Double Insulated - safe?

[Phil Allison]

"anon"
"Phil Allison"

** Last time I looked, air was a damn good insulator.

How would this qualify as "double insulation" ?


** Read my earlier post re " class 2 ".

Nothing applicable there.

** Then you are blind as well as a colossally stupid PITA.

Quote:

" The correct name is "class 2" .
The term "double insulation" is not a literal description. "

Look up a few references on the internet for an understanding
of what constitutes insulation for the purposes double-insulated
aka "class 2" equipment.

** Anonymous fools like you do not to tell me what to do.

Try reading a copy of the Australian Standard ( AS3100) that describes
Class 2 and "double insulation".




........... Phil

 

[David, not to be confused]

"anon" <invalid@nospam.com> wrote in message
news:437f5949_3@news.chariot.net.au...

A 6.0 mm air gap is all that is required to separate from the 240V mains.
Not
much is it, but as Phil pointed out, if there isn't any vibration, movement
or
contamination then a hazard won't suddenly appear.

If the standard specifies a minimum gap then it would need to be maintained
even under adverse conditions. That would mean either a physical barrier,
cable clamps etc to prevent movement.

As an example, in the case of where wiring terminates to a solder post, some
mechanical retention must also be used. Like hooking or wrapping the wire,
not just soldering (I think Phil mentioned this already). A barrier isn't
required
unless the spearation distance has to be reduced - in which case the separating
distance becomes the distance around the barrier.

BTW the equipment in question doesn't have even that separation - the two
mains wires (blue, brown) are lying over other secondary wiring and adjacent
to the transformer frame.

Insulated mains wiring lying over insulated secondary wiring would be OK if
both lots of insulation was rated for the highest working voltage. Most PVC
insulation used for the last twenty years is rated to at least 300V, including
extra low voltage wiring (its usually written on it). After all, its common to
see
mains and elv wiring in the same loom.

If the transformer frame is separated from 'safe' parts by 3.0 mm distance or
insulation as per the above, then its probably OK.

Even for "trained personel" working inside equipment where harazards
exist but are not identified or be easily identifiable could be considered
unreasonable.

Most (if not all) electronics service personel have received an electric
shock at least once in their career. Luckily most are not fatal - but could
have been. Would you consider such a state of affairs "reasonable" ?

Once the covers are off it is up to the 'trained personel' to apply their
knowledge and determine what areas are safe. Its not unreasonable
to expect trained people to exercise due care. There is a difference
between ignorance and negligence.

Safety laws don't specify what's "reasonable" so such things ultimately
have to be tested in court. Suffice to say, I wouldn't like to be an
employer, manufacturer or serviceman trying to defend a charge of
negligence.

There are legal tests for 'reasonable' but that could fill an entire
forum all on its own. However here are brief examples:

If a device has a mains cord that terminates in the equipment, it is
reasonable to expect the device to contain electrical hazards and
for a trained person to know this.

But, if a device is run from +24V DC, and has an internal inverter
producing 110V AC and 300V DC for use only inside the device,
then it may be unreasonable to expect a trained person to know
where the hazards are (UNLESS the trained person has had specific
training on that apparatus).

In the latter example, the internal hazardous parts may be
labelled or guarded to reduce the risk to service personel.

The stereo receiver in question has no points identified as being mains,
nor are there any hazard warnings inside or out.

The internal parts are not accessible to users and hazardous voltage
enters the equipment. A trained person would be aware that a danger
exists and through a combination of examination and measurement
determines areas that are safe. If you need tools to open it, and users are
not instructed to enter it, no warning is needed.

 

[Phil Allison]

"David, not to be confused with the other Davids."

Once the covers are off it is up to the 'trained personel' to apply their
knowledge and determine what areas are safe. Its not unreasonable
to expect trained people to exercise due care. There is a difference
between ignorance and negligence.

** The Sydney based " Jands Electronics " used to manufacture a high
powered stereo amp for sound system use called the SR3000. The designer
employed by Jands was Doug Ford, whom I knew fairly well and is my
informant here.

The SR3000 amp used a system of four DC supply rails for the output stages:

The voltages were: +150 +75 0 -75 -150

Also, to save weight and cost, the heatsink assembly was divided into
separate sections and isolated from the case - so the output transistors
were not insulated from the heatsink.

Yep, the various sub heatsinks were "live" at the four DC supply
oltages - with heaps of current available.

Aware this just might be a hazard to service personnel, Doug made sure the
fact the heat were live was boldly marked on some clear plastic covers that
directed air from the fans through the assembly.

But this was not *before* a very nasty incident in the Jands factory:

One morning, a number of freshly finished SR3000s were sitting happily on a
large trolley under going "soak testing".

Then, the boss of Jands ( David Mulholland ) decided to honour the factory
with his presence and inspect progress with the new whiz bang amplifier that
has just be put into production.

What do you reckon was the very first thing the boss did ?

What words do you suspect he immediately uttered ??

His loyal tech staff were all convulsed with hysterics.



.......... Phil