Tim W wrote:
Dave Osborne
wibbled on Thursday 17 December 2009 19:37


By introducing an earth wire (or indeed armour/screening) into the
interior of the unit, you would then effectively make it a Class I
appliance. You would then have to make an electrical safety reassessment
of the product design. Depending on the nature of the product this could
be straightforward or it could be virtually impossible to achieve
without extensive design reworking.

Yes - I agree with all of that in spirit. But I don't make formal electrical
assessments when I build mains devices from scratch, other than common
sense, sensible design and good workmanship. I've applied more consideration
in my devices to making sure that a SELV part cannot reasonably contact a
mains part (due to wire falling off) than I've observed in at least one
appliance I took to bits once which had all the relevant markings on it.

I've built Class I and Class II appliances and applied the relevant
engineering considerations to both.

My argument is that in this scenario, we have introduced an extraneous
metallic part - yes. But that part is earthed by design. I see very little
that can go wrong.

OK - if we were selling this to the general public, one would expect to make
a formalised assessment. However, as always, this is a one off for personal
use. It is of course down to whether the OP feels happy with doing such
stuff (this is a DIY group), but being aware of the cautions given I think
he's got reasonable grounds to make a judgement.

I'm merely presenting the engineering argument "what could go wrong"?

(As I said, I intended the SY flex to connect between the mains supply and a
recepticle in the car - but as we're starting to discuss the case of
modifying a Class II device, then why not - it's interesting.)

If we reduce the argument further, noone should put a plug on a class I
device because they might wire it incorrectly?

I'll just add, this is a "friendly argument" AFAIC as there's no way for you
to deduce that from the tone of my tapping ;-

Cheers

Tim


No real problem with anything you've said, Tim.

Unfortunately I can't take the discussion much further as I've never
worked for a company which mass-produces stuff to Class II, so it's all
personal opinion and speculation from here-on-in ;-)

I do suspect that when you say you've "scratch built to Class II" (to
paraphrase), you should probably say you've "scratch built without
earth". I think that you couldn't reasonably argue that you've
scratch-built a Class II appliance because the standard will likely have
all kinds of subtleties like creepage, clearance, dielectric strength of
plastics, flame-retardance, etc, etc. which you will not have been in a
position to consider (you don't have full specs for the materials used)
or test for (you likely don't have access to a proving laboratory with
all the required equipment). Also, I suspect that some of the tests are
destructive, so essentially Class II is a type approval.

I agree that if you're "scratch-building without earth" then it is
reasonable to apply good engineering practice and common sense. Very
often you're not reinventing the wheel, so you can confidently follow
standard practice. I would, however be very cautious about dismantling
an existing Class II appliance and re-engineering it because it may not
be obvious what abominations have been made to the design as part of a
value-engineering exercise which would not have been considered had the
item been an un-enclosed or Class I product.

For example, if I were building a piece of equipment into a 19" rack
enclosure and I had an SMPS which happened to be Class II, I would be
inclined to leave it in its own plastic case and install it in the rack
enclosure as a "black box" taking care to interface to the existing
inlet and outlet connections and fixing it down with (say) a cable tie
rather than screwing into the case.

If OTOH the SMPS was Class I, I would consider dismantling it and
discarding the case (particularly if I needed the physical headroom). In
this case I might well discard the inlet connector and the outlet cable
and wire to it as if I bought it as an unenclosed product.

Either way, the "whole thing" would be a Class I appliance.

Cheers,
DaveyOz