Transformer safety?

[GrimJosef]

Originally Posted by 1100 man

...
We, as repairers, have the responsibility to ensure any items used by other people are as safe as reasonably practical.

That's certainly a position we're free to take, but it's not the only position. What if the other people are as competent as we are ?

A case in point was recently a friend gave me a 1920's art deco heater he had bought."Can you fit a new mains lead"? I declined on the basis that there was minimal guarding to the exposed element and no mains switch. I assessed the shock hazard as being unacceptable. He would have used it without any worries as he, like most untrained people, is not competent to assess the dangers.

A critical component of any risk assessment is the context in which the activity will take place. If the heater was to be used somewhere where there was uncontrolled access, or the people who were allowed in couldn't be trusted to keep far enough away from the heater (e.g. toddlers) then I agree that wouldn't be very safe (although presumably there wasn't a 20% fatality rate when these things were in common use in the 20's). But it wouldn't require much training to keep an adult of sound mind from shocking themselves with this. I guess at that point it becomes a matter of looking at your friend and judging quietly whether he/she could understand and would follow the instruction "Don't touch the element with anything at all when this is plugged in".

Cheers,

GJ

[McMurdo]

The discussions here seem to have ignored both fault and power off conditions.

see post 23

[Herald1360]

Faults are just that, faults, and could happen to any equipment, new or old.

Live chassis issues under fault conditions would not seem to be any more dangerous than a live chassis under normal conditions which would have been perfectly acceptable for an ac/dc or autotransformer fed set in original condition. All the ones I have seen (admittedly not a comprehensive sample) have had clear warnings about not removing back panels or covers unless the set is disconnected from the mains supply.

[1100 man]

Originally Posted by GrimJosef

A critical component of any risk assessment is the context in which the activity will take place. If the heater was to be used somewhere where there was uncontrolled access, or the people who were allowed in couldn't be trusted to keep far enough away from the heater (e.g. toddlers) then I agree that wouldn't be very safe (although presumably there wasn't a 20% fatality rate when these things were in common use in the 20's). But it wouldn't require much training to keep an adult of sound mind from shocking themselves with this. I guess at that point it becomes a matter of looking at your friend and judging quietly whether he/she could understand and would follow the instruction "Don't touch the element with anything at all when this is plugged in".

Cheers,

GJ

The problem is, I can't control the environment in which the heater is used. Yes, my friend could be instructed in safe use, but what about his Girlfriend, the dog, visiting children etc? He could sell it on Ebay to some unsuspecting person. With no mains switch, it could be connected to a single pole mains socket with live & neutral reversed. The heater would not be 'on', but the element would still be live!
If I fitted a mains lead and made the heater functional, I consider, rightly or wrongly, that I am then responsible for it's use by unskilled people.

I assessed the risk as being unacceptable. On the other hand, the same friend
is destined to get the Volksempfanger radio once I have repaired it. With suitable precautions, such as new three core mains lead and earthed chassis, I consider that the risks of shock or fire are acceptable. I will just have to trust that he will not leave it unattended.
So in the end, it's down to our own judgement and what we consider acceptable.

Cheers
Nick

[Refugee]

That electric fire could be fitted with a hard wired RCCD. I am pretty sure I have seen such an item on sale in the past.
Like magic here is the link to such an item.
https://www.screwfix.com/p/masterplug-rcd-plug/44855

[1100 man]

That's an excellent idea. I've used them before for other things and they are not that expensive. I think it also had a 10ma trip current. I hadn't thought of one in this application though.
In fact, it would be a worthwhile addition to any vintage electronics that are to be given to other people, especially live chassis equipment.
I like that idea!

Cheers
Nick

[Refugee]

I would expect a more thorough search to bring up a 10ma one too judging by how easy it was to find that one.

[Synchrodyne]

Whilst Murphy’s exact reasons for choosing an autotransformer power supply for the A362 might remain obscure, evidently it was not to obtain maximum commonality with a corresponding AC-DC model. The A362 autotransformer included a 6.3-volt tapping solely to supply the EABC80 heater. Evidently Murphy saw some benefit (lower hum?) to doing this rather than using say a UABC80 in the series heater chain.

Having opted for the autotransformer with live chassis approach, which precluded protective earthing of the chassis itself, Murphy nonetheless saw fit to provide protective earthing for those parts of the receiver involved in external connections, namely the FM and AM aerials, both via stand-off resistors, and directly, one side of the speaker transformer secondary, from which the external speaker sockets were fed.

This required a three-core power lead. The schematic shows that that the earth wire from this went to the earth socket, which was connected to the other earthed items mentioned in the preceding paragraph. Primarily a protective earth, this would also have served as an AM RF earth in the absence of any other arrangements.

The provision of an earth socket allowed the connection of external AM aerials with two-wire feeders, such as dipoles or antistatic types. It also allowed the connection of a separate RF earth that was perhaps more direct and less noisy than the protective earth pathway. However, so doing would have created a “back stairs” (and probably very low current capacity) earthing pathway from the protective earthing system, which from the electrical side is generally regarded as something to be avoided. But it did not seem to bother the equipment makers back in the 1950s.

Additional to the protective earthing, the autotransformer also had a heat fuse, providing some protection against transformer failure modes that would result in overheating. So overall, Murphy appears to have made some effort on the safety side. In particular, the benefits of providing protective earthing to the “exposed” parts may have been seen as outweighing the possible misperception disbenefit, namely that the presence of a three-core power lead and a transformer could create the impression that this was a mains-isolated receiver with an earthed chassis.

Given that the autotransformer fed a half-wave rectifier, then its winding would have had to carry a DC component, and the latter would also have been fed back to the mains. (Putting DC on AC mains was one of the electric utility objections to AC-DC type equipment.)

It may be noted that Murphy used similar protective earthing arrangements (to those on the A362) for its contemporary AC-DC receivers, such as the U242. Thus these had three-core power leads. And its contemporary AC-only receivers with double-wound power transformers also had three-core power leads with the earth going to the chassis.

Perhaps Murphy had done more than some of the other setmakers in respect of protective measured. But if so, it then appears to have taken a step backwards when it came to its “400” series models. At least the schematic for the A482 AC-only model with double-wound transformer shows it as having a two-core power lead. If that was the case, then the only way of protectively earthing the chassis would have been via the earth socket. And the U472 AC-DC model also had a two-core power lead. The FM aerial was fully isolated by the input RF transformer, the AM aerial was connected to chassis by a standoff resistor, and there was no provision for an external speaker.

By way of contrast, the following, from the operating instructions for the Ekco A274, required suspension of disbelief upon initial reading:

“Connect the mains lead, by means of a suitable plug, to your electricity supply. The red wire should be connected to Line, the black wire to Neutral, and the green wire to Earth. If only a two-pin mains outlet is available, cut back the green wire so that it cannot enter the plug.”

And this was for a receiver that had abundant external interconnection possibilities, each with one side connected to chassis, and which were evidently intended for actual use, not just ornamental.

That Ekco felt that it had to cater for the possibility of power outlets without an earth connection for what was a 1956 model does suggest that the UK may have been rather late in the general adoption of protective earthing for domestic appliances, at least in comparison with “down-under” practice. Here in New Zealand, I don’t recall seeing other than three-pin power outlets, nor any locally-made radio receivers that did not have three-core power leads. Perhaps that was because the idea of protective earthing had a relatively early start. Multiple-earthed neutral (MEN) distribution was a national standard from 1920, and part of this was that each consumer had a local earth, with neutral bonded to that earth (by the MEN-link) in consumer switchboards. I can recall from my childhood early discussions with available/interested adults (back in the 1950s) about electrical matters that the importance of good earthing was frequently stressed.


Cheers,

[ukcol]

Originally Posted by Synchrodyne

Whilst Murphy’s exact reasons for choosing an autotransformer power supply for the A362 might remain obscure, evidently it was not to obtain maximum commonality with a corresponding AC-DC model. The A362 autotransformer included a 6.3-volt tapping solely to supply the EABC80 heater. Evidently Murphy saw some benefit (lower hum?) to doing this rather than using say a UABC80 in the series heater chain.

I started a thread about the strange mix auto-transformer, dropper resistor, 0.1A heater valves and 6.3v 0.3A heater valve (the EABC80) used in the Murphy A372 (and the A362) some time ago here:

https://www.vintage-radio.net/forum/...d.php?t=145497

Its amounts to a list of my and other peoples "guesses" as to the reason(s) but no definitive conclusion was arrived at.

[Sideband]

Originally Posted by 1100 man

Having played with TV's since I was a kid, live chassis is, to me, the normal state if affairs. There are very few TV's with mains transformers.

I feel the same way. I treat a live chassis set as just another set. It holds no fears for me. It must be the way we started and learned in the trade. ALL TV's had live chassis (with one or two rare exceptions that had isolation transformers....I never saw one). When I started in 1970 some of the sets we saw were getting quite old....there were plenty of early 60's sets still giving good service (and some older 50's sets as well). In the 70's, half-mains chassis sets were making an appearance and I thought these were much more dangerous since it didn't matter what way round the plug was, the chassis was still live.....

I started playing about with TV when I was about 14 and was fully aware of the live chassis situation. It was second nature to hold a neon screwdriver to the chassis and reverse the plug if necessary. Note that I have NEVER had a shock from a live-chassis set either TV or radio. They are no more dangerous to work on than an isolated chassis at least not to me. The first company I worked for didn't even have isolation transformers (OK they wouldn't get away with that now). I do use an isolation transformer on my bench at home but that is mainly so that I can connect earthed test equipment (signal generators , scopes etc) to live-chassis TVs and radios. Sometimes one of my restored TV's will develop a fault and I don't always lug the thing up on to the bench to give it an initial inspection so very often my initial checks are on non-isolated mains. I still check that the chassis is at mains neutral though.

Obviously it's up to the individual as to how they approach live-chassis sets. If you don't like them then avoid them but in general, if the chassis is at mains neutral, they are no more dangerous to work on than an isolated chassis.

Note that these are my opinions. Others may (and are entitled to) disagree!

[radiograham]

[QUOTE=GrimJosef;1078642]

Originally Posted by 1100 man

...
We, as repairers, have the responsibility to ensure any items used by other people are as safe as reasonably practical.

That's certainly a position we're free to take, but it's not the only position. What if the other people are as competent as we are ?

A case in point was recently a friend gave me a 1920's art deco heater he had bought."Can you fit a new mains lead"? I declined on the basis that there was minimal guarding to the exposed element and no mains switch. I assessed the shock hazard as being unacceptable. He would have used it without any worries as he, like most untrained people, is not competent to assess the dangers.

I would say in my experience of joe public you have to presume they are not to be trusted,i have never ceased to be amazed at the total ignorance of some people,i knew someone who tried to shave with a electric razor and couldnt understand why after he lathered up why it wouldnt shave him.There wasnt any shock hazard with that one of course but know of many examples where they were lucky not to end up on a slab.

[GrimJosef]

Originally Posted by radiograham

I would say in my experience of joe public you have to presume they are not to be trusted,i have never ceased to be amazed at the total ignorance of some people ...

I guess that's why the standards for retail electrical equipment are what they are. If you're selling a million sandwich makers a year and you have zero information about or control over who might buy them then you have to decide what level of risk is appropriate.

If, on the other hand, you are repairing just one vintage heater for someone you know well enough to call a friend, and if you can tell him how to use it safely as many times as necessary before you hand it back to him, then maybe the rules can be relaxed a smidgen ?

As for the untrustworthiness of the general public, the same logic that says we can't explain to them how to use electrical gear safely would also say that they shouldn't be trusted with a gas hob or with a lawnmower or (oh my lord !) with walking along a pavment with nothing at all separating them from fast moving traffic . I just don't know how there's anyone left alive !

The fact is, most people aren't daft. If they can learn to drive a car then they can learn to use a heater.

Cheers,

GJ

[Sideband]

Originally Posted by radiograham

I would say in my experience of joe public you have to presume they are not to be trusted,i have never ceased to be amazed at the total ignorance of some people,i knew someone who tried to shave with a electric razor and couldnt understand why after he lathered up why it wouldnt shave him.There wasnt any shock hazard with that one of course but know of many examples where they were lucky not to end up on a slab.

You have to be very aware that younger people have never even had to put plugs on leads....how many appliances are sold these days without plugs? There was a recent situation where a person in his twenties had suddenly got the 'retro' bug and bought a 70's style music centre, the type with separate speakers. He spent a while trying to get sound from the speakers and eventually asked on a forum. It transpired that he hadn't even wired the speakers up....he was used to wireless Bluetooth types as used with his computer and thought that all speakers were the same.....it hadn't occurred to him that there was no power lead for the speakers. There's more...having been advised what wire to get for his speakers and told how to connect them he still couldn't get sound.....the reason....he hadn't stripped the insulation off and just put the wire, with insulation under the spring clip.....that was only discovered after he sent in photos of his connections. It hadn't occurred to his advisors that he hadn't stripped the wire....

[Refugee]

That sounds like that story I was told by a TV engineer back in the day.
It was a very posh estate and a woman had called the power company out because the electricity was making a funny noise.
When the power sparky arrived he had a listen and realised that the electricity was indeed making a funny noise but only what was being consumed by the alarm on near by house that had been robbed.

[Skywave]

On the odd occasion when someone has asked me to repair an item of electrical equipment, the first thing I do is to give that item an initial inspection. A written report is then given to the owner of that item. On that report, the following appears:

Electrical Safety: Please note that it may be necessary to make design changes to any hardware that you supply to me in order to render such hardware electrically safe. Any such changes that are made will be fully documented and passed on to you.
If you do not wish me to make any such changes you must notify me, in writing, prior to any repair work taking place.
All electrical equipment that is serviced by me is electrically safety tested prior to return to its owner. You will be notified (in writing) of any possible short-comings arising from that test.
Please sign below to signify that you have read the above and are in agreement with it. Thank you.

The customer gets the original; I keep a copy.

There have been the rare cases when an item is in such a dangerous condition (due to age or its intrinsic design) that I will refuse to attempt any repairs: the owner has then always been informed accordingly.

Al.

[Top Cap]

As someone who lived through the 50's as a kid, an Earth connection was unheard of, except that there was a terminal on Dad's radio that was marked for that but Dad only fitted an Aerial. Everything at home was run from one 15A two pin socket, smaller 5A plugs accommodated with a simple multi-way adaptor bought from Woolies. Even Mum's iron was plugged into the light using a bayonet plug. It wasn't until the arrival of the washing machine that an earth wire was run for it, connected at that time to the lead water pipe. Later of course, my elder brother and I re-wired the place to full IEE regs. But I often think of how many two-wired systems there must have been around in the 50's and no doubt a lot of radio sets were made with this in mind. I have seen far more horrors, at the shop I worked at, they would sell heaters for the nurses in a local hospital to cook with. These were small square metal boxes with an exposed spiral element positioned under a simple saucepan rest, made of just a few metal rods. The world really was a far different place then.

[AC/HL]

No win no fee was a disaster still waiting to happen then!

[Panrock]

Originally Posted by 1100 man

Some years ago, a friend, who is very conversant with old tech, had left a 1950's woody radio on downstairs in the kitchen. He had been using it for some years and it had never given any problems. He had been upstairs for several hours when he became aware of a strange smell. Going to the top of the stairs, he could see a wall of smoke. Taking a deep breath, he plunged down in to the kitchen, to find the radio pouring acrid smoke!

The mains transformer had developed shorted turns and overheated. Fuses are usually no use in this case .

With this sort of scenario in mind, I've always fitted 60 deg.C thermal fuses in the mains feed of transformered sets, with the fuse itself bonded to the transformer body.


Steve

[GrimJosef]

The huge improvements in automotive safety standards over the years (I started to list them, but to be honest there have been too many) have led to a large and well understood fall in annual road injuries and fatalities, especially when corrected for the much larger volumes of traffic these days.

But I'd be really interested to know if there's been any substantial fall in casualties brought about by more and more strict electrical/electronic regulations. If I had to guess I might guess that fires caused by electrical equipment are less common (perhaps a lot less common) these days. But I wonder whether real injuries and deaths from electric shock were once very common and are now very rare ? I suspect that in fact they've actually never been very common. Modern equipment is so safe that you can treat it however you like and it won't hurt you. Decades ago it wasn't, so you had to be a bit careful, but if you were it wouldn't hurt you back then either. So perhaps the major result of all the safety improvements, in terms of electric shock at least, has simply been 'dumbing down' ? I honestly don't know if there's any truth in this though. Does anyone ?

Cheers,

GJ

[Skywave]

Originally Posted by Top Cap

As someone who lived through the 50's as a kid, an Earth connection was unheard of, except that there was a terminal on Dad's radio that was marked for that but Dad only fitted an Aerial.

I remember that - and those days - well. As you and I will recall, that era was one of 'make do or mend', so often the minimum was done to get anything to 'work'. An aerial (nearly always a 'long-wire') was erected to get the radio to 'work'. (Even if it was only a bit of wire on top of the picture rail).
"O.K., now it works: that'll do. So why do I need an 'earth' wire? Mother, put the kettle on; let's listen to Mrs. Dale's Diary".
Indeed, the world really was a very different place then.

Al.