Double Insulated - safe?

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I recently had to repair a stereo amp/receiver (big name brand,
approx 7 years old).  I was dismayed to find exposed solder tags,
bare pcb wire links etc carrying mains voltages.

I thought the practice of having exposed mains voltages inside
equipment was history.  In my 25 years of servicing professional
instrumentation (many brands) it was extremely rare to see
exposed mains under the hood.  But alas consumer electronics
don't seem to care.

More disturbing is that the above amp/receiver is not earthed
and displays the double insulated logo (two concentric squares).
So where's the so-called "double insulation" ?

As I recall, double insulation standards was supposed to mean
equipment had two levels of insulation - functional insulation
(e.g. the plastic coating on wires) and protective insulation (e.g.
a physical barrier between mains and secondary wiring).  The
idea being that even under catastrophic failure, the user would
still be safe.

In the amp/receiver in question, no attempt was made to separate
the mains parts from the secondary wiring.  All it would require is
for a mains wire connected to one of the exposed solder tags to
fall off and touch the nearby metal case and it becomes potentially
lethal.

If this is what electrical safety standards have now degenerated
to then I'm totally appalled.




Re: Double Insulated - safe?



"anon"
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** Very common in all sorts of equipment - service people have to be wary
and use an ELCB.


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** The correct name is "class 2" .

 The term  "double insulation"  is not a literal description.


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** For an appliance to meet "class 2" requirements there are many rules to
follow.

The most important ones relate to the AC supply transformer which must be so
constructed that no likely overload or failure can bring the primary and
secondary into contact.  Often this requirement can be met by adding a
thermal fuse to the primary so that power is shut off if ever the temp
becomes unsafe.

Usually , there is also fire proof and other high temp insulation fitted
between the primary and secondary windings.


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**  How much separation do you expect  ???

I bet there were at least two layers of plastic in every case.


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** Bet the wire ends are looped through solder tags prior to soldering.

Where an item is not likely to be subjected to high levels of vibration -
sleeving every cable termination is hardly essential.


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**  The fact is, the item you are whingeing about is not a "prescribed item"
so does not have to be inspected and approved by a registered lab before
going on sale.

The double square symbol is simply applied in the  (Asian ?) factory as
evidence that  "class 2"  rules have been followed and the user need to be
informed the device should not be earthed.




........   Phil



Re: Double Insulated - safe?



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This is still principle, but now (say the last fifteen years) there is also
reinforced
insulation.  That is one, homogenious (sp?) system that is equivalent to the
protection afforded by basic + supplementary insulation (double insulation).


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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.

There is no requirement for protection of service personel unless access is via
a
hazardous area.  Compare reaching into the bowels of a photocopier as opposed
to taking
the lid off the amplifier.  A trained service person is supposed to be able to
identify, either
visually, or by measurement, areas which are safe enough to handle. Guarding is
not
required if hazards are reasonably identifiable.



Re: Double Insulated - safe?



wrote in message
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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.

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.

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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" ?

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.

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




Re: Double Insulated - safe?



"anon"

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 **  An air gap is specified where there is no insulation  - ie PCB tracks
carrying  AC mains need to be separated from other tracks by a gap.

 Usually, internal AC leads are sleeved in clear plastic where they may
contact metal parts -  but this is not essential if the wires are coated in
a plastic of a sufficient thickness.


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  **  Yawn  -   blah blah blah .

   You are clutching at straws.


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  **  Tiny straws.


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 **  There is no hazard to warn a user about.  The internal parts that are
live in use are bleeding obvious to any tech qualified to work on mains gear
when it is powered up.

You are clutching at straws -  tiny ones.



...........  Phil




Re: Double Insulated - safe?


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Not
or

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.

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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.

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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.

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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.

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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.







Re: Double Insulated - safe?



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













Re: Double Insulated - safe?


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Hi Phil,

thats one of the funniest war stories yet....

Cheers
Terry

Re: Double Insulated - safe?



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Everyone has the right to safety whether it be user or repairer.
Other manufacturers have managed to design equipment without
exposed mains, why not consumer goods?

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Last time I looked at the Australian Standard (circa 1980) it required
segregation of mains and secondary wiring so that overheating or fire
wouldn't result in them coming into contact.

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No.  As I have indicated - numerous points of exposed mains (solder
tags, pcb links and pcb copper) with nothing to separate them from
the metal casing or secondary wiring other than air.

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AFAIK all electrical goods sold in Australia are subject to the relevant
Australian safety standards, irrespective of whether formal testing or
approval was required.


If anyone has any further information or encountered similar situations
with so-called "double-insulated" equipment, I'd be interested to hear.




Re: Double Insulated - safe?



"anon"
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 **   Blah blah  -   yawn......


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**  Pro gear is  FULL  of exposed bloody mains  -  you goose.


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


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 **  AFAIK,   that amp DOES comply.

 BTW

 I object strongly to the way you edited and selectively replied to my post
out of context in each case.




.......  Phil








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How would this qualify as "double insulation" ?

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Anyone reading this thread would have seen your original response
in full.  I disagree that anything said was "out of context".




Re: Double Insulated - safe?



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



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 **  All of it was.

  You are a pompous dope.


...........  Phil








Re: Double Insulated - safe?



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Nothing applicable there.

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





Re: Double Insulated - safe?



"anon"
 "Phil Allison"
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  **  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.  "



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 **  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








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I think you are mistaken about what double insulation can mean now.
If the PCB board is insulted to the case by plastic stands, this is the
first insulation. Then the second is the case.
So a TV can have live mains on the PCB with only the case stopping people
touching it because its double insulated. You will see this is practically
everything now.



Re: Double Insulated - safe?



"Dand"
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** Shame when the "case" is the metal box of a piece of hi-fi or video gear.

  BTW

  What rock did you crawl out from under ?



.........  Phil



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gear.

I thought you were a rock spider

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"FruitLoop"

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 **  No one here has any doubt that you are one.



...........  Phil



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Look in the mirror , thats if its not broken .

BTW since when did you constitute the whole NG ,  in your dreams ?


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Re: Double Insulated - safe?


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No layer of plastic lining the case under the pcb?

Bye.
   Jasen

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