Switched neutral?

[matthewhouse]

Many modern appliances switch the neutral, often seen on simple appliances. Live wire to thermostat, neutral wire to switch for example. Another very common example is live to fuse, neutral to switch. It simplifies manufacturing.

The switch in these cases is not provided for isolation, just for functional switching, as is the power switch on a radio. The switch on a socket outlet however is required to switch at least the live, as it could quite reasonably be used for isolating an appliance from the mains supply.

There is very limited danger from using serviced radios in normal use, with the back fitted and knobs in place. I wouldn't risk poking about inside an appliance that is plugged in, vintage or otherwise. It would be far easier to get a shock from an easily accessible toaster element or a bulb socket with the bulb removed, than from one of the radios. Even if the chassis is at mains voltage.

I'm more afraid of those dodgy Chinese mains adaptors...

[David G4EBT]

Don't know where I read it, but as I understand it, one possible reason for switching the neutral in AC/DC receivers is that switches (and relay contacts) when used on DC have about half the current carrying capacity than on AC and are prone to arcing, so by placing the switch in the neutral line, the effect of this is reduced. Don't know how much truth there is in that - I'm surprised that there isn't really a definitive answer - only speculation.

Attached is a portion of the circuit of a Portadyne Princess TRF set (Also the same as the 'Noble' TRF). I think these simple little sets, which perform surprisingly well, also appeared in kit form as surplus stock in the early '60s (Premier Radio? et al). The area highlighted on the circuit will remain live when the set is switched off, but as has been said, so long as no-one takes the back off and starts poking around, maybe that's no big deal.

I restored two of these sets and in both instances the switch was open circuit, which is problematic as the volume control, which also controls the reaction, has an anti-log track. Unobtainable, albeit Omeg would have made some for me, though with a minimum order level beyond what I would have needed. I dismantled the pots and managed to clean the switch contacts and reassemble them, then changed the wiring to switch the live. (The under-rated droppers had also burnt out in my sets and I had to replace them, but that's another story not relevant to this thread).

AS to safety shortcomings in older sets, I always put safety first, functionality second, and originality a poor third.

[ukcol]

Originally Posted by David G4EBT

Don't know where I read it, but as I understand it, one possible reason for switching the neutral in AC/DC receivers is that switches (and relay contacts) when used on DC have about half the current carrying capacity than on AC and are prone to arcing, so by placing the switch in the neutral line, the effect of this is reduced.

The current in the neutral will be the same as that in the phase or live whether or not the supply is DC, placing the switch in the neutral line, will not have an impact on contact wear.

Originally Posted by David G4EBT

The area highlighted on the circuit will remain live when the set is switched of........

....and most importantly the chassis will be live.

[David Simpson]

Its been my experience that most householders have a misconception about 13A 3 pin plugs. They somehow assume that every appliance just needs a brown 13A fuse. Most of the old radios which us guys work with will only draw less than 1A from the ac mains. Just a few hundred mA in fact. Therefore relying on wall socket connections isn't always that safe.
No doubt many of our vintage fraternity have their work areas/shacks/etc. out in garages or sheds. Does everyone have their supplies protected by 20mS RCD's(ELCB's) & fast acting MCB's ? I doubt it. I further doubt that many folk have dedicated workbenches protected by mains isolation transformers, and constant monitoring of V & I being supplied to suspect sets.
In view of this, why take further risks by following Mr Cole's, Mr Bush's, Mr Murphy's, etc. 1940's risky attitude to safety - live mains potential-wise ?
What happens to all the sets which pass through our hands ? We cant all hold huge collections. They get flogged on, probably thousands annually, - through this & other forum sites, though eBay, BVWS sales & auctions, and so on. How many times do we see labels warning the un-wary that some chassis' might be live? Very little, I suspect.
The vintage radio fraternity is aging, and probably shrinking. We need new blood - keen young folk to share & enjoy working with these lovely old sets & equipment. But I say to these youngsters - if some old codger tells you its safe to connect live mains to a chassis - tell him to sit on a screwdriver!

Regards, David

[Hartley118]

Originally Posted by David Simpson

........Certainly in Scotland, where PME LV & HV distribution conditions are just about universal - Neutral will always be at Earth potential in folks houses. So no need to fanny about with rf bypass capacitors.........

This rings true for me. I can't honestly say that I've ever found any useful interference reduction from an RF bypass capacitor on the mains input of a set or audio amplifier. Yes I suffer lots of interference on MF, some of it no doubt mains-borne, but a simple bypass capacitor doesn't seem to help at all.

Has anyone else had a more positive experience?

Martin

[Station X]

There's no PME here. No company earth terminal. You have to provide your own earth spike. There's about 5VAC between neutral and earth.

[ColinB]

Yes, in my (admittedly limited) experience of electrical installations, TT earthing is very common in rural locations. My parents place certainly has it, built in around 2003.

The normal setup is two overhead mains, an earth spike and a master 100mA RCD, with downstream 30mA RCD protection and then MCBs for the domestic circuits.

This would be the modern way, the older fasioned way was a voltage operated cct breaker for the whole installation, followed by rewireable fuses for each circuit. Many installations setup like this though are well overdue some extensive rewiring by now though... Be careful out there!

[Station X]

Yes. My house dates from 1929 and when I moved here in 1983 it still had an overhead electricity supply even though it's in an urban area. I think this dated back to the time when electricity was supplied by the tramway company. The wires were carried on steel poles and there were fuses in both sides of the supply indicating the original supply had been DC? There was also what I think was an ELCB. Earth was a thin red wire connected to a hot water pipe.

I eventually replaced the multitude of fuse boxes with a new consumer unit complete with RCD, ran an earth to a spike and bonded all services. You were allowed to do that in those pre Part P days.

We were converted to underground distribution about 15 years ago, but almost everyone in the street still has their main fuse and meter upstairs.

[David Simpson]

I do know that for many years now, under the terms of the "Buildings, Scotland Act" the IEE Regulations are mandatory up here. It would seem that you Sassenachs are getting a poorer deal down there.
Seriously though, it has been considered by many experts in electrical engineering that 30mS(the period of 1 & a half p - p waveforms of 50Hz mains) is the maximum anyone's cardio-vascular system should be exposed to. And 30mA of ac current, which is still a hefty whack. Don't forget that the p - p of mains waveforms is nigh-on 600V!
If folk cannot afford to upgrade their complete premises, then wee plug-in RCD's can be purchased cheaply from DIY stores. Then use them on your workbench. Workbenches can further be protected by installing individual fuse holders which will take fuses rated down to 60mA(Slow-blow, Anti-surge, & so on). They can be bought from RS or Maplins.
Anything that keeps yourself & your workbench as safe as possible will be welcomed by all of us, and all your dear ones.

Regards, David

[Station X]

I think IEE regulations are mandatory here too, but they're not applied retrospectively. Installations get extended or replaced from time to time and they have to meet the current rules. For instance two RCD's are now fitted, so that a single fault doesn't plunge the whole house into darkness. All consumer units must now be flame proof or enclosed in a flame proof enclosure. So the consumer unit installed in my workshop two years no longer meets standards, but I'm not going to replace it.

I have seen some dodgy advertising from electrical contractors suggesting that all plastic consumer units must be changed, be that's untrue.

[hannahs radios]

We have a TT system here the earth is the lead cable sheath also since we had a new fusebox the water and gas is bonded too. I would always modify a set to switch the phase line or if that's not possible add a warning notice on its mains lead to remind me I never leave vintage sets connected to live mains when I'm done all workshop power is off well except the small electric radiotor to keep damp out

[matthewhouse]

That's TN-S.

[hannahs radios]

Oh OK thanks for that I can never get my head round what's what

[BigClick]

I am working on a Regentone A.155 and the mains is switched on the neutral.

From what I have read so far i am thinking of doing the following....

Change the flex to a three core and connect the earth core to the chassis
Swap the switch from neutral to the live
Fit a 1amp fuse inline with the live input
Disconnect the external power output to "Gram"
Disconnect the external voltage selection strap (connect direct to the 230-250)
Maybe add equipment protection with a seperate 2amp fuse for each HT leg

I am not bothered as regards rear panel authenticity, I just want to ensure its safe because it is a present for my daughter and her family.

Can anyone comment if this is the wrong way to go?

[Herald1360]

Can of worms time again!

Restore it to "as new" functionally and fit a dedicated RCD plug to the two core lead. The two core lead can always be connected to give live side switching- that's no different from the 50/50 chance using the set as original from a two pin socket. Anything "better" ie modifications to the design will open that can in the event of any problem. An earth may be plugged into the E socket of course.

The sets were safe enough when they were new that no one killed themselves using them normally and the RCD will improve the situation under fault and poke about inside conditions which presumably your sister would leave to you!

My comments on...

From what I have read so far i am thinking of doing the following....

Change the flex to a three core and connect the earth core to the chassis Yes
Swap the switch from neutral to the live That will lead to more hum (live being near the volume pot) and anyway both neutral and live are considered 'live'.
Fit a 1amp fuse inline with the live input Yes
Disconnect the external power output to "Gram" Yes
Disconnect the external voltage selection strap (connect direct to the 230-250) Yes
Maybe add equipment protection with a seperate 2amp fuse for each HT leg Probably not worth it.

And secure/reinforce the back. Also educate the users about electricity and how to use it safely.

MM

[BigClick]

Originally Posted by Herald1360

Can of worms time again!

I hace looked and there are no worms

[Herald1360]

Ahhhh, just look for threads on safety of old sets and whether to try to bring them up to modern requirements for safety.


It's a legal timebomb set to explode shortly after anything unfortunate happens to a user of the set who isn't the person who modified it. It's an even bigger bang if it's been sold to a stranger

Have you a thick enough telephone book for your trousers?