Or even worse, have dielectric losses ....
G.
--------------------------------------------------
From: "Alan Melia" <[email protected]>
Sent: Monday, September 20, 2010 12:41 PM
To: <[email protected]>
Subject: Re: LF: RF and Combustion tests, and sparking, and TVI
Of course on of the problem is likely to be that flame retardants will
probably be banned under RoHS :-))
Alan
----- Original Message -----
From: "Scott Tilley" <[email protected]>
To: <[email protected]>
Sent: Monday, September 20, 2010 9:27 AM
Subject: Re: LF: RF and Combustion tests, and sparking, and TVI
Hi Andy
In Canada, to obtain a field electrical safety approval for an 'unknown'
polymeric or other non-metal enclosure the material must pass the
following test:
Using a paper match or butane cigarette lighter apply the flame directly
to the material at its most vulnerable spot for 15 seconds then remove.
If the material continues to burn after 5 seconds it is considered a
fail, if it self extinguishes within the 5 seconds it is a pass.
The above is good enough for insurance underwriters here and is codified
in the CSA "SPE-1000 model code for the field evaluation of electrical
equipment." I would suggest it be used in cases where fire is a concern
for amateur purposes.
Full product certification flame tests are far more rigorous but the
above is considered acceptable for one of's and prototypes...
73 es GL
Scott
On 9/20/2010 8:08 AM, Andy Talbot wrote:
>
> I've carried out a few tests to see why the LF loading coil / ATU
> housing caught alight and I'm now just about certain it was caused by
> sparking from the aluminium foil used to seal the joins in the B+Q
> Garden Store box and not RF being absorbed by teh plastic housing.
> Combustion:
> Placing pieces of the plastic housing in a gas flame, they caught
> alight almost immediately and continued to self-sustain burn with a
> yellow flame. The fumes given off were almost completel odourless so
> I'm pretty sure the material is polycarbonate - which on a reasonable
> assumption based on its name, contains only C, H and O, and explains
> why it burns moderately well. I was rather surprised how quickly
> the material caught alight, and then how slow but steady the resulting
> flame was.
> Incidently, I've tried burning clear polycarbonate roofing sheet in
> the past, and it barely self-sustains - so I suspect the latter has a
> fire retardant added. They're not going to bother with that on a
> cheap garden storage shed..
> In a microwave cooker:
> After several minutes of exposure, the material wasn't even warm. I
> tried pieces of the black base material and the green plastic
> housing, so it looks as if this material is not going to suffer any RF
> absorbtion effects.
> The 600 Watt beacon transmission had been going for about 30 minutes
> (at 33% WSPR duty cycle) before I noticed anything, so it may well
> have started burning right at the start to end up with the result
> shown in those pictures.
> An incident in the past lends weight to the sparking idea. Just
> once, a few months ago, digital TV reception was suffering
> intermittent interference and blocking and I realised the interference
> was correlated with the transmission sessions of my 150 watt 503kHz
> beacon signal. Checking in the ATU showed (smelled) nothing out of
> the ordinary, and I put the interference down to "EMC Related issues"
> perhaps along the mains. TV reception was OK the next day and ever
> afterwards. On reflection, 500kHz signals have no real way of
> affecting UHF, so I am wondering if minute sparking was going on just
> occasionally, with the resulting harmonics enough to cause QRN at UHF
> - with perhaps a session of dehanced propagation contributing.
> Conclusions.
> Cheap polycarbonate (probably) garden buildings from DIY centres are
> suitable for RF housings PROVIDED there is no sparking / direct heat
> exposure.
> Don't even think of using metal tape without soldering or properly
> bonding all joints - especially the sort not designed for electrical
use.
> Don't set light to garden sheds
> Andy
> www.g4jnt.com <http://www.g4jnt.com>
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