In article ,
N. Thornton wrote:
Let me see. You hold a phone receiver to your ear. And you do what with
headphones?

1. A telephone is satisfactorily insulated from the head, it is
designed to safely take not just ringing voltage but also voltage
spikes resulting from switching and inductance present in phone
systems. Headphones have no such rating.

And you've tested all available headphones, or have such data to hand? Or
are only guessing? And guessing what type of headphones the OP is going to
use?

2. The telephone system is designed to protect the phone user from
lightning strikes. This was introduced in 1930 in the UK due to deaths
from lighning strikes while subscribers were using their phones.

Quite a difference.

More straws. Perhaps you'd tell us all what precautions are taken with a
Hi-Fi system provided with a headphone jack where that system can be
connected to an aerial which might also be hit by lightning?


I also dont believe you can count on such devices to have
sufficient tested breakdown voltage for this app. I haven't
changed my mind any so far.

Well, if you can't trust a device made specifically for this
purpose,

wrong purpose.

Err, you'll just have to take it from me that an audio isolating
transformer designed for line isolating purposes will be perfectly
satisfactory for headphone use from the safety point of view.

If you showed you were familiar with and understood all the issues,
I'd consider it.

As oppose to thinking up the most outlandish situations possible? Do you
also think things should be proof to an invasion from Mars?


then I'd suggest you only ever use a battery operated self
contained headphone system - because there's just as much chance -
or more - of the mains transformer inside an isolated chassis set
breaking down and putting mains where it shouldn't.

A fair point to consider, but I'm going to disagree. Mains stereos are
either Class I, in which case the headphone socket is safety earthed,
or theyre class II, in which case a) the insulation is very highly
rated, and b) potentially dangerous wires are tied so that if one
comes loose it still cant touch anything and electrocute.

There are probably still some radios around and in use with a live
chassis, so the same would apply to modifying those.

True, only for live chassis ones. But antique radios with live chassis
are both rare and easily recognised, whereas live chassis TVs are a)
not recognisable, and b) common. Plus there is the EHT in TVs too,
circa 20,000 volts.

So what?


TVs designed to be
connected to external equipment will follow the same safety guidelines as
stereos.

yup, as far as their external ports are concerned. The big problem
comes when you start home modifying equipment that is not designed to
be so connected.

If it's done incorrectly. Hence the recommendation to use a suitable
isolation transformer...


With a live chassis TV and a phone transformer,
a) you dont have good enough insulation rating between mains and head

Care to prove that? A suitable transformer is flash tested to *way* above
any likely voltage.

Agreed, but there are also many that aren't suitable. Which would the
OP end up with? Who knows.

If he buys a line isolating transformer *as such* it will be fine. How
many times do you have to be told this, FFS? Not a valve filament
transformer, or an LOPT, but a line isolating transformer. It's its whole
purpose in life...


b) you dont have any of those backups inherent in class I or class II
either.

Which is why you use an isolating transformer...

Class I and II kit has an isolating transformer in them, in the power
supply, AND those additional safety measures. An iso alone, as you
propose, is not comparable at all.

Yes it is, as an interface. This is how you make safe a piece of untested
mains equipment - by using a mains isolating transformer.


c) with a percentage of home installs you'll also have inadequate
precaution against EHT arcing to the socket.

Eh? Why do straws and clutching come to mind?

EHT arcing is a routine fault in TVs. Maybe you haven't done a lot of
work on TVs. It is not a rare event, it is a routine fault.

Arcing to the audio output? You must have some very strange faults to deal
with. And you're saying that life threatening currents could be present
when this happens?

If you put a headphone iso in a TV it needs to be well away from the
EHT, or screened from it. Unknowing persons could end up installing it
next to the EHT lead. Cringe!

Sounds like you know all about this.

If you might get bitten via your finger, its no disaster, but if you
might get 20kV thru your head, its a whole different matter.


The result is a much higher level of danger, too high.

I wonder if you take the same precautions as you think necessary here in
everything?

Far from it, but I do take it seriously when someone discusses leading
the mains to someones head via a series of totally inadequate
insulation and safety precautions. In most other matters I wouldn't be
nearly so concerned.

If you look into medical equipment you'll find the legally required
minimum safety standards are higher than those I've been discussing here.

Just what has this got to do with it? Or are you suggesting that a TV
supplied by the makers with a headphone outlet is built to medical
equipment standards of safety?


If so, I wonder why you're reading a DIY group - you'd not be
able to even lift up a screwdriver.

In all honesty I dont think you really appreciate the issues. I am
qualified in this area. You have a lot to contribute, and I will be
glad to take your advice on matters you know well, just not on this
one.

I'm not going to bandy qualifications with you. but I've been around
taking audio feeds off unknown equipment for many a year with a prime
regard to the safety of those using or handling it. And if your ideas on
safety were implemented, no electrical instrument would ever be used in
broadcasting or recording.

--
*Someday, we'll look back on this, laugh nervously and change the subject

Dave Plowman London SW 12
RIP Acorn