Live chassis.

[Synchrodyne]

It's not a consequence of 'live chassis' though in practice they often go together, live chassis and half-wave rectification.

Agreed.  But the antipodean aversion to live chassis equipment was developed during the valve era, when half-wave rectification was probably universal.  (Somewhere there might be an exception – any bids on that….)  

But more recent equipment - TV's using switch-mode power supplies - may well use full-wave rectification direct from mains, chopper-regulated, supplying the rest of the circuitry without isolation. Admittedly, these will be built on a printed-circuit board and the supporting metalwork (as near a chassis as we'll get) could be either isolated, earthed, or connected to 0V line, which whizzes up and down according to the input bridge rectifier. Yet such a set would not have any DC component in its input current waveform.

By the time that type of equipment was on the scene, double-wound power transformers were pretty much de rigueur here.  I doubt that any setmaker would have ventured back into live chassis techniques at that stage.

The safety aspects of 'live chassis' are well known, of course, and it would be interesting to hear if Australia and NZ steered away from them in favour of the more expensive isolating transformer on safety grounds!

I’d say that safety played a big part in the avoidance of live chassis equipment, as well as the issue of putting DC on the (AC) mains.

I don’t know just how the NZ safety regulations were written, but if they were compiled essentially from a safety viewpoint, then one would expect that all live parts would have had to have been inaccessible, and that any metal parts, such as the chassis, which were accessible via ventilation holes/slots would need to be earthed.  This probably would have mandated the negative busbar approach for any AC-DC receivers, which was in any event a better way.  

But what about DC installations - or were there none? Or did they adopt the American practice of isolated/earthed chassis with a live busbar?

There was very little DC distribution in NZ, which settled on 230 V, 50 Hz AC quite early on.  (It was standardized by the Government of the day in 1920, planning for the North Island grid having started c.1916.)  I think that Auckland and Wellington both had some small amount of reticulated DC back in the day, but for Wellington at least that might have been industrial only.  I’d guess that where AC appliances needed to be connected to DC supplies, then converters would have been used, whether motor-generators, rotaries, vibrators or inverters.


Cheers,

Steve

[Synchrodyne]

"With hundreds of AC/DC radios & TV's only taking a half wave of each phase on a large circuit it must have upset the electric board a bit. What did they do about it?"

DC on the mains may have been less of an issue in the USA, with its split-phase supply system.  With AC-DC receivers randomly connected to either side of a split-phase system, there would have been some net DC cancellation back at the centre-tapped secondaries of the supply transformers.

AC-DC radio receivers were I think quite common in the USA, and the standard valve range accommodated this with a 150 mA heater series.  Also, there were quite a few valves, such as the 6BH6, 6BJ6, 6AQ6 and 12AT7 that could “swing both ways”.  In the early 1960s, Sylvania introduced a range of 100 mA series-string radio valves for use in compact AM radio receivers.

American AC-DC TV receivers had something of a peak in the mid-1950s.  In 1954, the industry introduced a range of 600 mA series-string TV valves with controlled heater warm-up times.  In 1956 this was supplemented with a smaller range of 450 mA and 300 mA controlled heater warm-up time valves for use in the smaller and portable TV receivers.  But I have read that some makers, after trying AC-DC TV receivers, reverted to the isolated type after a few seasons.


Cheers,

Steve

[colly0410]

 With hundreds of AC/DC radios & TV's only taking a half wave of each phase on a large circuit  it must have upset the electric board a bit. What did they do about it?

I'm wondering how many house's/flat's on average are connected to each urban/suburban transformer in UK, Australia & New Zealand? I know America has fewer house's/flat's per transformer because of lower voltage & use a lot of overhead lines. I'm going to America tomorrow so will be looking up at electric pole transformer's & counting how many cable's go to the house's, might get some weird look's. 

[ppppenguin]

My experience of US electrics (mainly suburban NY and a few years ago but I doubt much has changed) was that houses are routinely fed with split phase 120-0-120. Not sure if loads are shared between the split phases within a house but the 240V supply is used for big appliances such as washers, driers and stoves. Pole mounted transfomers are the norm except in very urban areas. From what I saw of larger appliances many could be used on 110V, 240V split phase or 208V 3 phase depending on how you connected them.

Live chassis is still used after a fashion for very small items such as central heating timers. Typically a capacitor dropper and some kind of voltage stabiliser. All in a plastic box with any connections either explicity live or isolated to Class II standard.

Thinking about large appliances makes me wonder about the new standard for 13A sockets designed for electric car charging. I'll start another thread.

[Synchrodyne]

A good summary of the Australian situation in respect of live chassis TV receivers was provided in an article in Electronics Australia magazine for 1975 August.


  EA 197508 p.64.pdf (Size: 687.74 KB / Downloads: 4)

The key issue was erosion of metal pipework and other structures by parasitic DC flowing via the earth path.  Evidently by 1975, much of Australia’s supply system was of the MEN (multiple earthed neutral) type, and measured DC via the earth path was quite high.  Hence the avoidance of appliances that fed DC back to the supply system.

The same arguments would have applied in New Zealand, although I have yet to find hard documentation.  Virtually all supply systems in NZ are MEN, which has been the national standard since 1920.

Regarding US practice in respect of the loading of split-phase household supplies, at least basis the house I had built in Texas back in 1990, the lighting and receptacle loads were approximately split between the two phases.  A 240-volt outlet was provided in the laundry room for the dryer, and another in the kitchen for the oven.  I am not sure if the island cooktop also used 240 volts, as the wiring was plumbed-in.  The distribution system was evidently multiple grounded neutral, and the neutral was bonded to ground and the ground rod at the switchbox.  The incoming telephone and cable TV services were also grounded to the main ground rod where they entered the house.  In fact, grounding rules were quite strict.  The structures of outdoor antennas had to be grounded, as did the outer screen of coaxial feeders, such grounds either returned to the main ground rod or if using separate rods, the latter had to be bonded back to the main ground rod.


Cheers,

Steve

[tv tony]

The French made an isolated chassis set. It was the Emo, valves were a swine to get, there was the Thorn 2000 that had an isolated chassis, I don't recall any others. Bush A823, Pye 693\697 and Decca Bradfords all used mains transformers but HT was formed by simple half wave rectification. SMPS's made things complicated as the supply could be be made either isolated or otherwise. On a different (slightly) topic, we had an engineer who thought ALL variacs provided mains isolation. I know they didn't but were there any that did? I was new to the trade, early 70's and did my time with a top company but because I was new and thought green he wouldn't listen to me. He's dead now, probably electrocuted. Again, was there any that did provide isolation. I never saw one. Good forum by the way.

[Refugee]

Isolated Variac transformers were very rare.
They very quickly became full mains with a transformer after them to provide the required ratio.
You can buy a box with a Variac and transformer inside it. I have seen one with a 500VA 25 volt transformer that fed 25 amps to the low voltage output.
We usually fit a 1:1 transformer for doing vintage radios.

[Mike Watterson]

Inherently a basic variac is a tapped auto-transformer using a wiper on the winding with insulation removed. I never in 50 years of electronics heard of an isolated one, though I've seen some rated for 115V mains. You only had fixed transformers for isolation, 1:1 for workshops, 1:1 centre tapped for 120V for bathrooms, if 240V in (< 20W), and centre tap to earth 110V on building sites.

[Refugee]

I did once see a photo of an isolated Variac.
It had a long E and I core with a primary wound on in several layers with a single layer low voltage isolated secondary over the top with a wiper on it.
They are very rare beasts indeed.

[Kalee20]

I've got one somewhere - 0 to 10V it is.