"mick"

The basic principle of Class II is *NOT* to rely on the mains earth for
user safety.

Two reasons:

1. If the mains earth connection is missing anywhere, the equipment
still operates ( with no warning to the user) and you have no safety.

2. If the AC outlet or equipment plug is miswired, active can appear on
the earth conductor and you have a lethal situation.


The principle of Class II is that there is NO REQUIREMENT to rely on
earthing for user protection at all,

** Not correct.

There must be NO earth connection made to the item.

The familiar notice reads:

" Double Insulated Do Not Earth. "
-------------------------------------


A very nasty example of scenario "2" occurs when the equipment plug has
earth and neutral wires reversed - which goes unnoticed until the day it
is plugged into an outlet that has active and neutral reversed.


Agreed. It's only relatively recently that domestic wiring in the UK has
been using earth-leakage protection. Older installations (including my
own!) don't have it - in fact, I'm still on rewirable fuses here!
Consequently we tend to be careful about wiring plugs & sockets up
correctly. Even when properties are being refurbished over here, there
is no legal requirement to change any existing distribution boards etc.
unless they are damaged. Consequently there are a lot of houses in the
same condition as mine.


** Much the same situation exists in Australia, though it varies from state
to state.

Interestingly, in the USA it has been compulsory to fit safety devices to AC
outlets in bathrooms for decades.

Originally they used mechanical "core balance relays" instead of the modern
electronic version.


.... Phil

In article ,
Phil Allison wrote:
The principle of Class II is that there is NO REQUIREMENT to rely on
earthing for user protection at all,

** Not correct.

There must be NO earth connection made to the item.

The familiar notice reads:

" Double Insulated Do Not Earth. "
-------------------------------------

Which means then in amplifier terms it cannot be connected to a device
where the signal ground is connected to main ground?

Something wrong there...

--
*I want it all and I want it delivered

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
Phil Allison wrote:
** Much the same situation exists in Australia, though it varies from
state to state.

Interestingly, in the USA it has been compulsory to fit safety devices
to AC outlets in bathrooms for decades.

In the UK until fairly recently, the *only* AC socket allowed in a
bathroom was a shaver socket fed via an isolating transformer. Also things
like light switches had either to be outside the room, or of an insulated
type like a pull cord one.

--
*If I agreed with you, we'd both be wrong.

Dave Plowman London SW
To e-mail, change noise into sound.

In article om,
mick wrote:
Agreed. It's only relatively recently that domestic wiring in the UK has
been using earth-leakage protection. Older installations (including my
own!) don't have it - in fact, I'm still on rewirable fuses here!
Consequently we tend to be careful about wiring plugs & sockets up
correctly. Even when properties are being refurbished over here, there
is no legal requirement to change any existing distribution boards etc.
unless they are damaged. Consequently there are a lot of houses in the
same condition as mine.

It's pretty well always been the case that if an installation conformed to
the regs of the day, it won't have to be updated to later regs. However,
once you start extending or modifying it things become less clear.
Although straightforward repairs are allowed.

As regards the overall costs of running and maintaining your house, the
cost of replacing the CU with a modern one isn't high.

And, of course, in the UK you can replace it yourself (obviously if
competent to to so, but hardly rocket science.)

In OZ, it seems only pros are intelligent/skilled enough to be allowed to
do such things.

--
*The older you get, the better you realize you were.

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (Nutcase TROLL)"
Phil Allison

The principle of Class II is that there is NO REQUIREMENT to rely on
earthing for user protection at all,

** Not correct.

There must be NO earth connection made to the item.

The familiar notice reads:

" Double Insulated Do Not Earth. "
-------------------------------------

Which means then in amplifier terms it cannot be connected to a device
where the signal ground is connected to main ground?

Something wrong there...

** Absolutely correct.

Class II items should ONLY be interconnected with other class II items,
however with audio and video gear, everyone breaks the safety rule. The
hazard of doing this is an X rated topic that one must never mention.

Class II items are inherently safe until you connect them to a class I
item - so when that class I item develops a safety fault, the whole shebang
becomes a death-trap.

With a class I system, each item grounds the others so it is also inherently
safe.

In the world of live music, I have come mixed class I / II systems that are
very scary.



..... Phil

In article ,
Phil Allison wrote:

In the world of live music, I have come mixed class I / II systems that
are very scary.

In UK TV studios etc, before the days of RCDs etc, every single item of
unknown equipment - like say a guitar amp etc supplied by a musician - was
fed via a mains isolation transformer. Expensive and time consuming but
very necessary.

--
*I'm planning to be spontaneous tomorrow *

Dave Plowman London SW
To e-mail, change noise into sound.

On Sun, 03 Aug 2014 10:16:40 +0100, "Dave Plowman (News)"
wrote:

In article ,
Phil Allison wrote:

In the world of live music, I have come mixed class I / II systems that
are very scary.

In UK TV studios etc, before the days of RCDs etc, every single item of
unknown equipment - like say a guitar amp etc supplied by a musician - was
fed via a mains isolation transformer. Expensive and time consuming but
very necessary.

We now have the annual PAT test. In my lab once a year a chap comes
round and tests the integrity of the grounding of everything that has
a mains plug, from the 100 grand Network Analyser to the kettle in the
kitchen.

Once he has gone all the oscilloscopes get their grounds removed once
again so they can make floating voltage measurements and not introduce
ground loop mess into normal measurements.

It doesn't actually make anything safer, but the company lawyers like
it.

d

"Dave Plowman (Nutcase TROL)"

In the world of live music, I have come mixed class I / II systems that
are very scary.

In UK TV studios etc, before the days of RCDs etc, every single item of
unknown equipment - like say a guitar amp etc supplied by a musician - was
fed via a mains isolation transformer. Expensive and time consuming but
very necessary.

** Really ????

Bet the Beeb et alia stopped doing that when they nearly electrocuted the
first guitarist.

1. The most common error in AC plug wiring is Neutral-Earth reverse - the
item works and there is no big problem until you find an AC outlet that has
Active and Neutral reversed.

2. Add a 240:240V iso-transformer that carries the supply ground through to
the outlet and it becomes instantly lethal. Draw it out on paper if you
doubt me.

FYI:

Supply earth goes direct to one side of the amp's AC tranny, then the
iso-tranny's 240V winding and back to the amp's chassis. The amp will not
run - but the chassis is 240AC live.

Oooouuucchhhhhh !!!!!!!!..



..... Phil

In article ,
Phil Allison wrote:
In UK TV studios etc, before the days of RCDs etc, every single item
of unknown equipment - like say a guitar amp etc supplied by a
musician - was fed via a mains isolation transformer. Expensive and
time consuming but very necessary.

** Really ????

Yes really.

Bet the Beeb et alia stopped doing that when they nearly electrocuted
the first guitarist.

Sigh.

1. The most common error in AC plug wiring is Neutral-Earth reverse -
the item works and there is no big problem until you find an AC outlet
that has Active and Neutral reversed.

Then that device would simply not work.

2. Add a 240:240V iso-transformer that carries the supply ground
through to the outlet and it becomes instantly lethal. Draw it out on
paper if you doubt me.

That's why they didn't.
FYI:

Supply earth goes direct to one side of the amp's AC tranny, then the
iso-tranny's 240V winding and back to the amp's chassis. The amp will
not run - but the chassis is 240AC live.

There is no 'live' and 'neutral' on the output of an isolating transformer.

--
*Warning: Dates in Calendar are closer than they appear.

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (****ing Nut Case TROLL )
Phil Allison

In UK TV studios etc, before the days of RCDs etc, every single item
of unknown equipment - like say a guitar amp etc supplied by a
musician - was fed via a mains isolation transformer. Expensive and
time consuming but very necessary.

** Really ????

Yes really.

** ********.

Bet the Beeb et alia stopped doing that when they nearly electrocuted
the first guitarist.

Sigh.

** Go **** yourself - pommy ****head.


1. The most common error in AC plug wiring is Neutral-Earth reverse -
the item works and there is no big problem until you find an AC outlet
that has Active and Neutral reversed.

Then that device would simply not work.

** Neither would the user - ever again - pommy ****head.


2. Add a 240:240V iso-transformer that carries the supply ground
through to the outlet and it becomes instantly lethal. Draw it out on
paper if you doubt me.

That's why they didn't.

** Didn't what - pommy ****head ??


FYI:

Supply earth goes direct to one side of the amp's AC tranny, then the
iso-tranny's 240V winding and back to the amp's chassis. The amp will
not run - but the chassis is 240AC live.

There is no 'live' and 'neutral' on the output of an isolating
transformer.


** That is the reason it becomes instantly lethal - pommy ****head.

Cannot draw a simple schem to save you life - can you ???



..... Phil