Testing for isolation

[Dave HalfordParticipant @davehalford22513]

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…

The joy of tag rash, plain 'local battery' was bad enough never mind ringing. That was one of the reasons I went to a repeater station instead of an exchange till one day I was jumpering (wiring) channel equipment to a TJF (very low power) and got a big flash, so smart arse thought he would see how long a spark he could draw – 1/4" later the tip of my best pliers melted.

[peak4Participant @peak4]

Posted by Dave Halford on 04/02/2019 11:35:17:

The joy of tag rash, plain 'local battery' was bad enough never mind ringing. That was one of the reasons I went to a repeater station instead of an exchange till one day I was jumpering (wiring) channel equipment to a TJF (very low power) and got a big flash, so smart arse thought he would see how long a spark he could draw – 1/4" later the tip of my best pliers melted.

Where were you then Dave, I was Sheffield, exchange and then transmission construction.

Bill (Though at work would have been known as Steve)

[Roger BParticipant @rogerb61624]

If you beleive you have isolated correctly and are prepared to touch it with your hand a short circuit with a screwdriver will be no problem. If you have made a mistake it may save your life. Belt and braces.

[Tim Taylor 2Participant @timtaylor2]

Ah yes, the venerable Tesla coil!

I built a table top version as a project for my high school physics class and displayed it as part of our annual tech show. For the demo I would hold a fluorescent tube in my hand and draw a corona streamer from the coil to the tube, lighting the tube – not that impressive until I also held a tube in my other hand away from the coil and it would light as well, though not as brightly.

Tim

[Former MemberParticipant @formermember32069]

[This posting has been removed]

[Mike PooleParticipant @mikepoole82104]

I suppose in most domestic situations the prospective fault current is going to be fairly low unless you live next to the substation so shorting the supply might not produce too big a bang. In a factory you are going to get a much more impressive bang and rather than save your life you might end it. Isolating and testing should be done properly and with proper equipment. If the proper process is followed then you should avoid having the “accident” if there is such a thing.

Mike

[Mike JosephParticipant @mikejoseph75242]

Our first day on our sparky course we were warned aginst using a neon screwdriver. An apprentice had dropped his gaffer's neon screwdriver into a puddle so that when used, there was a direct short across the neon and it killed the sparky.

A proper tester is the only way., I use a Dilog Combivolt.

You might make a mistake but electricity never does.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Mike Joseph on 05/02/2019 19:20:43:

Our first day on our sparky course we were warned aginst using a neon screwdriver. An apprentice had dropped his gaffer's neon screwdriver into a puddle so that when used, there was a direct short across the neon and it killed the sparky.

.

That puts the silly prank that I saw into perspective !!

TV Rental Company, in the early 1970s

Favourite trick amongst the 'engineers' was to replace someone's neon insert with a similar-looking fuse.

MichaelG.

[SamsarandaParticipant @samsaranda]

In a previous life I was an NDT technician in the Air Force and one of our tasks was to X-Ray any fuel couplings that had been broken down and rebuilt on Lightning Aircraft, not the current American Lightning now coming into service but the English model, the proper one. The reason for x-ray inspection was to check that it was correctly aligned internally, misalignment had been found to be the cause of a number of fuel leaks and subsequent engine fires resulting in the loss of aircraft and sadly some pilots. Once the coupling was ready for x-ray we would set up the portable x-ray equipment with the head aligned with the centre of the fuel coupling, as a check that everything was aligned correctly for the shot we used a metal retracting tape measure that could be held against the window on the tube and the blade used as a sighting line to the fuel coupling. One time when doing this, something we did hundreds of times, I ran out the metal blade and when the tip was about two inches away from the fuel coupling a blue flame with a loud crack jumped from the tape measure to the fuel coupling, It was a frightening experience, I hadn’t had it happen before and I felt lucky that I didn’t experience any electrical shock. Much discussion ensued as to what the reason was, the aircraft was jacked up in the hangar and had its own earth connected to a known true earth point independent of the hangar electrical systems. The portable x-ray equipment was powered from the normal hangar electrical systems, the equipment was capable of delivering 180 Kv albeit at only milliamperes, although when the spark jumped only the cooling circuits in the head were energised, the equipment was not generating x-rays, it was prior to the shot. Speculation settled on the probability of either an incomplete earth circuit in the equipment or the possibility of differential earths I.e. the aircraft and hangar circuits being connected to two divergent earths. There had been talk of deaths in the service from parallel earth problems concerning aircraft, how true this was I don’t know. Whatever it was that happened it was enough to drive a hefty spark, could it have been due to a build up of static discharging or something much more dangerous?

Dave W

[Tim Taylor 2Participant @timtaylor2]

Posted by Mike Poole on 04/02/2019 19:02:47:

I suppose in most domestic situations the prospective fault current is going to be fairly low unless you live next to the substation so shorting the supply might not produce too big a bang. In a factory you are going to get a much more impressive bang and rather than save your life you might end it. Isolating and testing should be done properly and with proper equipment. If the proper process is followed then you should avoid having the “accident” if there is such a thing.

Mike

Mike,

Distance from the substation is not a factor – I have seen downed 12.KV distribution lines lay on the ground and continue to arc for quite a while( and cutting a trench in the asphalt street in the process) until utility crews arrived to disconnect them

The fault current is based on the full load current of the power transformer and it's %Z (impedance). All transformers have impedance, and it's generally expressed as a voltage percentage. This is the percentage of normal rated primary voltage that must be applied to the transformer to cause full-load rated current to flow in the short-circuited secondary. For instance, if a 480V/120V transformer has an impedance of 5%, this means that 5% of 480V, or 24V, applied to its primary will cause rated load current flow in its secondary. If 5% of primary voltage will cause such current, then 100% of primary voltage will cause 20 times (100 divided by 5) full-load-rated secondary current to flow through a solid short circuit on its secondary terminals. Obviously, then, the lower the impedance of a transformer of a given kVA rating, the higher the amount of short-circuit current it can deliver.

Certainly the current rating of a transformer feeding a group of residences is less than would be found in an industrial environment, but it is still large enough to be a very significant hazard.

Tim

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