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Old 18th Feb 2019, 10:45 pm   #1
The Philpott
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Default Is a Jacob's Ladder likely to....

Is a Jacob's Ladder based on a 10kV Neon sign transformer (output limited to 50mA) likely to cause nuisance tripping on a consumer unit? The consumer unit and trips date to around 1995.

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Old 18th Feb 2019, 11:07 pm   #2
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Default Re: Is a Jacob's Ladder likely to....

The common type of neon sign transformer has a metal case with two ceramic insulators. The metal case is at mains earth in the sign body and often, the secondary winding is center tapped and connected to the transformer body too. So, with respect to ground, for a 10kV unit you have 5kV on each of the output terminals. But of course, any energy generated by the secondary windings, is by magnetic induction. The secondary is isolated from phase & neutral of the primary. So, any currents drawn from the secondary, between the two output terminals, or from one output terminal and earth, cannot create an unbalanced current in the primary circuit. Therefore, the transformer defeats the breaker box RCD (which all mains transformers do except auto-transformers). In terms of getting a fatal electric shock, from the secondary winding, which is possible with these transformers, the RCD provides no protection at all and the transformer should not trip the RCD whether the current flows from one terminal to ground (through you) or between the two output terminals say in a Jacobs ladder.
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Old 18th Feb 2019, 11:19 pm   #3
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Default Re: Is a Jacob's Ladder likely to....

That's what i had concluded, thanks for the thorough reply. I have great respect for the hazard posed by the potential output of such a device.

Dave
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Old 18th Feb 2019, 11:40 pm   #4
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Default Re: Is a Jacob's Ladder likely to....

I could have added that even if it is a type where it is encased in plastic and the secondary does not have a center tap, with the high voltage and the capacitance between the primary and secondary, some current could still flow from one output terminal to ground, and in that case, it is possible it might trip the RCD, but I have never tried it or had a transformer like this. I do not know if any of the plastic encapsulated case ones are configured as autotransformers, in that case the RCD function would be preserved, but nuisance tripping might be likely.

I got my first neon sign transformer from the dump when I was about 13 years old, it was a 1930's vintage Jefferson unit. I used auto ignition cable and would hold the wires in both hands and draw out long sparks, make Jacob's ladders with coat hanger wire etc. A large number of shocks of course, including one where a very long ladder wire swung across and hit the side of my head, of course the sparked jumped before it actually touched. The discomfort from those episodes paled into insignificance after I had a shock from a 400V DC supply that I was accidentally connected to for some seconds when I was about 15. I'm much more cautious these days.
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Old 19th Feb 2019, 12:02 am   #5
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Default Re: Is a Jacob's Ladder likely to....

In relation to shocks from DC supplies, many years ago one of the students in my school got accidentally electrocuted after a tropical storm felled a DC power line. He was trying to cross the fallen lines to help clear a building damaged by the storm. Initially it had no supply but suddenly the power came back. According to those who saw him, the DC trapped him like a magnet to the wires till he was dead. To this day I cannot forget how terrible that incident was. Apologies for narrating such a gruesome incident, but it just goes to show how careful we all should be when dealing with mains voltage.
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Old 19th Feb 2019, 12:10 am   #6
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Default Re: Is a Jacob's Ladder likely to....

Quote:
Originally Posted by The Philpott View Post
Is a Jacob's Ladder based on a 10kV Neon sign transformer (output limited to 50mA) likely to cause nuisance tripping on a consumer unit? The consumer unit and trips date to around 1995.
Hello Dave, I can't see any reason why it should do so. I had an NST about 15 years ago and had no issues with this. Not to derail this into a safety thread, but they are particularly deadly as they don't buzz audibly when live.

You may wish to build a simple timer/ relay to activate it and run it; this will protect the secondary a little from overheating, rather than you. Jacob's ladders are obvs a dead short and the transformer will heat up quickly.
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Old 19th Feb 2019, 9:21 pm   #7
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Default Re: Is a Jacob's Ladder likely to....

My only foray up Jacob's ladder involved an old car ignition coil, 24V dc supply for added grunt and a stupidly large surplus to work's requirement IGBT driven by a Wavetek function generator to drive the coil primary.

It would go up to 2-3" wide at the top of the bits of stainless steel welding rod used for the electrodes.
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Old 20th Feb 2019, 12:21 am   #8
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Default Re: Is a Jacob's Ladder likely to....

20 years ago one of the technicians at the company I was working at brought in an induction coil and fitted it with a Jacobs ladder. He demonstrated it in the lab impressing all who saw it with its rising spark. However it took out a PC in the adjacent room so any further demos were in our EMC chamber.

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Old 21st Feb 2019, 4:15 pm   #9
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Default Re: Is a Jacob's Ladder likely to....

If driving a Jacobs ladder from a neon sign transformer, it might be worth use of a lamp connected in series with mains supply.
So as to limit the mains current and avoid overheating the transformer when the output is nearly short circuited by the arc.

When the output is open circuit, the mains current will be low, and the voltage across the series lamp very low, and thus almost the full output voltage available to strike the spark.

Same principle as a lamp limiter used when testing a vintage radio, but you will probably need a larger lamp or a couple of lamps.

An electronic neon sign transformer may work better, these have very high frequency output which tends to ionize the sir and break down the air gap more readily.
The high frequency is a bit less dangerous than the line frequency from a conventional transformer, but is still potentialy lethal and GREAT CARE should be taken.
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Old 21st Feb 2019, 5:23 pm   #10
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Default Re: Is a Jacob's Ladder likely to....

Thanks for your comments, if a lamp limiter was viable it would also serve as a 'danger' indicator, with failure of the filament not being an issue- since this would de-energise the transformer..

I am conscious of over-running the transformer, I have powered this (quite simple) circuit up and having copied the dimensions of one made by a special effects guru (Clive!) it strikes reliably. I dare not run it for more than a minute as it stands, Al, as I have no way of telling how hot the winding deep inside is getting- A timer would certainly be a good idea. The initial strike gap is halved by a vestigial electrode in the centre that creates ionization. Centre electrode is fed from one of the main terminals via two 5meg 1w HV resistors (which have not yet overheated!)

I can confirm the consumer unit trips did not open during a 60 second test.
I will monitor the current through the primary next time to see how much I am stressing the secondary. (An Avo8 needle might not respond quickly enough- sounds like a job for an Avo 7 or 40.)

Dave
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Old 21st Feb 2019, 7:36 pm   #11
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Default Re: Is a Jacob's Ladder likely to....

A Dutch forum always displays a microwave transformer based Jacobs ladder on its stand on an annual electronics fair. They've put it inside a perspex case, since touching it could be deadly. There's a large button in front of the case that runs it for maybe 30 seconds upon pushing. The transformer seems to hold up well, but I wouldn't permanently energise it, even if the microwave transformer limits the current inherently just as a neon sign transformer does.

Since there's inherent current limiting, it might be sufficient to monitor the core temperature as that is probably the first to heat up.
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Old 21st Feb 2019, 7:39 pm   #12
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Default Re: Is a Jacob's Ladder likely to....

I made a Jacobs ladder years ago using an oil fired boiler ignition transformer, big fat spark and lots of noise.
 
Old 21st Feb 2019, 8:06 pm   #13
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Default Re: Is a Jacob's Ladder likely to....

The FART Neon sign transformer (no, really) is encased in a big resin block so assessing the temperature inside is just an educated guess. I will be interested to see how the current fluctuates on the 240v side.
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Old 25th Feb 2019, 10:19 am   #14
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Default Re: Is a Jacob's Ladder likely to....

Quote:
Originally Posted by The Philpott View Post
I dare not run it for more than a minute as it stands, Al, as I have no way of telling how hot the winding deep inside is getting-
Try measuring the winding resistance when cold and when hot (i.e. immediately after switching off). Assuming the windings are made from copper, a 20% rise in resistance would imply a roughly 50 degree celsius rise in temperature in the core. More than 20% and you may be pushing the transformer too hard.
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Old 25th Feb 2019, 1:11 pm   #15
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Default Re: Is a Jacob's Ladder likely to....

Now that simple test really was staring me in the face, Thanks David.

Incidentally removing the lid on this transformer cuts the power to the primary as an extra safeguard.

Dave
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