Was this safety feature any use?

[Bill]

Inspired by the live chassis safety thread:
When I started work at a big electronics company in 1969, I noticed that all the in-house built test gear, power supplies etc (and there was a lot) had two neon lamps on the front panel. One indicated the unit was switched on, as one might expect. The other lamp was wired between the live incomer and earth, before the on/off switch. it was therefore normally lit permanently as long as the unit was plugged in and some sort of earth continuity intact.
Some older parts of the factory still used unfused 5A round pin mains plugs.
Equipment that was getting on a bit had the neon very dim as it had become blackened with age, perhaps after 20 years or so.

Some gear was used by unskilled (in electronics) people , who as far as I'm aware did not know the significance of that "earth" neon. Maintenance staff,
would know and be able to see , in passing , if the unit was "safe".
So, was it a good idea or useless? i don't remember anyone receiving a dangerous shock, though there was plenty of reports of them, but always caused by static build up on nylon overalls etc!

[TonyDuell]

My worry would be that if the mains earth connection was not intact, the thing acts like a giant neon screwdriver. Touching the case could pass enough current through you (to earth) to light the neon. Whether that is dangerous depends on the neon's series resistor I guess.

Incidentally some versions of the Keynector test connector thing have the neon wired between live and earth. Mine does (and it's never been modified).

[Herald1360]

I would say that the neons wouldn't make the equipment inherently any less safe and would provide some (slight?) benefit so on balance a reasonable thing to do.....

[G8UWM-MildMartin]

It was a feature of Bridges Neonic electric drills. I wonder whether they had a patent.

[Brigham]

Neon indicators are often seen connected between line and earth, especially in 3-phase work, where a discrete neutral isn't provided.
I had a non-earthed bathroom light/heat lamp with a metal case, years ago. There was no simple way to get an earth to it, so I altered the neon to work between neutral and the metal case.
If the casing became live, the neon would glow, giving a warning not to touch it.
Same principle as the 'trolley standard live' indicator on an open-top double-deck tramcar.

[G6Tanuki]

In practice, a 'neon' between Live and Earth will probably light through general earth-leakage even if the Real Earth has gone-missing/and/or the effects of the 'half-supply' hovering centre-tap provided by the obligatory modern RFI X/Y capacitors.

I certainly wouldn't consider such a neon-arrangement to be a compliant-with-modern-safety-standards thing.

Comply with current safety-standards. They've evolved a lot since your 1969 workshop-experience.

[Heatercathodeshort]

This rings bells. Didn't the Viewmaster kit television have a similar arrangement to warn you if you had the chassis connected to the live side of the mains? John.

[Lucien Nunes]

a 'neon' between Live and Earth will probably light through general earth-leakage even if the Real Earth has gone-missing

This is the heart of the matter. To serve its purpose, an earth connection needs to be no higher than 1666Ω to trip an RCD without dangerous voltage appearing on the case (50V / 30mA), but needs to be as low as a few ohms to rapidly blow a plug fuse if there's no RCD. Therefore an 'Earth OK' indicator should only light when the loop impedance is of this order, if it is intended to show that the earthing is fit for purpose. The neon will however light with some hundreds of kilohms in series, which is a false positive indication of safety. The snag is that passing a heavy enough current from line to earth to give a more satisfactory indication would result in dangerous touch currents if the earth turned out to be O/C.

A similar problem applies to the basic (and older) socket testers with multiple neons. They will identify a completely broken connection in the socket as a missing earth, but will give a false 'correct' indication when any kind of flimsy earth is present that is of no use clearing a fault. It's a known issue and the modern, fancy ones actually contain a diminutive injection type loop impedance measuring circuit, to give a more useful and reliable indication of earth connection integrity without making anything live to the touch.

[Red to black]

And none of these indicators can show an Earth Neutral reversal either!

[dave walsh]

Some of this info, just about, goes a long way towards helping me understand the duel neon demand in that live chassis a thread.. I think
Thanks all

Dave W

[Bill]

I completely take the point that this set up doesn't comply with modern safety standards,Lucien has explained that very well,but it would be interesting to know where the original idea came from? Was it a recommendation in the 1950/60s wiring regs, something from IEE or some Company policy thought up by the electrical safety officer, if they even had one in those days?This was a large organization with many thousands of employees, sadly no more (the Company that is!)
Bill

[Lucien Nunes]

The origin of the practice is an interesting question. I am not aware of any authoritative recommendation to fit such indicators but have not specifically looked for one. They were not uncommon on entertainment lighting equipment too, where temporary installations and high leakage currents often coincide.

Thinking about the factory scenario, the earthing of the installation was likely to be reliable, with metal conduit, trunking and MICC being widespread and the main earth substantial and of low impedance. Therefore, the indicators may have been intended mainly to confirm continuity through the cordset and plug, where a clearer distinction would tend to exist between functional and open-circuit. In a world before widespread use of SMPSUs with their attendant filtering and capacitive leakage, they might have worked tolerably well for this purpose, other safety concerns aside.

[McMurdo]

About 20 years ago we had to collect all the Raford Labpacks from a college and alter the live-earth neon so it went from live-neutral instead, amongst other things.
Enclosed with the labpacks was a photocopied article from one the teaching magazines criticising the labpack's safety and we were asked to 'action' the remedies it suggested.

The boss once built a test rig using a neon to test when a push button was pressed at the end of a 3m cable. It was found that the neon lit even when the button was open circuit, we assumed due to the capacitance of the cable run.

[G6Tanuki]

Originally Posted by McMurdo

The boss once built a test rig using a neon to test when a push button was pressed at the end of a 3m cable. It was found that the neon lit even when the button was open circuit, we assumed due to the capacitance of the cable run.

Yes, neons will light with just the suggestion of voltage, or through a fre picofarads of capacitance-to-earth - which makes them totally inappropriate devices to indicate security of any earthing arrangement.

Even the currently-popular 'voltsticks' can't be trusted - walking under a 400Kv overhead line in damp weather can be enough for them to start chirping their false-alerts.

[McMurdo]

Even the currently-popular 'voltsticks' can't be trusted'

Well....IMHO they're more likely to give a false positive than a false negative; which is safer!
In fact I used one today to work on a machine where the regular electrician on site had been furloughed. The isolator said 'off' and the machine looked dead, but all the same I touched my stick all over.
My stick can be tested by clicking your fingers at the end or stroking it on a shirt sleeve.

I always wondered how the neonic drills actually worked. (mainly through seeing adverts in Practical Householder). Surely if you lost an earth then picked the drill up, it would just light up again? Or did it work differently?

[Bookman]

From an overall viewpoint lets us also not forget similar circuitry that was used on D.C. networks. Typically there would have been four lamps connected in series across +’ve and –‘ve and the central point earthed.
It follows does it not that if an earth occurred on either +’ve or –‘ve line then the corresponding two lamps associated with that pole would be shorted. Ergo only two lamps would be lit.
This application was typically helpful for systems where transient faults could occur so that some form of prevention could be sought.

[ex 2 Base]

Bookman re your contribution above. DC networks bring back some memories. Where I once worked we had three wire DC, 460 volts between Pos and Neg for motors and 230 from either Pos to Neutral or Neg to Neutral for lighting. We used the same light arrangement for locating earth faults. Later, I worked for BR servicing DMU with an all insulated 24 volt DC arrangement and used two lamp arrangement to locate earth faults. I had forgotten all about it until reading your contribution. Happy days .Modern electricians wouldn't know about such things. Ted

[Bookman]

Originally Posted by ex 2 Base

Bookman re your contribution above. DC networks bring back some memories. Where I once worked we had three wire DC, 460 volts between Pos and Neg for motors and 230 from either Pos to Neutral or Neg to Neutral for lighting. We used the same light arrangement for locating earth faults. Later, I worked for BR servicing DMU with an all insulated 24 volt DC arrangement and used two lamp arrangement to locate earth faults. I had forgotten all about it until reading your contribution. Happy days .Modern electricians wouldn't know about such things. Ted

Likewise your comment in respect of DC supplies for lighting circuits. In this I am referring to the use of gas discharge on such circuits but that would really send the topic off course!