Dave Plowman wrote in message ...
In article ,
N. Thornton wrote:

The *prime* reason that BT
insist (or used to) on a transformer between external equipment and their
line is to prevent mains getting onto it.

Yup. There's a difference in consequences between mains on a phoneline
and mains thru your head. Consequently there is a difference in what
level of rish is acceptable, i.e. to what voltage the insulation
should be rated, and what other precautions should be taken.

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.

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.


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.


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.


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.


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.


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.


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.

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!

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.


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.


Regards, NT