someone replaced while he was working on the circuit. After that, he always carried the fuse in his pocket!) or, if possible, use a lock out.

One hand in pocket should minimise risk of the current flowing across the heart and acting as a lethal defibrillator.

At least AC passes through Zero, regularly, giving you a chance too release grip; unlike DC

"Its the volts that jolts, it's the mils that kills" Otherwise those of us who have had a shock from a spark plug, or a TV flyback EHT, wouldn't be here. Prewar TVs had a separate mains power supply for the E HT, and they were lethal.

The more recent vehicle ignition systems can be lethal, so don't take the risk.

Howard

[Martin Harris 9Participant @martinharris9]

Posted by peak4 on 02/02/2019 16:16:57:

Anyone who's worked on live telephone stuff will be able to relate to a belt from ringing current. From memory, about 70v but @ 17Hz in the UK, and you don't half know when you've been shocked.

Telex was even nastier – plus and minus 80V DC gave the potential for 160V and that definitely made you jump! The main hazard while working on distribution racks was the legendary "tag rash" – you would get a belt from an exposed tag and jerk your hand away from it, scraping the back of your hand across the exposed tags on the next block – a leather "half glove" was often worn to protect the back of the hand…

[Ron ColvinParticipant @roncolvin83430]

I once manged to lay a spanner across the terminals of a 12v car battery. Lucky I was not holding on to it at the time. The cable insulation immediately begun to melt away. With the advent of the newest 48 VDC vehicle electrical systems, I should imagine that result of such a short would be more spectacular/lethal.

[MeunierParticipant @meunier]

Posted by Martin Harris 9 on 03/02/2019 10:41:28:

Posted by peak4 on 02/02/2019 16:16:57:

Anyone who's worked on live telephone stuff will be able to relate to a belt from ringing current. From memory, about 70v but @ 17Hz in the UK, and you don't half know when you've been shocked.

Telex was even nastier – plus and minus 80V DC gave the potential for 160V and that definitely made you jump! The main hazard while working on distribution racks was the legendary "tag rash" – you would get a belt from an exposed tag and jerk your hand away from it, scraping the back of your hand across the exposed tags on the next block – a leather "half glove" was often worn to protect the back of the hand…

And when jumpering on distr blocks in metal cabinets it only took holding on to cabinet frame with one hand and catching back of hand on a transmitting tag to let you know what 'tingle-tongue' was like !
DaveD

[Jon LawesParticipant @jonlawes51698]

One of my colleagues accidentally flashed some locking wire over a live bus in the back of an aircraft, flashed instantly into ball bearings which we then had to laboriously find… not to mention his flashgun eyeballs…

[Robert Atkinson 2Participant @robertatkinson2]

Flash-over can be deadly.
I know of one case when a technician was working on some "low voltage" (240V) instrumentation in a cabinet with a 44kV underground cable termination in it. The instrumentation was blowing a small sand-filled ceramic fuse. The technician ran out of fuses and replaced it with fuse wire. The fuse wire blew and the plasma created in the cabinet caused the 44kV feed to short out. The cabinet exploded and the technician was killed.

[Tim Taylor 2Participant @timtaylor2]

Posted by peak4 on 02/02/2019 16:16:57:

.

Lots of interesting videos on the internet, have a look at the sparks flying at the end of this one, so it works for people as well as birds.

Bill

Edited By peak4 on 02/02/2019 16:20:24

This is how utility companies clean & physically inspect high voltage transmission lines – they also use aerial IR inspections to look for loose connections.

The sparks seen are caused by corona on the transmission lines – the wand & clamps provide a way to discharge the energy so the worker can move between the helo and the lines without acting as the conduit.

Tim

[Tim Taylor 2Participant @timtaylor2]

Posted by Robert Atkinson 2 on 03/02/2019 21:12:55:

Flash-over can be deadly.
I know of one case when a technician was working on some "low voltage" (240V) instrumentation in a cabinet with a 44kV underground cable termination in it. The instrumentation was blowing a small sand-filled ceramic fuse. The technician ran out of fuses and replaced it with fuse wire. The fuse wire blew and the plasma created in the cabinet caused the 44kV feed to short out. The cabinet exploded and the technician was killed.

Arc flash can indeed be very dangerous, and it's not so much dependent on the voltage as it is the bolted fault current. There are several videos online of high voltage failures, which are pretty spectacular, but low voltage failures can be just as dangerous, often more so because they don't have the sophisticated protection circuits of the medium & high voltage systems.

I was part of an investigative team on an industrial arc flash incident on a 480V 3 phase system. An old circuit breaker failed to clear & then exploded. The energy released was approximately 13 megawatts in a period of about a third of a second, and completely destroyed the breaker panel. The resultant arc flash plasma exceeded 20,000 deg F ( a 480V arc flash can exceed 35,000 deg) and vaporized about 2 sq. feet of steel enclosure, sending droplets of molten metal as far as 30 feet. The pressure wave was calculated as approx. 35,000 psi and crushed the side of a steel motor control enclosure 4 feet away. Fortunately no one was in the MCC at the time or they would very likely have been killed.

Tim

 

Edited By Tim Taylor 2 on 04/02/2019 00:38:54

[not done it yetParticipant @notdoneityet]

Probably the worst case of improper, or rather non, isolation I’ve come across was three deaths due to a still energised electrostatic precipitator. Three men stepped inside, one after the other….

[SillyOldDufferModerator @sillyoldduffer]

Posted by Robert Atkinson 2 on 03/02/2019 21:12:55:

Flash-over can be deadly.
I know of one case when a technician was working on some "low voltage" (240V) instrumentation in a cabinet with a 44kV underground cable termination in it. The instrumentation was blowing a small sand-filled ceramic fuse. The technician ran out of fuses and replaced it with fuse wire. The fuse wire blew and the plasma created in the cabinet caused the 44kV feed to short out. The cabinet exploded and the technician was killed.

A similar effect can make ordinary mains dangerous in the right circumstances. Before firing up my home-made 600W Tesla Coil I swotted up on the risks. More dangerous than the very high-voltage they produce is the spark-gap : it has 6kV at 100mA across it, gets burn hot, and emits X-rays, ultra-violet light and Ozone. (Not too worrying unless the coil is operated a lot in a confined space. I keep about 15 feet away from mine and open the garage door.)

The spectacular high voltage streamers come off mine at about a megavolt, but the maximum current in them is less than a milliamp, ie the streamers aren't very dangerous in themselves. (It's volts that jolt and Amps that stamps…)

I could only find one fatality due to the very high-voltage output of a Tesla coil. A stage magician had an act involving direct contact with the streamers and had done it many times. He was killed when one of the streamers happened to hit a connector in the overhead lighting gantry. The tesla spark created a low-resistance pathway through the air allowing several amps of ordinary mains power from the lighting system to electrocute the magician 30 feet below. Like plasma from the fuse wire in Robert's example, the Telsa coil was just the starter – once several amps are flowing in a spark, heat sustains it, even after the Telsa Coil is switched off.

Can't find a photo of mine operating: this is a more powerful Australian example off the web.

Dave

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