Live Chassis Safety

[JohnBG8JMB]

Live Chassis Safety
Problem live chassis acdc radio with 2 wire reversible mains lead [2 pin Euro Plug].

Requirement: means of indicating when the live side of the supply is connected to chassis.

Don't say a Neon screwdriver! I need a permanent indicator lamp that lights when the chassis is live, and ideally another that lights when it is not.

There is no physical earth connection available

Take care all
John G8JMB

[Keith956]

Without more details I'd say use an isolation transformer, you're trying to solve the wrong problem.

[TonyDuell]

I don't see how the original problem could be solved. The definition of 'live' is really 'at a high voltage with respect to local earth'. If you are well-insulated from earth then surely no current will flow (and you will not get a shock) if you touch a live chassis (DO NOT try it!).

So if there is no physical earth connection available, I don't see how we can determine whether a chassis is at a high voltage with respect to earth. Voltmeters have 2 leads.

[rontech]

I would have thought that if you had mains power available in the first place, there would be an earth pin available on the wall socket. If you are running it from a lighting circuit then it must be ancient wiring.

[paulsherwin]

Why are you running a live chassis set from a Europlug?

[Lucien Nunes]

One solution I can see is to sense the voltage of true earth using the stray capacitance of an electrode. A plate mounted inside the case in such a way as to minimise its influence on the performance of the radio (and capacitance to the chassis), but presenting a large area to the outside world, would probably pass enough current to trigger a simple high-impedance voltage detector such as used in a Voltstick or similar. The sense electrode would need to be at (or very close to) chassis potential to prevent the radio itself triggering the indicator.

I agree however that this appears at first glance to be solving the wrong problem, however if the intention is (for example) for a non-technical user to operate the set in a country where non-earthed sockets and unknown polarity are the norm, then it could be a useful approach.

[dave walsh]

Are you wanting to run an AC/DC set without an insulated case John?
Why would you want to know when it's NOT live as well
I can't see how that easily works or is needed although Lucien probaly has a clever solution. Sledgehammer? Nut?

Just make sure ALL the metalwork is totally insulated from the exterior.
Establish LIVE on the power socket.
Mark that red and the corresponding side of the plug that has the wire NOT going direct to chassis.
If the reds line up then chassis is connected to negative/neutral and not live.
By definition an AC/DC set doesn't utilise an earth so it's absence seems irrelevant.

Perhaps I've missed something really obvious here-it's not impossible

Dave W

[eddie_ce]

Living in Germany where we have non polarised sockets, I have adopted the following solution.

I have a few isolation transformers, some 230V others 110V so where possible I can run sets on 110V to reduce dropper dissipation. To avoid confusion and fireworks these are wired with a BS546 2 Amp UK plug and socket individually protected by a cartridge fuse.

All distribution sockets connected to the mains have the same polarity, live on the pin on the right hand side. A small neon is connected between the neutral pin and earth so that this lights up should polarity be reversed. The current drawn by the neon is very low, thus avoiding nuisance tripping of the dedicated 10mA RCD.

Although the UK ring final circuit system is not universally loved, the BS1363 plug is in my humble opinion a great invention.

Just my 2 euro cents worth.

Both line and neural are considered "live", I wouldn't bet my safety on the neutral being near earth. With a properly made/maintained live chassis set it will be safe in both conditions.

[JohnBG8JMB]

Why are you running a live chassis set from a Europlug?
Paul
Eddystone 870A [live chassis insulated from metal CASE] in France - 2 -pin sockets and/or arbitrary LN connection. To several others- please don't try to redefine the problem.

Not essential, but nice to know if it's the right way round without getting a neon screwdriver out. That takes me back to 1956; first mains set I built - it had a live chassis [could not afford an HT trans, only the heater one] and I remember marking up the 2 pin 5A skt in my bedroom and the plug on the radio with polarity...I'm still here.

Lucien's idea sounds promising -for the 870A, perhaps could implement it as a neon from chassis to case

Now have I got any neons in deep storage?

73
John G8JMB

please don't try to redefine the problem.

There is no problem with a properly built/maintained "live chassis" set. The only thing I can see (or hear) is less hum in one way round, my DAC90A prefers live to chassis for less hum (hardly noticeable but I measured it after restoring the set). I didn't know about it until just now when I buzzed out the mains to chassis plug pins out of curiosity.

Thinking about it, the DAC90A switches the chassis end, so if it was wired "neutral to chassis" it would be "live to chassis" when off more "dangerous" when off than on.

[m0cemdave]

Originally Posted by merlinmaxwell

Thinking about it, the DAC90A switches the chassis end, so if it was wired "neutral to chassis" it would be "live to chassis" when off more "dangerous" when off than on.

I've seen that before, in an old National NC46 (1946 vintage "entry level" American SWL receiver).
Like the Eddystone referred to above there was a live chassis, completely isolated from the (normally earthed) metal enclosure which had a flip-up lid with two retaining screws.
The single pole mains switch was between supply and chassis. If the lid was up the chassis could be touched, but the off switch ensured it was no longer connected directly to the incoming incoming mains. The wiring associated with the other (unswitched) side of the mains was all under the chassis and couldn't be touched, and if it were on the "live" line the intervening circuitry would limit the current available via the chassis. Bear in mind that this was a 110V mains set, not 230V.
Made sense to me, although a 2-pole switch would have been preferable.

[stuarth]

I have an Eddystone 870 as a “conservatory radio”. I believe it is intended to be used with the case earthed. I would be uneasy using it with an unearthed case if I could not be certain that the chassis was neutral, not live, so I can see where the op is coming from. It’s not the same as a set such as the DAC 90 with a non conducting case.

My 870 is used with a UK plug, so the case is earthed and the chassis connected to neutral.

The neon (plus associated series resistor) should work, it’s not unlike a built in neon screwdriver. The neon might light through capacitance of the case to ground, but you may have to touch the case to improve “grounding”.

Will you be the person who checks the neon every time it gets plugged in?

Stuart

Mad thought, there is always a weeny bit of DC offset on the mains, build two detectors, one plugged into an earthed socket (your reference) and another in the set.

[David Simpson]

"Search" will reveal a helluva amount of thoughts & advice on "Live Chassis". Then more on safety & house earthing systems such as PME & TNC-S & so on.
In my book, there is no place in the 21st century for a mains radio chassis to be "live", or be fed by a 2 core flex, whether it be a vintage AC/DC set or a modern set. If folk are too un-skilled to be able to determine mains polarity, then they should stay well away from anything being connected to the house mains.
To those folk who think they know a bit about electrics, I say - if you power up something which obviously would fail a PAT Test in your house, & it's accessible to family members & visitors, then you are being irresponsible. Likewise "doing-up" something then flogging it on eBay, or selling to an individual.
For testing purposes, skilled vintage radio folk will obviously use isolation transformers, variacs, multimeters, meggers, etc. to keep themselves & family members safe.

Regards, David

For testing purposes, skilled vintage radio folk will obviously use isolation transformers

No, I don't, I use my brain to make it safe.

[Station X]

I just do a simple resistance check.

Let's not forget that working on something isn't the same as using something. With a chassis on the bench and connected to the mains I can touch any number of live points if I'm daft enough to do so. I can't do that if the chassis is in the cabinet with the back fitted.

I too would point out that a red light has never stopped anything.

I too would point out that a red light has never stopped anything.

Brilliant! (pun wasn't intended, but I like it). Light is used to slow down atoms, not stop them though.

[ms660]

Was in the R&TV servicing trade long enough to realize the benefits of using an isolating transformer when working on live chassis stuff.

Lawrence.