[chbot] Why galvanic isolation is important

[Charles Manning]

Apart from anything else, Volts don't kill. Current does.

I've been struck by lightning, thankfully through quite a high
inductance path, and nothing bad happened to me.

What really amazed me when I came to NZ twenty something years ago was
the lack of RCDs. In South Africa all houses are always fed through
RCDs. IIRC, that was mandated in the early 1960s. If that lady had
been charging her ithing through an RCD she's be fine now.


Risk management is surrounded by stupid polices because we have an
irrational understanding of risk. We are fearful of modern risks "What
if a UAV falls out the sky?" "What if the kids swallow powerful
magnets?" yet happily have matches, kitchen knives, blenders and
electricity in our houses. These were thankfully invented before
people became pathetic. You have a 100% chance of dying from
something.


On Mon, Jun 30, 2014 at 9:08 PM, Peter Harris <petes.username at gmail.com> wrote:
> Hi Volker
> I would like to thank you for bringing the issue of counterfeit SSRs to the
> club's attention, had you not alerted me I would merrily have installed the
> device putting myself at risk of electrocution and my house at risk of
> uninsured fire. Please be assured that I do take this subject seriously, I
> have had more than enough shocks and dealt with enough magic smoke in my
> career not to take unnecessary risks.
>
> My research into counterfeit Fotek SSRs indicated that although the units
> "work" the switching devices within the units are underrated and tend to
> burn out.
> Testing using a 2.2kW load with no heat sink on the SSR suggest that the
> device is capable of handling the required 3.5kW. Hopefully the isolation
> testing to be carried out at the next meeting will also give a good outcome.
>
> The final design will have no external low voltage connections so if mains
> should appear on the isolated side of the SSR it will be enclosed within the
> case. Grounding of the DC supply should cause an earth leakage trip under
> these conditions.
>
> I look forward to further advice and discussion.
> Best regards
>   Peter
>
>> Regarding Peter's SSRs it was good to see that people started to see the
>> isolation between the SSRs control inputs and load terminals as safety
>> critical, and thanks Mark for offering to bring in your isolation
>> tester. However I see this only as a good start, measuring isolation
>> once I don't think is really good enough to determine whether it will be
>> safe to touch the control terminals in the future. That approach may be
>> good enough for sparkies and copper cables, but integrated circuits
>> don't quite behave like that. Unfortunately there was a lack of interest
>> in discussing that further. The issue isn't restricted to SSRs, it shows
>> up much more often in stuff we fiddle around with.
>>
>> Volker
>>
>
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