Live Chassis Safety

[David Simpson]

For trained & skilled radio/electronics folk, its easy to be blaze about attitudes towards ones own testing procedures. However, I reckon its best to encourage newcomers to the vintage fraternity to take careful steps when starting to work on a mains supplied chassis etc.
In recent times, I've encouraged folk to seek mentoring from others in their locality. Or, vica versa, offer mentoring to folk who are struggling a bit. I've been told that there are now many many YouTube instructional topics on the go.
Us old uns can bang on from dawn to dusk about the pro's & con's of isolation transformers, variacs, & so on, but I reckon its wise to tell folk that the safety aspect of old time radio construction often left a lot to be desired.
If someone is given their grannie's old pre-war radio, which has lain in a damp loft for decades, its best that they assume that it may well be jolly unsafe, and may well have been got-at by some un-trained pillock.
No vintage radio these days should be powered up from the mains without a correctly fused modern 3 pin plug, using modern 3 core flex, & definitely not with a suspect possibility of a live chassis. Prior to powering-up it should be thoroughly examined & tested. This is the message I would recommend to any new entrant into the vintage radio fraternity.
If in doubt "keep it out" - - your finger. If in doubt "shout" - - seek help.

Regards, David

[AC/HL]

At the risk of ending up with a committee, with all it's attendant overtones, live chassis working (in our sphere of interest) is a throwback to the time of mixed AC/DC supplies, carried on unnecessarily for many years in the name of Scrooge. Goods were never intended to be tampered with by inexperienced people, the warnings and safeguards against that being similarly parsimonious. Fast forward to "vinyl" [SIC] and the university of YouTube we are now where we are. None of our members should have any illusions about the dangers and challenges posed by these outdated technologies, with or without any indication. That's not to say that it wouldn't be a good addition if it could be reliably implemented though. Best keep a neon handy just in case.

Edit: crossed with David.

[dave walsh]

Agreed re the previous comments. It's been pointed out that my old standby of a light bulb between chassis and earth might simply trip the board these days but a neon is always handy. Only just remembered that I've got an 870A [picked up at a Newhaven Rally]. I've even had it working

Dave W

[McMurdo]

Alot of bang-up-to-date 2-wire stuff is in metal cases with high voltages only millimetres away, eg DVD player with switchmode power supply board hovering on nylon spacers. Is there any reason that the eddystone radio chassis being live is likely to touch the case? If the insulation is in good order ie not relying on perished rubber bushes or wafer-thin sheets of card, then I suppose your real-world choices of protection are limited. Use an RCD adapter, or mark the correct orientation of the plug with a dot of paint, or install an earthed socket where it's going to be used. Or a portable isolation transformer as already suggested. Some american TV sets using mains voltage doublers were in fully metallic cases, the live chassis being supported on nylon spacers. And the yanks have a reputation of being extremely risk-averse where products are concerned!

[JohnBG8JMB]

"No vintage radio these days should be powered up from the mains without a correctly fused modern 3 pin plug, using modern 3 core flex, & definitely not with a suspect possibility of a live chassis"

It's all very well to pontificate but some of us live in the real world and not necessarily in the UK, If the UK 3 pin and ring mains are so good, how come the world does not use them? How come the IET keep upgrading THE WIRING REGS? If you look deep enough into the origins , you'll find saving copper at the bottom. 13 A plug is a monstrous thing physically and can be and is frequently incorrectly fused... I think not fit for purpose today; compare the size of 13A 6way dis board with a EU 2pin one, or even a EU 3 pin one. When I explained the system to a French sparky his comment translated to 'what were they on?'. If you want a hazard, consider a light socket - or PME.

It's sad how many have jumped on their h&s [ticklist?] bandwagons and ignored the original question.

rant over!

Take care all.

John

[pip5678]

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.

My DAC90A has a double pole switch - are there some that don't then or am I misunderstanding? Thanks

[JohnBG8JMB]

And the yanks have a reputation of being extremely risk-averse where products are concerned!

Not the wiring I saw in my hotel in NY city; the mirror over the handbasin had a Chrome plated metal frame with a light and metal toggle switch...

73
John G8JMB

[McMurdo]

To be honest John we're all here to help and your answers appear somewhat combative. A uk wiring system would ensure the chassis is not live, and you say the french system cannot, and yet you've just suggested the UK system is not fit for purpose! I give up!

PS we are all friends here

[bluepilot]

Originally Posted by McMurdo

A uk wiring system would ensure the chassis is not live, and you say the french system cannot,

French sockets are polarized.

[AC/HL]

The thread title (and line one of the first post) are about safety, so it's bound to feature strongly. The reason for the question only became apparent in post 10. The requirement seems to be impossible. Lucien's suggestion is the nearest so far, but could it be implemented reliably enough so as not to be potentially misleading and therefore unsafe. That's probably why it hasn't already been done.

[ekjdm14]

Originally Posted by JohnBG8JMB

"
It's sad how many have jumped on their h&s [ticklist?] bandwagons and ignored the original question.

I think the reason behind folk "jumping on the bandwagon" here is because, on an open forum where anyone can read it, it would be rather lax to suggest there's any way of making a non-polarised 2-pin plug & a live-chassis radio "safe".

For what it's worth, if the radio is in a good state of repair (including replacement of any perished rubber bushes & insulating card etc with modern materials) then I can't see it being any more or less dangerous regarding shock hazard than the before-quoted DVD player with it's SMPS and metal case. Or if safety is such a priority, then perhaps your house wiring ought to be updated to include polarised sockets in order that your radio never gets connected the "wrong" way.

[Lucien Nunes]

French sockets are polarized.

Plug vs. socket, yes, but historically there was no guarantee of which pole of the supply was connected to which contact in the socket. 2-pin sockets without earth, which do still exist widely around Europe and can still be installed in some places, lack not only the means of polarisation but also the availability of an earth as a reference from which to detect the polarity. Hence this thread.

Discussions about the relative merits of different countries' wiring systems rarely produce useful conclusions because many contributors only know one of them in any detail and even that sometimes not in enough detail. From experience designing equipment and installations to meet different electrical standards around the world, I would say there is more to this than meets the eye and it's not worth grappling with here. No one system is perfect.

The root of this question seems to me to be quite pragmatic. Whilst a live-chassis set should be constructed well enough that it is sufficiently safe regardless of polarity, in reality if it encounters some kind of incident that exposes the user to the risk of shock by direct contact with its innards, that risk might be lower if the chassis is connected to neutral rather than line. Therefore, as one of a suite of risk-control measures, a means of indicating whether it is indeed connected that way, without reliance on polarised plugs or protective conductors, might be desirable. Such an indicator does not remove the need to ensure that the insulation of the set is sound, just as the fitting of airbags in a car does not remove the need to have reliable brakes.

I can't see it being any more or less dangerous regarding shock hazard than the before-quoted DVD player with it's SMPS and metal case.

Except that the DVD player will have been manufactured and tested to the required standards for Class II equipment which are accepted today as affording sufficient protection against shock. The standards to which a live-chassis set's insulation was made may not have been as stringent regarding such aspects as creepage and clearance, dielectric strength, vibration resistance, heat resistance etc.

Anyway, I would expect that a Voltstick innards, which I think is little more than a FET input and perhaps one more stage to drive the LED, would work if the sense electrode were suitably arranged. Perhaps the user could wave their hand over the specified part of the case while watching the indicator, to provide a higher capacitance to earth. If repeated with the plug inserted in both orientations, only one of which lights the indicator, then the operation of the indicator is confirmed as well as the preferred polarity with which to operate the radio. The circuit could be powered across a 3V zener in the set's negative end, instead of the two AAA cells.

[David Simpson]

John G8JMB's original post was about safety. These days, safety must be paramount & take priority over the aesthetic appearance of an old AC/DC sets with a cloth covered twin vir rubber flex.
Before the war & for a decade or so afterwards, there was a hotch-potch of local power supplies throughout the UK, which the regional electricity boards inherited. Ever since the 1940's the ESA & the IEE Regs have endeavoured to protect industry workers & customers. Now in the 21st century we have PAT testing.
Many of the vintage radios that we're involved with come from those far off un-regulated days. PSCC, the 30mS(belt)requirement & fast acting MCB's are now the norm when considering electrical safety. Hence good earthing & bonding methods being required under IET Regs.
Jesus - cheapo DMM's, Insulation Testers, PAT Testers, RCD's & RCD Testers, & so on, are so easily available these days. Therefore there is no reason why any vintage radio enthusiast, be he a novice or an old hand, should not make every effort to ensure that his house supply is up to scratch, his chosen work area is safe, & the set he's working on is safe. I say bollox to anyone who insists that an AC/DC set, or winky-wanky un-earthed set, or whatever, must remain in the un-safe state it was sold in all those years ago.

Regards, David

[Station X]

Originally Posted by David Simpson

I say bollox to anyone who insists that an AC/DC set, or winky-wanky un-earthed set, or whatever, must remain in the un-safe state it was sold in all those years ago.

Regards, David

How do you make an AC/DC set safe then without major modifications? You obviously can't just fit a three core mains lead.

Should I be binning my Bush DAC90A and Ekco AD65?

[HamishBoxer]

Just use a small mains isolation transformer or not quite as good ,wire the mains lead direct so the chassis is always neutral.

The bin is available here if you are chucking out the AD65/AC90A!!

[Station X]

My DAC90A is permanently wired mains neutral to chassis. The AD65 still has a two pin connector, but it's marked for correct orientation.

No isolation transformer. All mains sockets served from RCD's.

[ionburn]

Perhaps a side issue here but relevant as it can have fatal results with the equipment we are discussing but on a good number of occasions whilst at work testing equipment in domestic homes I came across incorrectly wired mains sockets. The test equipment automatically tested the supply. Adding to that incorrectly wired extension blocks and I feel that on all occasions good isolation or fully checking would be prudent.

[Craig Sawyers]

That is the reason that I have a Martindale mains checker. Mine is an EZ150, now superceded by (it looks like) the EZ165.

I check extension blocks with that too, along with a periodic check around the wall sockets.

[dave walsh]

The thread is a bit of a runaway horse but that's not usually a problem and can often result in an interesting discussion. As Mcmurdo said we are all here to help [post 28] but that's not always so easy. John thinks we are missing his point but post ten is a bit late to begin to find out what the problem is. As "a bear of little brain" I'm still not very sure Some Diplomacy always helps I find!
I wouldn't have minded an answer to some of MY questions

Dave W

[lesmw0sec]

Reminds me of my very first job as an apprentice. I was advised to check if a chassis was live by applying the back of my hand to see if there was a buzzing feeling. Imagine the hysteria which would break loose with such advice today!